Ionization Methods for the Analysis of Gases and Vapors J. E. LOVELOCK Nafional lnstifufe for Medical Research, Mill Hill, London N.W. 7, England
b This paper considers the general physical basis of ionization methods of gas analysis and the technique of the method, including the calibration and testing of detectors. The physical basis, scope, limitations, and construction of the individual ionization detectors are also discussed as are their applications to gas chrornatography and gas analysis.
U
physical methods for the measurement of the concentration of a test gas in a n inert carrier gas were, if universal, insensitive, or if sensitive, limited in application to a few substances only. The development of gas chromatography by James and Martin (16) gave rise to the need for a physical method of gas analysis capable of responding to all concentrations of gases that might emerge in the carrier stream from a chromatographic column. Such a device, or detector, was expected also to be simple, reliable, rapid in response, and to provide information in the form of an electrical signal suitable for the operation of automatic recording equipment. Following the stimulus of this challenging need, no less than nine entirely novel methods of gas analysis have been invented during the past few years. Although no one of them meets this challenge completely, they are sufficiently complementary to satisfy among them almost all of its needs. Significantly all of these nen- methods NTIL RECENTLY
Figure 1. chamber S.
M.
162
A simple ionization
Source o f ions Apparatus for measuring current ANALYTICAL CHEMISTRY
depend on the properties of ionized gases for their operation. The use and the discussion of the performance of these new detectors have so far been restricted almost entirely to the field of gas chromatography. Although this is a n analytical technique of the greatest importance, i t seems timely to consider the potentialities of ionization methods in chemical analysis generally. I n many analytical problems the quantitative measurement of a compound or element can be made dependent upon the production of some convenient volatile derivative. The extreme sensitivity and n-ide range of response t o different substances provided by ionization methods represent a revolutionary advance and make possible measurements which otherwise would have been exceedingly difficult or impossible. Furthermore, even where their performance overlaps that possible with earlier methods, the fact that ionization detectors generate an electrical signal directly related to the mass or concentration of a test substance is a great convenience in the design of automatic analytical equipment. PHYSICAL BASES OF IONIZATION METHODS
The fundamental physical process underlying the operation of all ionization detectors is the conduction of electricity by gases. At normal temperatures and pressures a gas behaves as a perfect insulator; if, howerer, electrically charged atoms, molecules, or free electrons are present, their free motion in the direction of an electrical field renders the gas conducting. I n the absence of conduction by the gas molecules themselves, the increased conductivity due to the presence of very few charged molecules can be observed and this explains the great sensitivity of ionization methods for gas analysis. The conduction of electricity by gases can be observed by means of the apparatus illustrated in Figure 1. It consists of an insulating vessel through which gas can be passed and which has two conductors supported within it. The vesselalso contains some source of energy capable of splitting the otherwise electrically neutral gas molecules into ion pairs-for example, a flame or a source of ionizing radiation. Thp electrical cir-
cuit external to the chamber is completed through a n apparatus for the measurement of current flow to a source of potential. All practical ionization detectors can be considered to be derived from this simple chamber. With all gases at low ion densities the relationship between current flow and applied potential with such a chamber takes the form shown in Figure 2. As the strength of the electrical field increases the current tends toward a constant level, the saturation current, corresponding to the collection of all of the ions generated within the chamber. At high field strengths the current again increases and tends to infinity at some finite applied potential. This is due to the additional production of ions by various processes which obtain energy from the intense electrical field then present. I n qualitative terms the relationship in Figure 2 is valid for all gases when the total number of ions present is relatively small. The exact relationship, and in particular the length of the saturation current plateau, depend upon the nature of the gas and on the density of ionization. The factors determining the length of the saturation current plateau are important, because practical ionization detectors are expected to provide a current directly related to the concentration of test gas present within them. This condition is most easily achieved %-hen the additional ions from the test gas are collected without loss or gain, or in other ryords, a t some point upon the saturation current plateau. Ionization detectors usually operate at atmospheric pressure, where the principal process causing the loss of ions a t 100
r
100
I0
'000
F
Figure 2. Relationship between current, l, and field strength, F, in a simple ionization chamber
low field strengths is the recombination of negative and positive ions to regenerate neutral molecules. Some additional loss of ions may occur by diffusion to the vessel walls or by escape in the moving gas stream; these processes only become a significant source of loss at greatly reduced pressures. The probability of recombination betlveen a free electron and a positive molecular ion is between 10: and loqless than between oppositely charged molecular ions. It follows that the estent of recombination in a gas a t low field strength will depend upon the nature of the negative charge carriers; if these are free electrons, as in argon or nitrogen. recombination is less likely than in air or oxygen where free electrons are soon captured to form negative molecular ions. Sharpe (56) gives the derivation of equations relating the extent of recombination in various gases and with different chamber constructions. For a chamber n-ith plane parallel geometry, such as in Figure 1, the fractional loss, f , of ions \Then air is the gas present is as follows:
W i t r e a is the coefficient of reconibination, S the rate of ion production, d the distance between the electrodes A- and A+-the mobilities of the negative and the positive ions, respectively-and V , the applied potential. The equation suggests that losses \Till be least when the chamber is small and when the highest potential is used, consistent with the collection of ions without gain by field intensified multiplication. In a free electron gas, such as argon, recombination at a given field strength is much less than in air. The great disparity, however, between the mobiiity of the free electrons and the positive argon ions gives rise to another effect which is equally potent in preventing the collection of ions at high densities. I n argon, a t high ion densities, the free electrons are so rapidly collected a t the anode that a n excess of slowly moving positive ions remains in the chamber. This escess of positive ions collects as a cloud, or space charge. near the cathode and exerts a potential in opposition to that applied to the chamber. At high ion densities the space charge may so iinpcde the collection of ions t h a t even in a free electron gas. recombination again becomes a serious source of loss of ions. The form and derivation of the equation for space charge limitation are also given by Sharpe (36) and for a plane parallel geometry chamber are as follons:
The variables are the same as those given with the reconibination equation above. The value of N, the rate of ion
production, is that at which the space charge reduces the field at the anode to zero. As with the recombination effect the extent of space charge limitation is reduced by using small chamber dimensions and a high potential. The relationship between current flow and field strength at different densities, with air and argon for the chamber in Figure 1 is shown in Figure 3. These two gases are representative of the negative ion and free electron classes, respectively, and illustrate the effects of recombination and space charge losses, respectively. With both gases the length of the saturation current plateau decreases with increasing ion density until a t some finite ion density i t vanishes. This upper limit usually determines the maximum current which can be dran-n from a n ionization detector consistent with a linear response to vapor concentration. The use of high potentials to reduce the losses of ions by the processes just described is itself limited to the potential at which ion multiplication occurs. So far i t has been assumed that the field applied to the gas is uniform; in practical devices, however, there is always the possibility of small projections or sharp edges on the electrodes. At such places the local field may be intense and cause ion multiplication a t potentials which otherwise are suitable for the bulk of the chamber. The avoidance of sharp points and edges in practical devices is obviously important where the widest range of operation is desired. LOWER LIMITS OF MEASUREMENT
At Ion- ion densities the properties of gaseous conductors are highly favorable for the collection of all of the ions liberated by the presence of a test gas. The ultimate limits of detection are, therefore, determined by the following factors: the efficiency of the process of ionization employed, the extent of interference by the ionization of the carrier gas, and the practical limitations of the equipment for measuring small currents. I n theory a perfect method in which all of the test gas and none of the carrier gas are ionized should make possible the detection of single molecules. I n practice, although “noiseless” amplification methods exist by which a single ion could be observed, no detection method so far has a background ionization of carrier gas less than IO7 ion pairs per second and none is able to ionize the test substance completely. For the present, therefore, the detection of single molecules is still some way off; it is worth mentioning, however, that methods already exist in nhich ions from the test substance can be separated from those of the carrier gas. There is no reason in principle why this ultimate goal should not be closely approached.
I n the face of the high background ionization of the carrier gas i t is not feasible to count the number of ions liberated from a test gas. The rate of production of ions from the testJ gas can nevertheless be averaged over an interval of time and measured as a current flow in a direct current circuit. When this procedure is adopted the smallest rate of ion production observable depends primarily upon the random fluctuations of the background ionization current. Ion production and collection arc usually simple stochastic processm and consequently, the standard deliation of the rate of ion producwhere t is the time tion, A?, is (1Yt)1‘~ interval for observation. Under ideal conditions, nhere there is no other source of fluctuation of the background ionization current, the smallest rate of ion production from the test gas n hich could be measured n ith confidence is that equal to (.Vt)1’2. Detectors rarely achieve the complete ionization of the test substance; the minimum observable number of molecules is therefore (>Vt)l* E \There E is the ionization efficiency of the detector. Table I shows the background rate of ionization, the efficiency, and the minimum observable number of molecules under ideal conditions for the three most sensitive detectors: the flame ionization, the electron capture, and the triode argon detectors. I n practice there are other and greater sources of fluctuation of the background current; also, amplifiers are unable to measure at a response time of 1 second currents smaller than 10-14 ampere (approximately 106 ion pairs per second). I n these circumstances it is doubtful if any measurements have been made to closer than 10 times greater than those given in Table I for the ultimate limits of detection. Even so the sensitivity of ionization methods is very much greater than that of other physical methods for gas analysis. The properties of a gaseous conductor which set the performance limits to ionization detectors have, for simplicity, been restricted to the few important
Table 1. Theoretical Limits of Detection for the Three Most Sensitive lonization Detectors Flame Ioniza- Triode Electron tion Argon Capture
Background ionization of pyre carrier gas ions per second 108 Ionization efficiency
108
10--5 io-*
Minimum number of molecules in 1 second 108
106
VOL. 33, NO. 2, FEBRUARY 1961
1010
os 2
x
10’
153
10'
-
5cm~1e inlet capillary t u b e re51stcnce1
10'-
IO'
m8xmg v o l u m e or chromatograph
colurri
'y detcc:cr
-
' inlet /
preuure .
I tor
I
d!lutior~o r
contomi n c n l s
carrier
n 1. tziectiometor
...
s ht e l dud .
...
.. . .
,,
...
J
?-)I
1;
recorder or
Figure 3. Relationship between current, I , and field strength, F, at ion densities indicated with each curve for a simple ionization chamber A.
With air With argon in chamber Variables are all expressed in arbitrary units
164
a
ANALYTICAL CHEMISTRY
-
d
T
Figure 4. Schematic diagram of apparatus needed for ionization methods of gas analysis
6.
properties common to all methods. Specific detectors exploit some single property of a gaseous conductor for detection purposes and these will be considered separately in the description of each method. There are, however, two other important points arising from the behavior of a gaseous conductor which can affect the performance of an ionization detector. I t has been assumed so far, that the presence of the test substance does not appreciably modify the ionization properties of the carrier gas. While this is true of most test substances, there are, for example, some halogen compounds whose affinity for free electrons is so great that even a t low concentrations severe losses by recombination may occur. With early designs of ionization detectors the presence of this class of compound led to a reduction in sensitivity and dynamic range. Improvements in design have gone far to overcome this effect, but where the measurement of the concentration of compounds such as carbon tetrachloride is contemplated, some restriction in performance must be expected. This drawback can, of course, be turned to advantage by using an electron capture ionization detector in which the effects of ion recombination are deliberately exploited for detection. The presence of the test gas may also alter the mobility of the free electrons, but this disturbance is usually important only at high gas concentrations. I t may, however, with argon detectors, set the upper limit of observable concentration before the space charge and recombination limits are reached. Ionization detectors are not usually sensitive to changes in gas flow rate. This is because the drift velocity of the ions is large compared with the linear velocity of the gas flowing through the detector. If, however, the drift velocity of either the positive or the negative ions becomes comparable with
b backing o f f
the linear gas velocity a severe disturbance of performance will occur. The conditions of operation under which this effect is most likely to occur are with low applied potentials or when the space charge limit is approached. TECHNIQUE
Ionization methods are rarely suited to the static measurement of gas or vapor Concentration. Most of them are wholly or partially destructive to the sample presented to them and several respond to the rate of mass input rather than to the concentration of sample. Even without these characteristics the static measurement of very lon gas or vapor concentration is exceedingly difficult because of their adsorption on to the surfaces of the apparatus. Ionization methods of gas analysis are, therefore, almost always dynamic and measurements are expressed in terms of the total charge of ions collected in the detector during the passage through it of a slug of test gas in a moving stream of inert carrier. With a dynamic technique of measurement, adsorption effects, so long as they are reversible, only delay the time of passage of the sample through the apparatus. In gas chromatography, itself a dynamic method, differences in adsorption are used to separate the components of a mixture. Static concentrations can be measured provided a sufficient volume is available t o permit the continuous or interrupted sampling in a flow of carrier gas. Figure 4 shows a schematic diagram of the equipment forming the basis of all ionization niethods of gas analysis. There follows a description of the nature and practice of the technique proceeding from the supply of inert carrier gas to the apparatus for recording the analyses. Supply and Purification of Inert Carrier Gas. The commonly used gases, argon, helium, nitrogen, and
hydrogen, are seldom pure enough for use with these sensitive detectors. Contaminants can be considered in three classes: First, those to which the detector responds. These are objectionable because they cause an increased background current. With the flame ionization and argon detectors which give a signal related to mass input of detectable substance, such contaminants also render them sensitive to small fluctuations of gas flow rate. The most frequent contaminants of this class are the hydrocarbon gases which are present a t low concentration in most commercial gases. Contaminants of the second class are those to which the detector does not respond but which adversely affect its response by their presence. These are even more objectionable since their presence is not immediately noticed but can lead to a reduction in dynamic range and sensitivity. A common contaminant of this class is water vapor, whose concentration varies widely from gas cylinder to gas cylinder and with the temperature and pressure of the gas. Contaminants of the third class are those 15-hich have no effect, as, for example, nitrogen, which is frequently present in considerable amounts in other gases. Contaminants of this class are not harmful. It is most important to reduce the concentrations of contaminants of the first two classes to harmless levels. In practice the inclusion of a trap containing 10 to 20 grams of hfolecular Sieve is usually sufficient. When measurements of extreme sensitivity are contemplated, a similar trap a t -180' C. in a bath of liquid nitrogen may be used. Contamination of the carrier gas can also occur by the desorption of gases from the surfaces of the apparatus, or by diffusion through the walls of connecting tubing; this is particularly prone to occur with rubber or soft plastic tubing.
It is good practice to use the shortest possible connecting tubing, to take scrupulous care to avoid leaks, and wherever possible to employ polytet,rafluoroethylcne (PTFE) for the principal surfaces of the detector and associated connections. This material is not only impervious but also does not adsorb contaminants or tcst substances upon its surface. With methods that are capable of responding to concentrations as lon- as one part in 10l2by volume the importance of chemical "asepsis" can hardly be stressed too strongly. Regulation of G a s Flow Rate. K i t h most methods, particularly those which respond to t h e mass of substance prcsented rather than to its concentration. the precise regulation of carrier gas flow is rarely necessary. Flon- rate is conveniently controlled by a conventional pressure regulating valve at t h e head of t h e gas cylinder, and a resistance consisting of a fern feet of metal capillary tubing. The small advantages conferred by relatively complex methods of precise flow rcgulation are oftcn offset by the inrrcased contamination of the carrier gas which may result from their use. Sample Introduction. With t h e single exception of t h e cross-section method of ionization detection, detectors do not respond faithfully t o high gas or vapor concentration. Between 0.01 and 1.0% by volume is usually t h c upper limit of their linear dynamic range. It is essential, therefore, to ensure the sufficient dilution of the test substance in t h e inert carrier before its presentation to t h e detector. 'The method of sample introduction chosen depends upon the analysis contemplated, the state of the test material, and the quantity available. Illustrations of practical methods appropriate to different analyses will be given in the section on applications. Measurement and Recording of Signal Current. A11 ionization detectors are iniperfect in t h e sense tjhat they g e n t w t e some signal in t h e presence of pure carrier gas. This background current ranges froin IO-" t'o 10~-* amlxw with the various devices.
E
II
Figure 5. Apparatus for calibration of detectors by exponential decay method Inlet for carrier gas Mixing vessel Magnetically driven stirrer Detector Bypass valve
A. 6. C.
D. E.
The maximum signal in the presence of vapor consistent n ith a linear response is in the range to lo-* ampere. The apparatus for measuring current, if it is to be suitable for use with all of the detectors, must be capable of responding faithfully to all currents in the range lop6to 10-13 ampereand must have some provision for offsetting the background current of the detector in use. In practice these needs are met by a wries of good quality resistanc~s connected across the input of an electrometer amplifier, as in Figure 4. The background current is offset by a variable potential applied to the basc of the resistances. The design and construction of amplifiers for this range of currents are an expert problem. and where this eypcrtise is unavailable, it is essential to employ commercially available equipment. For the best results aild R here a response time not shorter than 1 second is sufficient, vibrating capacitor electrometers are recommended. For rapid measurements in the rangc l o + to 1 second a vacuum tube electrometer is needed and this type of amplifier is generally suitable except for the measurement of the smallest signals. The complete current measuring equipment usually includes some means of automatically recording the change in current during the passage of the sample. Even more convenient is the use of an
integrating amplifiw, or of a separate integrator which ('an sum the total charge of ions liberabed by the presence of the test substance. Potentionietric recorders are nom available which can respond to a current as small as lo-'* ampere (full scale deflection 4 X 10-1" ampere). For most purposes this is sufficiently sensitive and their use effects a considerable economy. High Voltage Supply. Only t h e i i smallest currents are d r ~ ~by~ ionization detectors and t h e supply of potential, which ranges from 10 volts with electron capture detectors t o 2000 volts with argon detectors, presents no special problems. Det,ectors such as 6he flame ionization and cross section, which operate in the saturation ciirrent region, have a response which is largely independent of potential o w r a ~ ~ i rang(.. de With these, batteries arc' cntirelj. suitable. For the argon arid c,lcctron capture detectors, whose response varies with applied potential, variahlc and stabilized source of potential is necessary. For precise measurement t'he stability of the potential supply should be &0.5%, which is well within the capacity of available equipment. Temperature Regulation. The reactions upon which ionization detectors depend for their operation are not sensitive t o small temperature changes. T h e range of temperature over which they may be used is largely determined by t h e materials of their construct,ion. At temperatures over 300' C. otherwise efficient insulators become electrically or mechanically unsta1)le. At room temperature or lower, water or test vapor may condense within or without t h e detector and disturb its response. It is good practiw to install detectors in a dry tempcrature-controllcd environment somewhat above room temperature. The precision of trJmperature regulation required rarely needs exceed =t1.0'
C. Sources of Ionizing Radiation. Sealed souIces of a- and &radiation are readily available for use in those
Table II. Sources of Ionizing Radiation
Isotope, a-Radiation SrgO
Pm14'
13
11 3
P B
Haz2@
Ra-D
dpproximate Quantity, mC. for Current of 3 x 10-9 Ampere 10 10
Range of Radiation from Source in Air at N.T.P. Cm. 10' 20
Half Life, Years 2.5 2.26 12.5
Maximum .4ccepted Body Burden, N c . 10-3 10-3
1.0
Suitable Detectors for Argon, cross section Argon, cross section Argon, rlect.ron capture Electron niobility (direct)
20 to 50
0 . 5 to 1
P
0.03
2.5
1620
10-4
Argon
(Y
0.10
2.0
25
10-4
ilrgon
01
P
VOL. 33, NO. 2, FEBRUARY 1961
165
detectors which need them. T h e active element is firmly embedded in a strip of inert metal, commonly silver, The @-emitting elements most frequently used are: Srw, PmI4’, and Ha; a-emitting elements available in sealed sources are: radium and radium D. The choice of element depends upon the needs of the detector, the conditions of operation, and the radiation safety requirements and regulations in force. Table I1 shows the quantities of the elements listed above needed, in a typical detector, to provide a primary ionization current of 3 X ampere. Also given are the ranges of the radiation, the maximum acceptable body burdens of the elements, and the types of detector and conditions of operation under which each radiation source may be used. Measurement of Detector Performance. TIYO dynamic methods for t h e calibration of detectors have been reported and are briefly described below. EXPONENTIAL DECAY METHOD. This was reported by Lovelock (22) and the apparatus needed is illustrated in Figure 5 . It consists of a glass vessel approximately 200 ml. in volume, containing a magnetically driven stirrer. The vessel is supplied with carrier gas a t a constant known rate, and the effluent from the vessel is led either wholly, or partially, to the detector under test. Measurements are made by introducing into the gas supply a known volume of test gas and observing the change of current flow from the detector as the gas concentration in the vessel decays with time. Where mixing is complete and no adsorption of test gas occurs onto the surfaces of the apparatus the gas concentration, C decays with time according to the relationship:
c = C,exp
B
i
-(:)
bi-here V is the volume of the vessel, U the gas flom rate C, the initial gas concentration, and t the time elapsed after introducing the test gas. If the current from the detector is automatically recorded and the logarithm of the recorder deflection plotted against elapsed time, a perfect detector liould indicate a log-linear decay of gas concentration with a slope equal to ( V / C). The decay can be followed from the highest gas concentration down to the noise level of the detector and departures from a linear response may be seen a t any concentration nhere they occur The method provides in one experiment a means of measuring the ultimate detectivity and linear dynamic range of a detector. I t is the most convenient method for the assessment of the effects of design change on the performance of detectors. CALIBRATION BY DIFFUSION DILUTION. The exponential decay method just described is excellent for the determina166
tion of dynamic range and ultimate limits of detection with a few limited substances, usually permanent gases. It is not suitable generally for the calibration of detector response to different substances. For this purpose an elegant method has been devised by Desty, Geach, and Goldup (6). It is based upon the diffusion of the vapor of a test substance along a short length of capillary tubing into a stream of carrier gas. The apparatus is illustrated in Figure 6 and consists of a capillary tube approximately 5 inches in length. The tube contains the test substance in liquid form and there is a gap between the meniscus of the liquid and the open end of the tube. The open end projects into a mixing chamber through which carrier gas can pass on its way to the detector. The whole apparatus is immersed in a bath whose temperature can be maintained constant within 10.1’ C. The rate of diffusion of vapor is best determined by observing the rate of fall of the liquid meniscus. Although the rate of diffusion changes with the distance betki-een the meniscus and the open end of the capillary, the rate of vapor diffusion a t any instant of time can be determined easily from a few
ANALYTICAL CHEMISTRY
I
I I
I I I
C.
I
I I I
! Figure 6. Apparatus for calibration of detectors by diffusion dilution method A. B. C.
Inlet for carrier gas Outlet to detector Capillary tube containing test substance
simple measurements. The theoretical basis of this method is described by the authors above and shown to be independent of the diameter of the capillary tube. Notation a n d Expression of Performance Characteristics. The relationship between signal current and quantity of test substance varies widely with t h e different methods. Some provide a signal related directly, and others exponentially, to gas concentration; yet others provide a signal related to the mass of test substances delivered t o the detector in unit time independent of its concentration. I n these circumstances it is hardly surprising that the expression and comparison of detector performance are confused and so far no system of units has been suggested which is suitable for the comparison of performance between all of the available methods. I n the technique of gas chromatography and with detectors whose response is directly related to vapor concentration, Dimbat, Porter, and Stross (7) suggested the sensitivity parameter, S. This quantity relates the area of a peak on a chromatogram to the quantity of test substance eluted from the column and is expressed in the units nig. ml mg-l. Although widely used and of great practical value in gas chromatography, this unit is, as pointed out by Ongkiehong (SO), inapplicable to detectors whose response is related to the rate of mass input of test substance, such as the flame and argon detectors. Furthermore. it is inconvenient to calculate from it the performance of a detector for some application other than in gas chromatography. The performance characteristics to be used in this paper are listed below; they are chosen to make possible the comparison of the many differing detection methods. The important sensitivity parameter is derived from the charge of ions collected during the passage of 1 mole of test substance through the detector and is called the “apparent ionization efficiency.” For detectors whose response is related to vapor concentration this unit nil1 vary inversely n i t h the rate a t nhich the sample is passed through the detector. For this class of drtector the quoted apparent ionization efficiencirs are given for a flow rate just sufficient to allow a dctector time constant of 1 second. This assumes that the time constant of the detector is determined solely by the rate a t which the test gas is swept from the sensing volume and for most practical purposes u-ill be the highest sensitivity possible. It is also assumed that all sensitivity figures quoted refer to that part of the detector response where there is a linear relationship between signal and quantity of test substance. In ideal circumstances i t is possible
to observe a signal equal to or even less than the random noise level. Under practical conditions it is usually considered that the smallest observable signal is one equal to twice the random noise level. In the interests of consistency this conventional limit a-ill be used in the definitions of practical performance characteristics which follow. APPARENT IONIZATION EFFICIEKCY, E. The ratio of the charge of ions, in coulombs, collected during the passage of 1 mole of test substance through the detector, to that of the charge expected from the complete ionization of 1 mole of test substance (9.65 X lo4 coulombs) is ampere X sec. per mole E = 9.68
x
104
LINEARDYNAMIC RANGE,R. This is the ratio of the maximum signal current consistent with a linear response to within 301,,to that of the smallest observable signal current-namely, one equal to twice the random noise level. RANDOM XOISE LEVEL, N . This is the standard deviation of the mean backgrouiid current of the detector when containing pure carrier gas and when the time constant of the entire equipment is 1 second. I t is assumed that with a time constant of 1 second there is a faithful response to varying signals a t all frequencies between zero and 1 cycle per second. SMALLESTDETECTABLEQUAKTITY, &. That quantity of test substance txhicli when presented in 1 second gives a signal equal to twice the random noise level. I t is assumed that both the values of N and of Q are measured under identical conditions. All of these perforniance characteristics, escept the random noise level, vary with the nature of the test substance used. TTherever possible comparisons will be made for a standard test substance, propane. DETECTORS
Several different ionizing reactions may be used for the measurement of gas or vapor concentration and, as with other reactions, there are many possible ways in which each of them can be exploited. The art of designing ionization detectors is young and few of the devices available a t present can be considerrd to utilize the basic reactions in the best possible way. There has been a regrettable tendency to praise, or deny, the virtues of the various methods from comparisons of the performance of single detectors. This practice has been a disservice to some of the best methods and to the technique generally: the complexities of ionization phmomena make it only too easy to construct, from any of the basic reactions, singularly inefficient devices. The detectors described in this section are those thought to be the
of the method is adequately described by Otvos and Stevenson (SI). Briefly, the passage of energetic radiation through a gas in an ionization chamber produces a steady concentration of ion pairs, i, according to the relationship : KP i--ZzQ RT
Figure 7. Ionization crosssection detector A. B. S.
Carrier gas inlet Gas outlet Source of ionizing radiation
best possible practical versions and in the comparisons among them it is understood that the comments refer to the device described and not necessarily to the reaction upon which it is based. Wherever possible comparisons TT ill be made to illustrate the function best performed by each device and to show that while none of them are perfect, their performances are complementary. Detectors are simple and inexpensive devices compared with the amplifiers, recorders, and other equipment and can easily be interchanged. It is good practice, therefore, to choose the detector best suited to the problem in hand rather than to try to force performance from an unwilling device. Cross-Section Ionization Detector. The measurement of gas and vapor concentration by the absorption of ionizing radiation was first suggested by Pompeo and Otvos (33) and several practical detectors based on this method have been described [Deal et al. ( 5 ) ; Boer ( S ) ] . Although the first ionization detector to be used, this method still possesses conspicuous advantages over its successors. It is the only method capable of measuring gas concentrations up to 100% within the detector. I t can be used with any carrier gas and for the measurement of all gases and vapors, Furthermore, the response to any chosen substance can be calculated from the published values of the atomic cross sections of its constituent elements, Its only disadvantage is a low sensitivity which has rendered it unsuitable for use in some applications of analytical gas chromatography. This single disadvantage has tended to diminish interest in its other desirable characteristics. PHYSICAL BASIS. The physical basis
where z& is the total molar fraction and cross section for ionization of the gas mixture, P the pressure, T the absolute temperature, R the gas constant, and K B constant determined by the geometry of the ionization chamber and the intensity of the radiation. This relationship is only true so long as the range of the radiation in the gas is considerably in excess of the path length it traverses in the detector. If the path length in the detector were greater than the range of the radiation, little or no increased ionization could occur in the presence of a more strongly absorbing gas. In general the denser polyatomic gases and vapors are more strongly absorbing than a light carrier gas such as hydrogen or helium. When either of these is the carrier gas, other gases and vapors when also present provide an increased current in proportion to their concentration. Any carrier gas may be used, but a t the expense of some loss of sensitivity compared with hydrogen or helium. DETECTORDESIGN. A practical cross-section ionization detector is shown in Figure 7 with the materials of its construction and its dimensions. The considerations determining the design of an efficient detector of this class are as follom: The volume of irradiated gas should be large enough R ith a conveniently sized radiation source to provide a sufficient ionization current for precise measurement. The volume, however, must not be so large that an excessive time constant is introduced by the holdup of vapor in its passage through the detector. A reasonable con~promiseis a volume of between 0.5 and 5.0 ml. To avoid loss of ions by recombination or space charge effects as described in the first section, the ratio of the diameters of the chamber and of the anode it encloses should not be greater than 3 to 1. The radiation source should be one which emits ,%radiation with a mean energy in excess of 0.5 m.e.v. such as 81-90 or Pmlr7. Weak p-emitters such as H3or a-emitting sources are not usually suitable because of the short range of this radiation in gases a t atmospheric pressure. K i t h a source containing 10 me. of @emitting element, the masimuni ionization current likely to be o b w v e d is in the region of 10-8 ampere. Provided the dimensions are within the range given and the applied potential is sufficient (300 to 1000 volts), no errors VOL. 33, NO. 2, FEBRUARY 1961
e
167
Table 111.
Performance Characteristics under Practical Conditions of Operation for Detectors Described in Text
Cross Section 2 x 10-9
Flame 10-:
Argon 10-3 to i o -- 2
Apparent ionization efficiency 106 3 x 106 Linear dynamic 104 range 3 x 10-14 10-12 10-12 Noise level, 10-11 to 10-8 10-9 t o 10-8 Background current, ampere 10-1% 3 x 10-12 4 x 10-13 Minimum detect2 x 10-7 propane propane able quantity and propane test substance used, g./sec. 10 -11 10 -12 Minimum detectlo-' able concentration by volume Carrier gas HP Hz A Substances detectAll All organic Most organic and able inorganic Applications Precision anal. at low Analyt. gas hnalyt. gas chrosensitivity and pre- chromatog., particuparative gas matog. larly at extreme chromatog. sensitivity
-
bh+
168
ANALYTICAL CHEMISTRY
10-6
x 102 3 x 10-11 3 x 10-5
103
3
10- 1 2 3 3
x
10-8
x 10-1' cc1, 10-13
10-9
co2
10-6
Electron Mobility, Direct 2 x 10-6
x 103
3
10-13
3
x 10-10
10-I1
co,
10-8
A He -4 Halogen and 0 Permanent gases Permanent gases and compds. organic Qnant. and qual. Permanent gas Permanent gas anal. anal. anal. at high sensitivity
due to space charge or recombination intensively studied of the ionization effects are likely to occur. With some methods. Reports on its performance and some suggestions for improvement of the earlier designs of this detector small diameter anodes and low applied have been made by the following authors: Ongkiehong (30), Desty, potentials were used. In these cirGeach, and Goldup ( 6 ) , and Condon, cumstances severe recombination losses Scholly, and Averill (4). with strongly electron capturing comPHYSICAL BASISOF METHOD. The pounds would be expected and were a process leading to ion production in source of complaint. PERFORMANCE AND APPLICATIONS. flames of hydrogen and in hydrogenThe performance characteristics of the containing carbon compounds remains detector illustrated are compared with obscure. The flame temperature is too low to account directly for the extent those of the other methods in Table of ionization observed, approximately 111. The cross-section detector is seen to complement the other methods. one molecule in 105. Stern ($7) sugIts dynamic range coniniences a t the gested that thermionic emission from carbon particles present in hydrocarbon higher concentration levels mhere most flames might account for the extent of other detectors are approaching the ionization observed, and this theory upper limit of their response. It is best suited to precision measurement has been extended to explain the where large quantities or high concenoperation of the flame ionization detector. While some ionization might trations are available, such as in the occur from this effect in a rich hydroanalysis of permanent gases and in carbon flame, i t seems most improbable preparative gas chromatography. The in a weak mixture-for example, methconstancy of the response of the detector anol in hydrogen; furthermore, carin the face of changes in flow rate, applied potential, and contamination make it excellent for use with automatic -I COAXIAL SOCKET analytical methods. P T F E I N S U L A T I N G R I NG Flame Ionization Detector. A hydrogen flame burning in air, or oxygen, AIR O U T L E T produces few ions; the introduction of a volatile carbon compound into the flame, however, greatly increases t h e production of ions. This effect COLLECTOR was applied t o t h e design of a detector GLASS WINDOW by hIcWilliam and Dewar (18) and by Harley, Nel, and Pretorius ( I S ) . The PLATINUM JET flame ionization detector has proved t o be one of the simplest and most - SI N T E R E D D I S C satisfactory of the sensitive ionization detectors. The attractions of its simAIR I N L E T plicity of construction, stability of brass performance, and disregard of the nature of the carrier gas used, or of the PTFE presence of such common contaminants Figure 8. Flame ionization detector as mater vapor, have made i t the most
4
Electron Mobility, Indirect
Electron Capture 10-8 to 0 . 3
bon particles could only be formed in a flame by a condensation process; in such a process the density of carbon particles would not be directly related to vapor concentration as is, in fact, the ion density. -4 more probable explanation is the pooling of stored energy between the excited states of molecules, radicals, and free atoms on their collision in the flame. Cumulative ionization processes of this type have been described by Druywsteyn and Penning ( 8 )with other gases. DETECTORDESIGN.Figure 8 shows the flame ionization detector designed by Desty, Geach, and Goldup (6) with its dimensions and thc materials of its construction. -4lthough not greatly different from that described by the inventors of the method, the design shov-n has features which make it one of the most satisfactory practical versions so far. The hydrogen is supplicd to the flame through a small orifice in an otherwise solid block of platinum-iridium alloy. The heat conduction from the block to the metal of the chamber is adequate to maintain its temprrature well belon the level at which thermionic emission could occur. Care must always be taken in the design of flame ionization detectors t o avoid thermionic emission from metal surfaces. This effect is not only a potent source of noise but also leads to anomalous responses with certain compounds. The air supply for combustion is through a porous metal diffuser which provides a laminar flow of air within the chamber. The chamber of a flame ionization detector is always filled with air containing water vapor. From the considerations in the first section, therefore, the losses of ions a t high densities will be determined entirely by recombination effects, as both water
vapor and oxygen readily capture free electrons to form negative molecular ions. The general response of the detector to variations of ion density and of applied potential will be in the form shown in Figure 3,A. The response is linear with vapor concentration SO long as the ion density is not so high that a saturation current can no longer be established. With the design illustrated the upper limit of current flow a t which the loss of ions by recombination does not exceed a few per cent is in the region lo-’ to 10” ampere. PERFORMANCE AND APPLICATIONS. The general performance characteristics for the response of the detector to the standard test substance propane are listed in Table 111. The outstanding features of this detector are a wide linear dynamic range and a useful response extending to relatively high vapor concentrations, approximately 1.0%. The detector is also remarkably insensitive to the presence of such contaminants as air or water vapor in the carrier gas stream. The background current ~ i t hpure carrier gas is ampere) so very lou (10-l’ to that in spite of its low ionization efficiency (10-9, very small quantities can be measured provided an amplifier of sufficimt quality is available. The drawbacks and uncertainties concerning this detector are mostly in connection with its relative response to different substances. Considerable changes in design and operating conditions may have little effect on the dynamic range and sensitivitj of the device. I t appears, honever, that changes in geometry and in the flow rate and composition of the gases supplied to the flame altcr the relative response to different compounds. For precise analyses nith a giver1 detector it is necessary to calibrate for each compound and then ensure that the conditions of calibration are maintained throughout the analysis. The detector responds to all organic compounds except formic acid In general the response is greatest n i t h hydrocarbons and diminishes n ith increasing substitution of other elements such as 0. N, and halogens. I s TI ith other ionization detectors, thc response to highly halogenated compounds is further restricted by electron capture phenomena and the performance characteristics quoted are not applicable to this type of compound. Apart from the vapors of the elements in groups I and I1 of the periodic classification, the flame ionization detector does not respond to inorganic compounds. The detector will respond to mixed organic-inorganic compounds such as alkyl inorganic compounds. Where, however, solid particles are formed during the combustion of the test substance, excessive
ment, and the ionization of vapor molecules by the transfer to them of the energy stored in the metastable atoms. When an ionization chamber contains argon and a source of free electrons, the addition of vapor causes an increase in current flow, I , related to the vapor concentration, C,as follows:
Figure A. 6. C.
S.
9. Argon detector
Inlet for carrier gas Inlet for scavenge gas G a s outlet Source of ionizing radiation
noise and anomalous responses may occur. Argon Detectors. If the crosssection detector illustrated in Figure 7 is used with argon as the carrier gas and with a relatively high applied potential, the signal given by most substances is greatly magnified. The first version of the argon detector was no more than this. It served with the flame ionization detector to introduce sensitive ionization methods to gas chromatography. I t was soon found that this simple version suffered the drawbacks of a limited dynamic range and represented a singularly inefficient form of a class of detector with considerable potentialities for evolution. Recent advances in the theory and design of argon detectors were reported by Lovelock (92). There follows a brief account of the physical basis of the method and a short description of the best tried, and a t present preferred, version of the detector. PHYSICAL BASIS. Argon detectors depend for their operation upon two reactions: the excitation of argon to its metastable state by electron bombard-
where A , B, a, and b are constants, V is the applied potential, z the primary electron concentration, and y the initial metastable concentration. The derivation of this equation from the t m o basic reactions above has been described by Lovelock (fzS,2’4). Provided the recombination and space charge limits are not exceeded, the relationship suggests, and experiments confirm, a rapid increase in current with increasing vapor concentration tending to an infinite current a t some finite vapor concentration. Practical argon detectors always include some means of limiting this large rise of current with increasing vapor concentration. In the first versions a resistance in series with the chamber and the source of potential served for this purpose. In the recent versions the primary current and chamber dimensions are so chosen that a space charge is always presmt within the chamber, The greater ionization in the presence of vapor increases the space charge density and so neutralizes the tendency to an exponential growth of current with rising vapor concentration. With an appropriate geometry and primary current the negative feedback introduced by the internal space charge provides a linear dynamic range exceeding lo5. An alternative mode of operation is to choose a geometry and primary current such that no space charge can develop. If the detector is then connected to a source of constant current the potential across it will fall in a log-linear manner with rising vapor concentration. The production of metastables by electron bombardment and the ionization of vapor molecules by collision with the metastables are both efficient reactions. Each primary electron is capable of generating 104 metastables and the probability of ionization by collision with a metastable approaches unity. The high ionization efficiency of the more advanced argon detectors 10% has been observed) is, therefore, readily explained. The reaction is applicable t o all molecules n ith an ionization potential equal to, or less than, the stored energy of the metastables (11.7 e.v.). The response to different substances is primarily determined by the frequency of collisions between the test molecules and the metastables and the general factors affecting this have bern deVOL 33, NO. 2, FEBRUARY 1961
* 169
c
U
0.9
C
U v
\
1 -11
14
lo-,lo
quantity
16:
of
IO.’
IC6
lo.5
p r o p a n e , gms.
Figure 1 0. Relationship between ionization efficiency, E, and sample size for detector shown in .Figure 9
10.
100.
1000.
IO
4
.
Z,voIts/cm.
Applied potentials in kilovolts are shown against each line
Figure 1 1 . RelationshiD between mean enerav of free electrons in various gases at different field strengths “ I
scribed by Lorelock ( 2 3 ) . I n practicc the response to most compounds is fairly closely related to the mass of substance introduced and is independent of the molecular species. With the smaller moleculm, molecular weight less than 100, collisions with the metastables are more frequent and a greater signal for a given mass would be expected. This is found with some compounds, but with others, particularly rrhere the ionization potential approaches 11.7 e.v., the expected increase in response may be offset by a decreasing probability of ionization. I n general. calibration is needed with the lighter molecules. DESIGKAND COKSTRUCTION. Figure 9 shows the construction of a space charge limited argon detector Kith its dimensions and the materials of its construction. Usually the sample is introduced through the anode, which is a hollow tube. a t flow rates up to 20 cc. per minute. The body of the chamber is s w p t with a stream of clean dry argon a t a flow rate of 50 to 100 cc. per minute. For flow rates of the sample stream in excess of 20 cc. per minute, it is best to dispense 155th the scavenge gas flow and introduce the sample through the scavenge gas inlet at the base of the chamber. The source of ionizing radiation is chosen to provide a saturation current of between 1.0 and 2.0 x ampere when the detector is filled with nitrogen. It is not possible, because of the space charge which is present, to observe a saturation current plateau when the detector is filled with argon. The dimensions and magnitude of primary current shoxn have been selected from 170
ANALYTICAL CHEMISTRY
experiments to provide the greatest possible efficiency and linear dynamic range. The proper operation of this detector depends upon the establishment of a space charge just sufficient to neutralize the otherwise nonlinear characteristics of a simple argon detector. Any drparture fcom the design given n-ill lead t o an impaired response, particularly in the direction of a restricted dynamic range. Another important design feature in this detector is the inclusion of a diffuser consisting of three layers of fine nietal gauze a t the base of the chamber. This provides a laminar flow of scavenge gas through the space charge region near the cathode. A high linear gas velocity or turbulence in this region impairs the performance. The primary electron stream approaching the anode of the detector has been found by direct observation to be confined to a narrow tubular channel. This may ne11 be attributable to the focusing effects of the positive ion space charge similar t o that deliberately employed in early designs of cathode ray oscilloscope tubes. The confinement of the electron stream to a small tubular volume projecting from the tip of the anode implies that the sensing volume of the detector is similarly confined, as the ionization of test substances can only occur where the inetastables are generated. This observation explains why the effective volume of the detector is very much smaller than its physical dimensions and makes possible its use for rapid
~
measurements. I t also probides the basis of a further improved version, the triode argon detector. I n this the positive ions produced in the reaction between the metastables and the test substance are collected a t a ring electrode mounted between the anode and the chamber. The triode detector, therefore, achieves the separation of the background current with its random fluctuations from the signal current and makes possible an even further increase in sensitivity. The addition of the third electrode alters the field distribution within the detector and it may be some time before the optimum dimensions for the triode detector are determined. An interim version has been described by Lovelock ($8); even if this is not the best possible design it is considerably more sensitive than the diode version above. PERFORMANCE. Figure 10 sho\ys the ionization efficiency for different applied potentials and sample size with a standard test substance. propane. Other performance characteristics are reported by Lovelock (2%’)and compared with other detectors in Table 111. The outstanding performance features of argon detectors are their high ionization efficiency and ability t o respond to almost all volatile compounds both organic and inorganic. Their disadvantages are that the maximum vapor concentration they will observe is in the region of one part in IO3 to 105 by volume and that their performance is seriously impaired by the presence of air or water vapor in the
carrier gas. The place of argon detectors among the hierarchy of ionization methods is clearly for quantitative measurements a t extreme sensitivity and nhere a rapid response is desired. It is possible, with the flame ionization detector, for example, to achieve the same order of detectivity as that with the detector described above. The ionization efficiency of the argon detector is, however, approximately lo3 times greater, so that for the same rate of sample input, the signal provided will be greater in the same proportion. hkasurements a t extreme sensitivity can, therefore, be made with relatively simple amplifiers, or even by connecting the detector directly to a high impedance recorder. Also, where very fast measurements are contemplated, such as described by Scott (55), the large signal output is well suited to the design of an amplifier input circuit with a short response time. The measurement of large samples and of samples contaminated with air or water vapor is less convenient with argon detectors but can be made by diluting the sample stream in an excess of clean dry argon before its presentation to the detector. The measurement of strongly electron-capturing substances such as those containing halogens or NO2 groups must always be made a t the highest applied potentials and with small or diluted samples if errors due to recombination effects are to be avoided. The two reactions forming the basis of argon detectors can be applied in several different ways. The justification of the plural designation is the existence of five different practical versions, each v i t h a particular construction and performance. The performance of the version shown in Figure 9 is far better than that of the first described model and there is good reason to believe that still further improvements are possible. In theory it is possible by using highly purified helium or neon as the carrier gas, to detect all gases and vapors. This is because the stored energies of helium and neon metastables are 20.8 and 16.6 e.v., respectively; these are higher than the ionization potentials of many other gases or vapors. Apart from work of Berry ( 2 ), who used an elaborate method with painstaking care to purify helium, little success has attended attempts to use these gases in argon detectors. It is possible that the methods used to remove ionizable contaminants from the helium or neon were insufficient. There is reason to believe also that the excitation of metastables by electron bombardment in helium is much more difficult to achieve than in argon. It is probable that an effective helium detector, if it could be made, would be different in design from an argon detector.
Electron Capture Ionization Detector. With most ionization methods, recombination phenomena interfere with, or set a limit, to their faithful response. A detector specifically designed to exploit recombination effects for the measurement of compounds having a n affinity for free electrons was reported by Lovelock and Lipsky (27). A much improved form of this detector will be described below; it is applicable to the quantitative analysis of halogen compounds generally and to the qualitative analysis of volatile compounds. PHYSICAL BASIS. The rate of recombination b e h e e n positive and negative molecular ions is between 106 and lo8 greater than between free electrons and positive ions. The presence of a gas, or vapor, capable of capturing free electrons to form negative ions is, therefore, readily observed in a free electron gas in terms of an increased rate of recombination. In practice an ion chamber containing a free electron gas is maintained at a potential just sufficient for the collection of all of the free electrons produced. The introduction of an electron-capturing gas or vapor causes a decrease in current flow related to the concentration of test substances as follows: I = I , exp( -Kcs)
where I , is the saturation current in the pure carrier gas, I the current in the presence of test gas a t a concentration c, K a constant related to the field strength and electron affinity of the test substance, and z a factor related to the dimensions of the ion chamber, The relationship bears a resemblance to Beer's law for light absorption and the general effect further resembles absorption spectrophotometry as follows:
F-4
...................... ...................... ......................
. . . . . . . ...... . . . . . . . . . . . . ............. .. .. .. .. . . . .
scale,
cm
brass
0 Figure 12. detector A. 6. C.
PTFE
Electron capture ionization
Inlet for carrier gas and anode Diffuser made of 100-mesh brass gauze Source of ionizing radiation D. G a s outlet and cathode This is also the design of the direct electron mobility detector, only the gas flow is reversed
The electron affinity of a given substance varies with the energy of the free electrons in much the same way as the absorption of light varies with the energy of the photons. The electron energy in an ion chamber has a mean value determined by the effective field strength and by the nature of the gas present. A comprehensive account is given by Townsend (58) of the free electron energies a t various field strengths in different gases. These data are summarized in Figure 11. Briefly the mean free electron energy a€ a given field strength is highest with the monatomic gases and lowest with the polyatomic gases and varies a t moderate field strengths from 10 e.v. in argon to 0.1 e.v. in carbon dioxide. These electron energies correspond to photon energies in the short ultraviolet and the infrared regions, respectively. Simply by choosing an appropriate carrier gas and applied potential electron, absorption a t any mean energy in this wide range can be observed. In practice the detector can be rendered selectively sensitive to a wide variety of classes of organic and inorganic compounds. The cross section for the capture of free electrons by even strongly absorbing gases is small, approximately 10-18 sq. cm. HoRever, during the passage of an electron through 1 cm. of gas a t atmospheric pressure as many as 106 collisions with gas molecules occur. Thus the capture probability is very high and the effective ionization efficiency approaches 100% for the strongly capturing gases and vapors. For certain specific compounds this is undoubtedly the most sensitive ionization detector available; furthermore, its selectivity means that there is little interference by common contaminants and makes the realization of its ultimate extreme sensitivity a simple matter. DESIGNAND CONSTRUCTION. Figure 12 shows a recent version of detector for this method with its dimensions and materials of construction. I n this design the supply of carrier gas is arranged to flow in opposition to the motion of the negative charge carriers. Where these are free electrons their drift velocity is so much greater than the linear gas velocity that their collection is unimpeded. Negative molecular ions, however, drift slowly to the anode and the gas flow increases their time of transit across the chamber and their probability of encountering a positive ion. This modification increases the sensitivity of the method and is a considerable improvement over the use of a simple cylindrical ion chamber as first described. -41~0the plane parallel arrangement of the electrodes renders the field within the chamber uniform and improves the resolution of the available spectrum of electron energies. Jl'ith this design it is VOL 33, NO. 2, FEBRUARY 1961
171
important to keep the ion density relatively low. A saturation current of 3 X IOF9 ampere should not be exceeded. At higher ion densities the small applied potential, 10 to 100 volts, is all too readily neutralized by the development of a positive ion space charge. With this design the greatest sensitivities are obtained when noble carrier gases such as argon or helium are used. With argon other modes of detection may also occur to some extent and may confuse the results obtained. The most generally useful gases giving the widest range of discrimination between classes of compound are helium, nitrogen, and hydrogen. For the observation of electron absorption at very low mean energies carbon dioxide or argon containing 1% of carbon dioxide is found best. PERFORMANCE AND APPLICATIONS. The basis of operation and performance of this detector is still under investigation. Some tentative general conclusions from experimental observations can, however, be made as follows: The capture of slow electrons by compounds requires the presence within them of some element with an affinity for free electrons. If the sum of the energy released by the formation of a negative ion and that supplied as kinetic energy during the collision is greater than the energy required for the dissociation of the molecule, then usually electron capture occurs. Carbon and hydrogen have little or no affinity for free electrons so that, in general, hydrocarbons do not readily capture free electrons. Oxygen and the halogens capture electrons readily to form stable negative ions and several electron volts of energy are released by the reaction. Hydrocarbons substituted m ith these elements, therefore, capture electrons and the affinity of the molecule appears to bear a relationship to the ease of dissociation of the heteroelement from the compound. With oxygen, for example, ethers capture weakly or not a t all, but anhydrides, peroxides, and 1,2diketones capture very strongly. Similarly, with the halogens the intensity of electron absorption goes in the order of the ease of dissociation of the halogen from the compound. Thus the order of affinity in halogen-substituted hydrocarbons is I > Br > C1 > F. Also a compound such as chlorobenzene from which the halogen is not readily removed captures weakly, whereas benzyl chloride, in which the halogen is labile, captures strongly. Although, in general, hydrocarbons possess a weak affinity for free electrons, there are some exceptions such as cyclo-octatetrene, anthracene, and other aromatic hydrocarbons. It has been suggested that the solubility of sodium in ether containing anthracene and other aromatic hydrocarbons is attributable to 172
ANALYTICAL CHEMISTRY
the affinity of the n-orbital system of these aromatic compounds for free electrons. A great convenience of the method is that the response t o each of the weakly capturing classes of compound can be abolished in turn simply by increasing the applied potential so that the detector can be rendered selective in its response. I n general, all members of some given class-for example, esters-cease to respond a t a well-defined applied potential, but the more strongly capturing classes, such as ketones, can still be observed. The precise potential required depends upon the geometry of the detector, the conditions of temperature, pressure, and the carrier gas employed; calibration is therefore necessary. Yevertheless, once calibrated, the response of a detector is consistent. For quantitative analysis this detector is best suited to measurements at high sensitivity of organic and inorganic halogen-containing compounds and also certain oxygen-containing compounds such as volatile nitrates, ozone, and oxygen; such substances are difficult or impossible to measure a t high sensitivity with other methods. Although this detector has a limited dynamic range, its response to different concentrations obeys Beer's law closely enough to permit calibration or calculation of the quantities measured when these are outside the linear range. The dynamic range can be varied by altering the applied potential. Perhaps more important is its use in qualitative analysis, particularly for the identification of the functional groups of the compounds emerging from gas chromatographic columns. One of the recent advances in the technique of gas chromatography is the use of high resolution capillary tube columns by Golay ( I O ) . The maximum sample load which can be handled by these columns is approximately 1.O pg. The collection and identification of the lesser components of a mixture emerging from such a column by conventional chemical methods are obviously exceedingly difficult. It is, however, well within the capacity of the electron capture detector. The study of the structure and electronic state of organic and inorganic molecules in terms of their electron affinity also has promising possibilities in theoretical chemistry. Electron Mobility Detectors. With few exceptions the permanent gases cannot be measured a t low concentrations with the detectors described so far. Sensitive ionization methods for the analysis of permanent gases, as well as other volatile substances, have been described; all of them are based upon changes in the mobility of free electrons in the noble gases, Then other gases are present, They include indirect methods such as those described by
Willis (SQ), Johnson ( I @ , and Ellis and Forrest (9); also a method depending directly upon electron mobility changes described by Lovelock (366). PHYSICAL BASIS.I n the noble gases the collisions between electrons and gas atoms at low field strengths are elastic. The mass of a free electron is between 10-6 and that of the rare gas atoms; consequently, any energy gained by an electron during its fall through a n electrical field is but slightly shared with the gas atoms during collisions. The mean energy of the electrons is, therefore, much greater than that of the gas atoms and their mean velocity between collisions is yery high indeed. When another gas is present in the rare gas the collisions betreen the electrons and the molecules of this gas are nonelastic and the high energy of the electrons is absorbed and their mean velocity lowered. A reduction of electron velocity decreases their ability to excite, or ionize, the noble gas aiid also increases their bulk drift velocity in the direction of the field. The methods of 7Nillis (39),Johnson (181, and Ellis and Forrest (9) all use a conventional argon detector such as that illustrated in Figure 9. The carrier gas is either argon deliberately contaminated with an ionizable gas or impure helium. The detector is operated a t a high potential so that the contaminants are heavily ionized. When a permanent gas is introduced into the carrier stream entering the detector there is a fall in mean electron energy by the process described above. This results in a decrease in the proportion of noble gas atoms excited to the metastable state and in turn a decrease in the number of contaminant molecules ionized. The presence of test gas is thus observed as a fall in current flow through the detector. Using contaminated argon this method is limited to those gases which are not themselves ionized by collisions with argon metastables. With helium as the carrier gas almost all gases and vapors can be observed; this is because the electron energy needed to excite helium atoms is so high that a small amount of any other gas is sufficient to reduce the extent of excitation. In the method described by Lovelock (do'), pure argon is used as the carrier gas. The argon is ionized in a localized region of an ionization chamber by radiation \?hose range in the gas is small, such as a-radiation or n-eak @radiation from a tritium source. T o an anode, placed some distance from the ionized region, short pulses of electrical energy are applied; the duration of the pulse is chosen so that in the pure argon there is inwfficient time for electrons to drift to the anode and only the smallest current flows. When any other gas is present in the argon the mean agitation velocity of the
lqq
c , c,
detector
I
A
J-
1
-
A
A
&
Figure 13. Schematic diagram for connections to a direct electron mobility ionization detector
CI,CZ = 10-9 forad C B = 10-1" f a r a d
RI, Rz, 45 = 106, 1 OB, 10' ohms, respectively
0.1.
0.2.
0.3.
04.
05.
06. 0 %
frequency, MC/s electrons is Ion-ered and this causes an increaqe in their bulk drift velocity tonard the anode. There is then time, during the application of the pulse, to collect a t the anode a proportion of the free electrons related to the concentration of other gas present. The :tbility of different test gases to reduce electron energy by nonelastic collisions depends upon the complexity of their molecules. I n general, the more atoms in the molecule the more effective is the gas in reducing electron energy. DESIGN. ~ X D CONSTRUCTIOK. The indirect methods require simply an argon detector; the version illustrated in Figure 9 is entirely satisfactory for this purpose. The carrier gas can be argon, to which a few parts per lo8 by volume of ethylene, or propane, have been added or commercial helium hich always contains ionizable contaminants. The detector is operated a t a potential of between 750 and 1250 volts, sufficient to provide a current f l o ~in the region 3 to 5 times 10-8 ampere. The detector is connerted to the amplifier and recording equipment in such a manner that the large background current is offset and the reduction of current flow in the presence of test gas is observed as a positive deflection. The drift velocity method requires a detector identical with that illustrated in Figure 12 for the electron capture method. As with the electron capture method, the most satisfactory radiation source is tritium and i t should be present in sufficient amounts to provide a direct current saturation current in pure argon of between 3 and 5 X 10-9 ampere. I n the drift velocity method i t is important that the gas flow is introduced through the cathode; othervise the detector may function with electron-capturing gases as a n electron capture detector and provide a confused response. The electrical connections are illustrated in Figure 13. The detector can be seen t o function as a
Figure 14. Relationship between signal current and frequency of applied pulses with electron mobility ionization detector Curves are drawn for pure argon ond for argon containing 0.1 b y volume of gases indicated
rectifier whose efficiency depends upon the concentration of test gas present in the argon carrier, PERFORM.4SCE .4KD APPLICATIONS. The indirect methods, although simple and capable of responding to nearly all permanent gases, are, compared with other ionization detectors, relatively insensitive. The response to different gases varies considerably and calibration is necessary. Where equipment employing a n argon detector is available, the analysis of permanent gases at moderate sensitivity requires merely a change in the supply of carrier gas. It is in these circumstances that the indirect method is most useful. It would appear that the use of commercial helium as a carrier gas is more convenient than that of contaminated argon and also provides a greater sensitivity and range of response. The detailed performance characteristics of the drift velocity method are still under investigation. It is, however, more sensitive than the indirect methods, largely because the background current and noise level in the absence of test gas are very low. Some interim performance characteristics are listed in Table I11 for comparison with the other detection methods. One of the interesting features of this method is that the detector can be rendered insensitive in turn to the diand tri- and polyatomic gases simply by decreasing the duration of the applied pulses. So far, however, it is found that the response of the device is linear with gas concentration only when the duration of the pulses is just short enough to give no current with pure argon. The effect of pulse duration on
%
the response to different gases is illustrated in Figure 14. The simplest and most convenient pulses to apply to the device are halfwave rectified sine waves. The source impedance and capacitance of the pulse generator and its connections to the detector must be low if the distortion of the pulses is to be avoided. With the detector illustrated in Figure 12 the pulse amplitude should be between 50 and 100 volts and the frequency of the parent sine waves between 0.2 and 1.0 Mc. The drift velocity detector is sensitive to nearly all permanent gases and volatile substances. As nould be exprctrd the response to other rare gases is slight or negligible. Also the detector is relatively insensitive t o nitrogen and i t fails to respond to strongly electron-capturing gases, such as the halogens and their compounds. 'l'hc use of a detector which is highly sensitive to most of the gases present in air is an exacting practical problem. Scrupulous care is needed to ensure that the carrier gas is clean, that there are no leaks in the apparatus, and that the surfaces of the apparatus are not releasing volatile material previously adsorbed upon them. Although less developed than the other ionization methods so far described, the drift velocity method is mentioned because it is a t present the only available method for the measurement of permanent gases and compounds such as water vapor at high sensitivity. The maximum concentration of test gas consistent with linear response is less than 0.1% by volume in the argon carrier; the method is not, therefore, recommended for VOL. 33, NO. 2, FEBRUARY 1961
173
gas analysis at high concentrations. I t s special place appears to be in the analysis of permanent gases by gas chromatography using capillary tube columns and for the trace analysis of substances such as water vapor, COS, and CO which are otherwise difficult or impossible to measure. Other Ionization Methods. I n addition to those listed so far, other ionization methods of gas and vapor analysis have been suggested. Important among them are: the glow discharge detector, Pickethly (32); Harley and Pretorius ( 1 2 ) ; the radiofrequency corona discharge detector, Karmen and Bowman (19, 10); the electron impact ionization detector, Ryce and Bryce ( 3 4 ) ; Hinkle et al. ( 1 4 ) ; Guild, Lloyd, and ilul ( 1 1 ) ; and the photoionization detector, Lovelock (26). In their present forms these methods are either relatively untried, such as the photoionization detector, or for one reason or another, have been unable to compete with those already described. They will therefore be described only briefly but again it must be emphasized that any drawbacks found with a single version of these other detectors does not mean that the reactions upon which they are based lack promise. Also the present versions may for certain specific applications possess advantages lacked by the better known methods; wherever possible this feature will be mentioned. Glow Discharge Methods. The electrical properties of a glow discharge a t low pressures depend greatly upon the composition of the gas present; the complex physics of the glow discharge is comprehensively reviewed by Druyvesteyn and Penning (8). The response of a glow discharge in a given carrier gas to the presence of a test gas or vapor does not depend primarily upon some single ionization phenomenon but rather upon a combination of the familiar effects of recombination, space charge limitation, and electron mobility. Also a glow discharge is usually maintained by the emission of electrons from the cathode by a number of processes, including photoelectric emission, positive ion bombardment, and collisions with excited atoms. A change in any of these processes when test gas is present will alter the current flow through the discharge. The glow discharge method is, therefore, universal in application, although the prediction of the nature of its response to a given substance is virtually impossible. I n addition to responding to all gases and vapors, the device provides a large signal output capable of operating recording equipment without further amplification. With many substances also it possesses a wide linear dynamic range. Its disadvantages are twofold. First, it 174
ANALYTICAL CHEMISTRY
b
9. i. ?. scale, cm
Figure 15. A. B. C. D. E.
Photoionization detector
Gas inlet to source of ultraviolet radiation and discharge cathode Discharge anode Sensing chamber cathode Carrier g a r inlet and anode of sensing chamber Outlet to suction pump
requires low pressures for operation, which may be inconvenient in some practical applications. Second, with many substances and with most versions so far tried, the passage of vapor through the device causes long term, or permanent changes, in the emission of electrons from the cathode. In all high efficiency ionization detectors organic compounds are decomposed to their constituent elements; the deposition of carbon on the surface of the cathode following such decomposition can profoundly alter its emission of electrons. In spite of this, claims have been made for a consistent response during long periods of operation by Pickethly (32). I t is, therefore, possible that with some designs and an appropriate choice of cathode material a stable and reproducible version of this detector may become available. At present, however, the use of this device would appear to be limited to the high sensitivity analysis of those permanent gases which do not adversely affect its long term response. Radiofrequency Corona Discharge Detector. Karmen and Bowman (19, 20) observed that the rectification of a radiofrequency corona discharge in helium a t atmospheric pressure was affected by the presence of other gases in the helium. With most organic compounds the current decreased; with air an increase in current was observed at low concentrations but a t high concentrations the current decreased. Their detector consisted of a cylindrical metal chamber containing within it a fine wire supported coaxially. Radiofrequency energy a t 40 Mc. mas applied to the central wire through a capacitor and the direct current potential developed between this wire and the surrounding metal chamber which was grounded, was observed. R7ith cylinder helium in the chamber a potential of 50 to 60 volts was developed across a resistance of 1 megohm. The exact basis of operation of this device has not been explained, although i t bears a resemblance to the indirect electron mobility methods. Unlike the glow
discharge detector, this device is not unduly affected by the deposition of small amounts of decomposition products upon its surfaces. The only disadvantage reported of the method is that the current flow in the device is sensitive to changes in temperature and in use more care is needed in temperature regulation than with the other ionization methods. In its present form it is less sensitive than the hydrogen flame and argon detectors. I t s place would appear to be where the use of radioactive sources or of hydrogen and flames is thought to be undesirable or hazardous; perhaps also for the analysis of permanent gases. Electron Impact Ionization Detectors. The ion source of a mass spectrometer is another form of ionization detector. In it, gases and vapors are ionized by the impact of fast electrons. I n the complete mass spectrometer the ions of the test substance can be separated from those of the carrier gas by the difference in their trajectories through an electrical and a magnetic field. The mass spectrometer is the most highly developed and versatile of all of the ionization methods. It is, however, a complex and expensive instrument compared with the other detectors and also considerably less sensitive. The drawback of expense and complexity has to some extent been overcome by the development of simplified mass spectrometers and also by the use of an electron impact ion source without the addition of means for separating the ions of the test substance from those of the carrier gas. Ryce and Bryce (34) first suggested the use of an electron impact ion source in which the electron velocity was maintained a t a level insufficient to ionize a refractory carrier gas such as helium, but sufficient to ionize the other gases and vapors. In its simple form the electron impact detector consists of an evacuated vessel containing a thermionic source of free electrons and an anode for the collection of the primary electrons and secondary electrons generated by the impact ionization of the test gas. This device suffers drawbacks similar to those of the glow discharge detector. It requires low pressures for operation, and the emission of primary electrons from the hot filament is subject to interference by the deposition of decomposition products from the test substance. The advanced forms reported by Hinkle et al. (14) and Guild, Lloyd, and Aul (11) overcome this second drawback by a system for maintaining the electron emission constant in the face of contamination. This is done by monitoring the primary electron current and, by an appropriate feedback circuit, adjusting the filament temperature so that the emission remains constant. Provided expense, complexity, and the lack of
to detector
sample i n l e t
capillary
tube
throttle
I
0
scale, cm,
Figure 16. Injector pump for continuous sampling of air and other gases to measure their content of detectable vapor
extreine 3ensitivit.y are no objection this is an excellent method n-ith a wide linear dynamic range and an ability to respond without much prejudice to nearly all gases and yapors. Photoionization Detector. The specific ionization of the test gas or vapor in an ineit carrier gas can be achieved by the irradiation of the mixture with photons of appropriate energy. A detector based on this method has been described by Lovelock (W6), and is illustrated in Figure 15. I t consists of a chamber containing a glow discharge, within a hollow cathode, which is supplied continuously with clean gas. The ultraviolet light from this discharge illuminates an open ionization chamber into which the stream of carrier gas and test gas is introduced. Suitable gases for the discharge are helium, argon, nitrogen, or hydrogen. If the discharge is maintained by direct current it is necessary to operate the device a t pressures less than 100 mni. If, however, radiofrequency is supplied to the discharge and helium used, operation a t atmospheric pressure is possible. When gases from commercially available cylinders are used in the discharge, the more energetic photons appear t o be absorbed by the common contaminants present. In these circumstances the device does not respond to the permanent gases, water vapor, and the few other substances Ivith high ionization potentials. Nearly all polyatomic organic and inorganic gases and vapors can nevertheless be detected. Although relatively new, this d e tector shows considerable promise even in its first version. It has a low background current (10-10 ampere), a n ionization efficiency intermediate between that of the flame and argon detectors, and a wide linear dynamic range. It may be used with any carrier gas which is not itself ionized and is particularly well suited to the direct measurement of ionizable gases and vapors in air. The disadvantage of low pressure
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Figure 17. Chromatogram of separation of 10 PI. of a mixture of HB,02,and CO using a 1-foot column of Molecular Sieve (Linde 5A) Gas flow rote, 120 ml. per minute of argon and using direct electron mobility ionization detector
operation from a practical viewpoint is to some extent offset by the fact that pressure regulation is unimportant and the device works well with a conventional laboratory n-atrr pump. At low pressures also, the anomalous results obtained with other detectors in the analysis of strongly electron-capturing gases and vapors, such as halogen and nitro compounds, do not occur. The principal uncertainty concerning the potentiality of this method is in the long term stability of the source of ultraviolet radiation. So far, however, this appears to be commendably stable even in the face of gross contamination of the surfaces of the device. APPLICATIONS
So far ionization detectors have been used almost esclusively in conjunction Lvith gas chromatography columns. This results partly from the historical background of their devclopment but also because the column and sensitive detector complement one another to a high degree. The detectors are for the most part nonselective in their response to different substances and the column forms an ideal filter which presents in sequence the pure components of a mixture, or removes interfering substances from the one to be measured. There is a rapidly growing literature of analytical methods employing gas chromatography columns and sensitive ionization detectors. I t
is not intended in this section to rex+\\ this subject in detail, although a few specific applications rvill be described briefly, chosen to illustrate the way in which the various detectors may best be used. The direct a p plication of ionization methods to chemical analysis is still largely untried ; there are, nevertheless, many circumstances, such as those requiring rapid or continuous measurement. where the use of a column ahead of a detector 1s a disadvantage or impossible. Means of applying ionization detectors to this type of measurement will be suggested and described. Preparative G a s Chromatography. Gas chroniatography may be used for the isolation of pule compounds in quantities up to 1 or more grams. I n this application the vapor concentration emerging from the column tends to be too high to permit the use of the more sensitive detectors; furthermore the flame and argon detectors are n-holly or partially destructive, respectively, to the sample passing through them. The most suitable detector for this application is the crosssection ionization detector; the gas flow from a large preparative co umn may be passed through such a detector without fear of false response or harm to the substances to be isolated. I n those instances. such as the isolation of relatively nonvolatile compounds, whcre in spite of large sample loads the vapoi concentration is too low for VOL. 33, NO. 2, FEBRUARY 1961
175
detection by the cross-section detector, the following alternative procedure is useful: A small proportion of the gas stream from the column is diverted into a second stream of pure carrier gas and so diluted t o a level appropriate for the more sensitive detectors. The apparatus illustrated in Figure 16 is convenient for this purpose. Analytical Gas Chromatography. PERMANENT GASES. The analysis of permanent gases by gas-solid chromatography is well established and the general theory of the method is reviewed by Janak (I?’). So far this important application has received less attention than it merits, probably because of the poor performance of the thermal conductivity detectors which are most frequently employed. It represents a challenging field for the application of ionization detectors and the place of the different methods is as follows: For the precision analysis of gases st moderate to high concentration, the cross-section detector is best. For the analysis of traces or of small volumes of gases the various electron mobility methods are needed. The potentialities are illustrated by the chromatogram, Figure 17, of the analysis of 10 11. of a mixture of HS, 02, and CO. This TTas made with a Molecular Sieve column and the direct electron mobility detector in the form illustrated in Figure 12. The high resolution and rapidity of capillary columns have not so far been applied to the separation of permanent gases, although in principle there is no reason why they should not so be used. The sensitivity and response time of the electron mobility methods are adequate for this need and \vi11 provide the detectors of choice when eventually the column technique is established. VOLATILE ORGANIC SUBSTANCES. In the analysis of mixtures of volatile substances, using packed columns, the sensitive ionization detectors are steadily replacing the previously used but less efficient thermal conductivity detectors. The advantages attendant upon the use of ionization detectors arise from their sensitivity, stability, and wide linear dynamic range. Sensitivity is important, not only in the analysis of trace quantities, but also because the resolution of columns improves as the load is decreased. Furthermore, in the analysis of relatively nonvolatile substances the vapor concentration emerging from the column may be low even with comparatively large sample loads; this is found in the important application of the analysis of fatty acid methyl esters. The most commonly used in analytical gas chromatography are the flame and argon detectors. Xeither are perfect and some of the earlier models of these detectors were inefficient and unreliable. 176
ANALYTiCAL CHEMISTRY
Figure 1 8.
Chromatogram of separation of isomers of tetramethylbiphenyl
Column load, 10-8 gram Column, 100 feet o f 0.01 -inch cupro-nickel coated with Apiezon 1 stationary phase Column operated a t 195’ C. with inlet pressure of 1 atm. Detector, triode argon
For good performance argon detectors must be constructed exactly as described by Lovelock (22) and their performance is adversely affected by the presence of air or water in the carrier gas. The flame detector, although less critical in the dimensions of its construction, usually needs three separate gas supplies and the careful regulation of gas flow rate is important in the maintenance of a consistent response. None of these drawbacks seriously disturbs the precision of analysis so long as the appropriate precautions are taken to minimize their effects. These two methods will undoubtedly be used on a considerable scale for some time to come. I t is possible that the photoionization detector which appears to suffer none of the disadvantages of the other two methods may in time become a serious contender for use in analytical applications. In general, the advantages of using sensitive ionization detectors with packed columns in gas chromatography are significant; it should not be forgotten that the gas density balance described by hlartin and James (d9) still compares favorably with the ionization methods where high precision and moderate sensitivity are needed. With the recently developed capillary tube columns, which demand very small sample loads and a rapid response time, there is no alternative to the sensitive ionization detectors. Figure 18 illustrates an analysis, made nith a capillary column and a triode argon detector, of the isomers of tetramethylbiphenyl n.hen the column load was only 10-8 gram. Figure 19 illustrates the simultaneous use of qualitative and
quantitative detectors to identify and measure the components of a mixture. I n this analysis 10-8 gram of a mixture of cyclohexane, fluorobenzene, chlorobenzene, and the isomers of di-, tri-, and chlorobenzene were applied to the column. The upper chromatogram illustrates the proportion of the various components and the lower chromatogram the differentiation of these in terms of their respective electron affinities. Such a combination of quantitative and qualitative detection is of the greatest potential value in the measurement and identification of the components of a complex mixture. The combined information provided by column retention time and electron affinity go far toward the identification of an unknown substance. Complementary analyses of this type can be made upon as little as 10-9 gram of a separated component. Gas Chromatography. T O L A T I L E ISORGANICCOIIPOUNDS. N o s t elements possess volatile compounds which may be separated by gas chromatography-for example, halides, hydrides, nitrates, carbonyls, and compounds containing organic radicals. Inorganic gas chromatography is relatively new, although the separation of metal chlorides was reported by Keller and Freiser (22) and of interhalogen compounds by Iveson and Ellis (15). In this field the argon detector can be applied to the detection a t high sensitivity of most volatile inorganic substances. All of the volatile hydrides of groups T’ and VI of the periodic classification, including P u ” 3 and H2S, respond normally with this detector. In group IV
the simple tetrahydrides of carbon, silicon, and germanium have ionization potentials too high for detection, but as with the hydrocarbons the more complex hydrides are detectable. The response to boron hydrides is unknown, but i t is almost certain that the more complex are detectable. Many volatile halides can also be detected but with some the dynamic range of the detector may be restricted by electron capturr. The detection of inorganic halides is brst performed by the electron capture detector; this method is also valuable for the detection of the halogen gases, interhalogen compounds, and t h e fluorocarbons, which otherwise are difficult to detect at low concentration. I n general, as soon as the column techniques are established for the separation of inorganic compounds, ionization methods are available for their detection. Continuous Measurement of Gas or Vapor Concentration. M a n y applications require t h e continuous measurement of t h e concentration of a single gas in another gas or mixture of gases of constant compositionfor example, t h e measurement of t h e concentration of potentially toxic or flammable substances in air, t h e measurement of ventilation or of mass transfer using a volatile tracer substance, and leak detection and vapor pressure measurement. The only report of the use of ionization detectors in this type of application is t h a t of Andreatch and Feinland ( 1 ) . The only detector capable of sampling air directly for the measurement of low vapor concentration is the photoionization method and at present this is still under development. With the other devices i t is necessary to use some means of introducing a steady stream of air into the carrier stream proceeding to the detector. The most convenient method for the continuous sampling of air and one which can be used with any detector is that shon-n in Figure 16. It consists of a small injector pump n hich is driven by a supply of carrier gas a t 10 to 20 p.s.i. pressure and at a flow rate of between 50 and I50 cc. per minute. The pump develops a small npgative pressure, between 2 and 24 inches of water pressure. The air to be sampled is drawn into the negative pressure port of the pump and there diluted in the rapid stream of pure carrier gas which drives the pump. The extent of dilution is controlled by a throttle; i t is convenient to make the throttle of some chosen length of capillary tubing connected between the pump and the air. I n spite of the dilution which occurs in the pump, it is possible, by using an appropriate detector, to measure the concentration of almost any vapor in air from a molar fraction of unity down to 10-9,and mith
some vapors even less. The method may also be used to monitor the concentration of a single component in some other gas to which the detector chosen does not respond. The selectivity of the different ionization detectors also makes possible the simultaneous use of two or more of them for the continuous observation of
compounds by combustion; and in the analysis of metals for their content of permanent gases. KO reports of this approach are known and i t is mentioned to draw attention to the potentialities of ionization methods outside the field of gas chromatography. ACKNOWLEDGMENl
I am indebted to S. R. Lipsky and to A. Zlatkis for the stimulus of their suggestions. I also thank B. %I. Wright for the design and construction of the injector pump shown in Figure 16 and P. Simmonds for his valuable technical assistance. LITERATURE CITED
(1) Andreatch, A. J., Feinland, R., .~KAL.
Figure 19. Simultaneous quantitative and qualitative chromatograms using argon and electron capture detectors, respectively Column load, 1 pg. of a mixture containing: 1. Cyclohexane 2. Fluorobenzene 3. Chlorobenzene 4. m- and p-dichlorobenzene 5. o-Dichlorobenzene 6. 1,3,5-TrichIorobenzene 7. 1,2,4-TrichIorobenzene 8. 1,2,3-TrichIorobenzene Column, 100 feet of 0.01 -inch cupra-nickel coated with Apiezon 1 stationary phase Operating conditions, 1 2 5 ’ C. and 1 atm. inlet pressure Electron capture detector, lower chromatogram, was supplied with a potenlial of 3 5 volts, and carrier gas was argon
a multicomponent mixture. Although untried, this approach would appear to possess potentialities in the following applications: the continuous observation of the composition of respiratory gases; carrier gas methods of chemical analysis, such as the determination of the elementary composition of organic
CHEW32,1021 (1960). (2) Berry, R., Nature 188,578 (1960). (3) Boer;)H., “T’apour Phase Chromatography, D. H. Desty, ed., Vol. 1, Butterworths, London, 1957. (4) Condon, R.(D., Scholly, P. H., Averill, W.)’ Gas Chromatography,” R. P. Vi-. Scott, ed., Butterworthe, London, in prres. (5) Deal, C. €I., Otvos, J. W., Smith, V. N.,Zucco, 1’. S.,ANAL. CHEW 2 8 , 1958 (1960). (6) Desty, D. €I., Geach, C. J., Goldup, A., “Gas Chromatography,” R. P. IT. Scott, ed., Butterworths, London, in press. (7) Dimbat, XI., Porter, P. E., Stross, F. H., ANAL CHEK 28, 290 (1956). (8) Druyvestryn, 11,J., Penning, F. 31, Reels. Modern Phys. 12, 87 (1940). (9) Ellis, J. F., Forrest, C. W.,Anal. Chim. A c t a , to he published. (10) Gol~>y, bf. J. E., “Gas Chromatography, D. H. Desty, ed., Vol. 2 , p. 36, Butterworths, London, 1958. (11) Guild, L. V , Lloyd, 11. I., Aul, F., 2nd Biannual Intern. Gas Chromatoeraphy Symp., Instr. SOC.Ani., Lansini, Mich., 1959. (12) Harley, J., Pretorius, V., Zbid., 178, 1244 (1956). (13) Harley, J., Sel, W.,Pretorius, V., Xature 181, 177 (1958). (14) Hinkle, E. A, Tucker, H. C., Wall, R. F., Combs, J. F., 2nd Biannual Intern. Gas Chromatography Symp., Instr. SOC.Am., Lansing, Mich., 1959. (15) Iveson, G., Ellis, J. G., “Gas Chromatography,” D. H. Desty, ed., Vol. 2, p. 300, Buttervwrths, London, 1958. (16).James, -4.T., Martin, .4. J. P., Bzochem. J . 50, 679 (1952). (li) Janak, J., Ann. A‘. 1‘. ilcad. Sei. 72, 606 (1959). (18) Johnson, R. E., Symp. Instr. SOC. Am.. Montreal. Quebec. 1960. (19) Karmen. A,’. Bowman. R. L.. Bnn. iT.Y . Acad. Sei. 72, 714 (’1959). ’ (20) Karmen, A,, Bon-man, R. L., 2nd Biannual Intern. Gas Chromatography Symp., Instr. Soc. Am., Lansing, Mich., 1959. (21) Keller, R. A,, Freiser. H., “Gas Chromatography,” R. P. W. Scott, ed., Butterworths. London, in Dress. (22) Lovelock, J. E., Zbid., in‘ press. (23) Lovelock, J. E., J . Chromatog. 1, 25 (1958). (24) Lovelock, J. E., .Vafure 181, 1460 (1958). (25) Zbid., 185,49 (1960). (26) Zbid., 188,401 (1960). ~
\ -
VOL. 33, NO. 2, FEBRUARY 1 9 6 1
177
(27) Lovelock, J. E., Lipsky, S. R ,J . Am. Chem. SOC.82,431 (1960). (28) McWilliam, I. G., Dewar, R. A., “Gas Chromatography,” D. H. Desty, ed., Vol. 2 , p. 142, Butterworths, London, 1958. (29) Martin, A. J. P., James, A. T., Biochem. J . 63, 138 (1956). (30) Ongkiehong, L., “Gas Chromatography,” R. P. FT’. Scott, ed., Butterworths, London, in press. (31) Otvoe, J. W.,Stevenson, D. P.,
J . Am. C h e m SOC.78,546 (1956). (32) Pickethly, R. C., ANAL.CHEM.30, 1309 (1958). (33) Pompeo, D. J., Otvos, J. W. ( t o Shell Development Co.), U. S. Patent 2,641,710 ( 1953). (34) Ryce, S. .4.,Bryce, W. A , Can. J . Chem. 35, 1293 (195’ (35) Scot;, R. P. W.,‘)“Gas Chromatography, R. P. W. Scott, ed., Britterworths, London, in press. (36) Sharp?, J., “Siiclenr Radiation De-
tectors,” pp. 130-4, blethuens, London, 1955. (37) Stern, O., quoted by Lewis, B.. Von Elbe, G., in “Combustions, Flames and Explosions of Gases,” p. 206, Academic Press, New York, 1951. (38) Townsend, J., “Electrons in Gases,” Hutchinsons, London, 1947. (39) Willis, Y.,-Vuture 183,1754 (1959). RECEIVEDfor review November 3, 1960. Accepted December 8, 1960.
Adsorption Characteristics of Some Gas-Liquid Chromatographic Supports EVERETT M. BENS Research Department, Chemistry Division,
b Different solid supports were studied to understand more completely the causes for tailing in gas-liquid chromatographic separations. The adsorption of some common solvents on several solid supports was studied by measurement of the retention volumes of the solvents on the supports at several temperatures. The retention volumes of some aliphatic and aromatic hydrocarbons, alcohols, and ketones on C-22 firebrick, glass spheres, and Tide were plotted against inverse temperature; all produced linear isotherms. Effects of small quantities of residual solvent upon the retention time and its effect upon the above plot are discussed. A graphical comparison of the various solid supports is made so that the effects of adsorption may be evaluated when separations are made using small quantities of stationary phase. Rapid evaluation of the change due to different treatments of the solid support is readily made.
T
of gas-solid chromatography has been retarded by three major disadvantages: chemical changes induced by certain adsorbants, serious tailing effects due to nonlinearity of the adsorption isotherms, and displacement effects due to the dependence of individual isotherms upon the nature, number, and concentration of the components in the sample. While the chemical changes have been particularly prevalent in gas-solid chromatography, Vilkas and Abraham (29) have recently reported isomerism of 8-pinene when a nonpolar substrate was used on either firebrick or Celite. When more polar substrates were used the active sites of the support were deactivated but reactivated after aging. HE WIDESPREAD USE
178
ANALYTICAL CHEMISTRY
U. S.
Naval Ordnance Test Station, China lake, Calif.
The importance of this characteristic has been recognized by the British suppliers of gas-liquid support materials (20) who devised a special test of the catalytic action of their supports. The nonlinearity of adsorption isotherms nhich causes tailing has been a problem in both gas-solid and gasliquid chromatography. Knight ( I S ) improved the symmetry of elution peaks for hydroxyl and amino compounds by saturating the carrier gas with a polar material similar to the sample, thus reducing the activity of the support. Other investigators of gas-liquid chromatographic supports have used varying amounts of stationary phase to eraluate the effects of the support upon separation and tailing. I n one such study, Johns (16) found that a decrease in particle size of the support increased the retention time. I n addition, repeated sampling tended to increase the peak height observed for polar materials, an effect IT-hich may be ascribed to decreased adsorption due to saturation of active sites. Most workers h a r e used chemical treatment of support materials to remove the active sites and, thereby, reduce the tailing effects. It has become common practice to acid-wash, or caustic-treat solid supports so that tailing of the more polar materials is reduced. -4novel treatment by Ormerod and Scott (21) consisted of the deposition of an equal weight of silver upon the support and in this manner they reduced tailing successfully. Another worker (10) has evaluated the effect of varied amounts of support and stationary phases upon relative retention values. He found the retention times of hydrocarbons to be least affected by the support material, while the more polar materials such as
water, alcohols, amines, ketones, and aldehydes 17-ere changed considerably. A better understanding of the part played by the solid support may nom be obtained from a thorough report prepared by Ottenstein (26) on the more commonly used solid support materials. He discusses the differences between the Chromosorbs, firebrick, and Celite 545 as \vel1 as reviewing the attempts of other workers to deactivate these materials, either by chemical treatment to remove the active sites, or by saturation of the active sites with a more polar inaterial in the stationary phase. A brief discussion of other solid supports such as glass beads, metal helices, Tide, and Teflon n a s included. The use of glass beads and small amounts of stationary phase has been particularly applicable for the separation of some solid organic compounds (12). A recent study (3) of the adsorptivity of Silocel firebrick was made in which the adsorption isotherm, determined by frontal development, was correlated with the elution method using uncoated firebrick. Emphasis was placed on the chemical treatment of the active adsorption sites, believed to be the hydroxyl groups of the siliceous material, nith hexamethyldisilizane to modify the support and thus prevent tailing. Suppression of any remaining activity with a trace of polar material (polyethylene glycol) was recommended. Since the affinity of many materials has been studied in relation to the solid support and some stationary phase, it would appear that further study of the support material alone would be beneficial. I n such a study the retention volumes of any material on the support should be the same as that of any inert gas if there were no adsorption effects. By such a study the effect of chemical
