Sensitive lo nizatio n Cross-Section Detector for Gas Chromatography J. E. LOVELOCK, G. R. SHOEMAKE, and A. ZlATKlS Department of Chemistry, University o f Houston, Houston, Texas
b The ionization cross-section detector i s recognized as the only catholic ionization detector which i s precise, reliable, robust, and linear in its response. The only drawbacks are lack of sensitivity to smallest quantities and a weak signal to unit mass of detectable substance. The sensitivity of this detector has been increased more than 100-fold by the simple expedient of reducing the sensing volume of the detector. The improved small volume ionization crosssection detector retains all of the good qualities of the method and i s potentially valuable in all applications of gas chromatography where reliability and precision at moderate sensitivity are required.
T
ionization cross-section detector of Pompeo and Otvos (6) is in all respects but one the closest approach to the perfect gas chromatography detector. It has a response which is precise, catholic, linear to 1 0 0 ~ ogas or vapor concentration and which, for any given molecule, can be calculated from the known properties of its constituent atoms. It is also rugged, insensitive to changes in carrier gas flow rate, and does not require a precisely regulated source of polarizing potential. The only weaknesses are insensitivities in terms of the smallest quantity detectable and in terms of the signal output given for unit mass of detectable substance. The lunar and planetary probe experiments of the Xational Aeronautics and Space Administration (7) contemplate the use of gas chromatography for gas and vapor analysis. There is a pressing need in these experiments for a rugged detector with almost ideal performance characteristics. This need has stimulated the exploration and re-exploration of detection methods. One of the more promising approaches to the problem appeared to be the improvement in the sensitivity of the ionization cross-section detector. Martin ( 4 ) suggested that the sensitivity of a gas chromatography detector could be improved by reducing the mass of gas within the sensing volume of the detector. This could be achieved either HE
460
ANALYTICAL CHEMISTRY
by increasing the temperature or by decreasing the pressure or volume of gas within the detector. Examples of the practical use of the detector in gas chromatography problems are given, for it appears to have potentialities in gas chromatography generally as well as in the special problems of extra-terrestrial gas and vapor analysis which stimulated its development.
by recombination or other causes; the rate of ion production is therefore directly related to the observed current flow. I n practical gas chromatography, i t is usual to offset the current flow associated with pure carrier gas and observe as a signal only the increase due to the presence of test gas or vapor. The current, I , accompanying the presence of a vapor concentration (in molar fractions), s,is
PROCEDURE
Physical Basis. An ioni7,a t'ion crosssection detector consists of a n ionization chamber connected t o a source of potential a n d some means of measuring the current flow within it. T h e gas within the chamber is irradiated with ionization radiation from either a n internal or a n external source. T h e ionization current when the chamber contains a light gas such as hydrogen or helium is small but increases on the addition of any other gas or vapor. The increase in current is due to the fact that to a first approximation the total cross section for ionization of a gas is proportional to the total number of electrons in each gas molecule. Gases and vapors other than hydrogen and helium are comparatively richly endowed with electrons. A comprehensive account of the physical basis of ionization cross-section methods of chemical analysis is given by Otvos and Stevenson (6). These authors report that the total molecular cross section for ionization by a given radiation is the simple addition of the constituent atomic cross sections of the molecule for the same radiation and is almost completely independent of the nature of the chemical combination. It follows t h a t the passage of radiation through a gas in a n ion chamber of volume, V , produces ion pairs a t a rate, i, as
i
=
PV KZxQ RT
(1)
where z& is the total molar fraction and ionization cross section of the gas. P , R, T are the pressure, gas constant, and absolute temperature, respectively, and K is the proportionality constant determined by the chamber dimensions and radiation intensity. I n practice, the potential applied to the chamber is always sufficient to collect the ions without significant loss
Where Q Z and Qc are the molecular ionization cross sections of the test compound and carrier gas, respectively. These two relationships assume ideal conditions for t'he collection and production of ions within the detector. I n particular it is assumed that only a small proportion of the energy of the incident radiation is absorbed by the gas; also that no significant loss of ions by recombination, or other cause, takes place. I n practice these itleal conditions are not difficult to meet. The factors affecting the lower limit of detection by the cross-section niet'liod are apparent in the relationships above. There nill be a background current attributable t,o bhe ionizabion of the carrier gas and this will fluctuate with changes in ambient temperat'ure and pressure. The production and collection of ions are stochastic processes and will also cause random fluctuations in the current f l o ~ .The sum of these random changes in the detector current constitutes the noise level and sets the limit for the least detectable signal. The noise due to temperature and pressure changes is to a considerable extent controllable and, so long as it is less than the noise from the production and collection of ions, can be disregarded. The noise level from temperature and pressure changes will, however, increase directly with increasing ionization; whereas the stochastic noise from ion production and collection increases only mith the square root of the rat,e of ion production. It follon.; that the performance of the detector will improve as the radiation intensity is increased until the noise from temperature and pressure changes is as great as that from ion production.
iss
Figure 1. Small-volume ionization cross-section detector A.
Chamber electrodes are radiation sources
Practical experience indicates that this point is reached a t an ion current in the region of 10-7 ampere. It is assumed that the detector geometry and applied potential are appropriate for the efficient collection of all of the ions produced; this is not difficult to achieve with a detector of the type described later in this paper. The esperimental conditions corresponding to this ideal limit of detection for the cross-section detector include an ionization current of lo-' amp. in hydrogen mould, in the most favorable circumstances, be ascociated with a nosie level of 10-l~ amp.; a noise level of similar magnitude would be generatcd by a pressure fluctuation of one microbar per second or by a temperature change of 10-3" C. per second. The total noiqr level nould be in the amp. so that a region of 3 X signal of lo-'* a.mp. should be discernible, such a signal would be given by COS, for example, a t a concentration of one part per million. Equation 2 indicates that if the radiation intensity is adjusted to maintain the ionization current constant, then the pressure or volume within the detector can be decreased without change in performance. This implies t h a t although the lower limit of detectable concentration is unaltered, the lower limit of mass detectable can be iniproved. Thus, a detector with a volume of 1 pl. a t atmospheric pressure or of 1 ml. a t a pressure of 1 millibar could see as little as gram of C 0 2 (one part per million in this reduced volume). These conditions are not far beyond practical reach; they imply a n ionization efficiency of 0.05yo and a detectivity a t least as good as t h a t which can be achieved with the flame ionization detector usually considered to be highly sensitire, The calculations highlight the paradox between vapor mass and vapor concentration detectivity. Carried to the ultimate limit, a detector with a volume so small t h a t only one molecule could
be accommodated at a time n-ould be both uniquely sensitive and totally insensitive according to the units of measurement considered. Fortunately, most of the recent advances in gas chromatography, such a'. capillary tube and lightly coated columns, present their small vapor samples to the detector a t higher concentration. than formerly. They need from the detector an ability to see small masses and a fast response time. The small Cross-section detector readily satisfies these needs. The only practical problem opposing the construction of a small volume ionization cross-section detector is in the choice of the source of radiation. For the lowest possible noise level we need a radiation source and detector geometry such that the least number of ion pairs is generated by each particle of incident radiation. Because a particle of ionizing radiation produces and collects ion pairs effectively and instantaneously, the fluctuations in ion current will not depend on the number of ions collected in unit time a t the electrodes, but on the number of primary generating particles in unit time. A particle generating 105 t o 106 ion pairs nould be for the same ion current very much more noisy than one generating 1 t o lo2 ion pairs. cr-Radiation, R-hich generates a t atmospheric pressure lobion pairs per centimeter of travel through a gas, would be efficirnt in providing a high current with a low radiation intensity but intolerable from the viewpoint of noise level. ?-Radiation a t high energies would produce no more than one ion pair per centimeter of travel but a n ion current of 10-7 amp. might need a hazardously large quantity of radioactive material. 0-Radiation seems to provide the best compromise between these extremes. The proper function of the detector requires t h a t the particles of radiation lose only a small proportion of their energy in the gas. For this reason i t is usual t o recommend the use of high energy @-particles such as those from P. X o w that a considerable reduction in chamber volume or pressure is contemplated, the use of weak 0-particles becomes feasible. A cross-section detector with a l-cm. path a t atmospheric pressure would function poorly with P-radiation from H3, for these have a range of only 1 to 2 cm., even in hydrogen. A path of 1-mm. would be acceptable with this radiation, for then only a small proportion of the radiation would be totally absorbed in the detector chamber. Tritium-containing, sealed sources of p-radiation are readily available and were convenient to use in the experimental development of the detector. The experiments to be reported in this paper were made entirely with tritiumcontaining sources. Alternative and more stable radioactive sources for high temperature or other severe conditions could always be used.
B I
Figure 2. Experimental arrangement for measurement of detector performance A. B.
C.
D. E.
F. G.
H.
Carrier gas inlet Test gas inlet Three-way valve Mixing chamber Detector Battery Electrometer Recorder
The relationships describing the function of an ionization cross-section detector predict a linear response t o vapor concentration over the entire dynamic range of the detector; they also suggest that the response to a known concentration of any given substance can be calculated from the cross sections for ionization of the condituent atoms of its molecules. The validity of such predictions is discussed at length in the work of Deal et al. ( 2 ) and of Boer ( 1 ) . Briefly i t can be said that provided the Conditions for the efficient production and collection of ions are obaerwd, the relationships express n ith conbidprable accuracy the performance of the detector.
Construction. Figure 1 ~ h o w st h e dimemions and materials of construction of t h e experimental detector. T h e ion chamber is formed b y a rectangular section Teflon ring of 1-cm. internal diameter. This ring is closed between two parallel sheets of metal foil, each 1.2 e m . long and 0.8 em. wide. T h e small gaps between t h e inside of the ring and t h e edse of t h e foil permit the passage of gas through the chamber. The foil is held firmly in place against the ring by two stainless steel disks and the whole assembly mounted inside a closed-end cylindrical Teflon tube. T h e stainless steel disks each include a hollow coaxial tube which passes through the outer Teflon body of the detector: these tubra serve the double purpoqe of gac and electrical conduction. The sheets of foil enclosin t h e detector chamber are cut from tain stainless steel sheet onto which is coated a thin film of titanium or zirconium containing occluded tritium ; they serve therefore both in the production of ions as the radioactive sources and in the collection of ions as the chamber electrodes. The plane parallel design for the detector chamber was initially chosen because of the ease with which VOL 35, NO. 4, APRIL 1963
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PLATE S E P A R A T I O N
Figure 3. and N2
changes in the thickness or diameter of the Teflon ring would permit equivalent changes in the electrode separation of chamber volume. It was soon found however that it possessed other more important advantages. K h e n used with two identical electrodes, the chamber is electrically symmetrical so that the development of spurious signals from contact potentials, which can occur with dissimilar electrodes, is no longer a problem. EXPERIMENTAL
The performance of the detector was investigated using the experimental arrangement shown in Figure 2. An accurately known flow of carrier gas was passed into a 250-ml. mixing vessel containing a magnetically driven stirrer; both the vessel and the stirrer were made from Teflon. The outlet from this vessel passed directly to the detector. The ionization current of the detector mas measured, using a vibrating reed electrometer, in terms of the potential developed across a known high resistance. At the commencement of a calibration experiment, the supply of carrier gas was temporarily disconnected. A test gas passed into the mixing vessel until the steady signal from the detector indicated the presence of undiluted test gas throughout the system. The supply of test gas was then cut off and the carrier gas flow restored; this flow was maintained until the detector, by its steady signal, indicated the presence of uncontaminated carrier gas within the detector volume. During the progressive dilution of the test gas, the detector signal was recorded graphically by a potentiometric recorder. At any time, t, after the commencement of a n experiment the concentration of test gas, C, remaining in the mixing chamber s given by C = Co exp
462
mm.
Relationship between ionization current and electrode separation in
-
L7t/V
ANALYTICAL CHEMISTRY
(3)
Hz
where Co i R the initial concentration, namely 100%; T’, the chamber volume, and C7, the gas flow rate. From the recorded signal, the response of the detector to any test gas concentration can readily be discovered. A more detailed description of this calibration method and the precautions necessary for precise measurement are given by Lovelock ( 3 ) . Measurements were made using the following test gases: .4r, CHI, CF4, NP, 0 2 , GO,, SFG, COS, CIHS, C2H6, and CIH10, with hydrogen and helium as the carrier gases. The effects of variations of the electrode separation and applied potential of the response of the detector to different gases were also determined by this method. Electrical Characteristics. Figure 3 shows the ionization current in hydrogen and nitrogen a t S T P for various electrode separations, all a t a n applied potential of 300 volts. Figure 4 shows the ionization current in different gases a t various applied potentials. Here t h e electrode separation was constant a t 1 m m . I n both the above experiments the quan-
tity of tritium in the sources was between 100 and 200 me. With as close a separation as 0.5 mm., no field intensified ionization is observed, even with an applied potential of 300 volts (6000 volts per centimeter). Also in the strongly electron absorbing gas, SFG,no loss of ions by recombination is seen above 100 volts applied potential. I n practical gas chromatography, recombination losses as severe as those t o be expected in undiluted SF6are not likely to be encountered. It may safely be assumed, therefore, that a polarizing potential of 100 volts is sufficient t o avoid errors from this cause. The chamber volume of 0.5-mm. electrode separation is only 40 pl. The ion current, nevertheless, even with the modest quantity of tritium still exceeds 10-9 ampere in hydrogen. An even smaller detector is obviously practical, especially by reducing the internal diameter of the chamber. Sensitivity. Table I L t s the general perforniance characteristics of the detector. They were measured with a n electrode separation of 1 mm., a polarizing potential of 300 volts, and n i t h hydrogen as the carrier gas. The apparent ionization efficiency of the detector, assuming a f l o rate ~ of 80 pl. per second, is one in 107. This is small but is at least 50 times greater than that ivith previous version> of this detector. The detectivity for small vapor concentrations, independent of detector volume, has increased modestly in the range of from 3 to 10 times. This improvement is attributable to the improved signal-to-noise ratio accompanying the use of a more intense source of radiation. The detectivity for small masses shows the greatest improvement and a t 10-9 gram per second is beginning to encroach upon the province of the more sensitive ionization detectors. Dynamic Range. T h e results of “eYponentia1” dilution experiments with different test gases are shown in Figure 5. The detector signal is given i n logarithmic units so t h a t a linear response t o varying gas concentration will appear as a straight line. It can be seen that even with a “heavy” gas, such as SFG or CsHs, the
Sfl
-
N1
n1
0
10
50
100 VOLTAOE
1so
200
300
Figure 4. Relationship between ionization current and applied voltage for detector with a 1 -mm. electrode separation and when filled with Hz, Nz, and SFB
300
,
,
1 1
loo
30
10
3
1
.3
CONCENTRATION, vi
Figure 5. Calibration of detector with a 1-mm. electrode separation by exponential decay method
response is linear t o a concentration of 60% by volume and an increasing, although nonlinear, response is retained to 100%. This slight departure from linearity above 60% is attributable t o the total absorption of a n increasing proportion of the tritium 6-particles in the heavier gas. Figure 6 illustrates the effect of electrode separation on the concentration at which a nonlinear response occurs. At 0.5 mm., the response is linear nearly to 90% by volume, n-hereas at 2 mm. a departure from linearity is apparent at 5oyO,.A more energetic radiation
Table I. Performance Characteristics of Small ionization Cross-Section Detector Compared with Those of Typical Ionization Cross-Section Detector
(Measurements made with a band pass of 0 to 1 cycle per second) Small Typical Characteristic detector detector Ionization efficiency 10 -7 2 x 10-9 Linear dynamic range 3 x 10: 104 Noise level 10-13 (ampere) 10 - 1 3 Background current in hydrogen (ampere) 5 x 10-9 5 x 10-11 Minimum detectable quantity (gm. per sec.) 10 --B 3 x 10-7 Minimum detectable concentration by vol. 2 x 10-6 2 x 10-5 Carrier gas Hz H2 Detector volume 0.08 5 (ml.) Substances detectable All All
source than the very weak H3 6-particles would, of course, completely avoid this small departure from linearity at high gas concentrations. I n practical gas chromatography, a nonlinear response a t concentrations above 5oY0 is hardly of consequence. Vapor concentrations this high are rarely if ever observed. Radiation Sources. Tritium-containing sources of 0-radiation were used in all of t h e experiments here reported. Under extreme conditions, such as temperatures over 225” C. or possibly in space vacuum, t h e occluded tritium may escape from t h e source. However, for t h e general practice of gas chromatography, t h e long life, safety, a n d convenience of tritium sources are sufficient t o make them the radiation source of
choice. For space probe use, reliability is a paramount need and a n y one of the many possible more stable radiation sources are preferable. Carrier Gases. Carrier gases, other t h a n hydrogen or helium, have relatively large molecular ionization cross sections. T h e cross section of K2, for example, is 6.40 comparr3d with Qc equals 2 for HP. The upe of a carrier gas with a larger molecular cross section for ionization increases the background current and noise level and decreases the signal given b y compounds whose own cross sections are not greatly different from that of the carrier gas. h change from hydrogen to KZor Ar as a carrier gas decreases sensitivity by a factor of 3 to 10 times according t o the conditions and compounds to be measured. The rare gases Ar and H e should never be used as carrier gases in their pure or partially pure state if accurate measurements are desired. The radiation in the detector will produce sufficient metastable H e or Ar to cause t h e detector to function simultaneously as both a cross section and an “argon” or “helium” detector. I n such circumstances, high sensitivity would be found b u t a t the price of an unpredictable response to different substances. If the applied potential is relatively high (above 100 volts), these anomalous effects with the rare gases will be greatly increased due to the field intensified multiplication of metastables. Helium and argon saturated with water vapor seem to be free of these objections to their u4e as carrier gases in the cross-section detector. Effects of Temperature Pressure and Gas Flow Rate. The current flow in the detector varies directly with t h e pressure and inversely with t h e absolute temperature of t h e gas within it. \Then used with hydrogen or helium as t h e carrier gas, t h e detector is not a sensitive thermometer or manometer; with t h e detector described in this paper, the temperature and pressure fluctuations
TIME MIN.
Figure 6. Effect of electrode separation on linearity of response of the detector Exponential decays of N2 as test gas
VOL. 35, NO. 4, APRIL 1963
463
As expected, the molecular and atomic cross sections for ionization by tritium radiation are different from those found with other electron energies. Nevertheless the total molecular ionization cross section is still simply the sum of the constituent atomic cross sections. Table I1 lists the atomic ionization cross sections for tritium 0-particles relative to hydrogen which is arbitrarily taken to be unity. Boer ( 1 ) discussed the relationship between the response of the detector to equal masses of different compounds in terms of the molecular and atomic ionization cross sections. H e showed that a simple practical response factor, f, was given
i
Figure 7. Chromatogram of separation of low boiling organic compounds
Column 200-ft. of 0.03-in. diameter stainless steel tube coated with polyglycol stationary phase. Column load, 10 pg.; temperature, 25'C.j inlet pressure, 3 p i . ; carrier gas, hydrogen
which would equal the stochastic noise level are 0.03" C. and 0.1 millibar per second, respectively. These are comparatively high rates of change of temperature and pressure so that in normal use it is unnecessary to use a compensating ionization chamber t o offset noise or drift. The detector is unaffected by changes in gas flow rate unless these are accompanied by turbulence sufficient to generate temperature and pressure fluctuations in the detector. Response Factors. O t r o s and Stevenson (5) measure molecular ionization cross sections using impacting electrons with mean energies of approximately 0.1, 30, and 600 k.e.v. The ionization cross section varied for a n y given compound with the energy of the impacting electron] but for each of the three different electron energies the total molecular cross section was consistently the sum of the constituent atomic cross sections of the molecule. The mean energy of tritium 0-particles is 5 k.e.v. which is different from the electron energies previously studied.
Table II. Relative Atomic Ionization Cross Sections for Tritium Radiation (Cross section of hydrogen atom taken as
Atom H C N 0
F
S
Ar
464
unity) Relative ionization cross section 1.00 3.69 3.20 4.56 4.08 8.75 9.98
ANALYTICAL CHEMISTRY
il i.
ii,
J
30
5
2
3
A1
Figure 8. Series of chromatograms from various quantities of air applied to a 6-ft. 5A Molecular Sieve column
less with these than with other carrier gases. No adverse effects on the precision of measurement result, however, from their use. DISCUSSION AND RESULTS
The three chromatograms, Figures 7 to 9, illustrate the use of the detector in some of the more severe problems of gas chromatography. Figure 7 , a capillary column chromatogram, illustrates that the detector volume is small enough, its response fast enough, and sensitivity high enough for it to function in a region commonly thought to be the special province of sensitive and fast responding detectors. Figure 8 is a series of chromatograms illustrating the separation of oxygen and nitrogen from air on a Molecular Sieve column. The column loads range from 1 to 30 pl. Figure 9 IS a chromatogram of the separation of four steroids at 240" C. on a 12-foot SE 30 stationary phase column. Here the detector was operated, a5 the noise level shows, near the limits of detection. Severtheless a few micrograms only of the steroids provided a chromatogram from nhich an estimate of the loss of steroid in the system could be made. No other detector a t present available could have provided such information. Both the gas density balance and the crois-section detector have a responae n hich can, for any substance, be calculated from the known properties of its constituent atoms. All other detectors require calibration. It is not possible to calibrate a detector using a substance n hich is to an unknown extent lost during its passage through a chromatographic column. The gas density balance in the forms so far described is not sufficiently sensitive for the chromatography of steroids. The small
Carrier gas, hydrogen
where Qz and QC are the molecular ionization cross sections for the compound and for the carrier gas, respectively, and JI, the molecular weight of the compound. Boer's results were with YN radiation but as discussed above are in practice applicable to tritium radiation also. This relationship predicts that in a homologous series the response factor ail1 tend toward a constant value when the ionization cross section of the compound is large compared with that of the carrier gas. With hydrogen or helium as the carrier gas this constant response is reached for relatively simple compounds. W t h other carrier gases such as Nq, Ar, or COZ the difference, (Qz - QJ, will for simple compounds be much smaller or even negative. This means that the detector sensitivity is
Figure oids
9.
The separation of four ster-
Column, 12 ft. of 1% SE 3 0 stationary phase; temperature, 240' C.; carrier gas, hydrogen
cross-section detector remains the only one available for the assessment of such losses. The results show that the sensitivity to small masses of the ionization crosssection detector can be improved 100fold without any loss in its other desirable characteristics. There is also no reason, in principle, against a further 100-fold improvement in sensitivity which m-ould set the limit of detection in the region 10-1‘ to 10-12 gram per second. The present version provides a precise and reliable detection method for all applications of gas chromatography except extreme dilution trace analysis. For this, a detector with a high sensitivity to low gas or vapor concentrations is needed. It is important to note, however, that a catholic detector with a
high sensitivity to lo^ vapor concentration is very difficult t o handle. The contamination of the system with ITater vapor in the range 1 t o 100 parts per million is almost impossible to avoid and would came noise and drift with such a detector. The fact t h a t the small cross-section detector is sensitive to small masses rather than to small concentrations is for most purpose., therefore, a n advantage. The detector, differing perhaps in its mode of construction and radiation qource, would seem well suited to the needs of lunar and planetary gas chromatography. LITERATURE CITED
(1) Boer$,H., “Vapor Phase Chromatography, D. H . Desty, ed., Trol. I ,
Butterworths, London, 1957.
( 2 ) Deal, C. H., Otvos, J. W., Smith,
s.,zucco, P. s., ANAL.CHEY.28, 1958 (1956). (3) Lovelock, J. E., in “Gas Chromatography 1960,” R. P. W. Scott, ed., Butterworths, London, 1960. (4)Martin, A. J. P., “Vapor Phase Chromatography,” D. H. Desty, ed., Butterworths, London, 1957. (5) Otvos, J. W.,Stevenson, D. P., J . Am. Chem. SOC.78, 546 (1956). (6) Pompeo, D. J., Otvos, J. W. (to Shell Development Co.) U. S. Patent 2,641,710 (1953). (7) Wilson, E. M., Vango,,?., Oyama, V., “Gas Chromatography, N. Rrenner, ed., Academic Press, New York, 1962, in press. RECEIVED for review September 7 , 1962. Accepted February 5, 1963. This work was supported by a grant from the National Aeronautics and Space Administration (XSG 199-62). Presented a t the International Symposium on Advances in Gas Chromatography, University of Houston, Houston, Texas, January 21-24, 1963. V.
N e w Radioactive Gas Chromatographic Detector for Identification of Strong Oxidants B. J. GUDZINOWICZ and W. R. SMITH Monsanto Research Corp., Boston laborafories, Everett 49, Mass.
b Recent investigations conducted a t this laboratory using a krypton-85 quinol clathrate as a source of radioactivity have shown that some gaseous inorganic oxidants eluted from gas chromatographic columns do react with the clathrate, releasing krypton atoms, which subsequently can b e detected b y a Geiger ratemeter. Comparison of the chromatogram and the ratemeter trace identifies the oxidants in a gas mixture. In addition to fluorine, which could b e detected quantitatively a t a 1 to 20 p.p.m. concentration level in air, qualitative results of exploratory studies have shown the following gases to possess high reactivity with the Kr-85 clathrate: Br2, NO2, OF2, C12, N02F, and N02CI.
F
OR ~ . 4 m YEARS, the thermal conductivity cell (thermistor and filament wire) has been the most nidely used detector in gas chromatography. However, new analytical problems have required development of ne,v concepts and designs of highly specific, sensitive detection systems based on ionization by @-emitter, electron capture, and flame. The Lovelock electron capture detector (6, 7 ) is specific for some polynuclear aromatics and halogenated materials. Flame and argon ionization
devices (5, 8 ) have also been evaluated; the linear range, detectability limits, and response of various organic compounds with characteristic functional groups incorporated in their structures (11) have been reported. I n the organic and biomedical fields of research, new techniques and detectors for liquid and gas chromatography permit radioassay of tagged components in a n effluent stream by direct, on-stream monitoring. We have shown that some gaseous inorganic oxidants eluted from gas chromatographic columns react TT ith a krypton-85 quinol clathrate to release the radioactive krypton atoms, which subsequently can be detected b y radioactive counters. This is a new analytical approach offering a F a y qualitatively to identify oxidants in gaseous mixtures based on their reactivity with the clathrate. Simultaneous records derived from both the chromatographic and radioactivity detectors can be made. The system also seems amenable t o quantitative work. EXPERIMENTAL
Apparatus a n d Reagents. T h e gas chromatographic instrument employed for these basic investigations was t h e Pcrkin-Elmer 154C Vapor Fractometer operated a t 20” C. a n d
equipped with a 20-foot long b y l/c-inch diameter copper column packed with 33% by weight KO.3 Kel-F polymer oil (Minnesota Mining and Nanufacturing Co.) as stationary liquid phase loaded on 35- t o 80-mesh Chromosorb W (Johns-Manville Co.). The preparation of this column packing has been previously reported ( 3 ) . Reproducible 0.25-cc. and 1-ec. smiples of gases are introduced into the chromatograph through a gas sampling valve accessory constructed of stainless steel and Teflon plastic for corrosion resistance. The inlet pressure and flow rate of the helium carrier gas a t one atmosphere and 20’ C. were 15 p.s.i. and 41 cc. per minute, respectively. The components of the krypton-85 quinol clathrate detection system are identified in Figure 1. The krypton45 clathrate is prepared by crystallization of hydroquinone in the presence of gaseous krypton435 (9). As the crystals form, krypton atoms are trapped within the structure. illthough the clathrate is very stable a t room temperature, i t is temperature sensitive (IO), with rapid release of the caged component above 165” F. At room temperature, Chleck and Ziegler (4) estimated that leakage of Kr-85 from the clathrate structure is no greater than a few parts per million per day. Krypton-85 is a nearly pure @-emitter having a half life of 10.3 years. Important in its use is the fact t h a t krypton is a n inert gas and cannot be stored or metabolized VOL. 35, NO. 4, APRIL 1963
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