implies uniformity of the flow pattern around the thermistor bead, If the pressure is increased, the density of the flowing gas is increased and the flow rate must be reduced in order to get back t o the same flow pattern. If we consider the geometry of the microdetector, the thermistor bead (diameter 0.84 mm) is contained in a narrow channel of diameter approximately 2.4 mm. The bead itself is spherical. When the gas from the column reaches the narrow channel of the detector, its linear velocity must increase considerably because of the decrease in cross-sectional area available for flow. We estimate that at a volumetric flow rate of 100 ml/minute a t column outlet, the linear flow rate through the detector is 40 cm/second. This relatively high linear flow rate around the bead leads to a Reynolds number of approximately 18 (at 100 ml/minute and 2 atm). At this Reynolds number, a stationary object in a gas stream will have a well developed “wake” behind it and this wake will grow with increasing mass flow rate (4). The heat transfer characteristics of the bead will change as the wake occupies a greater proportion of the downstream area and, if the wakes on both sides of the detector are not equally developed, variation in detector response is inevitable. In the case of nitrogen, these variations appear to occur in the region where peak inversion requires only a very small change in heat transfer, thus the variations produce positive or inverted peaks for small changes in flow rate. With methane, however, these variations all occur on the positive side of peak response. The results for methane at various pressures also fall on one curve when plotted against mass flow rate. These two
curves will show maxima and minima a t about the same flow rates provided that the differences in viscosity and density of the two gases are allowed for in accordance with the definition of Reynolds number. A few runs were carried out with ethane as carrier gas and it appears that the data obtained would also show the same maxima and minima if p and 9 are allowed for. This is a possible qualitative explanation for the anomalous response a t moderate flow rates although, when the first packed column was exchanged for another of rather different permeability, the results obtained still seemed to fit on the curve shown, in spite of the fact that the flow pattern on the reference side of the detector must have changed somewhat. Probably this will need more investigation. Microdetectors, when used on conventional columns, inevitably involve fairly high linear flow rates, and in a thermistor detector with straight flow-through design the heat loss from convection is bound to be high (5). However, with a knowledge of the response curve, the high sensitivity of these detectors need not be lost as one can work in a region of high positive response. This could be achieved either by use of low flow rates or by splitting flow and taking only a proportion of it to the detector. As this study has indicated, the pressure of the flowing gas must be taken into account and the flow rate reduced accordingly to reach low mass flow rates.
(4) L. Rosenhead, “Laminar Boundary Layers,” Clarendon Press,
(5) A. B. Littlewood, “Gas Chromatography,” Academic Press, New York, 1962.
London, 1963, p. 102.
RECEIVED for review July 10, 1967. Accepted September 15, 1967.
Direct Current Discharge Spectral Emission-Type Detector Robert S. Braman’ and Alexander Dynako IIT Research Institute, 10 W . 35th St., Chicago, Ill. 60616
A direct current discharge in helium carrier gas was studied for use in detection and qualitative identification of materials in the vapor state eluted from gas chromatographic columns. Emission spectra obtained, optimum operating conditions, limits of detection, and the influence of various factors on detector response were studied. The dc discharge detector can be operated using a small power source with either selected interference filters or a spectrometer to yield limits of detection in the to gram per second range. Atomic emission for F, CI, Br, and I was obtained from halocarbons with limits of detection in the gram per second range. The influence of compound structure on CN, C2, CH, and C relative emission intensities was briefly studied.
USE OF ELECTRICAL DISCHARGES in detection devices or for analysis is not new. Both ionization and emission processes have been utilized. An argon ionization-type detector was devised by Yamane (1, 2 ) who used a subsidiary electrical 1 Present address, Department of Chemistry, University of South Florida, Tampa, Fla. 33620.
(1) M. Yamane, J . Chromatog., 9 , 162-72 (1962). (2) Zbid., 11, 158-72 (1963).
discharge to produce metastable argon species. This detector was probably similar to the photoionization detector reported by Lovelock (3) and the dc discharge detector of Karmen and Bowman (4). Ionization is measured in all of these detectors when metastable argon atoms collide with organic compounds. More recently, a very similar commercial model direct current discharge ionization-type detector has been reported based upon measurement of ionization produced when organic materials pass through a corona discharge in helium (5). This is also an ionization-type detector of high sensitivity and of low selectivity. The study of emission spectra of organic vapors is quite old. The first work was done in the 1920’s by Stewart and coworkers (6). Similar work continues today. In the early work, attempts were made to use emission spectra for qual(3) J. E. Lovelock, Nufure, 188,401 (1960).
(4) A. Karmen and R. L. Bowman, “Gas Chromatography,” N. Brenner, J. E. Callen, and M. D. Weiss, Eds., Academic
Press, New York, 1962, p. 189. ( 5 ) C. H. Hartman and K. Thompson, Varian Aerograph, research notes, Spring 1967. (6) A. W. Stewart and C. L. Wilson, “Recent Advances in Physical and Inorganic Chemistry,” 7th ed., Longmans, Green and Co., London, 1946. VOL 40, NO. 1 , JANUARY 1968
95
itative and quantitative analysis of vapors by observing emission from fragments larger than diatomic molecules or atoms (7). It was concluded, however, that this was not feasible. Attempts to observe emission from the larger mass fragments had generally resulted in failure, mainly because all of the several types of discharges employed fragmented the molecules to too great an extent. It is also possible the emission intensities of the larger fragments were weak or obscured by diatomic molecule emission. McGrath et al. (8) have reviewed the early and more recent work on the analysis of gases and vapors by spectroscopic techniques. They also concluded that the analysis of organic compounds seemed improbable. The analysis of inert or permanent gases by spectroscopic techniques has proven to be feasible. The earliest work is probably that of Mouren and Lepape (9) who determined krypton in xenon. Since then devices based on observation of emission bands or lines have been described for detection of Nz in He (IO), N2 in Ar (11), and several other similar simple gas mixtures. The most effective use of emission spectra for analysis of both organic and inorganic compounds is a more recent development associated with the technique of gas chromatography. Sternberg and Paulson (12) and Grant (13) were probably the first to use emission for detection. They, respectively, developed gas chromatography detectors based upon observation of total emission from a Tesla coil discharge and hydrocarbon fuel flame. Nevertheless, no attempt was made in either case to use wavelength selection and very little study of the detectors was reported. The first work of any extent on the use of spectral emissiontype detectors in gas chromatography was carried out by McCormack, Tong, and Cooke (14), Juvet and Durbin (15), Braman (16), and Bache and Lisk (17). Juvet and Durbin used a hydrogen flame for observing chromatographed acetylacetonates of transition elements. Braman studied the use of the hydrogen-air flame emission for detection of chromatographed organic compounds. He also studied the use of dual-flame emission-flame ionization detection and the influence of structure on relative emission band intensities for component identification. McCormack, Tong, and Cooke (14) were the first to report the use of diatomic molecule and atomic line emission spectra in a microwave-stimulated plasma, Good selectivity and low limits of detection were obtained using this plasma. These authors mention briefly the attempted use of both a direct current discharge and a hollow cathode source unit as a basis for an emission detector. No data were presented and these approaches were apparently discarded by them in
(7) J. B. Austin and I. A. Black, J . Am. Chem. Soc., 52,4552, 4775 (1930). (8) W. V. McGrath, R. J. Magee, W. F. Pickering, and C. L. Wilson, Talanta, 8, 892 (1961). (9) C. Mouren and A. Lepape, Compt. Rend., 174, 908 (1922). (10) P. Emmett and R. E. Wilson, Talanta, 11, 1003 (1964). (11) H. Fay, P. H. Mohr, and G. A. Cook, ANAL.CHEW,34, 1254 (1962). (12) J. C. Sternberg and R. E. Paulson, J . Chromatog., 3, 406-10 ( 1960). (13) D. W. Grant, "Gas Chromatography 1958," D. H. Desty, Ed., Academic Press, New York, 1958, p. 185. (14) A. J. McCormack, S. C. Tong, and W. D. Cooke, ANAL. CHEM.,37, 1470 (1965). (15) R. S. Juvet and R. Durbin, J. Gas Chromatog., 1, 14 (1963). (16) R. S. Braman, ANAL.CHEM.,38, 734-42 (1966). (17) C. A. Bache and D. J. Lisk, Zbid.,37, 1477 (1965).
96
ANALYTICAL CHEMISTRY
favor of the microwave-stimulated emission (MSE) plasma for a detector. Nevertheless, the direct current discharge was considered by us to have several distinct advantages as a plasma for an emission detector. For example, the apparatus required can be made more compact in size than the microwave waveguide required for the MSE detector and the large microwave power source can be replaced by a smaller power supply. Consequently, a study of the direct current discharge (DCD) detector was undertaken. EMISSION PROCESSES
Emission-type detectors are based upon observation of electromagnetic radiation produced by excited species from materials introduced into a suitable plasma. The intensity of the radiation is dependent upon the population of the excited species present in the observed plasma. The wavelength of the radiation is dependent upon the nature of the atomic, diatomic, or poly-atomic species being excited in the plasma. The number and identity of the species produced is dependent upon the composition and structure of the molecules introduced into the plasma. Thus, for example, fluorobenzene in vapor form in helium carrier gas when introduced into the helium plasma of the DCD detector is fragmented into C, H, F atoms, Cz, and CH diatomic species at least. These atoms and diatomic molecules are excited by bombardment in the helium plasma and characteristic line and band spectra may be observed. The mechanism of molecular degradation and excitation of the observed species may be attributed to ionized or metastable helium atoms, to electron bombardment of the molecules, and possibly to thermal excitation processes. The discharge is initiated by bombardment of helium atoms by electrons. When the first resonance level of helium is reached (21.2 eV) and at slightly higher energies (24.6 eV) helium ions, He+, may be formed. Metastable helium may also be formed (19 eV). All of these species have sufficient energy to break molecular bonds and to excite all of the other elements. Thus, for example, atomic fluorine emission (14.5 eV) is observed when fluorine-containing compounds are present in the plasma. The metastable helium atom is likely the main species responsible for excitation, It must lose its energy by collisions. Metastable states have a longer average life (10-3 second) than normal excited states (10-8 second). Resonance states of helium may also be involved because the helium emission spectrum decreases in intensity as impurities in the helium increase. Although helium was the only gas extensively used with this detector, argon and the other inert gases probably could be used. Similar excitation mechanisms would be involved but excitation potentials would be lower, for example, 11-15 eV for argon. Spectra of various organic compounds and air in the helium plasma obtained by using the spectrometer were similar to those observed by many others (6, 7, 14). Air quenched the spectra of organic compounds. EXPERIMENTAL
Apparatus. The component parts of the DCD detector as used in a gas chromatographic analysis arrangement are shown in Figure 1. Because of the requirement for separation of air and other gases except helium or argon from detected components, a gas chromatographic column or similar apparatus must be used with the DCD detector. Suitable gas sampling systems are required to inject samples into the separation column.
PHOTOMULTIPLIER TUBE ASSEMBLY TUBING
MM ENTRANCE SLIT
PLASMA LENGTH I TYPICSALLY
I :I
Figure 1. Apparatus arrangement for DCD detector system
Gas sampling was performed by use of a Loenco, 6-port gas sampling valve in this study Sample loop volumes up to 5-10 ml could be injected although I-ml size samples were usually taken. Liquid samples were prepared in hexadecane and injected by means of a microliter syringe. Interference filters or a prism or grating monochromator may be used for wavelength selection. A Jarrell-Ash, 1-meter Czerny-Turner mount plane grating spectrograph was used in all of the studies reported here, unless indicated otherwise. A slit width of 400 p was generally used unless indicated. The power supply for the photomultiplier (PM) tube must be regulated to eliminate noise through changes in gain of the PM tube. A Keithley Instruments Model 242 regulated high voltage power supply was used, stability =tO.O1 after warmup and load regulation 1 0 . 0 0 5 x . A smaller regulated power supply would suffice for development of more compact equipment. A Keithley Instruments Model 417 picoammeter was used to provide for current amplification. This electrometer amplifier has internal suppression which was convenient for zeroing the output signal. Currents used normally to 10-9-A range. were in the A Westronics 0-2.2 mV/ll-inch strip chart recorder was used. A voltage divider network was used on the input of the recorder to more closely match its range with the electrometer output. The recorder was set to provide a fullscale deflection (10 inches) for a 2 0 z full-scale reading of the picoammeter. Detector Cell Design. A direct current discharge can be produced and maintained at atmospheric pressure in helium or argon carrier gas. Consequently, it was a simple matter to design and construct an emission detector cell suitable for use in gas chromatography. The detector cell designs used in these studies are shown in Figure 2. Hollow platinum
x
PM voltage, dc 1800 1600 1400 1200 loo0
B
A
Figure 2. Designs of the DCD detector cell tubing electrodes may be used instead of solid electrodes as shown, but most work was carried out with the solid electrodes (Figure 2 4 . Quartz capillary tubing, 1- or 2-mm id., and either 0.25inch or 6-mm 0.d. were used. Quartz tubing was superior to borosilicate glass because of its superior heat resistance and light transmission in the UV region. In later work, it was found that 6-mm quartz tubing, 2-mm id., blown out to 5-6-mm i.d. for a 3/4-in~hsection could also be used as a chamber. Less heating of the capillary walls or less reaction of components with the chamber walls was expected but not definitely confirmed by experiment. The heat from the discharge is confined largely to the glass walls of the capillary tube. Swagelok-type fittings were convenient to use for end connection to the quartz capillary. There was no indication of decomposition of the Teflon ferrules which were heated to no more than 60-70" C under operation at the usual 15-30 W. Platinum electrodes were silver soldered to the brass fittings. The Pt tube lead-in was assembled as shown in Figure 2B. One of the problems in using this detector cell is electrical insulation of the high voltage from the metal columns and tubing. This was accomplished by the use of a nylon or Teflon fitting where the column joined the detector cell. No effluent of vapors from these plastic materials was detected in the experiments conducted. Designs for the detector cell should provide for the quick and easy change of capillary tubing. Overloads of organic material cause a coating of carbon to form on the inside of the quartz tubing in the plasma zone.
Table I. Signal and Noise as a Function of Photomultiplier Tube Voltage S-20 tube, quartz window, dry-ice cooled Noise, normalized," cm Continuum Normalizeda Dark current Band 5165 A -5175 A response, cm2
...
S/Nb
254 435 x 103 1710 91.5 254 X 108 2370 68.4 6.35 25.4 63 x 103 1910 10.2 15 x 103 1180 2.29 5 . 1 3 x 103 2240 2.16 0.56 a All data converted to the same electrometer gain, full-scale recorder, 10 inches, was 0.2 X A. * Noise data assumed to be cm2assuming the noise had a bandwidth of 1 cm which was -35 sec. S/N is response/band noise. 254 107 33 12.7 2.28
VOL 40, NO. 1, JANUARY 1968
97
Table 11. Operating Parameters for a Small, Unregulated DC Power Supplya Current, V, W, Carrier gas Gap, inches mA plasma plasma 100% He 0.25 36 314 11.3 0.28 0.38 0.52 0.63 0.81 9 5 z He 5 % Ar
a
0.13 0.33 0.41 0.50
30 26 22.5 18 11
34.5 31 29 24.5 Carrier gas at room temperature, 22” C.
433 565 690 840 1075
13.0 14.7 15.6 15.1 11.8
310 430 500 635
10.7 13.3 14.5 15.6
Platinum slowly vaporizes from the metal electrodes during use and coats the capillary near the electrodes, but not in the discharge zone. This does not adversely influence the optical characteristics of the detector cell. In fact, the Pt coating shields the electrode ends which can glow red or white hot on occasion producing an undesirable black body radiation. Photomultiplier Operating Characteristics. A photomultiplier tube of the S-20 cathode response type, an EM1 9558QB (Quartz window), was used throughout the study. The quartz window perpits operation of the S-20 kube from approximately 1800 A to approximately 8000 A. This gives the S-20 tube with a quartz window the largest wavelength capability of all photomultiplier tubes available The tube was dry ice cooled to keep noise low. Since photomultiplier tubes can be operated over a range of voltages, the S-20 tube used was studied to determine the optimum voltage, if any. This was done by determining the dark current noise level over the range of voltages of interest and the response of chromatographed duplicate samples of n-hexane under the same 5xperimental conditions except PM tube voltage. The 5165-A CZ baad emission response was observed. The noise at the 5165-A CZband was determined with pure carrierogas. The noise in the spectral region just above the 5165-A C Z band was also determined. Data are given in Table I. Noise values are given here in centimeters and are actually a measure of the height of transient or random noise. Dark current noise apparently contributes appreciably to the noise above 1400 V dc. Although there appeared to be no optimum PM voltage below 1400 V dc, 1000 V was selected as suitable because high intensity emission would be less likely to overload the tube at this voltage and dark current noise was low. It was noted in this study that noise near an emission band was almost the same as the background noise on the band. Noise from fluctuations in concentrations of impurities in the carrier gas must be small as must be the concentration of impurities. The actual noise observed then is attribcted to fluctuation in the plasma or tooa continuum noise. Note that the dark current noise at 5175 A is a factor of 4 to 6 times lower than the other noise values at 1400 V and below. The dark current noise here includes contributions from the recorder, picoammeter, and photomultiplier tube electronics, Power Supp!y. An advantage of the DCD detector is its potential small physical size; hence a small compact power supply for the discharge would be desirable. In this study four different supplies were used. Two were large, 0-5000-V dc laboratory models capable of delivering 2000 V dc at 100 mA. A third was a smaller, 0-2000-V dc, 0-100-mA, Hewlett-Packard dc voltage or current regulated power supply. The fourth, a best selection, was a compact unregulated Ferrotran, Inc., Model SU 1000 power supply rated for 1000 V dc at 20 mA. This was a small packaged 98
ANALYTICAL CHEMISTRY
power source, 36/8 X 41/4 X 5 inches in size and weighed approximately 2 lb. The operating wattage of the detector depends upon the electrode gap and plasma gas composition. Consequently, a study of operating wattage was made. Data are given in Table 11. The power supply operated best with a 0.5-inch electrode gap. The data indicate that there was little effect for electrode gap on plasma power level. The He-5% Ar gas mixture did not greatly alter the operating power level. A load resistor must be used as ballast if such is not provided internally in power supplies. A 25-w, 12,000-ohm resistor was used for most of the work with the unregulated power supply. The operating wattages shown in Table I1 could be varied by using a Variac transformer to change the ac line input into the Ferrotran power supply. There was generally no difference observed in experimental data when going from one discharge power supply to another. Unless indicated, only the Hewlett-Packard power supply was used through the study. Procedures. CARRIERGAS. Several commercial grades of helium were used as the carrier gas from time to time. Because of the high sensitivity of the detection technique, impurities in the carrier gases were readily observed. Bands were observed arising from NB,0 2 , H20, COZ, and probably trace hydrocarbons in all helium sources employed. Air components, light hydrocarbons, and CO and COz are the major impurities listed for the commercial helium. Some of the helium sources had comparatively high levels of impurities. The Jarrell-Ash scanning spectrometer was suitable for detecting impurity levels- in the carrier gas for the discharg:. The region 3900-3870 A was scanned at a rate of 25 A/ minute. In purifitd carrier gases this region shows only the very intense 3888-4 helium atomic line with no banding near the sensitive 3883-A C N band. As the impurities increased, the He line decreased and C N banding increased. “Zero gas,” “high purity” and “ultrapure” helium grades were tested for impurity levels. By far, the best was the ultrapure grade, and unless otherwise specified, helium carrier gas was used throughout the study. It was necessary in all cases to remove impurities from helium by cold trapping in liquid nitrogen. A coconut charcoal, 8-14 mesh, was packed into a 3/8-in~h diameter copper tubing, 10 feet long and coiled. This coil was placed in a large Dewar flask and cooled with liquid nitrogen during operations. Long lead-ins were used to keep the cryogenic temperatures far from the nearest fittings. Swagelok fittings were used throughout the carrier gas-gas chromatography column apparatus. The cooled, charcoal gas absorption trap was found suitable for several days’ use without regeneration. To regenerate the coil it was removed from the Dewar flask and heated by hand for 5 minutes with a hot air gun. This was observed to drive off moisture and hydrocarbon impurities. The cryogenic cooling technique was sufficient to remove all Nz, CN, CH, and CZ banding from the ultrapure grade, but not from the other grades. Even so, OH, 0, and NH bands were always observed and very likely may be due to persistent impurity levels in the discharge chamber itself. No systematic attempt was made to study the removal of the OH, 0, and NH bands. Carrier gas flow rates were measured by means of a soap bubble flow rate apparatus conventionally used in gas chromatographic work. GAS CHROMATOGRAPHIC COLUMNS.Five types of gas chromatographic columns of various lengths were used throughout the experimental work. These are listed below. 5, 10, 30% Apiezon N on Chromosorb W, 60/80 mesh 20 % Carbowax 20M Chromosorb W, 60/80 mesh 10% Versamid 900 on Chromosorb W, 60180 mesh
1 4 z Kel-F on Gas Pak F, approximately 80 mesh Uncoated hollow copper tubing All of the above columns were used successfully except Versamid. Despite preconditioning at 250" C for 4 hours, the Versamid column continued to elute detectable levels of carbonaceous materials even at room temperature, 22 O C. Hollow copper tubes were used in the response linearity experiments with 2-bromopropane because they exhibited a lower column bleed than most of the other packed columns. Samples of the hexadecane-2-bromopropane mixtures were simply injected into the tubing through an injection port. Response peaks for the 2-bromopropane tailed somewhat but were suitable for obtaining response data. The Apiezon N columns were used in work with hydrocarbons and the halocarbons. Carbowax 20M columns were used with the alcohol series. Kel-F on Gas Pak F columns were used with the more polar compounds. Except for the use in the column temperature effect study, all columns were usually operated at 22" C but no higher than 50-60" C. The Apiezon N, Carbowax 20M, and Kel-F columns exhibited very little to no column bleeding up to 50-60" C. Because of the low limits of detection of the detector, column bleeding or the slow elution of previous sample components can be a problem. Care must be exercised to select columns having low bleed characteristics especially at higher column operating temperature. Backflushing would appear to be desirable. Column lengths varied from 6 inches to 20 feet; most work was performed with 3-foot long, 1/4-inch diameter packed columns. SAMPLE PREPARATION AND INJECTION.Solutions of volatile organic compounds were prepared in hexadecane. Samples were injected using 0-10-pl syringes. Air samples or gas mixtures were injected by means of a gas sampling valve. Liquid samples above approximately 0.1 pl in size cannot be passed through the discharge without adversely affecting the detector operation. The discharge is extinguished by liquid samples this size or larger. Although the discharge reinitiates after passage of most of the large component peak, considerable deposits of carbon form on the inside of the detector tube. These deposits block subsequent emissions from the plasma and, of course, change the response of the detector. In some cases the deposits of carbon could be removed by injecting air or oxygen samples but this is still not satisfactory for routine gas chromatograpic work. Rejection of solvent peaks was suitable. Stream splitting may also be suitable but this was not tried. Commonly available reagent grade chemicals of sufficient purity were used. The hexadecane solvent was distilled to remove light impurities.
>
1.71
I
I
I
I
I
I
I
I
1
I
-I
0
23.3 WATTSIINGH L
0 4 0 80 120 160 200 2 4 0 2 8 0 320 360 4 0 0 HE FLOW, HE, mllmin
Figure 3. Effect of carrier gas flow rate on response 1-mm x 0.75-inch plasma, CZ band response, 1.38-rg nhexane samples
with % Ar in He ranging from 0 to 15. Data are presented in Table 111; 5-6% Ar appears to be an optimum. Above this value the discharge becomes "thin." Below this value the response decreases somewhat. A gain in response is observed from addition of 5 6 % Ar to He. The gain is, nevertheless, small and consequently the main advantage in using 5-6% Ar in He is quenching of the 3888-A He atomic line. Quenching was observed to be nearly complete when 5-6% Ar was present in the carrier gas. Most of the subsequent studies were carried out using only He carrier gas because of the convenience of using a single carrier gas which avoids problems in cryogenic cooling of mixed gases to trap out residual impurities. Effect of Flow Rate on Response. The effect of carrier gas flow rates on detector area response at 5165-A (C,band) was studied by gas chromatographic analysis of 1.36-pg n-hexane samples over a range of flow rate values. Data are presented in Figure 3 which indicates an optimum flow rate near 140 ml/tninute. Small changes around the optimum flow rate will have small but significant effects on response. Flow rates should be carefully regulated if the most precise quantitative work is to be carried out. Thereason for the appearance of this optimum is not readily apparent. Higher helium flow rates may increase the gas pressure in the plasma and decrease the range of active helium
RESULTS AND DISCUSSION
Effect of Carrier Gas Composition on Response. Mixtures of argon in helium were studied as a possible improved carrier gas for the DCD detector. The argon would have the beneficial effect of quenching the He 3888-A line and thus permit use of the nearby 3883-A CN band for detection without requiring a high resolution monochromator. Pure argon gave a narrow diameter plasma discharge much less stable than the wider diameter plasma discharge obtained with helium. Nevertheless, mixtures of argon in helium gave plasmas of suitable size if argon were kept below approximately 10%. A series of experiments was performed for the purpose of obtaining quantitative data for evaluation of the use of argon in the helium carrier gas. Duplicate samples (1.36 pg each) of n-hexane were chromatographed using the various carrier gas mixtures. Carrier gas flow and plasma power were kept as constant as possible. Response data were obtained at CZ,CN, and CH band emission wavelengths
Table 111. Effect of Carrier Gas Composition on Response (1.36-pg n-hexane samples, 0.5-inch plasma length)
ReWavelength, A sponse,a and band cm2
Argon
Power, W
Flow, ml/min
5165 Cz 5165 Cz 5165 Cz 5165 Cz 4312 CH 4312 C H 4312 C H 4312 CH 3883 CN 3883 C N 3883 C N a
13.0 0 13.1 23.5 2.0 13.6 25.6 5.4 13.1 2.4 12.3 13.9 23.9 0 13.1 30.9 2.0 13.6 34.9 6.4 13.2 9.7 12.3 13.9 20.1 2.0 13.6 23.1 6.4 13.2 7.4 14.8 13.9 Normalized for 3 x 10-6-A electrometer range.
VOL. 40, NO. 1, JANUARY 1968
84 86 91 90 84 86 92 90 86 92 93
99
t
BI f m
5
1.0-
w ul z
0.5 In PLASMA 104rn1HEIMIN. 34WATTS/INCH
CN
a
9
a a NORMALIZED RESPONSE, cm*
Figure 5. Effect of electrode gap on response 1.38 pg of rz-hexane, 4312-ACH band . 2 - ' 1 ' 1 40 60 80
1
1
'
100
1 ' 120 T ,' C
1 1 140
1
160
I
I 180
I
200
Figure 4. Effect of column temperature on response 1.5
pg
of n-octane
atoms. This could decrease the number of collisions with carrier gas components by decreasing the effective plasma volume and subsequently decrease the number of excited diatomic molecules. The lower helium flow rates should by analogy continue to increase the population of excited diatomic species. An optimum is observed, nevertheless, at least for the CZband. Its decrease in response at much lower helium flow rates is tentatively attributed to dissociation with deposition or possible migration of atomic carbon or carbon particles out of the plasma zone. The residence time of sample in the plasma may be calculated from the flow rate and plasma volume which was approximately 15 pl. At the optimum flow rate, the residence time of the sample is 7.5 mseconds. Effect of Column Temperature on Response. Previous work with the microwave-stimulated emission detector by others (14) indicated that a column temperature effect on response may be expected. To investigate this, the response of the DCD detector to individual samples of n-octane chromatographed over a range of column temperatures was determined. Identical sample sizes were chromatographed and peak area determined for each column temperature used. Response data were obtained at CN, CH, and Cz band wavelengths. Results are plotted in Figure 4. Although temperature effect was observed, variations in response are small for purposes of isothermal gas chromatography. Because of the temperature effect, it is obvious that for the most precise quantitative analytical work, a constant temperature (& 2-3" C) will have to be maintained. A decrease of response from the optimum was experienced with increase in temperature, the reverse of the data reported by McCormack, Tong, and Cooke (14). Effect of Plasma Gap Length on Response. A study was run to determine the effect on detector response as the gap between the electrodes was varied. Samples of 1.38 pg of nhexane were chromatographed and the emission responses observed at the 4312-A CH band. A constant power density (watts per length of plasma), carrier gas flow rate and slit width (400 p ) were used. Results (Figure 5 ) indicate that response is not a critical function of gap length. With the exception of the 0.25-inch gap, the increase in response with longer gaps is approximately linear. A gap length of 0.5 inch was selected for most work because it was long enough to 100
ANALYTICAL CHEMISTRY
avoid the decreased response with short gaps and short enough to keep total power consumption down. Gaps longer than 0.75 inch were not studied because their plasma image would extend above and below the 2-cm entrance slit height of the spectrometer. As the gap is lengthened an increase in signal per unit sample size is obtained, but at the same time the background or plasma noise is also increased linearly. Therefore, any further increase in the gap length will not change the limit of detection. Influence of Power Level on Response. The power level in watts per inch of plasma length was one of the characteristics considered to be of importance in optimizing experimental conditions. A brief study was carried out for detector response from a fixed sample size at constant carrier gas flow rate and constant plasma length. Emission area responses from chromatographed 1.38-pg n-hexane samples were obtained using the 3883-A CN and 5165-A Cz bands. Results are plotted in Figure 6. An optimum power density of 30 W per linear inch of plasma was observed. It was also noted that at the optimum power level, small changes in plasma power would not greatly influencethe response. The lower response at lower power levels is attributed to fewer active helium atoms. The lower response at higher wattages higher than the optimum power level is tentatively attributed to the dissociation of the diatomic species observed. Limit of Detection and Linearity of Response-CN, CZ,CH Bands. The compound 2-bromopropane was selected for study of responses, and limits of detection at the main emission bands obtained with all organic compounds and for the heteroatom bromine. It was selected because of its low volatility and the presence of a heteroatom bromine in its structure thus permitting observation of response linearity for both band and line emission spectra. Data were obtained by the gas chromatographic technique. Known sample solutions were prepared by syringe injection of known volumes of pure 2-bromopropane into hexadecane in small volumetric flasks. Standard dilutions were made from these solutions. Hexadecane was selected as a sample solvent because of its low volatility which produced long retention times at the temperature at which the samples were chromatographed. No buildup of hexadecane was observed eluting from the chromatographic column used, an 8-foot long X 0.25-inch diameter copper tube with no packing. The column was run at room temperature, approximately 22" C. A helium carrier gas flow rate of 80 ml/minute was used. The plasma was 0.5 inch long and operated at 27 mA (17 W).
Table IV. Noise at Selected Wavelengths, 18-Second Peaks
VI I-
2 61
Wavelength, A
a a
2a
5-
a I-
8 4LT a
x x x x x 0.19 x
9.5
3.6 5.6 7.8 6.6
Dark current
J
u7
HIn
Noise, A-sec (area)
7000 5165 4312 3876 2478
W
10-11 lo-" lo-" lo-" lo-" 10-11
3-
W
Table V. Limits of Detection for 2-Bromopropane
a
Limits of detection gram g-sec-1 a
Wavelength, A
0
2
4
6
8
IO
12
14
16
WATTS (
18 20 22 24 26 28 30 32
4 -INCH GAP)
4.1 X 5 . 3 x 10-11 1 . 4 X lo-" 6 . 0 x 10-12
7348 Br 5165 Cp 4312 CH 3875 CN
2 . 3 X 10-11 2 . 9 X 10-12 8.0 x 10-13 3.3 x 10-14
18-second peaks.
Figure 6 . Influence of power level on response 1.38-pg n-hexane samples
Chromatograms tailed but were suitable for obtaining response data. Response data were obtained by planimeter for peak area determinations. Noise area data for determining limits of detection were obtained at the several wavelengths of interest by the technique of Johnson and Stross (18). With only pure carrier gas passing through the column and plasma, the electrometer amplifier was set at a sufficiently high gain to obtain easily observable fluctuations in the base line. Time-scale segments, 18 seconds long (corresponded to chart divisions), were sectioned off and areas measured. The noise areas were calculated. The standard deviation in the noise areas was used to determine the limits of detection (LD) from the equation:
where unAis the standard deviation in noise area at the wavelength, A, of interest. R is the response in area per gram of sample and tIl2 is the width of the peak at half peak height in units of time. The resulting LD is in units of grams seconds-', a standard expression used. Ten independent sets of areas were used to determine the standard deviation in noise area. The 18-second intervals approximated the peak widths of the 2-bromopropane. The selection of an 18-second interval and noise calculation by this technique suppresses the effect of noise (also present) of shorter or much longer frequency. The shorter or much longer frequency fluctuations obviously do not unduly distort or prevent observation of peak-shaped responses 18 seconds in duration. If peak-shaped responses significantly shorter or longer than 18 seconds had been observed, then recalculation of noise areas and standard deviation based on the different peak widths in time units would have been required. Noise data are given in Table IV. The ampere-second units correspond t o peak areas because the recorder records current as a function of time. All data were obtained with helium carrier gas flow of 80 ml/minute, 1000 V dc PM tube voltage, and 400-p wide and 2-cm long slit on the Jarrell-Ash (18) H. W. Johnson, Jr., and F. H. Stross, ANAL.CHEM.,31, 1206-11 (1959).
spectrometer. A dry-ice cooled S-20 quartz PM tube was used. The noise value at 7000 A appears to be a bit high. The higher and lower wavelength (above 4200 A and below 3000 A) regions are generally lower in noise because fewer bands are present. Only small differences in noise level were observed from day-to-day throughout the experimental work, Differences observed were attributed to differences in carrier gas impurity levels present. The dark current noise level in this series of data was 20 to 50 times lower than the observed background or plasma noise with the slits open. Limits of detection for 2-bromopropane at the four wavelengths studied are given in Table V. Linearity of response for 2-bromopropane at various wavelengths was also determined. The data obtained are plotted in Figure 7. Response linearity was observed for all of the emissions observed for a 0.04-2-pg sample weight range. It is likely that response linearity will be observed from the limit of detection sample size to approximately the 2-pg range. Samples sizes should be below 10 pg for best operation of the DCD detector.
0.a 0
1
' I " " '
I
1
,
I
I,
,
I
I
I
lI181,:
I
IO' 101 IO IO' RESPONSE, in* INORMALIZEDI 11.8 x IO" amp-rec/inzl
,
I , l ,
10'
Figure 7. Response linearity for 2-bromopropane at specific wavelength assignments VOL 40, NO. 1, JANUARY 1968
101
Table VI. Limits of Detection for Selected Halocarbon Compounds Wavelength, A, and Response associated element A-sec pg-1 6902.5 F 9.35 x 10-8
Formula and compound C6H5F Fluorobenzene Same 6856.0 F CkLClz 7256.6 C1 Dichloroethane C3H7Br 7348.6 Br 2-Bromopropane CaH7I 5464.6 I 1-Iodopropane Same 6082.5 I LD’s based upon noise level standard deviation of 2.7 X lo-’* amp-sec. * LD for 20-second peak widths are 8 X 10-l2 to 7.5 X lo-’* g-sec-l.
LD%bg compound 2.9 x 10-ll
18.6 x 10.5 X
1.5 X 2.6 x lo-”
17.2 X
1.6 x lo-”
6.2 x
4.4 x 10-11
1.72 x
1.6 X lo-’’’
0
Limits of Detection Study-Halocarbons. Limits of detection for various other halogen-containing organic compounds were also determined by the gas chromatographic technique. The response of F, C1, Br, and I-containing compounds was determined at wavelengths found in preliminary work to give the greatest response. Limits of detection are given in Table VI and here are expressed in units of grams. Values of peak widths at half height, tl/*,were variable (approximated 10-20 seconds) because several different compounds were used. Limits of detection obtained using the halogen atomic emission lines were approximately 10 times poorer than those of organic compounds at the major CN, CH, or Czband wave-
Table VII. Response Selectivity of Halogens in Organic Compounds Response, Wavelength, A, A-sec/pg (x Ratio Compound Element A. FLUORINE SELECTIVITY 9.35 1 Fluobenzene 6902.5 F 0.14 67 Dichloroethane 0.21 45 2-Bromopropane 0.15 62 1-Iodopropane 0.06 156 n-Hexane 1348.6 Br
7256.6 C1
6082.5 I
5464.6 I
102
B. BROMINE SELECTIVITY 0.96 28 Fluobenzene 0.41 64 Dichloroethane 26.4 1 2-Bromopropane 0.11 240 1-Iodopropane 1.18 22 n-Hexane C. CHLORINE SELECTIVITY 1.4 8 Fluobenzene 10.5 1 Dichloroethane 0.21 50 2-Bromopropane 0.15 70 1-Iodopropane 0.14 75 n-Hexane D. IODINE SELECTIVITY 1.95 1.2 Fluobenzene 1.44 1.6 Dichloroethane 2.22 1 2-Bromopropane 2.30 1 1-Iodopropane ... ... Fluobenzene 3.11 2 Dichloroethane 5.4 1.1 2-Bromopropane 1 1-Iodopropane 6.2 2.5 2.5 n-Hexane
ANALYTICAL CHEMISTRY
lengths. The limits of detection for Br, C1, and I, as shown in Table VI, are two to three orders of magnitude lower than obtained by Bache and Lisk (19), who used a microwavestimulated helium plasma. The difference is attributed in part to the lower cell operating pressure for their detector. A lower operating pressure results in a lower density plasma with fewer excited species present in the observed plasma. The emission wavelengths selected by Bache and Lisk (19) also were different from those used in this work as was the photomultiplier tube. Selectivity was studied by obtaining response values for all compounds at all of the halogen emission wavelengths selected. The response data are given in Table VI1 together with a calculated response ratio. Fluorine, bromine, and chlorine emission lines exhibited fair selectivity, factors of approximately 40:1 to 70:l. Iodine emission lines studied proved to be poor for selectivity. The best line for iodine is probably the 2062 I line in the ultraviolet region as reported by McCormack, Tong, and Cooke (14). Each halogen element has many emission lines in the “red” region. Examination of those listed in a table of spectrum lines (20) permitted selection of a few listed as having the highest intensity. Preliminary tests of these narrowed the selection to those wavelengths used as shown in Tables VI and VII. The iodine wavelengths had the strongest line and band interferences which probably accounts for the observed poor detection selectivity for iodine. Effect of Slit Width on Response. The argument occasionally arises that interference filters give poorer limits of detection than spectrometers with narrow slits because the bandpass of interference filters is much larger than the width of emission lines and pass background noise. The filter bandpass in addition to the lines presumably would include any background noise. Nevertheless, this is not a problem if emission bands are observed. Bands such as CZ,CN, CH, and p a n y others exhibit fine structure which extend over 1050 A, Thus the fine structure should add to the response signal if a wider bandpass filter or spectrometer slit $ used. Interference filters are readily available with 10-50-A bandpass, and some smaller. Experiments were run in order to verify the influence of bandwidth on response for bands and lines. The hydrogen 6562-A line and the 5165-A Czband were selected. Since the Cz band fine structure at 5165 A extends into the 5129-A CZ (19) C. A. Bache and D. J. Lisk, ANAL.CHEM., 39, 786-9 (1967). (20) A. N. Saidel, V. K. Prokotiev, S. M. Raiski, “Tables of Spectrum Lines,” Veb Verlag Technik, Berlin, 1955.
bandhead, it is useful for a wide bandwidth response study. Samples of n-hexane in hexadecane were chromatographed using several for the 5165-A Cz band data slit widths. The column used was a 3 feet long X 0.25 inch, 10% Apiezon N, on acid-washed Chromosorb W, 60-80 mesh. A 1-mm i.d. quartz detector tube containing a OS-inch length plasma was used. One series of data was taken at 26.5 W and 30 ml of helium carrier gas per minute, another at 17.0 W and 45 ml of helium carrier gas per minute. The hydrogen line data were obtained in the same manner using a 0.5-inch length plasma at 18.4 W, 80-100 ml/minute helium carrier gas flow rate and the same column at room temperature (22' C). Area response data were calculated with a planimeter and then normalized to the same electrometer gain. The results are shown in Table VIII. The H-line response as shown in Table VI11 increases in a linear manner with increasing slit width as expected. One of the Cz band series (17 W) of responses increases nonlinearly with increasing slit width while another (27 W) increases linearly. Nevertheless, it can be seen that while the H-line response increases a factor of 6.6 when going from 100- to 400-p slit widths, the Cz response increases a factor of 10. Thus, the wider slits for bands give an improved response through addition of emission of adjacent fine structure to the bandhead emission. Emission Response of Selected Series of Organic Compounds-Cz, CH, CN, H, C Emission. A study was performed for purposes of obtaining emission response data from several selected series of compounds with specific characteristics. These series of compounds included four aliphatic alcohols and eight hydrocarbons. Of interest were responses at Cz,CH, CN, bands and the C and H lines. It was expected that emission responses at these wavelengths would depend upon the manner in which the organic compounds broke down in the plasma and ultimately upon structure of the analyzed compounds. Thus, for example, benzene and cyclohexane would be expected to exhibit different emission responses at all the wavelengths. The procedure employed was to chromatograph known amounts of compounds in hexadecane solvent and to determine the response at all the wavelengths selected. The alcohol series was carried out using a 3-foot X 0.25-inch diameter 20% Carbowax on Chromosorb W column at 33' C with a carrier gas flow of 107 ml/minute. .The plasma gap was 0.50 inch in a 2-mm i.d. capillary. The detector was operated at approximately 12 W. Response areas were determined by means of a planimeter. Data were calculated in terms of ampere-seconds (area). Data for the alcohols-ethanol, 1-propanol, 2-propanol, and 1-butanol-are presented in Table IX. Differences in response between the compounds on both a weight and molar basis are not large but are significant. A number of comparisons may be made. As the carbon and hydrogen number of the compound in a homologous series increases, the response per mole increases. Bands generally follow this experience but there are marked differences. For example, the Ct band emission for 1-propanol is greater than for 1butanol and ethanol. However, the CH band emission on a molar basis increased with carbon number. The experience with the 4221-A and 3875-A C N bands is not accounted for. The C N response at 4221 A should parallel the 3875 A C N response for a series of compounds. The fact that the 2propanol data are somewhat out of line is tentatively attributed to experimental error. A series of hydrocarbons was studied in a similar manner.
Table VIII.
Effect of Slit Width on Response
H-Line response,^
a
Slit, p
cmz
25 50 100 200 300 400
0.46 1.8
11.4 30.9 52 75
CZband response,h cm2
C2band response,e cm2 ... ...
6.3 30.3 93.7 396 741 930
...
163 333 510
Normalized to 1 X 10-7-A scale; 18.4-W plasma. Normalized to 1 X lO-'-A scale; 17-W plasma. Normalized to 1 X 10-7-A scale; 26.5-W plasma. Table IX.
Wavelength and assignment 6562 A H-line 5165 A C2-band 4312 A CH-band 4221 A CN-band 3875 A CN-band 2478 A C-line
Wavelength Effect on Response for Selected Alcohols 12-
Ethanol 14OQ (6.4)h 24 (1.1) 330 (15) 65 (3.0) 2420 (110) 230 (10)
Propanol Propanol 1-Butanol 155 (9.3) 44 (2.64) 400 (24) 88 (5.3) 2880 ( 170) 300 (18)
120
(7.2) 77 (4.64) 410 (25) 100 (6.2) 2300 (140) 250 (15)
Units, A-sec X lo-* pg-1 Units, A-sec X pmole-1
Benzene and toluene were chromatographed using the same Carbowax column as described for the alcohols. A carrier gas flow of 67 ml/minute, a 0.5-inch gap plasma at 11.8 W, and a 400-p slit (3.3-A bandwidth) were used. n-Pentane, nhexane, and cyclohexene were chromatographed using a 3foot 10% Apiezon N column at room temperature. A 0.75inch gap plasma at 11 W and a 1-mm quartz capillary were used with a carrier gas flow rate of 82 ml/minute. Hexene-2, n-heptane, and n-octane were chromatographed using a 3-foot 10% Apiezon N on 60-80-mesh Chromosorb W column at 43" C. The plasma cell was the same and was operated at 11 W. A carrier gas flow of 136 ml/minute was used. Data for the hydrocarbon series of compounds are given in Table X. Variations in the responses with wavelength are sufficiently large to use the various emission wavelengths for qualitative identification. The difference in experimental conditions, nevertheless, between the three sets of compounds presented are sufficient to prevent making some comparisons. Response data for n-heptane, n-octane, and hexene-2 appear to be higher than the response of other hydrocarbons at the same wavelength. It is possible to compare ratios of C2/CN, Cz/CH, etc.. . emission between compounds. The most striking differences were noted in comparing the CN/CH ratio of aromatic compounds to those of aliphatic hydrocarbons as shown in Table XI. A regular decrease in the CN/CH ratio as the carbon number increases for aliphatic compounds indicates a decreasing molecular fragmentation to give C N diatomic species. VOL 40, NO. 1, JANUARY 1968
103
Table X.
Response for Selected Hydrocarbons A-sw X 10-8/rg-'
Wavelength, A and assignment
+Pentane
+Hexane
n-Heptane5 n-Octane'
Hexene-2' Cyclohexene Benzene
... ... ... ... ... 6562 H-line ... 5165 G-band 132 82 329 205 389 67 4312 CH-band 392 304 907 773 837 227 ... ... ... ... ... 4221 CN-band ... 3875 CN-band 863 541 869 614 919 340 2478 C-line 212 257 1480 984 673 245 a Data obtained at different experimental sample flow rate than others. b Limits of detection range from 2.05 X g (benzene, CN) to 9.1 X lo-" g (toluene, H). c Estimates from noise at nearest wavelength.
The CN formation is much greater for the aromatic compounds. Note that addition of some aliphatic character to the benzene nucleus in the form of a methyl group to form toluene shifts the CN/CH ratio closer to that of aliphatic com-
Table XI. Response Ratios Weight basis, dimensionless
Compound n-Pentane n-Hexane Hexene-2 Cyclohexene n-Heptane n-Octane Benzene Toluene n-Propanol 2-Bromopropane 'Suspect low.
CN/CH 2.2 1.78 1.1 1.5 0.96 0.79 4.57 3.25 7.2 3.4
CH/C 2.97 2.70 2.15 3.39 2.75 3.78 2.82 4.54 9.12 5.5
Table XII.
4312 A CH 3875 A CN 2478 A C 2781 A CC1 2781 A CC1 2781 A CC1 LD LD (CC1) (5.9 W)
4
0.0095~ 0.0036 0.0056 0.0056c 0.0078 0.0066
pounds. The presence of an O H functional group appears to markedly charge the ratio. The CH/C2 ratio for all compounds appears to be comparatively constant regardless of hydrocarbon type for those compounds studied. Propanol and 2-bromopropane give quite different ratios. The CH/C ratio appears to give a regular decrease with the homologous series. Effect of Compound Structure on Response-Chlorinated Methane Series. To further study the effect of structure on emission response patterns, data on the series CCL, CHCl,, and CHzClz were obtained. Samples were from 0.638 to 0.528 pg compound in hexadecane. A 10% Apiezon N on Chromosorb W column at 38" C was used. The carrier gas flow rate was 81.6 ml/minute helium. A 0.375-inch gap and 1-mm i.d. quartz capillary tube was used. Area data were calculated by means of a planimeter. Table XI1 gives the data for the series. Responses in terms of ampere seconds per microgram and per micromole are provided. All data appear to be consistent and n number of observations can be made.
Chlorinated Methane Series Response
CHCla
CCla
Watts
19.3 (2.31) 106 (12.7) 1130 (135) 1370 ( 164) 91 (10.7) 130 (15.5) 67 (8.0) 4.8 X 10-l2 g (C) 5 . 1 X 1@l1g
8.5 (1.31) 28 (4.3) 776 (120) 1160 ( 179) 96 (14.8) 130 (20) 54 (8.3) 5.7 x 10-11 g (C) 5.1 x 10-11g
11.3
Response, A-sec X per pg. Response, A-sec X 1W6per pmole.
104
104 153 693 179 2320 486
31.1. (2. 64)b 273 (23.2) 1990 ( 169) 1620 (138) 85 (7.22) 112 (9.52) 65 (5.52) 4.1 X 10-12 g (CN) 5.9 x lo-" g
Noise data: 2478 A 6.6 X 10-11 A-sec 2781 A 6.6 X 10-ll A-sec 3875 A 7.8 X lo-" A-sec b
540
Noise Level A-sec X 10+b
CHzClz
Wavelength 5165A C2
CH/C 1.85 1.18 1.24 0.93 0.616 0.78 1.54 1.42 1.34
117 295 832 383 3810
Toluene
ANALYTICAL CHEMISTRY
11.3 11.3 11.3 11.3 5.9 17.6
HE
OUTLET H V (-)
SAMPLE
TO PM
GAS SAMPLING VALVE
Table XIII.
A n COLUMN (+) AND MOUNT STRIP HEATERS
Figure 8. Gas chromatograph assembly using a small interference filter with the DCD detector cell
The CH emission increases with increase in number of H atoms in the molecule. Note that the CH emission doubles from CHC12 to CH2C12. There is apparently some H available (residual in the carrier gas) even in the case of CC14 because a small response is observed for C H emission. The carbon line emission at 2478 8, increases with increase in degree of chlorination and is a maximum with CC14. It would appear that in the case of CH2C12decomposition of the molecule does not go as completely to form free C atoms as in the case of CC14. The C N band emission, moreover, increases with decreasing chlorination. It is difficult to reconcile the opposite behavior of C N and C, but the following reactions are offered as possibilities (nitrogen being available as an impurity in the carrier gas).
+ N2-+ C N + N H or NH + C H C N + Hz (or 2H) CH
+
The NH emission has always been strong even in the best carrier gases and therefore this species must be present. The reactions below are also possible sources of CN but are probably less important than the above reactions.
+ N2 2C + 2NH 2C
-+
-+
2CN 2CN
+ H2 (or 2H)
Some data on the effect of power level on CCl band response is provided. The optimum power level is lower than observed in earlier studies for CZand C H bands. Chlorinated species optimum power is 15.7 W per inch or less while the optimum power for C1 and CH was in the 30 W per inch range. This observation of improved response for lower power levels for CHCla and CCld is in agreement with work employing the microwave-stimulated plasma emission-type detector (14). Interference Filter Detector System. It is not convenient or necessary to use a spectrograph for wavelength selection, Interference filters are acceptable and compact substitutes. The interference filter bandpass half-widths depend up!n the wavelength region. Quite n!rrow half-widths (3 to 50 A) can be obtained nabove 4000 A. The bandwidths obtainable below 4000 A become wider and below 3000 8, the filters may have 100-200-A bandwidths at half-peak transmittance, Several interference filters were obtained. H?wever, only one, 4315 8, (35zpeak transmittance) with 25-A bandwidth at half-peak transmittance was used. A small gas chromatograph with an interference filter DCD detector was constructed with the design as shown in Figure 8. The apparatus arrangement of the interference filter detector was essentially the same as shown in Figure l except
Sample (0.658pg)
Limits of Detection for an Interference Filter Type Detector (4312 A CH Band) Carrier gas flow rate ml/min Power, W InchesZ
n-Hexane
78
12.1
n-Hexane +Hexane n-Hexane n-Hexane n-Hexane n-Hexane n-Hexane n -Hexan e
36 24 78 78 78 78 78 78
12.1 12.1 11.0 12.0 7.1 8.6 10.3 10.3
2.47 2.40 3.68 3.84 2.20 1.05 6.00 12.84 22.0 15.0 15.0
LD,*g 1.6 X
10-11
1 . 1 x 10-11 1 . 0 x 10-11 1 . 8 x 10-11 3 . 8 x lo-" 6 . 6 x 10-l2 3 . 1 x 10-12 1 . 8 x 10-12 2.6 X 10-12
Normalized to 1 X 10-5-A range. Based upon noise area value of 0.06 inch2at 1 X 10-*-A range. These are convertible to g-sec-1 values by dividing by 45 sec. (1
that the spectrometer was replaced with an interference filter. A S-11 response photomultiplier tube (Du Mont 6291) was used at room temperature. A Ferrotran, unregulated 0-1000 V at 20 mA dc power supply was used for the detector power source. The gas chromatographic column was short, only 6 inches long. Helium purified by passing through the liquid nitrogen-cooled charcoal trap was used as the carrier gas whose flow regulation system was provided external to the GLC detector system, The sample loop volume was l/2-2 ml, depending upon the desired sample size. Limits of detection for the interference filter-based detector were determined using the 4312-8, C H band filter and nhexane samples. The 2-mm i.d. quartz capillary was operated at several power levels for the 0.25-inch gap. A helium carrier gas flow rate of 78 ml/minute was used with a 6-inch long 10% Apiezon N on Chromosorb W column. Limits of detection data are given in Table XI11 and are comparable to the data obtained using a spectrograph for wavelength selection. Data presented in Table XI11 include studies of variations in carrier gas flow rate and operating power level. Although LD values improve with a decreasing carrier gas flow rate, the gain in sensitivity is small, at least for the flow rates employed. Optimum power level appears to be near 10 W. CONCLUSION
This study has demonstrated the operating characteristics of the direct current discharge emission-type detector. The comparatively small discharge power supply required and small size of the detector cell make this detector well suited for use in gas chromatography instrumentation. The sensitive detection of fluorine by its atomic line emission attests to the general applicability of the detector. Data also indicate that the CN, CH, Cz, C, and H-band and line emission intensities bear a relationship to structure of the compounds being detected, and qualitative identification of eluted components is suggested based upon relative emission intensities. A further examination of this is currently under way. VOL 40, NO. 1, JANUARY 1968
105
Difficulties in the use of this detector stem from its high sensitivity and low limits of detection. Column bleeding must be avoided. Operation at elevated temperatures accentuates the column bleeding problem. Although not yet demonstrated for operation at temperatures above 50-60” C and on compounds with higher molecular weights (200-300), there would appear to be no reason why such applications could not be made, providing column bleeding is minimized. Suitable means must be taken to prevent passage of no more than approximately 0.1 mg of sample through the detector plasma at any single injection.
ACKNOWLEDGMENTS The authors gratefully acknowledge the assistance of Guy F. Origlio of the U.S.Army Engineering Research and Development Laboratories, Fort Belvoir, Va. RECEIVED for review August 16, 1967. Accepted November 14, 1967. Presented at the 18th Annual Mid-America Spectroscopy Symposium, May 16, 1967. Program carried out under Contract No. DA 44-009-AMC-1576 (T) from the U. S. Army Engineering Research and Development Laboratories.
Specific llnteractions Affecting Gas Chromatographic Retention for Modified Alumina Columns David J. Brookman and Donald T. Sawyer Department of Chemistry, University of California, Riverside, Calif. 92502
The gas chromatographic retention of hydrocarbons has been studied on salt-modified aluminas. The results establish that the retention volume is dependent upon a composite of nonspecific and specific adsorptive interactions. The latter includes effects due to the pi character of the sample molecule and to its planarity. For aromatic hydrocarbons, substituent groups affect the gas-solid interactions, and for olefins the cis isomer is retained more strongly than the trans isomer. A quantitative evaluation of these effects and of different modifying salts permits the design of columns for the selective separation of cistrans isomers and of mixtures containing paraffinic, olefinic, and aromatic hydrocarbons with similar boiling points.
ACTIVATED ALUMINA has been used for many years as an adsorbent in liquid-solid adsorption chromatography. Its use as an adsorbent in gas-solid elution chromatography (GSC) has not been widespread, however, because the resulting chromatographic peaks are asymmetrical due to the nonlinear adsorption isotherms for solutes (1). Much of the asymmetry is eliminated if the adsorbent is partially deactivated with silicone oil or water (2), or with inorganic salts (3, 4). Comparatively weakly interacting adsorbates can be easily and symmetrically eluted from such modified adsorbents (4-6). The sensitivity of ionization detectors permits operation on the linear portion of the adsorption isotherm (7) such that the retention volume is independent of sample size. The study of the adsorption mechanism for various ad(1) A. B. Littlewood, “Gas Chromatography,” Academic Press, New York, 1962, p. 108. (2) C. G. Scott, J. Inst. Petrol., 45, 118 (1959). (3) C. G. Scott, “Gas Chromatography 1962,” M. van Swaay, Ed., Butterworth’s,Washington, 1962, p. 36. (4) C . G. Scott and C. S. G. Phillips, “Gas Chromatography 1964,” A. Goldup, Ed., Institute of Petroleum, London, 1965, p. 266. (5) A. V. Kiselev, “Gas Chromatography 1962,” M. van Swaay, Ed., Butterworth’s, Washington, 1962, p. xxxiv. (6) J. F. K. Huber and A. I. M. Keulemans, “Gas Chromatography 1962,” M. van Swaay, Ed., Butterworth’s, Washington, 1962, p. 26. (7) J. C. Giddings, ANAL.CHEM., 36,1170 (1964).
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sorbents by GSC is of considerable interest. By varying the adsorbent and comparing the adsorption changes resulting for different adsorbates, the adsorptive effect of the different moieties present can be determined. Kiselev (8) has shown that interactions may be divided into nonspecific and specific types from his work with graphitized carbon. A nonspecific interaction is representative of paraffinic hydrocarbons, in which every bond is a sigma bond. In contrast pi-bonded systems can exhibit specific interactions because of the presence of a region of high electron density. The existence of specific interactions presupposes a substrate that couples with the specific center of the adsorbate. Hence, graphitized carbon shows only nonspecific interaction for all compounds while silica or alumino-silicates, for example, can exhibit nonspecific adsorption for aliphatics and a combination of nonspecific and specific adsorption for olefins. Kiselev also notes that the usual conception of polar and nonpolar adsorbates and adsorbents fails to explain the nature of physical adsorption. Other workers have found that a polarity effect exists and that the polarizability of a molecule affects its adsorptione.g., the large retention volumes of olefinic materials on polar adsorbents ( 4 , 9 ) . The present paper summarizes the results of a detailed investigation by gas-solid chromatography of specific adsorptive interactions for several modified aluminas. The effect of increasing polar character while pi-bond and hence polarizability remain constant for a series of adsorbates has been of particular interest. EXPERIMENTAL Analabs Type F-1 activated alumina was used as the adsorbent. In most cases this material was acid washed in 6F HC1 overnight to remove iron contamination. The acid washed material was then washed with distilled water, dried at 120” C, and the 100-110 mesh portion separated by means of ASTM sieves. The salt used for coating was weighed and (8) A. V. Kiselev, “Gas Chromatography 1964,” A. Goldup, Ed., Institute of Petroleum, London, 1965, p. 239. (9) B. T. Guran and L. B. Rogers, ANAL. CHEM., 39,632 (1967).
