Response of a Thermal Conductivity Microdetector in Gases of Low Thermal Conductivity Robert L. Pecsok and Malcolm L. Windsor Department of Chemistry, UniGersity of California, Los Angeles, Calg. 90024 A study has been made of the response of a micro thermal conductivity detector (for gas chromatography) with two carrier gases, nitrogen and methane, both having low thermal conductivity. Anomalous response was found over the whole of the flow rate range studied, leading to peaks not being detected at all at certain flow rates. This work extends an earlier study by Bohemen and Purnell to microdetectors with thermistor elements, finds peak inversion in both nitrogen and methane carrier gases, examines the effect of carrier gas pressure and cell current, and offers a possible explanation for the response variations a t moderate flow rates.
ALTHOUGHTHERE ARE many excellent detectors available for gas chromatography, thermal conductivity detectors remain as popular as ever for general purpose instruments. Because of their relatively large size, the early detectors were suitable only for packed columns. Recently microdetectors have become available which are suitable for use with capillary columns, but their high sensitivity makes them useful in many applications. The factors which determine the response of this detector, however, are only qualitatively understood and it is not often possible to predict its response to a given sample under given conditions. Bohemen and Purnell(1) found that with nitrogen as a carrier gas, peak distortion and inversion could be produced by a n increase in filament temperature or by an increase in flow rate of carrier gas through the detector. In the present study we have extended this work by examining the response of a microdetector with thermistor elements in two carrier gases of low thermal conductivity, nitrogen and methane. EXPERIMENTAL
A flow diagram of the gas chromatograph is shown in Figure 1 . It was constructed to operate over a range of carrier gas inlet pressures from about 1.5 atmospheres to 10 atmospheres. The column outlet pressure is generally not more than 1 atmosphere below inlet pressure; the major pressure drop to atmospheric pressure occurs not across the column but across a high pressure needle valve a t the outlet. Except for the glass soap film flowmeter and the glass manometer, the flow system is constructed of 0.25-inch 0.d. copper tubing connected by Swagelok fittings. The detector is the Carle Micro-Thermal Conductivity System (Model 1000, Carle Instruments, Inc., Anaheim, Calif.). The sensing elements consist of two thermistor beads with a diameter of 0.013 inch in a cell with a volume of approximately 58 ~ l . The thermistors have a cold resistance of 10 kohms and are supplied with a constant voltage current from eight 1.35V mercury cells. Carrier gas inlet pressure is measured by a gauge (Helicoid Test Gauge, 0-200 psi), accurate t o 10.5z, and the pressure drop across the column by a differential mercury manometer made of heavy wall glass tubing. The flow rate is determined by the pressure drop across the column which is controlled by means of the needle valve mentioned (1) J. Bohernen and J . H. Purnell, J . Applied Chem., 8,433 (1958).
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above. The flow rate is accurately measured by a sealed soap film flowmeter (2), constructed from heavy wall glass tubing to withstand the pressure. Both the manometer and the flowmeter are contained in a heavy wall Lucite box. The columns are 0.25-inch 0.d. copper tubing packed with 2 0 z by weight of squalane (Wilkens Instrument and Research, Inc.) on Chromosorb W (60/80 mesh, non acidwashed). Samples are injected as a vapor by means of a twoway sampling valve. The columns, detector, and most of the flow system are contained in the same water bath maintained a t 25.00” i. 0.01” C. The reference side of the detector is fed by a reference packed column (Figure 1) similar to the column used for the samples. Both are supplied by the same inlet pressure so that the flow rate and pressure in the two sides of the cell are about the same and any fluctuations originating in the inlet regulating system will tend to cancel out. Except where otherwise indicated, the sample injected was n-pentane (Matheson Coleman & Bell “spectroquality reagent”). A constant sample size was used equivalent t o approximately 2 MI of liquid. Methane was Matheson Co. “Ultra Pure,” nitrogen was Liquid Carbonic “Hi-Pure.” Peaks were measured at a convenient signal attenuation and corrected t o attenuation X1. Carrier gas flow rate was measured at column outlet and, after small corrections for the water vapor in the soap film flowmeter and for the difference between column temperature and the flow measurement temperature, was corrected to mean column pressure in the usual way. RESULTS
Detector Response in Nitrogen. In the first set of experiments a constant current of 20 mA was used for the detector. The inlet pressure was held constant a t 2 atmospheres and the outlet valve adjusted so as t o vary the flow rate from 5 ml/minute to approximately 120 ml/minute. The pressure a t the detector, which is situated at column outlet, varies from about 1.9 atmospheres at the lowest flow rates to 1.2 atmospheres a t the highest flow rate. When the flow condition was steady, a series of from three t o six samples of n-pentane were injected. The areas of the resultant peaks were taken as the peak width a t base times half the peak height. The mean value was taken noting whether the peaks were positive or inverted-Le., o n which side of the base line they occurred. In order t o decide the polarity of the detector, small samples of helium were injected. The thermal conductivities of nitrogen, helium, and n-pentane are approximately 58, 348, and 36 (cal gram-’, sec-l “C-I). Thus a sample of helium passing through a cell where nitrogen is the carrier gas will tend to cause cooling o n the sample side, whereas a sample of n-pentane will cause heating. We assume then that a sample of n-pentane will normally appear as a peak on the opposite side of the base line t o a helium peak. For this reason we have called such peaks “positive,” whereas (2) A. J. B. Cruickshank, M. L. Windsor, and C . L. Young, Proc. Roy. SOC.London, A. 295,271 (1966).
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those occurring on the same side of the base line as helium peaks are considered t o be inverted. The results are shown in Figure 2 as a plot of peak area (at attenuation XI) against carrier gas flow rate at mean column pressure (inlet pressure 2 atmospheres). At low flow rates the peaks are positive but the area decreases with increasing flow rate. At high flow rates they are inverted with the area of the inverted peak increasing with flow rate. At flow rates higher than those shown, detector noise becomes quite severe and the location and measurement of peaks becomes very difficult. At the highest flow rates measured, however, peaks were still inverted. At moderate flow rates, peaks are positive or inverted depending on the flow rate; the curve for nitrogen shows two maxima and two minima. Thus at a flow rate of 60 ml/minute, a small but positive peak is obtained whereas at 68 ml/minute a small but inverted peak is obtained. Near the cross-over region between positive and inverted peaks, the peaks are often misshapen, frequently W-shaped. The distortion of the peaks always starts from the peak base and not a t the maximum, in agreement with the results found by Bohemen and Purnell (I). Finally, t o check that the curious shape of the curve was not a peculiarity of n-pentane, samples of 2,2-dimethylbutane were injected and the whole flow rate range briefly examined. The same shape of curve was obtained with maxima and minima in response a t about the same flow rates, this in spite of the fact that the vapor pressures and retention volumes of the two hydrocarbons differ considerably. Effect of Variation of Cell Current. Detector response in nitrogen carrier gas was again studied over a range of flow rate a t various cell currents-namely, 12, 16, and 24 mA. I n Figure 3 the results are shown for 12 mA together with the data a t 20 m A for comparison. Thermistor cells show a maximum in sensitivity at a current depending on various factors including the size, shape, cell temperature, and thermal conductivity of the carrier gas (3). The maximum is usually a t fairly low currents, and in this case the response a t 12 mA is greater than that a t 20 mA. However, maxima and minima for the two plots occur at the same flow rates. If a peak is positive a t 20 mA, it is more positive at 12 m A ; if inverted, it becomes more inverted.
(3) C. B. Cowan and P. H. Stirling, “Gas Chromatography,” Academic Press, New York, 1958, p. 165.
Sample: rz-pentane. Current: 20 mA. Inlet pressure and sample size constant
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Effect of Variation of Carrier Gas Pressure. The results described so far were all obtained with nitrogen carrier gas a t a n inlet pressure of 2 atmospheres. The flow rate was again studied with the inlet pressure a t 1.61 atmospheres and 2.74 atmospheres. The results are shown in Figure 4. The maxima and minima occur at widely different flow rates. F o r example, a t a flow rate of 50 ml/minute, a run a t 1.61 atmospheres will produce a positive peak of area approximately 100 cm2, whereas a run a t 2.74 atmospheres will product an inverted peak of approximately the same area. The effect of carrier gas pressure on detector response was also studied with methane and response was measured a t inlet pressure of 2.70, 3.38, 4.06, and 4.73 atmospheres. These results are mentioned later. Detector Response in Methane. Methane has a fairly low thermal conductivity, 72 cal gram-’ second-’” C-I,but one higher than that of nitrogen which is 58. (On the same scale, helium is 348.) Detector response was measured over the flow rate range at a n inlet pressure of 2 atmospheres, and the results are shown in Figure 2. I n keeping with the higher conductivity of the gas, one finds a greater positive response for the sample than in nitrogen. The shape of the curve is the same with maxima and minima at about the same flow rates. The peaks in methane are not inverted until very high flow rates are reached. For example, a peak VOL 40, NO. 1 , JANUARY 1960
93
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for n-pentane at a flow rate of 50 ml/minute has an area of approximately 80 cni2 in methane and is positive; in nitrogen its area is approximately 50 c m 2and it is inverted. DISCUSSION In the ideal gas chromatographic experiment, the peak area would be a direct measure of the concentration of sample passing through the cell. The results given here show that this is very far from being the case when carrier gases of low thermal conductivity are used. The carrier gas flow rate is seen to have an enormous effect on the detector response. Indeed, with nitrogen at 2 atmospheres there are five flow rates at which one obtains no signal at all or, at best, a grossly distorted peak. In methane, large variations in response exist throughout the flow rate range and peak inversion occurs a t high flow rates. (In helium and other gases of high thermal conductivity we would not expect inversion as described here t o occur a t the flow rates normally used for GLC.) In discussing variation of response as measured by peak area, it is as well to ascertain if it is the peak height or the peak width that is behaving anomalously. The peak height is mainly a detector function whereas the peak width depends on the column and on the recorder chart speed. Plotting peak height and peak width separately against flow rate shows that it is certainly the height which is behaving anomalously. (Peak width shows the steady decrease with increasing flow rate which would be expected due t o the sample spending less time in the detector.) Bohemen and Purnell (I) made a detailed study of the behavior of a thermal conductivity cell with nitrogen carrier gas. They used a glass cell with tungsten wire filaments and they found a steady decrease in peak area with increasing flow rate leading t o complete peak inversion, in some instances, at higher flow rates. No indication of the carrier gas pressure is given, however, and the sample sizes used were approximately 10 times larger than in this work. The sensitivity of a thermal conductivity cell may be considered a linear function of the change in the rate of loss of heat, Aq, from the filament or thermistor when the carrier gas is replaced by a mixture of sample and carrier gas. Bohemen and Purnell gave the following expression t o account for heat loss in a wire filament thermal conductivity cell, in which the first term on the right-hand side takes account of the 94
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ANALYTICAL CHEMISTRY
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difference ( A m between thermal conductivities of pure carrier gas and mixture, while the second term allows for heat loss by forced convection and involves the molar flow rate (m) and the difference (ACp) between heat capacity of pure carrier gas and sample plus carrier gas mixture. =
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where A T is the difference in temperature between filament and surrounding cylinder, AT' is the difference in temperature between incoming and outgoing gas, L is the filament length, and ro rlare the radii of cylinder and filament, respectively. The signs of A K and A C p are generally different so that sensitivity is reduced to a level below that anticipated on grounds of conductivity alone. Indeed, at high flow rates the second term can equal or exceed the first; then one will obtain distorted or inverted peaks, This would account for the final inversion of peaks in nitrogen and methane a t high flow rates, but not for the curious variations in response at moderate flows. The results in Figure 4 showing the effect of carrier gas pressure on response in nitrogen are useful in this respect and become more rational if plotted in the following way. Instead of carrier gas volumetric flow rate, one can plot a mass flow rate by multiplying each inlet flow rate by the inlet pressure (this gives a mass flow rate relative t o nitrogen at 1 atmosphere pressure). Second, one can allow for the diluting effect of high carrier gas pressure on the sample by multiplying the response, as measured by peak area, by the outlet pressure (because the detector is situated a t column outlet). Figure 5 shows the results plotted in this manner for nitrogen. Results at all three pressures studied fall approximately on one and the same curve, so that one could predict the response a t any pressure and flow rate. Plotting the data in this way is equivalent t o plotting peak area us. the Reynolds number, R, of the thermistor bead.
R=-
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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. I n the case of nitrogen, these variations appear t o 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 o n 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 t o 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 t o 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 t o 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, London, 1963, p. 102.
(5) A. B. Littlewood, “Gas Chromatography,” Academic Press, New York, 1962.
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 S t . , 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 t o 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 a n 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. I n the early work, attempts were made t o 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
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