trated in Figure 19 for component 3 will occur. Thus, for bands of constant width, their separation can be varied to yield thereby different steady state concentration values. P R O G R A M M E D TEMPERATURE
The application of programmed temperature with various input functions will not be considered in detail in this paper, but its application in connection with the square wave input functions (Figure 19) is of interest. The designation of 1, 2, and 3 may represent increasing temperatures for a single component. Obviously, the amplitude of the square wave response will vary with temperature and if several sample components are present, the square wave response will increase with increasing temperature, the increase in
response occurring rather suddenly a t temperatures characteristic of each component. LITERATURE CITED
(1) Boeke, J., “Gss Chromatography,” 1960, ed. by R. P. W. Scott, Butterworth, Loddon, 1960, pp. 88-103. (2) Boeke, J., Parke, N. G., Gas Chromatography Symposium, East Lansing Meeting, June, 1961. (3) Bosanquet, C. H., “Gas Chromatography,” 1958, ed. by D. H. Desty, Butterworths, London, 1958, pp. 107115. (4) Bosanquet, C. H., Morgan, G. O., “Vapor Phase Chromatography,” 1956, ed. by D. H. Desty, Butterworths, London, 1956, pp. 35-51. (5) Claesson, S., A ~ k i vKemi, Mineial. Geol. 23A, No. 1, 133 p. (1946). (6) Eberly, P. E., Jr., dmberlin, C . N., Jr., Trans. Faraday Soc. 57, 1169 (1961). (7) James, P. H., Phillips, C. S. G., J . Chem. SOC.1066 (1954).
(8) Knight, H. S., ANAL. CHEW 30, 2030 (1958). (9) Mayer, S. W., Tompkins, E. R., J. A m . Chem. Soe. 69,2866 (1947). (10) Orr, C. H., ANAL. CHEM. 33, 158 11961). (11) -Schay, G., “Theoretische Grundlagen der Gaschromatographie,” Veb Deutscher Verlag der Wessenschaften. Berlin, 1960. (12) Schay, G., Szekely, Gy, Szigetvary G., Acta Chim. Acad. Sci. Hung. 12, 309 (1957). (13) Tiselius, A., Arkiv Kemi, Mineral. Geol. 14B, No. 22, 5 pp. (1940). (14) Tiselius, A.,Zbid., 16A, No. 18, 11 pp. (1943). RECEIVED for review November 14, 1961. Accepted June 22, 1962. Division of Analytical Chemistry, 141st Meeting, ACS, Washington, D. C., March 1962. Research supported by the United States Air Force through the Office of Scientific Research, Air Research and Development Command, Contract No. AF 49(638)-333, and the Advanced Research Projects Agency, Contract No. SD-100.
Evaluation of Several Integrators for Use in Gas Chromatography DONALD T. SAWYER and JAMES K. BARR Department of Chemistry, University o f California, Riverside, Calif,
b Four types of integrators were studied and evaluated for use in gas chromatography. A bail-and-disk integrator, a 6-volt fixed-field d.c. motor integrator, a 1 0-mv. voltage-to-frequency electronic integrator, and an electrochemical integrator were calibrated and, with the exception of the latter, used under chromatographic conditions in the separation of four ketones, The ease of operation, precision, linearity, and relative merits of the four integrators are discussed.
Q
analysis by gas chromatography requires accurate measurement of the areas for the elution peaks. Numerous efforts (10, 12) have been made to ensure that the peak area is linearly related to the amount of a component (either by weight or by moles) and this aspect of the problem is not considered here. However, the possibilities for integrating the differential response signal of a gas chromatographic detector range from weighing the cutout peak t o , voltage-tofrequency converters with high speed counters attached to print-out equipment (4). Janak (6) has discussed a statistical evaluation of methods for integrating gas chromatographic peaks. The ball-and-disk type of integrator UANTITATIVE
has been discussed by Perrine (IS), who has indicated that it is capable of accuracy to 0.1 or 0.2%. The application of a low-inertia motor integrator to gas chromatography has been suggested by both Keulemans (6) and Dal Nogare, Bennett, and Harden ( 2 ) ; they indicate an accuracy of about 0.3% for this system. Simmons (4, 14) has presented impressive gas chromatographic integrations using a voltage-tofrequency converter plus a frequency counter; the accuracy appears to be good to about 0.1 or 0.2oJ,. If each of these three integrators is considered independently, a rational assessment of their comparative utility and accuracy is almost impossible. Each group has used different gas chromatographs, samples, recorders, operating conditions, and criteria of evaluation. The promotional literature from the manufacturers is of little additional help, particularly in terms of determining how satisfactory ‘;he integrators will be for gas chromatographic analyses. Thus, a direct comparison of these types of integrators under the same conditions of application seemed desirable, and has led to the present study. The comparison is concerned with the linearity and accuracy of the integrators for known rectangular voltage-time integrals, and also with their accuracy and reproduc-
ibility for a four-component sample mixture under actual gas chromatographic conditions. Four integrators have been considered: a ball-and-disk type on a strip-chart recorder, a low-inertia d.c. integrating motor activated by a retransmitting slide-wire on a strip-chart recorder, a voltage-to-frequency converter with counter, and an electrochemical current integrator. With the exception of the last system, each of the integrators has been tested with a range of gas chromatographic samples to determine linearity and reproducibility under realistic conditions. EXPERIMENTAL
Equipment. A Sargent Model SR strip-chart recorder equipped with a new ball-and-disk integrator (Model 204, Disc Instruments, Inc.) was used to determine the performance of this type of integrating system. The recorder had a full-scale range of 1 mv. and a full-scale response time of 1 second; a full-scale deflection gave 6000 c.p.m. (using 10 counts per smallest division). The full-scale range could be changed to 100 mv. by replacing the 1-mv. range plug; this was done for the calibration of the recorder. The low-inertia integrating motor, Model 916/4, was obtained from Electro Methods, Ltd., Stevenage, England, and had a counting rate of 1000 c.p.m. VOL 34, NO. 10, SEPTEMBER 1962
1213
Table 1.
Calibration of Recorder
Relative Input, Mv.
Indicated, Mv.
3
Deviation
i o
40.0 50.0 60.0
iO.0 80.0 90.0 100.0
39.4 49.8
59.3 69.0 79.0 89.2 99.0
-1.5 -0.4 -1.1 -1.4 -1.2 -0.9 -1.0
for an input of 6 volts (using 10 counts per smallest division). The counting rate is approximately linear with input voltage, with the motor drawing approximately 0.9 ma. for a 6-volt input signal. Because of the rather large current drain, direct connection of the motor to the gas chromatographic detector was precluded. By using a M inneapolis-Honeywell Model Y 143X ( 5 7 ) strip-chart recorder with 1-second response and 1-mv. full scale sensitivity and equipping i t with a 200-ohm retransmitting slide-wire obtained from the manufacturer, current drainage on the detector was prevented. A heavyduty 6-volt dry cell (marine type) was applied across the 200-ohm retransmitting slide-wire; thus the current drain by the integrating motor was made negligible compared to the 30 ma. going through the slide-wire. -4 high input impedance voltage-tofrequency converter, recently described by Bell and Chuinti (I), was evaluated by attaching it directly to the thermal conductivity detector. This system produces 60,000 c.p.m. for a 10-mv. input and registers the counts on 6 decades of counting tubes. Any commercial frequency counter could be used in place of the counting tube circuit. (The Vidar Corp., Mountain View, Calif., markets a commercial version of a voltage-to-frequency converter which is satisfactory for gas chromatographic integrators, Model 260 .) The electrochemical current integrating system was fabricated according to the circuit of Lane and Cameron (7); the electrochemical diode (Solion) used in the circuit was obtained from the Sational Carbon Co., New York, N. Y. By using the same retransmitting slidewire system as mas used with the low inertia motor, but with a 60,000-ohm resistor in series with the electrochemical integrator, it was possible to cause a 100-pa. signal to result from a full scale deflection of the recorder. Thus, the system converted, by use of the retransmitting slide-wire, a 0- to 1-mv. signal to a 0- to 100-pa. signal, which then could be integrated by the current integrator. The four integrating systems were calibrated and evaluated with rectangular test signals obtained from a variable voltage and current source. This sys1214
ANALYTlCAL CHEMISTRY
tem, which has been described by DeFord (3), waa constructed from Philbrick operational amplifiers and provides currents and voltages accurate to within &O.l%. G&s chromatographic measurements were made with a gas chromatograph equipped with a thermal conductivity detector (Model A90, Wilkens Instrument and Research Co.). The chromatograms were recorded with either a Minneapolis-Honeywell Model Y 143X (57) 1-mv., 1-second, strip-chart recorder or a Sargent Model SR 1-mv., 1-second strip-chart recorder. The former was equipped with a 200-ohm linear retransmitting slide-wire obtained from Minneapolis-Honeywell, while the latter was equipped with a ball-anddisk integrator. The stainless steel gas chromatographic column, 1/4 inch X 10 feet, was prepared with 42- to 60mesh acid-washed Chromosorb P coated with G.E. SF-96 (20% by weight) and was operated a t 91". A helium flow rate of 49 ml. per minute was used with a bridge current of 150 ma. in the detector. A 10-111. Hamilton syringe, equipped with a Chaney adapter, was used to introduce samples. Chemicals. The G.E. SF-96 liquid phase used for coating the column was obtained from Wilkens Instrument & Research, Inc. The four-component mixture used for testing the performance of the integrators was prepared from reagent grade acetone, methyl ethyl ketone, diethyl ketone, and methyl isobutyl ketone. Equal volumes of each ketone were accurately measured and combined to give the mixture used in the study. RESULTS AND DISCUSSION
Although a recent study has indicated that the ball-and-disk type of integrator is capable of high precision (IS), careful adjustment and periodic checks are necessary to realize such performance. This or any integrator can be no more accurate than the recorder with which it is associated (11). Therefore, the recorder used for the study of this integrator was checked for accuracy a t 12 points over its range. Table I summarizes these, using a full-scale range on the recorder of 100 mv., and indicates that this particular recorder is inaccurate by up to 4% for certain parts of its range. The calibrations indicated in Table I, of course, are for only a single recorder; however, they are believed to be representative of the performance which is realized for many recorders used in the actual praotice of quantitative gas chromatography. The studies of Orr (11) for a recorder-integrator combination tend to support this conclusion. To test the performance of this particular ball-and-disk integrator further, a series of constant voltages was applied to it for known periods of time. The applied volt,a,ges were such as to
give from 5 to 100% of the full-scale deflection for the recorder (a 100-mv. range plug was used to permit more accurate measurement of the input voltage). The average of 17 separate integrations gives an integrator constant of 0.9913 =k 0.005 count per mv.second (99.13 =ZI 0.5 counts per mv.second with a 1-mv. range plug). The deviations are about the same over the entire 5 to lOOyo range. The constant supplied with the instrument is 1.000 count per mv.-second and using this would give integrals approximately 1% low. The constant deviates too much to allow accurate integration of signals below Dal Nogare and coworkers (2) and Keulemans (6) have discussed the perforniance of a low inertia d.c. motor integrator for gas chromatography. I n Dal Nogare's circuit a starting voltage was applied t o the integrator by a bridge circuit t o prevent the errors and nonlinearities caused by low-level signals. A full-scale voltage for the retransmitting slide-wire was used, which was twice the rating of the integrator, as a means to increase its linearity further. Because this approach and its reliability are somewhat dependent upon the practice of the operator, an alternative approach was employed in the present study. Lingane (8) has shown that the dynamic range of the motor integrator can be extended through the use of the empirical relation f E d t = kN + B t
(1)
where k and fl are constants characteristic of the individual d.c. motor, N is the registered counts, and tis the elapsed time for integration of a peak. To evaluate the constants, a series of rectangular integrations using different known voltages and times was made with a 6-volt motor integrator; the voltages ranged from 0.050 to 12.000 volts. A least squares evaluation of 43 different integrations gives a value for k of 0.3582 volt-second per count and for fl of 0.00967 volt. For input signals for 0.5 to 12 volts the average integration error, using Equation 1, is less than 0.1%. For inputs below 0.5 volt the average integration error increases from 0.1% a t 0.5 volt to about 0.5oJ, a t 0.05 volt. By applying a 6volt marine battery across a 200-ohm retransmitting slide-wire on a 1-mv. recorder [Minneapolis-Honeywell Model V 143(57)], the motor integrator can be energized and caused to integrate gas chromatographic peaks. 'Thus, under such conditions the counting rate is equivalent to 16.67 counts per mv.second. The third integrator to be considered is the voltage-to-frequency type with a digital counter. This integrator h m been shown by both the developers (1) and our calibrations to be accurate to ap-
r IO
4
4
Methyl is0 butyl ketone
‘6
l I8
a cn w
Q
Figure 1. Typical chromatogram for four-ketone mixture used in study of integrators
4 0K
Sample size, 2 pl.
(L
0 -2 0
w
L;, a
20
16
12
T I M E,
8
systems are summarized in Table 111. Because of the difficulty of introducing accurate, known sample volumes, the results have been tabulated a~ the percentage of the total integral for the four peaks. Each value represents the average of three or four separate sample injections. The same gas chromatograph and the same ketone mixture were used for all three tests and the analyses were made, as closely as possible, under identical conditions. Thus, the disagreement between the results for the three integrators is probably real and due to the characteristics of the integrators themselves. The results with the voltage-to-frequency integrators are believed to be the most accurate because of its high degree of accuracy and linearity, and its independence of recorder errors. This type of integrator has the distinct advantage of being directly connected to the detector, made possible by its high input impedance. Thus the errors due t o the recording system are not included or accumulated by
4
0
Min
proximately 0.2% for inputs from 1 to 10 mv.; at 10 mv. the integrator gives an average of 1000 c.P.s., with slight deviation from linearity for signals below 1 mv. The fourth integrator, the electrochemical diode, was studied by applying to it known constant currents, from 1 to 100 pa., for known periods of time. These tests established that at best this is a crude integrator of limited usefulness to gas chromatography with the circuit used. The reproducibility for identical integrals is only about 3% and the integration constant deviates approximately 5y0,in a random fashion, for inputs from 1 to 100 pa. Because of this limited accuracy and a tendency to drift, no additional studies have been made for this integrator. However, very recent work by Martin and Cox (9) has shown that a considerable improvement in accuracy and temperature drift is obtainable with thermistor compensation in combination with a 3E 110 Solion tetrode cell. A reported accuracy within 1% indicates that further investigations of this integrator may be profitable. The performance of the four integrators for rectangular inputs is summarized in Table XI. Although the ball-and-disk integrator could not be conveniently calibrated independent of the recorder, the other three have been calibrated in this fashion. I n the case of the low-inertia motor, the range of the recorder is indicated for the case of 6 volts applied to the retransmitting slide-wire. On the basis of the results in Table 11, the first three integrators were further evaluated for integration of gas chromatographic peaks. A four-component mixture of ketones was used for this, and its chromatogram is shown in Figure .1. The peaks have some tailing, which is the most difficult portion of a peak to integrate, especially for the ball-and-disk type and the lowinertia
motor; thus they should provide a realistic test of the integrators. The results of the analyses of the ketone mixture by the three integrating
Table 11.
Summary of Calibrations and Performances for Four Integrators Using Known Rectangular Signals
averCounts/Mv. Dews/9 Calibrated Range Sec. tion 5 to 100% of full scale on recorder 99.1 f 0 . 5 0 . 5 (1 mv.) 2. Low-inertia d.c. motor 0.50 to 6 volts (8 to 100% of full 16.67 f 0 . 0 2 0 . 1 scale of recorder, 1 mv.) 3. Voltage-to-frequency 1 to 10 mv. 100.0 f 0 . 2 0 . 2 4. Electrochemical diode 1 t o 100 pa. 26.5 f 1 4.5 Integrator 1. Ball-and-disk
Table 111.
Peak Areas for Ketone Mixture as Determined by Three Integrators
Sample Volume, pl. A. Ball-and-disk 0.8 1.8 2.8 3.8 Av.
Peak 1
% ’ of Total Area for Entire Mixture Peak 2 Peak 3 Peak 4
27.29 f 0.68 27.45f0.18 27.37 f 0 . 1 4 27.25 f 0.19 27.34 f 0 . 3 0
25.73 f 0 . 1 6 24.81f0.36 25.08 f 0 . 2 1 25.56 f 0 . 7 4 25.30 f 0.37
24.77 f 0 . 2 0 25.10f0.12 25.10 f 0 . 0 3 24.67 f 0 . 3 0
25.9 f 0 . 3 26.1 i 0 . 3 26.0 f 0 . 3 26.4f0.3 25.8f0.2 26.0 f 0 . 3
24.2 f 0 . 2 24.4 f 0 . 2 24.5 f 0 . 3 24.4f0.1 24.5f0.1 24.4 f0.2
25.8 f 0 . 2 25.6 i 0 . 4 25.6 f 0 . 4 25.3iz0.3 25.6fO.O 25.6 f 0 . 3
24.0 f 0 . 6 24.0 f 0 . 3 23.9 & 0 . 4 23.8f0.1 24.0f0.1 24.0 i 0 . 3
27.22 f 0 . 1 8 27.11 i 0.00 27.27 f 0.17 26.80 f 0 . 1 6 27.00f0.12 26.86 f 0 . 1 4 27.04f0.13
24.71 f 0 . 0 8 24.75 =t0 . 0 5 24.81 f 0 . 0 7 24.62 f 0 . 0 7 24.70f0.06 24.53 f 0.10 24.69f0.07
24.82 f 0.10 24.88 iz 0 . 0 4 24.95 f 0 . 0 3 25.26 f 0 . 1 9 24.94f0.04 25.02 f 0 . 1 0 24.98f0.08
23.26 f 0.10 23.26 f 0 . 0 8 22.97 f 0 . 2 2 23.32 f 0 . 1 9 23.35f0.14 23.59 f 0 . 1 5 23.2910.15
22.23 f 0 . 7 6 22.64f0.37 22.44 f 0 . 1 7 22.51 f 0 . 2 7
24.91 f 0.16 22.45 f 0 . 3 9
B. Low-inertia motor
Av. C. Voltage-to-frequency 1 2 3 4
5 6 Av.
VOL. 34, NO. 10, SEPTEMBER 1962
1215
such integrators. A second advantage is that the area is registered by a digital counter to four or five digits rather than by an oscillating pen blip or by a radial dial as is the case for the ball-and-disk and the low-inertia motor integrators. The ball-and-disk type of integrator has the distinct advantage of presenting the integral information directly on the chart along with the chromatographic peak. The results for the ball-and-disk integrator are in agreement with those for the voltage-to-frequency integrator, but the average deviations for the areas obtained with it are approximately three times as great. The low values for the first two peaks obtained with the low-inertia motor prolxihly can be attributed to inadequate response speed for the fast-rising, early peaks of the gas chromatogram s h o \ w in Figure 1. Thus, the integrator probably has accounted for less than the total area for each of the first two peaks and as a result high values are determined for the areas of the last two peaks (where relative per cent composition is the analytical approach). The voltage-to-frequency integrator gives a counting rate of 100 counts per mv.-second, which is the same as the ball-and-disk integrator with a 1-mv. recorder a t full scale deflection. However. with a 10-mv. recorder, the ball-and-disk integrator gives only 10 counts per mv.-second, while the voltage-to-frequency integrator still
gives 100 counts per mv.-second because of its independence of the recorder. Thus, the latter integrator permits a less expensive 10-mv. recorder t o be used to monitor the gas chromatogram without sacrificing any sensitivity in terms of integration. This type of integrator also is easily adaptable to automatic datahandling systems and printers (4). The low-inertia d.c. motor has a much lower counting rate and thus cannot give areas to more than three significant figures. If a retransmitting slide-wire with 6 volts across it is used on a 1-mv. recorder, the counting rate is the equivalent of 16.67 counts per mv.-second (equivalent of full-scale deflection of recorder), which is one sixth of the best t o be realized with either the ball-anddisk integrator or the voltage-to-frequency integrator. Although the motor integrator has the lowest counting rate and probably the least accuracy, it is entirely satisfactory for many applications. Furthermore, i t requires a fraction of the investment necessary for most other types of integrators and it is readily adapted t o the recorders used by most gas chromatographers.
ACKNOWLEDGMENT
The authors are grateful to the Bell and Howell Research Center for a grantin-aid in.support of this work, which in-
cluded a graduate fellowship t o one of the authors (J.K.B.). LITERATURE CITED
(1) Bell, S . W.,Chuinti, V., Electronics 35 ( l l ) , 64 (1962). ( 2 ) Dal Sogxre, S., Bennett, C. E;:
Harden, J. C., “Gas Chromatography, V. J. Coates, H. J. Noebels, I. S. Fagerson. eds.. 1). 117. -4cademic Press, Sew York, 1958. (3) DeFord, I). D., Division of Analytical Chemistry, 133rd Meeting, ACS, San Francisco, Calif., hpril 1958. (4!, Infortronics Corp., Houston, Tex., CRS-1 Digital Chromatograph Integrator,’’ Bull. CRS-102. (5) JFnak, J., J . Chromatog. 3, 308 (1960). (6) heulemans, A . I. M., “Gas Chromatography,” p. 209, Reinhold, New York, 1957. ( 7 ) Lane, It. S., Cameron, D. B., Electronics 32 (9), 53 (1959). (8) Lingane, J. J., -4nal. Chim. Acta 18, 349 (1958). (9) Martin, ,J. W., Cox, J. R., Electronics 35 (12), 46 (1962).
(10) Foebels, H. J., Wall, R. F., Brenner, S . , ”Gas Chromatography,” Academic Press, S e n York, 1961. . (11) Orr, C. H., ANAL. CHEM.33, 158 (1961). (12) Pecsok, R. L., ed., “Principles and Practice of Gas Chromatography,” Wiley, Xew York, 1959. (13) Perrine, W. L., “Gas Chrornatography,” H. J. Noebels, R. F. Wall, N. Brenner, eds., p. 119, Academic Press, Yew York, 1961. (14) Simmons, hl. C., Second Annual Research Conference on Gas Chromatography, University of California, Los Angeles, Calif., January 1962. RECEIVEDfor review April 26, 1962. -4ccepted June 20, 1962.
CaIcuIation of Relative Molar Response Factors of Thermal Conductivity Detectors in Gas Chromatography E. G. HOFFMANN Max-Planck-lnstitut fur Kohlenforschung, ML’lheim (Ruhr), Germany
b In spite of the introduction of new detecting devices, the thermal conductivity cell is still the most commonly used detector in gas chromatography when GC is used in quantitative analysis. Relations have been derived from the kinetic theory of gases for calculation of sensitivity factors of thermal conductivity cells as detectors in gas chromatography. They have been applied to 18 substances in the gases Hz, He, and Nz as carriers. The calculations could b e verified in part by related measurements with thermistors as sensitive elements and an electronic integrating device. 1216
ANALYTICAL CHEMISTRY
N
INVESTIGATIONS have stated t h a t the signal strength coming from a sample in a thermal conductivity cell and related to equal molar concentration is considerably dependent on the nature of the substance. As Messner, Rosie, and Argabreight (28) pointed out, the signal strength within a homologous series increases nearly proportional with the molecular weight. I n 1958 we reported (13) this to be a consequence of the elementary kinetic theory of transportation phenomena in binary gas mixtures. We stated that the different influence of the substance vapors on the thermal conductivity of
UMEROUS
the carrier gas is mainly determined by the different cross sectional areas of the substance molecules by which the heat transport of the carrier gas is impeded. Later on Littlewood (26), using some derivations from the more rigorous Chapman-Enskog theory of transport properties, confirmed this result. I n the meantime we could extend our former results and apply them to other carrier gases (12). Earlier, some authors expressed the opinion (9-11,22,23, 29, 31) that the use of light carrier gases like H e or HPshould lead to signal strengths nearly independent of the nature of the materials
