Analytical Accuracy in Gas Chromatography Using Thermal Conductivity Detectors SIR: Several investigators ( 2 , 6 , 7 , 8) have reported pressure and flow rate changes upon sample introduction using elution techniques. These changes were ascribed to viscosity differences between carrier and sample (2, 8),and were acknowledged to affect accuracy with thermal conductivity detectors (8). Similar changes causing spurious peaks were investigated using a Molecular Sieve column (S,4) and argon carrier. The results indicate that an adsorption phenomenon, rather than viscosity differences, is primarily responsible for the pressure and flow rate changes. The results also suggest that the method of calibration with pure substances (8) leads to systematic analytical errors, because of the mutual effect that the components of a mixture have on one another's peak parameters. Such errors decrease with decrease in sample size, and are independent of base line stability. PROCEDURE
The apparatus used is shown in Figure 1. The stainless steel pretzel-type thermal conductivity cell was Model 9285, TE 111, manufactured by the Gow-Mac Instrument Co., Madison, N. J. The Wheatstone bridge was that described by Dimbat, Porter, and Stross ( I ) , modified to permit positive recording of peaks using either helium or argon carrier. The recorder was a Minneapolis-Honeywell Electronik of 0- to 1-mv. range.
The No. 5 8 Molecular Sieve column was prepared by filling a degreased 15foot length of 0.12-inch inner diameter stainless steel tubing with 30- to 70mesh material (Linde Air Products Co., Tonawanda, N. Y.). The column was then coiled to shape and dried by heating to 240' C. in a vacuum oven for 24 hours. The 6-foot silica gel column was prepared in similar fashion from 0.18-inch inner diameter tubing and 30- to 70mesh material of unknown origin. It was dried for 24 hours in a vacuum oven a t 160' C. -411 runs were made a t room temperature, with a chart speed of l/2 inch per minute. Gas samples of 99 volume % ' purity or better were introduced into the gasometer. With all stopcocks and valves in the positions shown in Figure 1, simultaneous evacuation of the sample tube to 0.5 micron and warmup of the chromatograph occurred. Needle valve B was opened wide, while needle valve D was adjusted until pressure readings of 72 and 29.2 p.s.i.g. were registered a t gages -4 and E, respectively. Flow rate of argon was 70 ml. per minute. The sample was then admitted to the sample tube at the desired pressure. The tube was brought up to column pressure with carrier before sample introduction, thus avoiding pressure changes from this source. A 700-mm. sample of carbon dioxide was run by this procedure on the silica gel column. The pressure a t gage A was 73 p.s.i.g. and that a t gage E ww 7.4 p.s.i.g. The flow rate of argon remained a t 70 ml. per minute.
RESULTS
The peaks caused by pressure and flow changes accompanying sample introduction, and subsequently referred to as adsorption peaks, are shown in Figures 2 and 3. Adsorption peak height was proportional to the magnitude of pressure change. The normal gas sample peaks for helium, hydrogen, nitrogen, methane, carbon monoxide, and oxygen had approximate retention times of 11/*, 2, 8, 12, 32, and 4 minutes, respectively. The adsorption peaks for helium, hydrogen, and oxygen include, a t their left extremities, small portions of the normal gas peaks. Typical changes in flow rate a t flowmeter M and pressure a t gage E were the following. I n Figure 2, for hydrogen, the pressure rose from 29.2 p.s.i.g. a t point 1 to 30.5 p.s.i.g. a t point 3. It then fell to 28.7 p.s.i.g. a little after point 4, and finally rose back to 29.2 p.s.i.g. about 15 minutes after sample introduction. The flow rate gradually rose from 70 ml. per minute a t point 2 to 85 ml. per minute. It remained a t this level until somewhat beyond point 4, after which entrance of hydrogen into the capillary flowmeter obscured further immediate changes. With positive adsorption peaks the
E K
, HELIUM
in
MOLECULAR SIEVE COLUMN To
HIGH VACUUM LINE
ARGON TANK
MANOMETER
Figure 1 .
..ii
Schematic of gas chromatograph
A.
Reducing valve g a g e Hoke needle valve C. Three-way Demi valve, George Dah1 Co., Bristol, R. I. D. Hoke needle valve E. Bourdon gage, range 0 to 50 p.s.i.g. F. 4-mm. stopcock G, H. 4-mm. three-way vacuum stopcocks J. 6-mm. three-way vacuum stopcock K, I. Three-way Demi valves M. Capillary flowmeter
B.
288
ANALYTICAL CHEMISTRY
6
4
2
0
T I M E , MIN.
Figure 2. Adsorption peaks on Molecular Sieve column 700-mm. samples
phenomena were simpler. K i t h carbon monoxide, for example, the pressure fell from 29.2 p.s.i.g. a t point 1 to 28.1 p.s.i.g. a t the peak maximum. It then slowly increased. The flow rate changed, as before, some 5 to 10 seconds after point 1, decreasing to about 65 ml. per minute. The flow rate then gradually rose. The height of the adsorption peak varied linearly with sample pressure for all of the gases. Measurements have not been made for helium, oxygen, nitrogen, and methane below 175 mm., for hydrogcn below 90 mm., or for carbon monoxide below 5 mm. Pressure and flow rate changes upon introduction of oxygen samples were too small to measure accurately. The small adsorption signal consisted of two separate peaks, which were poorly reproducible. Reproducibility of the heights of all other adsorption peaks was about i10% for consecutive runs. Figure 4 shows the adsorption peak obtained with the silica gel column and carbon dioxide sample. .4t point 1 a large negative signal appeared which did not correspond to any pressure or flow rate change. The pressure fell from 7.4 p.s.i.g. a t point 2 to 6.1 p.s.i.g. a t point 4. A decrease in flow rate began a t point 3. The retention time of the normal carbon dioxide peak FT-as about 12 minutes. A large pressure and flow rate decrease occurred with 700 mm. of carbon monoxide on the Molecular Sieve column using helium as carrier. However, the adsorption peak was small (0.03-mv. height). DISCUSSION
Changes in pressure and flow rate upon sample introduction have been ascribed to viscosity differences between carrier and sample (2,8). On this basis, only pressure decreases (and/or flow rate increases) could be expected to occur here. This is because argon has a higher viscosity than any of the sample gases (8),and flow rate is inversely proportional to viscosity. The observation of both positive and negative changes, and the dependence of the magnitude and directions of these changes on the retention times of the samples indicate that an adsorption phenomenon is prirnarily responsible for these effects. The adsorption phenomenon causing a pressure decrease inay be visualized by considering a column through which a n unadsorbed carrier gas is flowing. A plug of pure sample gas is introduced into the carrier stream. It is assumed that this gas can be completely adsorbed. When it passes into the column, it is removed from the gas phase and a pressure drop ensues. The pressure decreases continuously during sam-
4
-
TIME, MIN.
Figure 3. Oxygen adsorption peaks on Molecular Sieve column Numbers indicate millimeter sample pressures; 1 7 5 A run immediately after 700 and 350; 1 7 5 8 run after shutdown of argon now and re-equilibration of column
ple introduction as more and more of the sample is removed from the gas phase. Because no gas is flowing down the column during adsorption of this sample, and because of the pressure decrease, a flow rate decrease is registered. To visualize pressure increases, consider an adsorbed carrier. When a steady state is reached, as much carrier is adsorbed in the column as is desorbed. A 10-cc. plug of carrier flowing into the column results in 10 cc. of column void volume being occupied. If 10 cc. of a completely unadsorbed sample is now injected into the column, it will replace an equal volume of carrier from the void spaces. However, some of the carrier adsorbed on the column fill will desorb, in order that the distribution coefficient of the carrier remain constant-Le., that equilibrium be maintained. This desorbed carrier causes the pressure to increase. The flow rate also increases because of the increased forecolumn pressure and increase in the mass of unadsorbed gas in the column. These phenomena can affect analytical accuracy in two Fi-ays, when using thermal conductivity detectors and large samples. First, the adsorption peaks distort the base line and can render the measurement of small peaks difficult if not impossible ( 2 ) . Such behavior can be avoided by several means -e.g., by including a large buffer volume between the needle valve and the column. Pressure disturbances are also kept to a minimum. Even with such a buffer, however, flow rate changes must take place because of mass conservation. A crude but simple analogy illustrating this may be drawn by considering the column to be a vessel filled with sodium hydroxide solution through which helium carrier is bubbling. A 10-cc. plug of pure car-
6
4 - 2 TIME, MIN.
0
Figure 4. Carbon dioxide adsorption peak on silica gel column 700-mm. sample
bon dioxide is int,roduced into the stream of helium. I n the time interval during which the carbon dioxide is absorbed, no gas will escape the vessel and the flow rate downstream will be zero. I n chromatography, this flow change affects the gas velocity down the column in a complex way. The magnitude and character of such velocity changes will depend on the retention time (or distribution coefficient) of a component, as compared to that of the carrier. The magnitude decreases with decrease in sample size. The second and more important type of analytical interference arises from the velocity change described above, if a chromatograph is calibrated using pure substances. Consider, for example, the analysis of 10 cc. of a mixture of 10% hydrogen in carbon monoxide, using the argon Molecular Sieve system. During calibration with 1 cc. of hydrogen alone, a small flow rate increase takes VOL. 32, NO. 2, FEBRUARY 1960
289
place down the column. However, during analysis of the mixture, an initial flow rate decrease takes place as a result of adsorption of carbon monoxide. The nornial gas peak obtained for hydrogen in the mixture m-ill be different from that obtained in the calibration, as the column conditions are completely different (8). Such interference has already been reported for another system (6). It occurs regardless of the method used for peak measurement, as both peak height and area are affected by velocity changes (8). For optimum operation n.ith thermal conductivit: detectors, pressure changes should be kept small. Such changes augment flow changes and incrcase errors from this source. Measures for rernedying base line instahilitj which do not minimize pressure changes are satisfactory for the first type of interference, but conditions for analysis may still be far from optimum. Errors caused by the second (velocity) type of interference can be further
minimized by using small samples, and can be eliminated completely by using either a n integral detector or one which records constant peak areas regardless of gas velocity. Other interesting aspects of the argon Molecular Sieve system will be reported elsewhere. Although undesirable from an analytical viewpoint, the adsorption peak phenomenon may prove of considerable value in studying the kinetics of coniplex adsorption and desorption processes.
ACKNOWLEDGMENT
The author is indebted to P. L. Walker, Jr., and H. B. Palmer of this department, and to J. R. Lotz, 11. H. Bnrsky, and L. W. Littau of the chemistry department for discussion and review of this paper. He is also indebted to A. Roeger, 111, and L. J. Duffy of this department for their technical assistance.
LITERATURE CITED
(1) Dimbat, bl., Porter, P. E., Stross, F. &I., A N A L . CHEM.28, 290 (1956). ( 2 ) Harrison, G. F., "Vapour Phase Chromatography," D. H. Desty, ed., p. 332, Academic Press, New York, 1957. (3) Janak, J., Ilrejci, X , Dubsky, H. E., Collection Czechoslov. Cheni. C o r n i m n .
24,1080 (1959). (4) Kyryacos, G., Boord, C. E., BYAL. CHEX 29, 787 (1957). ( 5 ) Nodop, G., 2. anal. Chem. 164, 120 (1958). (6) Smith, R. E.,Sminehart, J., Lesnini, D. G., -4s.4~.CHEM.30, 1217 (1958). (7) Sullivan, I,. J., Lotz, J. R., Willingham, C. B., Ibid., 28, 495 (1956). (8) Van de Craats, F., "Gas Chromatography 1958," D. H. Desty, ed., p. 248, Academic Press, New York, 1958. ALLANWEISSTEIS
Department of Fuel Technology Pennsylvania State University University Park, I'n. RECEIVEDfor review April 17, 1959. Bccepted October 5, 1959. Excerpted from a thesis to be presented to the faculty of the Pennsylvania State University in partial fulfillment of the requirements for the Ph.D. degree.
Gas Chromatographic Separation of Metal Halides by Inorganic Fused Salt Substrates SIR: Freiser ( 2 ) has recently reported the separation of the low boiling tetrachlorides of tin and titanium (boiling point, 114' and 136" C., respectively) using a n-hesadecane column a t 102" C. The separation of metal halides using organic stationary liquid phases, hon ever, is frequently not practical, because organic compounds are generally too volatile to be used a t the temperatures required for many inorganic separations. &o undesirable reactions often occur betn een active metal halides and conventional organic liquid phases. Metal halides may be separated by partition gas chromatography using fused salts as the stationaryliquid phase.
A chromatograni of titanium (IV) chloiide (boiling point, 136" C.) saturated a t room temperature nitli antimony(II1) chloride (boiling point 225" C.) is shown in Figure 1. A 35-pl. sample was injected with a hypodermic syringe. I n quantitative work hydrolysis must be prevented a t
the tip of the syringe. Dry nitrogen, flowing at a rate of about 30 ml. per minut'e, served as carrier gas. The column was packed with Johns-hfanville C-22 insulating brick (Sil-0-Cel) ground to 30/GO mesh and coated with a n eutectic mixture of anhydrous bismuth tricliloridc and lead chloride (89 moly 5 I(iCI.l, ntrlting point, 217" (3,).
-i c
0 .c 0
An all-glass column and detector cell was constructed from borosilicate glass tubing 6 mm. in outside diameter. Vycor tubing is required for column temperatures above approximately 520" C. Stainless steel or copper tubing cannot be used because of reaction with the fused salt and with the metal halides being separated. The apparatus consists of a flash vaporizer; a 12-foot borosilicate glass column in the form of two concentric helices, 5 inches in height and 3 inches in outer diameter; and a i)latinuni filament thermal conductivity detector cell ( 1 ) forming one arm of a conventional Wheatstone bridge arA 1x11 insulated, highrangement. vnttage fi\:ed resistor n a s used as the rciference "cell." 290
ANALYTICAL CHEMISTRY
a, c
a
c3 L Q)
B
0 a,
CK
0
I
I
I
5
IO
15
Figure 1 . Gas chromatographic separation of titanium tetrachloride solution saturated a t 27" C. with antimony trichloride Column, BiCI3-PbCI:: eutectic mixture on C - 2 2 firebrick; column temperature, 2 4 0 ' 5 1 ' C.
