ing, the plug velocity was irregular and, furthermore, it increased gradually from 2.1 to 2.6 cm/second-i.e., a relative increase of 24%. On exit of the plug from the column, the gas velocity in the column increased abruptly from 2.6 to 5.1 cm/second. The flow pattern obtained without the use of the liquid brake (Figure 3, B ) probably illustrates the conditions prevailing in the more commonly used coating procedures. Under such conditions, the formation of a uniform film may be hampered by different factors. As the film thickness under certain conditions has been found proportional to the plug velocity (6), the gradual increase in plug velocity during the coating process should result in a gradually increasing film thickness along the column. The rapid acceleration of the plug during its exit from the column should result in a considerable increase in film thickness in this region of the column. The abrupt increase in gas flow that occurs after emergence of the coating plug may result in the formation of small new plugs of coating solution at various locations in the column; these plugs would travel at considerable velocity and result in an uneven film thickness along the column. Finally, it may be difficult under such conditions to obtain a slow and uniform rate of evaporation of the solvent after exit of the coating plug, which is considered an important factor for obtaining a uniform film (6). As pointed out by Bernhard (9) and Zlatkis (IO), a certain degree of smoothing of film irregularities undoubtedly occurs (9) R. A. Bernhard, ANAL.CHEM., 34, 1576 (1962). (10) A. Zlatkis, Department of Chemistry, University of Houston, Texas, private communication, September 1966.
during the conditioning of the column, particularly if the highest permissible temperature for the liquid phase is used. It may be assumed, however, that the smoothing is effective only in localized areas in the column and that it would be difficult by this procedure to correct for a gradually increasing film thickness throughout the entire column as caused by plug acceleration during coating. Also, the beneficial effecb of the smoothing upon column quality may not be entirely predictable. The flow pattern obtained with the use of the liquid brake (Figure 3, A ) should facilitate the deposition of a uniform film because the plug velocity remains constant during the coating, and the gas flow after plug emergence remains constant and at the same low velocity as that of the coating plug. The reported flow stabilization device provides for a coating procedure that is well defined, reproducible, and simple to perform. Four columns which were prepared by this procedure have all performed satisfactorily. No columns were prepared without use of the liquid brake for possible comparison of the column efficiencies. Other types of flow restrictors, such as a narrow-bore capillary tubing with the nitrogen gas flowing through it to create the required back-pressure, could probably with equal effect replace the liquid brake system, provided the restrictor were placed downstream from the column to be coated. RECEIVED for review July 21, 1967. Accepted November 14, 1967. Presented at the 49th Annual Conference of the Chemical Institute of Canada, Saskatoon, Sask., Canada, June 1966. Work supported by Distillers Corp. Ltd.
Gas Chromatographic Determination of Hydrogen in Helium Carrier Stream John C . MacDonald Department of Chemistry, Fairfield University, Fairfield, Conn. 06430
GASCHROMATOGRAPHY with most substances yields a linear detector response with concentration, but hydrogen is a well recognized exception when helium is the carrier gas. The response of a thermal conductivity detector for increasing amounts of hydrogen injected into a helium carrier stream is given in Figure 1 . This effect has been investigated by Schmauch and Dinerstein ( I ) who attributed this nonlinear response to the nonlinear relationship of thermal conductivity and hydrogen mole fraction. This was confirmed later by the calculations of statistical mechanics and the experimental data of Hansen, Frost, and Murphy (2). The gas chromatographic determination of hydrogen in this split peak region has in the past been accomplished by substituting nitrogen, nitrogen-helium or helium-argon (3) as the carrier gas and by adjusting the hydrogen sample size ( 4 ) . In routine analysis where continual adjusting of sample size or (1) L. J. Schmauch and R. A. Dinerstein, ANAL.CHEM., 32, 343 (1960). ( 2 j R T S I Hansen, R. R. Frost, and J. A. Murphy, J . Phys. Chem., 68, 2028 (1964). (3) N. Hara, H. Shimada, and M. Oe, J . Chem. SOC.Japan, Indust. Chem. Section, 64, 772 (1961). (4) H. Pauschrnann, Z . Anal. Chem., 203, 16 (1964).
INCREASING HYDROGEN/HELIUM R A T I O
Figure 1. Dependence of recorder response upon hydrogenhelium ratio carrier gas may be undesirable, a method for hydrogen in helium carrier gas in the split peak region is desirable and the following procedure was developed. The method is useful, VOL 40, NO. 2, FEBRUARY 1968
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for example, when nitrogen analyses are to be made simultaneously or when analytical sensitivity demands helium carrier gas for a multicomponent analysis which includes the determination of hydrogen.
All data were obtained on a Beckman Model GC-2A gas chromatograph equipped with a 10-cc gas sampling valve. The column was 12-feet X 0.25-inch stainless steel packed with Molecular Sieve, 5A, 42/60. Column temperature was constant at 40' C; helium was at an input pressure of 23 psi and the gas flow rate was 120 cc/minute. The detector current was 150 mA. The data were obtained by filling the sample valve to a measured pressure and injecting at that pressure. DISCUSSION OF RESULTS
The instantaneous pen position of the recorder reflects the instantaneous thermal conductivity of the hydrogen-helium mixture at the detector. The base line indicates zero mole per cent hydrogen. As hydrogen appears at the detector an ordinary positive peak occurs if and only if the hydrogen does not exceed about 13 mole per cent, which is the minimum in the thermal conductivity curve of hydrogen-helium mixtures (2). At concentrations of hydrogen greater than about 13 mole per cent (2), the thermal conductivity of the mixture increases relative to helium and the peak travels in the negative direction. At concentrations of hydrogen greater than about 27 mole per cent (2), the thermal conductivity of the mixture is greater than that of helium. A reversal in signal sign results, and the recorder travels below the base line. The extent to which this valley of the split peak travels in the negative direction is a function of the concentration of hydrogen injected. The technique used in measuring these so-called peak heights is shown as ( A B ) in Figure 1. The collected data for all peak heights are shown in Figures 2,3, and 4. Under these injection conditions a positive peak results at hydrogen injection pressures less than 70 torr. Injection pressure greater than 200 torr gave essentially negative peaks although the split peak
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Figure 3. Peak heights at 0-200 torr
Figure 2. Peak heights at 0-700 torr
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