flow rates of 0.5-3.0 cma mine’. A discharge time of 20 seconds gave a hole of 220-ptm diameter and the capillary was used to measure flow rates of 50-200 cma min-1. The flowmeter must be calibrated by normal methods after it is assembled.
1tDRILLED HOLES
CONCLUSIONS
A WIGH*FREQUWCY COIL
Teflon capillaries, prepared as above, should have a number of uses in addition to their use in differential-manometers. Two obvious uses are in dropping mercury electrodes (3), and as controlled leaks in mass-spectrometer inlets.
ACKNOWLEDGMENT B
The authors acknowledge discussions with R. N. Whittem and practical assistance from J. H. Levy.
Figure 2. Diagrams that show how Teflon capillary is prepared capillary so formed can be used to measure higher flow rates. If several capillaries of different hole size are prepared by using different discharge times, flowmeters can be constructed to cover all flow rates encountered in normal laboratory work with fluorine. A discharge time of 1 second produced a hole of 70-gm diameter and this capillary was used to measure
RECEIVED for review March 2,1972. Accepted May 22,1972. (3) A. Bond, T. A. O’Donnell, and A. B. Waugh, ANAL. CHEM., in
press.
Coulometric Calibration of a Thermal Conductivity Detector for Oxygen and Nitrogen W. Gary Williams and Dayton E. Carrittl Nova University Oceanographic Laboratory, 8000 North Ocean Drive, Dania, Fla. 33004 A TECHNIQUE HAS BEEN DEVELOPED which enables the quantitative introduction of either oxygen or nitrogen into the sample stream of a gas chromatograph. The method has facilitated the calibration of a thermal conductivity detector for a continuous range of sample sizes of these gases. Either oxygen and hydrogen, or nitrogen and hydrogen, are generated in a coulometer within the gas chromatograph. The calibration gases are desorbed into the sample stream by bubbling the carrier gas through the electrolyte for a five-minute period commencing with the end of electrolysis. A molecular sieve column separates the gases and allows oxygen and nitrogen retention times sufficient to prevent interference from the desorption pressure peak. In these studies, sample sizes of oxygen and nitrogen between 0.25 and 5.00 micromoles were generated and injected, although it certainly should be possible to extend this range in either direction. The accuracy of calibration curves determined by this technique is confirmed by the correct analysis of two different standards. EXPERIMENTAL
Apparatus. A Varian Aerograph 90 P-3 gas chromatograph modified for the analysis of dissolved gases in aqueous solutions is used (Figure 1). The desorption chamber (Figure 1, insert) is like that described by Swinnerton, Linnenbom, and Cheek ( I , 2) in their papers on injecting dissolved gases by stripping the gas-liquid mixture with the carrier, except for Present address, Institute for the Study of Man and His Environment, A-305 Graduate Research Center, University of Massachusetts, Amherst, Mass. 01002. (1) J. W. Swinnerton, V. J. Linnenbom, and C. H. Cheek, ANAL. CHEM., 34, 483 (1962). (2) Ibid.,p 1509.
the addition of a pair of one-centimeter square bright platinum electrodes one centimeter above the glass frit (3). The constant current source is a Leeds and Northrup Coulometric Analyzer (Satalog No. 7960). The 0.25-in. X 30-ft molecular sieve 5 A column, which is operated at - 17 OC, was originally designed for the separation of argon and oxygen. The molecular sieve is activated by heating the column to 400 “C while purging with dry nitrogen for 12 hours. With a helium carrier flow rate of 60 ml/min, retention times for hydrogen, oxygen, and nitrogen are 10,24, and 180 minutes, respectively. A Leeds and Northrup Speedomax-W equipped with a Disc Integrator records and integrates the signal from the hot wire thermal conductivity detector. The precision of the gas chromatograph is f1 as measured by the relative standard deviation of five air injections from a gas sample valve. Procedure. The procedure described by Lingane (4) regarding oxygen-hydrogen and nitrogen-hydrogen coulometry is followed. A 0.5M K2S04electrolyte is used for the generation of oxygen and hydrogen and 0.1M hydrazine sulfate for nitrogen and hydrogen. The overall electrolysis reaction for the hydrogen-oxygen coulometer is
If the minimum current density of 50 mA/cm2is observed, the experimental yield of oxygen is 0.2498 gmole/pequivalent. For the nitrogen-hydrogen coulometer, N2H5+ is oxidized at the anode
+ 5H+ + 4e-
N2H5+ + NZ(g)
(2)
(3) W. G . Williams, “A System for the Analysis of Dissolved Oxygen, Nitrogen and Argon in Natural Waters,” M.S. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1968. (4) J. J. Lingane, “Electroanalytical Chemistry,” 2nd ed., Interscience Publishers, New York, N.Y., 1968, pp 452-7.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
2119
atmosphere
Figure 1. Modified gas chromatograph with a desorber-coulometer
course
frit
&toggle valve LEGEND k c o n s t a n t flow c o n t r o l
and hydrogen ion is reduced at the cathode. The complete electrolysis reaction is NzH,+
+
N P ( ~ ) 2H2(g) f HT
(3) and for current densities greater than 7 mA/cm 2, the experimental yield of nitrogen is 0.2493 pmole/pequivalent. To begin calibration, about 10 ml of electrolyte is drawn into the desorber which is then connected into the apparatus with Swagelok fittings. The purge helium is passed through the desorber and vented to the atmosphere to degas the electrolyte and remove any air in the system. A ten-minute purge at a flow rate of 100 ml/min reduces the concentration of air in the system below the limits of detection. Then the purge helium is halted and the carrier is directed through the desorber, ensuring that the electrolyte is well stirred, and current is applied to the electrodes for the time required to generate the desired quantity of gas. Once electrolysis has been terminated, the carrier continues to pass through the electrolyte for 5 minutes at 60 ml/min to desorb the newly formed gases. Longer stripping times do not significantly increase the area of the elution curves. In these experiments, the electrolysis current was 64.3 mA, and for example, the generation of 4 pmoles of oxygen required about 25 seconds. The very different retention times of oxygen and nitrogen lead to the development of individual procedures for producing calibration series. In the case of oxygen, an injection is made and the elution curve recorded before the next injection is commenced. For a nitrogen series, four consecutive injections are made at intervals of about 45 minutes to prevent peak overlap. It is not practical to use the hydrogen peak, since its retention time is so short that the peak appears upon the tail of the desorption pressure peak. Also, the thermal conductivities of hydrogen and helium are similar which result in a very small response factor for hydrogen. The hydrogen peak could be eliminated completely by using a hydrogen carrier. It should be noted that the procedures for calibration series described above are specific to the gas chromatograph and particularly to the column which has been described. There was no attempt to reduce the retention time of the nitrogen
2120
+
peak. This could be accomplished by either reducing the length of the column or by raising the operatiqg temperature. The literature on the use of molecular sieve 5 A in the separation of fixed gases should be consulted in the design of a system to meet specific requirements (5-7). Another possibility would be to calibrate upon the unseparated nitrogen hydrogen or oxygen hydrogen peak, in which case the experimental yield is 16.77 pl(STP)/pequivalent and 16.78 kl(STP)/ pequivalent, respectively.
+
+
RESULTS AND DISCUSSION Calibration curves for nitrogen and oxygen were determined from five coulometric injections for each gas. These curves are linear to within =t1 between 0.25 and 5.00 pmoles, the range of both injection series. One test of the calibration curves was made by analyzing three dry air samples injected with a 250-microliter gas sample valve. Within the system precision, the measured mole per cents of oxygen and nitrogen are the same as the values reported by Gleuckauf (8). A second test involved the determination of oxygen and nitrogen concentrations in nine samples of distilled water saturated at known temperatures and pressures. The measured concentrations agree with the values from the tables of Murray et al. (9, IO).
RECEIVED for review February 28, 1972. Accepted May 30, 1972. It is with appreciation that we acknowledge the support of the Office of Naval Research through Contract No. 1841(74)and NOOOl4-67-A-0386-0001. (5) E. W. Lard and R. C. Horn, ANAL.CHEM.,32, 878 (1960). (6) G. W. Heylmun, J. Gas Chromatogr., 3, 82 (1965). (7) B. D. Gunter and B. C. Musgrave, ibid., 4, 162 (1966). (8) E. Gleuckauf, in “Compendium of Meteorology,” J. F. Malone, Ed., Reinhold Publishing Corp., New York, N.Y., 1959, p 85. (9) C. N. Murray and J. P. Riley, Deep-sea Res., 16, 311 (1969). (10) C. N. Murray, J. P. Riley, and T. R. S. Wilson, ibid., p 297.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
