absence of gas phase terms in higher than the first power of velocity. It is thus valid for use with all known rate equations except those involving interfacial mass transfer terms. It applies, therefore, whether C: represents a radial gas mass transfer term (4, a radial inhomogeneity function independent of capacity factor, k , ( 5 ) , a composite of both, or again, if C,O = 0. The second prerequisite for thc validity of the method is that the prwsure corrections to the rate equation are as stipulated herc. The reproducibility of the representative data implies that these conditions are met. A very rough estimate of the quantity (C: Clf) from the high velocity assymptote to the nitrogen curve in
+
LITERATURE CITED
Table
I.
Evaluation of Ci from Data in Curves of Figure 1
A H , Cm.
Cm./Sec.
Ati,
104 cI,
0.005 0.008 0.013 0.0165 0.018 0.019
6.08 10.96 15.09 18.60 21.86 24.07
8.2 7.3 8.6 8.9 8.2 7.9
Sec.
Figure 1 (-2.2 X indicates that, for nitrogen elution, C,O is about 1.8 X 10-3 sec., since f-0.5. This value is somewhat greater than would be expected of radial gas mass transfer effects alone.
( 1 ) Bohemen, J., Purnell, J. H., J. Chem. soc., 1961, 360. (2) Zbjd., p. 2630. (3) Giddings, J. C., Seager, S. L., Stucki,
L. R., Stewart, G. H., ANAL.CHEM. 32,867 (1960). ' (4) Golay, M., "Gas Chromatography," D. H. Desty, ed., p. 36, Butterworths, London, 1958. (5) Golay, M., preprints, 2nd International Symposium, I.S.A., p. 5, Lansing, Mich., June 1959. (6) Jones, W. L., ANAL. CHEW 33, 829 (1961). (7) Khan, M. A., Nature 186, 800 (1960). (8) Kieselbach, R., ANAL. CHEM.33, 23 (1961). R. H. PERRETT J. H. PURNELL Department of Physical Chemistry University of Cambridge Lensfield, Cambridge, England
Effect of Air on Hot Wire Thermal Conductivity Detectors SIR: We have found that large air peaks show prolonged tailing (Figure 1) on a four-element, tungsten, hot wire thermal conductivity detector (GowMac filaments, Type 9225, 20 ohms cold) when the latter was operated near maximum sensitivity (100 t o 300 ma. bridge current) in a detector temperature range of 200" to 300" C. T o our knowledge this effect of air or oxygen has not been pointed out in gas chromatography literature. The effect was first observed on a process automatic chromatograph which was being used to analyze a hot air stream for small amounts of naphthalene. The effect was so serious that the process chromatograph could not be operated satisfactorily. Peak height response for naphthalene was too high because of the nonreturn to base line of the air peak. To operate the instrument with 2-ml. air samples, a n automatic vent valve was used following the column so that the air peak could he diverted and not pass through the detector. A comparison of the chromatograms of 2-ml. samples of air and nitrogen is shown in Figure 1. The F&M Model 500 Gas Chromatograph (F&M Scientific Corp., Avondale, Pa.) instrument was used with the following chromatographic conditions: detector temperature, 300" C.; detector bridge current, 190 ma.; column, 7 feet, 1/4 inch, 15% silicone-oil (Dow Corning 200) on Fluoropak 80-sized fluorocarbon polymer a t a column temperature of 180" C.; helium flow rate, 50 ml. per minute and I-mv. recorder with full bridge output (IX). The tailing portion of the air chromatogram required up to one hour
[i
, 5
-
10
I5
4
20
25
TIME, HIN
Figure 1.
Effect of air on base line
before returning to the original base line. Oxygen gave a similar effect. Other vapors of equivalent sample sizes-namely, acetone, benzene, wet nitrogen, hydrogen, and S02-showed a normal return to base line in 2 to 3 minutes. h'ew filaments and an empty column showed the nonreturn to base line with an air sample; hence, the effect was probably not caused by organic material or carbon deposits on the filaments. The magnitude of the effect depended on operational variables that result in a high hot wire temperature, The base line displacement using the F&M 500 gas chromatograph and the above conditions reached a saturation level of about 1 mv. on repeated additions of Zml. samples of air or oxygen. The effect was less with lower bridge currents and was hardly noticeable a t 100 ma. However, with the detector at 300" C., and bridge current a t 100 ma., the effect was again observed, With the process chromatograph, the effect was obtained with the detector at 180" C. (oven temperature) and a bridge current of 290 ma.
Surface oxidation of the tungsten wire seems to be a plausible explanation for the effect. It is well known (1, 8) that tungsten filaments will oxidize at about 500" C. The temperature of a hot wire filament will increase momentarily during the passage of a large air sample because of the poor thermal conductivity of air as compared with helium, and the temperature may be high enough to give some oxide coating on the filament of the hot wire using the conditions previously mentioned. The oxide coating might prevent the filament from returning to its original equilibrium value. Eventually the oxide coating may slowly decompose so that the filament returns to the original base line. Certain tungsten oxides (several forms are possible) are reported (3) to be unstable. It would seem advisable that large gas samples containing oxygen should be chromatographed a t lower detector temperatures (also less bridge current) where oxygen has no effect. If higher sensitivity is desired (higher hot wire temperature), a diversion valve can be used between the column exit and the detector for venting the air peak. LITERATURE CITED
Xolthoff, I., Elving, P., "Treatise on Analytical Chemistry," Part 11, Vol. 1, p. 272, Interscience, New York, 1961. (2) Langmuir, I., J. Am. Chem. SOC., (1)
35, 105-27 (1913). (3) Li, K. C., Wang, C. Y., "Tungsten," 2nd ed., ACS Monograph Series, p. 248, Reinhold, New York, 1947. J. S. PARSONS W. B. PRESCOTT H. C . LAWRENCE American Cyanamid CO. Bound Brook, N. J. VOL. 34, NO. 10, SEPTEMBER 1962
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