liquid was continuous through the intersection with the interface, and that no change in slope occurred at this line. This appears to be apprOximatelY justified by the Photo@aPhs, though we expected that the requirement Of equilibrium of the surface and interfacial tensions would require a change of slope.

It is clear from these considerations that the calculation of

ACKNOWLEDGMENT

This work was performed while the author was a guest of the Chemistry Department, University of California, Berkeley. For their hospitality, and for the generous assistance of paul L. Chambrb with the calculations, he is most grateful,

R. A. STAIM surface tensions from the form of frozen drops is fraught with dangers. The established method (3) involving photography Department Of Chemistry of liquid sessile or pendent drops is to be preferred.

(3) W. D. Harkins and A. E. Alexander, in “Physical Methods of Organic Chemistry,” A, Weissberger, Ed., 3rd ed., Inter- science, New York, N. Y., 1959, pp 805-809.

Trent University Peterborough, Ontario Canada

RECEIVED for review May 3, 1971. Accepted June 5, 1971.

I AIDS FOR ANALYTICAL CHEMISTS

New Use for a 0.5-Nanometer Molecular Sieve Gas Chromatography Column

W. A. McAllister and W. V. Southerland Department of Chemistry, East Carolina University, Greenville, N . C. 27834

.

THE USE OF 0.5-nm molecular sieve columns in gas chroma- tography for separation of diatomic gases is routine. At this laboratory, however, it has been used for the separation of COS and NzO at 250 “C. This has been a valuable asset, allowing the use of a simple gas chromatograph for the sepa- ration and detection of CO, NP, NO, COZ, and NPO without a column change or modifications to the chromatograph. The literature (1-6) and commercial firms all propose using series columns, parallel columns, or elaborate column switch- ing devices to separate this group of gases. These particular gases result from our studies of some catalyzed oxidation- reduction reactions between CO and NO at 1 atmosphere. The detection limit is satisfactory in that a 0.5-ml sample provides a detectable limit of about 1 part per thousand on the 3-ft X l/,-in. column.

EXPERIMENTAL

The gas chromatograph used in these measurements was a Gow-Mac Model 69-500 with a 1-mV recorder. Helium flow gas was used with a flow rate of 60 ml per minute and the detector current was 150 mA. The 60-80 mesh 0.5-nm molecular sieve was obtained from Fisher Scientific Company. It had been refined by Coast Engineering Laboratory and was from their lot 81170. It was packed manually in ‘/,-in. copper tubing (approximately 4 grams per foot) and was conditioned for 1 hour at 250 “C by purging with helium.

RESULTS AND DISCUSSION

One can only obtain complete separation of the diatomic portion of the gaseous mixture a t 250 “C by using an inordi-

(1) W. M. Graven, ANAL. CHEM. 31, 1197 (1959). (2) E. Heftmann, “Chromatography,” Reinhold Publishing Corp.,

NewYork,N.Y., 1961. (3) P. G. Jeffery and P. J. Kipping, “Gas Analysis by Gas Chroma-

tography,” The Macmillan Company, New York, N. Y., 1964. (4) E. W. Lard and R. C. Horn, ANAL. CHEM. 32,879 (1960). (5) R. Stock and C. B. F. Rice, “Chromatographic Methods,”

Reinhold Publishing Corp., New York, N.Y., 1963. (6) D. H. Szulczewski and T. Higuchi, ANAL. CHEM., 29, 1541 (1957).

Table I. Retention Times for OS-Nanometer Molecular Sieve GC Columns

Temp, 3-ft 6-ft

0 1 0:30 0:42 N2 0:53 1:12 NO 1 :20 2:00 co 2:25 3:45

Column length

“C Gas Retention time, min:sec

100

NzO 2:50 4:05 con 4:oo 5: 35 250

nately long column. However, complete separation is not absolutely necessary for our reaction studies because the ratios of peak heights for the diatomic species on the three- and six-foot columns are sufficient to give an approximate indication of how the reaction is proceeding. The tempera- ture can be lowered to 100 “C occasionally to obtain accurate diatomic concentration from peak area measurements.

The data for typical operation are given in Table I. An unsuccessful attempt was made to separate SOZ and NOz. Ballistic temperature programming was also unsuccessful, apparently because of the slow rate at which the temperature of the column increases. A more sophisticated instrument or a modification of this instrument oven with an additional heater should work if the temperature rise is rapid enough. The 250 “C temperature seems to be the minimum for sepa- ration of the triatomic gases and must be reached fairly quickly while still allowing time for greater separation of the diatomics at lower temperatures. Six feet seems to be the minimum column length for this mode of operation.

RECEIVED for review May 13, 1971. Accepted June 14, 1971. This work was supported in part by the National Air Pollution Control Administration, Consumer Protection and Environ- mental Health Service, Public Health Service Grant Numb& AP01085-01,

1536 ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971