ically. A column stored under ambient conditions for long periods no longer functions properly. Recently Yang and Wolf (3) have reported using 1-hexadecanol on (2-22 firebrick to separate ammonia and methylamine. We have tried this column under the same conditions as Yang and Wolf (except for the flow rate) for the separation of amine mixtures. The retention volumes found are: methylamine, 258 cc.; dimethylamine, 390 cc.; ethylamine, 533 cc.; flow rate, 17 cc. per minute. Thus,
on the basis of these data, this column promises to be as good as or better than the o-toluidine columns. On the basis of the results with the o-toluidine columns, we suggest that solubility data offer a semiempirical method for the choice of a column material which has some advantage over the hit-or-miss methods so frequently used. LITERATURE CITED
(1) James, A. T., Biochem. J. 52, 242 (1952).
IT.,Martin, A. J. P., Smith, G. H., Ibid., 52, 238 (1952). (3) Yang, J., Wolf, A., J. Am. Chem. SOC. (2) James, A.
82,4488 (1960).
ALEXANDERR. AMELL PHILIPS. LAMPREY ROBERTC. SCHIEK
Department of Chemistry University of New Hampshire Durham, N. H. WORKsupported in part by the Atomic Energy Commission under Contract AT(30-1)1911 with the University of New Hampshire.
Separation of Nitrogen Trifluoride from Carbon Tetrafluoride by Gas Chromatography SIR: The separation of CF4 and NFa by gas chromatographic techniques has been reported (d) but required long columns held at low temperatures and, for the best results reported, 70 minutes to complete an analysis. Recent work in this laboratory has shown that excellent separation of CF, and NF3 can be accomplished in less than 10 minutes using a 10-foot column operated a t 25" C. As little as 2% NF3 in mixture with CF4 has been detected. The column consists of 60- to 80-mesh silica gel (Davidson Grade 12) coated with 10% by weight of Halocarbon Oil (Series 13-21, 68.7 centistokes a t 100" F.) available from the Halocarbon Gorp., Hackensack, N. J. Similar results were obtained using a 16-foot column prepared in the same manner. In this case, analysis time was increased to approximately 18 minutes, EXPERIMENTAL
Conventional gas chromatographic equipment was used for this work. The detector consisted of two matched 2000-ohm thermistors (Veco A-111). The detector block was held a t 40" C. for all runs. Helium was used as carrier gas. The silica gel WRS screened to obtain the 60- to 80-mesh fraction and used without further activation. The oil was weighed directly onto the pre-
Table 1.
determined weight of silica gel and mixed gently but thoroughly until the material was free flowing and without a glomerates. Columns consisted of 'I-inch O.D. copper tubing packed in the conventional manner and coiled to fit a 1-gallon Dewar jar which contained water a t the desired temperature. Samples were introduced by means of a Perkin-Elmer gas sampling valve fitted with a 1-cc. sample loop. Mixtures of CF4 and NFa were prepared manometrically to give samples with the compositions by volume shown in Table I. Operating conditions were varied to determine the effect of column length, amount of liquid phase, temperature of column, and flow rate of helium. Trace amounts of air were well separated from the CFa which eluted before NF3 under the conditions used. These results are summarized in Table I. Peak areas were determined by the triangulation method: peak height X width a t half height. The area per cent of each component was then determined from the total area measurements. Those runs made a t the same helium flow rate showed excellent reproducibility of total area, independent of the sample composition. The separation factor, azv,has been defined by the Committee on Nomenclature (1) as the ratio of corrected retention volume of two components
a+, =
where t, = retention time of second of two
components
tZ = retention time of first of two
components t, = retention time of air
In the analyses reported here the separation factor of the CFd/NFa mixtures calculated to 1.3 under most conditions tried. This compares favorably with a reported 1.05 separation factor for the difficult pair, m- and p-xylene (3). LITERATURE CITED
(1) Coates, V. J., Noebels, H. J., Fagel;; son, I. S., eds., "Gas Chromatogra hy, ISA Sym osium 1958, Academic fress, New Yorf: (2) Nachbaui, E., Engelbrecht, A., J. Chromatog. 2 , 5 6 2 4 (September 1959). (3) Wiseman, W. A., Nature 185, 841-2 (1960).
Ex erimental Station
ADAHB. RICHMOND
E.?. du Pont de Nemours & Co. Wilmington 98, Del. CONTRIBUTION No. 694, Central Research Department, E. I. du Pont de Nemours & Co., Inc.
Sample B CF, NFa
Sample C CF, NFa 10 90
Sample D CFI NFs
98 25 75 50 50 Actual Comoosition, vol. % .. 10-ft. column, 10% liquid, 25" C., 30 cc./min. He 28 72 50 50 85 15 10-ft. column, 10% liquid, 37" C., 30 cc./min. He 50 50 16-ft. column, 10% liquid, 25' C., 30 cc./min. He 51 49 88 12 95 16-ft. column, 10% liquid, 37" C., 30 cc./min. He 28 72 96 IO-ft. column, 10% liquid, 25" C., 45 cc./min. He 28 72 51 49 90 10 97 A 16-ft. column with 15% liquid at 25' C. and 30 cc./min. helium showed no improvement. Increase of temperature to 49' C. did not improve se aration on either 10 or 15% liquid columns. Increase in helium flow improved peak shape a t 25' but did not improve actual separation factor.
8.
ANALYTICAL CHEMISTRY
tz
Calculated Composition in Area Per Cent of Synthetic Samples
Sample A CFI NFa
1806
- 1. - tn
ty
2 5 4 3
Sample E CF, NF: 75 74 72 77 77 77
25 26 28 23 23 23
