~ [ gidss p
in use, it is kept at 125 “C with a molecular sieve filtered N, flow through it. At the temperature of the dry ice-isopropyl alcohol bath, NzO is still a gas, but is adsorbed onto the silica gel. A liquid Nz bath was tried, ensuring a solidified NzO,but the valves leaked and the additional cost of better quality valves was not warranted. Analysis System. The temperature of neither the oil bath nor the gas chromatographic column was critical. The first three peaks in Figure 2 are a result of the transfer technique. The first two peaks were from dead air above each U-tube valve, and the third peak was from switching the gas sampling valve. Reproducibility and Accuracy. In a further series of tests, three sets of four parallel atmospheric samples were taken. Since each set was taken at different times, the average of each set was normalized to give the three sets a common average. After normalization of the 12 samples, the per cent standard deviation from the average was calculated to be 2 . 0 x . This value is higher than the average changes in the samples taken during the three-day period (Table 1). The quantitative reliability of the method was determined by using a mixture of NzO in Nz prepared by Scott Research Laboratory (SRL). (SRL was unable to provide the requested analysis accuracy of f1 % if air was used as the diluent.) Five 15-1. samples of this standard were trapped, transferred, and analyzed according to the previous procedures. The average value was 285 ppb with a standard deviation of j = 5 ppb. This showed an excellent correlation with the SRL value of 294 ppb with an accuracy of *3 ppb. The values reported here are in the area of other reported levels; the authors feel confident that the Table I data are close to the true concentration. Because of the very good precision, accuracy, and simplicity of the method, this technique can be used to monitor NzO trends in the atmosphere and to study NzO sources and sinks.
U-xibe!;of Rock and Schiitz could possibly de-
~l (4). The results are summarized in Table I. The discontinuity in the THF-diglyme contraction between - 120 and - 130 "C was found to be a reproducible effectprobably caused by solidification. Our results show some discrepancies with those cited in the references. These may well be due to a better temperature control in our case.
Table I. Degree of Contraction of 4 Solvents at Different Temperatures Contraction (in
Methanol
Methylcyclohexane
Temp, "C
EPA
glycerol
+ isopentane
25 20 10 0 - 10 - 20 - 30
0 1.1 2.8 4.4 5.6 7.2 8.3 10.6 11.7 12.8 13.9 15.6 16.7 17.8 19.0 20.6 21.9 23.3 24.5 25.6 27.0 28.3 29.6
0 1.1 2.4 3.6 4.4 5.6 6.7 7.8 8.9 10.0 11.1 12.2 13.3 14.4 15.6 16.7 17.8 18.9 20.0 21.1 22.2 23.3 24.4
0 1.1 2.2 3.6 5.0 6.7 8.1 9.4 11.1 12.2 13.3 15.0 16.1 17.8 18.9 20.0 21.7 23.3 24.4 25.6 27.2 28.9 29.4
t
-40
- 50 -60 - 70 - 80 -90 - 100 -110 - 120 -130 - 140
(1) W. J. Potts, Jr., J . Chem. Phys., 21, 191 (1953). (2) R. Passerini and I. G . Ross, J . Sci.Instrum., 30,274 (1953). (3) K . Rosengren, Acta Chem. Scund., 16, 1421 (1962). (4) E. Hardin Strickland, J. Horwitz, and C. Billups, Biochemistry, 8, 3205 (1969).
Z,relative to volume at 25 "C)
- 150 - l -M --
- 170 - 180 - 190
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
THF
+
Diglyme 0 1.1 2.2 3.3 4.4 5.6 6.7 7.8
8.9 10.0 11.1 12.2 13.6 14.7 15.8 16.7 23.3 24.4 24.4 25.0 25.6 26.1 26.1
1119
