NOTES
Generation of Accurate Halocarbon Primary Standards with Permeation Tubes Hanwant B. Singh" and L. Salas Stanford Research Institute, Menlo Park, Calif. 94025
D. Lillian', R. R. Arnts2, and A. Appleby Rutgers University, New Brunswick, N.J. 08903
A number of permeation tubes for generating low concentration primary standards are tested for 18 halocarbons, and the permeation rate data are presented. In most cases, permeation tubes offer a satisfactory technique for the generation of accurate primary halocarbon standards. Because of the serious environmental impact of ambient halocarbons (1-3), it has become imperative that the atmospheric concentration levels of these pollutants be determined with utmost accuracy. Halocarbons are a relatively new group of pollutants generally present in the clean atmosphere at an extremely low concentration range of about 1-1OOO ppt ( v/v) ( 3 ) .Generation of known low-level concentration mixtures for use in development and calibration of instruments is a critical part of this research. In a recent workshop on halocarbon measurements, it was discovered that interlaboratory calibrations were uncertain by as much as a factor of two, which was considered unacceptable ( 4 ) . However, in almost all cases, halocarbon primary standards a t ppt levels were generated from pure materials with subsequent multiple dilutions. On the basis of the experience in our laboratories, at extremely low concentrations such procedures are tedious and inaccurate. The errors are due to a number of reasons, but surface sorption and heterogeneous reactions are probably the most significant a t very low concentration levels. In a continuing research program to determine the atmospheric fates of halogenated compounds, permeation tubes (5) were used to obtain primary halocarbon standards at an error of 10% or less. Results presented here are based on experimental data obtained at SRI over a four-month period.
5 L/min over the permeation tubes without causing any heat transfer problems. All tubing materials were either aluminum or glass. Fittings were either brass Swagelok or all glass ball joints. Permeation tubes were weighed at least once a week and in many cases twice a week on a semimicro ( 10-5 g) balance. The water bath was filled to a constant level every week with additional water a t 30-32 "C to make up for evaporation losses. Results a n d Discussion Table I lists the 18 halocarbons for which the permeation tubes were tested. This table also provides information on the wall thickness used for each tube, its permeation rate in ng/ min and ppb/min/L (ng = v/v) of diluent g; ppb = gas, its performance, and the conditioning time. In all cases, a conditioning period of up to six weeks was required before a constant permeation rate could be established. As a general rule, for a given compound the conditioning time increased with increasing wall thickness of the permeation tube. The permeation rates were obtained statistically as slopes of the least-squares error line used to fit the weight vs. time data. The error criterion used was the standard error of the slope using the t distribution chart at the 95%confidence limit (6).The permeation rate was defined as Permeation rate (ng/min) = b l f {[~(bl)]'/~t1.,,/2(N - 2))
Experimental Permeation tubes (3.2 in. long) for 18 halocarbons of interest, constructed from standard F E P Teflon tubing of varying thicknesses, were obtained from AID Inc:(Avondale, Pa.). The methods of manufacture and sealing of permeation tubes are proprietory (AID, private communication) but are very similar to those used by O'Keeffe and Ortman ( 5 ) in principle. Each permeation tube was contained in a specialized glass holder (Figure 1)which was held in a 37-L water bath (18 X 12 X 10.5 in.) maintained a t 31.0 f 0.05 "C. Permeation holders were flushed' constantly with a purge gas flowing through at a rate of approximately 15 mL/min. The purge gas used was prepurified helium and was further passed through a sequence of traps containing charcoal, anhydrous calcium sulfate, and molecular sieve. The permeation holder inlet coil length was adequate to allow a helium flow rate of as much as Present address, U.S. Army, Environmental Hygiene Agency, Ab,erdeen Proving Ground, Md. 21014. - Present address, LJS.Environmental Protection Agency, ESRL, Research Triangle P a r k , N.C. 27711.
MIXTURE OUT
PUR I FlED HELIUM IN
t
1 -
l i 4 - i n TUBING
,3---__- --8
'
9-1'/4"
314-1n. O.D., 1i2-1n. I.D.
6"
GLASS SIEVE PLATE (Permeation tube stands vertically on this plate)
Figure 1. Permeation tube holder Volume 11, Number 5,
May
1977
511
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Table 1. Permeation Rate Data for Halocarbon Primary Standards Compound
CH3CI CH2C12 CHC13 CC13F (Fll) CH3Br CH31 coc12
C2HsCI CHpCHCl CH2CICH2CI CHCICHC12 CClFpCClFp (F114) CClpFCClFp (F113) cc12cc12c
CC14d CH3CC13d CH2BrCH2Brd CCI2F2 (F12)"
Tube wall thickness, a In.
0.125 0.030 0.030 0.030 0.030 0.030 0.125 0.030 0.062 0.030 0.030 0.030 0.030 0.030 0.030 0.030 0.030 0.125
Permeallon rate, nglmln-9.5 % contldence limit
Mean permeation rate, ppblLlrnin (25 "C, 1 atm)
1350.0f 80.4 580.0f 34.2 132.0 f 12.7 1050.0f 78.4 2020.0 f 102.2 256.6f 9.6 2460.0 f 183.6 460.0 f 37.8 1170.0f 64.1 71.3 f 6.0 246.0f 20.1 14160.0f 240.0 480.0f 49.6 64.8f 26.1
653.6 166.8 27.0 186.9 520.2 44.2 607.6 174.4 457.7 17.6 45.7 2025.6 62.6 9.5
... ... ... ...
... ... ...
...
Statusb
Conditioning time, weeks
S S S S S S S S S S S S S
