1728
ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
Preparation of Refillable Permeation Tubes Angelika Teckentrup and Dieter Klockow * Department of Chemistry, University of Dorfmund, P.O. Box 50 05 00, 0-4600 Dortmund 50, German Federal Republic
Permeation devices. originally described by O'Keeffe and Ortman ( I ) , offer a simple method of preparing test atmospheres with low concentrations of gaseous materials. Known concentrations of a variety of air pollutants may be prepared by passing clean dry air, nitrogen, or another gas over permeation tubes which contain the compounds under study in a condensed form. These compounds are emitted a t a constant rate when the tubes are held at constant temperature. A wide range of concentrations can be obtained by varying the flow rate of the carrier gas. the temperature, the surface of the permeation membrane, or its thickness. During past years different versions of permeation devices have been described (2-5). Especially the methods of sealing the tubes have been improved. The permeation device presented in this paper exhibits the following characteristics. The liquid phase is not in contact with the permeation membrane but in contact with a glass tube (or other material), which is not permeable to the gas. Thus the permeation rate is independent of the height of filling (4). T h e tube can easily be closed with a new type of a Polytetrafluoroethylene (PTFE) plug which does not only represent a removable seal but also, at the same time, is a permeation membrane, the design of which is variable with respect to length and wall thickness. This kind of seal can be used even for gases with high vapor pressures (H2S: 19 bar, 298 K). The construction of the permeation device is shown in Figure 1. It is filled by condensation of a cylinder gas (e.g., SO2, H2S, or NH3) at 195 K under a protecting argon atmosphere. After that, the plug .4is pressed into the glass tube C by a plastic screw cap B (see Figure l b ) . The glass tube C is a modified part of a vacuum stopcock commercially available (6). T h e device is operated in a vertical position, so that only the vapor of the condensed compound is in contact with the permeation area. At a given inner diameter di, different permeation rates can be obtained by varying the length 1 and the wall thickness d of the upper part of plug A. The size of the storage "tank? may be adapted to the problems under study. An empty tube can easily be opened for refilling by removal of cap B (see Figure 1). An O-ring is added to the cap to prevent plug A pulling loose from the screw cap when the tube is opened. A steady state of permeation is reached within approximately one day after resealing the filled tube. The permeation device described is used in our laboratory for the continuous generation of ppm concentrations of ammonia and hydrogen sulfide in nitrogen. The permeation rates were determined gravimetrically for two NH3 and one H2Spermeation tubes with d, = 1 mm. At a length / = 7 mm (10 mm) and a d l thickness d = 1.5 mm (1 mm), the permeation rate for NH3 a t 293 K is 0.81 pg/min (1.52 pgimin). T h e permeation rate of a H2S-tube with 1 = 3 mm, and d = 3 mm is 0.69 pg/miri at 293 K. This tube is always kept under
0003-2700/78/0350-1728$01 0010
B ing bulge
A I
O-ring
C
c-
1
1c m
Figure 1. Permeation tube. (a) Open, (b) closed
dry nitrogen to prevent H2S from being oxidized within the P T F E walls. The permeation rates were determined during a period of seven months. No trend could be observed.
LITERATURE CITED (1) A. E. O'Keeffe and G. C. Ortman, Anal. Chem.. 38. 760 (1966). (2) F. P. Scaringelli, A. E. OKeeffe, E. Rosenberg. and J. P. Bell. Anal. Chern., 42. 871 (1970). (3) 6. E. Sattzman, W. R. Burg, and G. Ramaswamy, Environ Sci. Techno/., 5 , 1121 (1971). (4) F. Lindquist and R. W. Lanting, Atmos. Envifon. 6 . 943 (1972). ( 5 ) A. E. O'Keeffe, Anal. Chem., 49, 1276 (1977). (6) J. Young, Sclentlfic Glassware Ltd., Acton, London, England.
RECEIVED for review May 8, 1978. Accepted .July 10, 1978.
C 1978 American Chemical Society
