Stabilization of a standard gas mixture generator with diffusion tubes
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Stabilization of a Standard Gas Mixture Generator with Diffusion Tubes L. Torres*/J. Mathieu/M. Frikha Laboratoire de Chimie Analytique Appliqu~e, Ecole Nationale Sup~rieure de Chimie (Institut National Polytechnique), 118, route de Narbonne, 31077 Toulouse C~dex, France J. Namiesnik Institute of Chemical Engineering. Technical University of Gdansk U 1 Majakowskiego 11/12, 80-952 Gdansk, Poland Key Words Diffusion tubes Standard gas generator Summary The generator contains diffusion tubes of known length and internal diameter. Once produced, the mixture of the carrier gas and acetone, the reference material, is sent through a chromatographic flame ionisation detector. Its signal reaches a constant level after a time corresponding to the stabilization time of the generator. The tech- nique described is simple and gives repeatable results. The preparation of standard gas mixtures constitutes a very important step in the development of equipment whose final application is monitoring the atmosphere. These mix- tures are also necessary to study the efficiency of processes such as adsorption, desorption and oxidation. The methods used for preparing these gas mixtures can be divided into: (i) static methods [1-3]. The gas or vapours of the com- pound to be analysed are introduced in a very small but known amount into a reservoir containing a known volume of diluting gas; (ii) dynamic methods [4-6]. A very small stream of the gas to be analysed is mixed with a stream of the carrier gas or diluting gas. In the equipment most frequently used, diffusion [7-10] or permeation [11-14] tubes are used to generate stable diffusion rates of the com- pound to be analysed. The principle of the diffusion tube method has been defined by M. McKelvey et al. [10]. The diffusion rate for a given compound can be calculated using a very complica- ted expression, however, in practice these rates are obtained experimentally by weighing at 24 h intervals the ampoule containing the test compound. This experimental technique allows rates as low as 1 tag min -1 to be determined with precision. On raring both the dimensions of the diffusion * Author to whom all correspondance should be addressed. tube and the temperature, a large range of diffusion rates can be obtained, and it is therefore quite easy to obtain the desired concentration of the test compound in the stable carrier gas stream. The use of diffusion tubes has, however, two major inconveniences. The time requiredfor them to be calibrated is relatively long and especially their stabilization for very small diffusion rates is extremely slow. Even though this method is used by a relatively large num- ber of workers, the stabilization time for the generator is not known with any precision; values between several hours and several days can be found in the literature depending on the diffusion rates. The aim of this work is to determine the stabilization time of the generator to standard gas mixtures. Acetone has been chosen as the standard and the diffusion rates examin- ed are in the range 25-200 tag rain -1 . Experimental The stabilization times were measured using the apparatus shown in Fig. 1. The nitrogen (flow rate 50 cm a min -1) used as carrier gas, was purified by passing over heated copper (at 300 ~ (3) and dried over KOH pellets (4). The nitrogen then flows into the thermostated generator of the upper mixing chamber (6) which is connected via the Teflon diffusion tube (8) of known internal diameter and length to the ampoule (7) containing acetone, whose vapour diffuses at a constant rate through the diffusion tube into file mixing chamber. The gaseous mixture formed then passes into the chromatographic FID. Throughout the experiment the temperature of the generator was maintained at 20.0 -+ 0.2 ~ Seven diffusion tubes, of the same length (40 ram), of internal diameters 1 to 7 mm were examined. To determine the diffusion rate for acetone for each one of the tubes, the generator is flushed by a purified nitrogen stream for 6 h. This time is vastly in excess of the longest stabilization time for the generator. The ampoule was then disconnected, weighed on a microbalance, reconnected to the generator, and again weighed after 24h. The loss in weight divided by the time lapse between the two weigh- ings gives the diffusion rate for the diffusion tube for acetone. 712 Chromatographia Vol. 14 No. 12, December 1981 Short Communication 0009-5893/81/12 0712-02 ~ 02.00/0 1981 Friedr. Vieweg & Sohn Verlagsgesellschaft mbH
Transcript of Stabilization of a standard gas mixture generator with diffusion tubes
Stabilization of a Standard Gas Mixture Generator with Diffusion Tubes
L. Torres*/J. Mathieu/M. Frikha
Laboratoire de Chimie Analytique Appliqu~e, Ecole Nationale Sup~rieure de Chimie (Institut National Polytechnique), 118, route de Narbonne, 31077 Toulouse C~dex, France
J. Namiesnik
Institute of Chemical Engineering. Technical University of Gdansk U 1 Majakowskiego 11/12, 80-952 Gdansk, Poland
Key Words
Diffusion tubes Standard gas generator
Summary
The generator contains diffusion tubes of known length and internal diameter. Once produced, the mixture of the carrier gas and acetone, the reference material, is sent through a chromatographic flame ionisation detector. Its signal reaches a constant level after a time corresponding to the stabilization time of the generator. The tech- nique described is simple and gives repeatable results.
The preparation of standard gas mixtures constitutes a very important step in the development of equipment whose final application is monitoring the atmosphere. These mix- tures are also necessary to study the efficiency of processes such as adsorption, desorption and oxidation.
The methods used for preparing these gas mixtures can be divided into:
(i) static methods [1 -3] . The gas or vapours of the com- pound to be analysed are introduced in a very small but known amount into a reservoir containing a known volume of diluting gas;
(ii) dynamic methods [ 4 - 6 ] . A very small stream of the gas to be analysed is mixed with a stream of the carrier gas or diluting gas. In the equipment most frequently used, diffusion [7-10] or permeation [11-14] tubes are used to generate stable diffusion rates of the com- pound to be analysed.
The principle of the diffusion tube method has been defined by M. McKelvey et al. [10]. The diffusion rate for a given compound can be calculated using a very complica- ted expression, however, in practice these rates are obtained experimentally by weighing at 24 h intervals the ampoule containing the test compound. This experimental technique allows rates as low as 1 tag min -1 to be determined with precision. On raring both the dimensions of the diffusion
* Author to whom all correspondance should be addressed.
tube and the temperature, a large range of diffusion rates can be obtained, and it is therefore quite easy to obtain the desired concentration of the test compound in the stable carrier gas stream. The use of diffusion tubes has, however, two major inconveniences. The time requiredfor them to be calibrated is relatively long and especially their stabilization for very small diffusion rates is extremely slow.
Even though this method is used by a relatively large num- ber of workers, the stabilization time for the generator is not known with any precision; values between several hours and several days can be found in the literature depending on the diffusion rates.
The aim of this work is to determine the stabilization time of the generator to standard gas mixtures. Acetone has been chosen as the standard and the diffusion rates examin- ed are in the range 25-200 tag rain -1 .
Experimental
The stabilization times were measured using the apparatus shown in Fig. 1.
The nitrogen (flow rate 50 cm a min -1) used as carrier gas, was purified by passing over heated copper (at 300 ~ (3) and dried over KOH pellets (4). The nitrogen then flows into the thermostated generator of the upper mixing chamber (6) which is connected via the Teflon diffusion tube (8) of known internal diameter and length to the ampoule (7) containing acetone, whose vapour diffuses at a constant rate through the diffusion tube into file mixing chamber. The gaseous mixture formed then passes into the chromatographic FID. Throughout the experiment the temperature of the generator was maintained at 20.0 -+ 0.2 ~ Seven diffusion tubes, of the same length (40 ram), of internal diameters 1 to 7 mm were examined.
To determine the diffusion rate for acetone for each one of the tubes, the generator is flushed by a purified nitrogen stream for 6 h. This time is vastly in excess of the longest stabilization time for the generator. The ampoule was then disconnected, weighed on a microbalance, reconnected to the generator, and again weighed after 24h. The loss in weight divided by the time lapse between the two weigh- ings gives the diffusion rate for the diffusion tube for acetone.
712 Chromatographia Vol. 14 No. 12, December 1981 Short Communication
0009-5893/81/12 0712-02 ~ 02.00/0 �9 1981 Friedr. Vieweg & Sohn Verlagsgesellschaft mbH
l 2 3 5
|
Fig. 1 Diagram of apparatus for determination of stabilization times.
(1) Carrier gas bottle, (2) pressure regulator, (3) reactor containing Cu heated to 300 ~ (4) absorber filled with KOH pellets, (5) three- way taps, (6) mixing chamber, (7) ampoule containing reference sample, (8) Teflon diffusion tube, (9) thermostatic bath, (10) chro- matographic F I D.
300.
200.
,00.
Stablhzat ion t ime
( m in)
5b 1~o ,~o Fig. 3
Stabilization time as function of diffusion rate.
2'00 '- Di f fusion rote
(i.l g ~rain - t)
Tm T ime
Fig. 2 Typical experimental recorder output showing stabilization time Trn.
Results and Discussion
Fig. 2 shows an example of a tracing for the recorder obtain- ed for our stabilization time experiments.
At the beginning of each experiment only the pure carrier gas flows through branch B of the gas circuit (Fig. 1). This corresponds to the absence o f a signal at the recorder. As soon as the carrier gas is sent through the generator, acetone is carried to the detector and is characterized on the re- corder by the rapid increase in signal which then slowly decreases to a stable value after a t ime Tm corresponding to the stabilization time for the generator.
Fig. 3 shows the variation of the stabilization time, Tm, as a function o f the diffusion rate. For an increase in the diffusion from 25 to 2 0 0 ~ g m i n -~ a decrease in the stabil- ization time from 260 to 30ra in is observed. These two parameters are inversely related to one another.
The method described here to determine the stabilization time is very simple and gives repeatable results, and can be applied equally well to both diffusion and permeation tube generators.
R e f e r e n c e s
[1] R. Villalobos andH. G. Gill, JSATrans. 9,51 (1970). [2] A. S. Mikhaitenko and T. Ya. Loginova, Zavod. Lab. 45,
313 (1979). [3] D. Cornish, Chem. ind. April, 264 (1978). [4] A. Brassens, Analusis 3, 576 (1975). [5] Y. Hashimoto and S. Tanaka, Environ. Sci. Technol. 14,413
(1980). [6] J. Vejrosla andJ. Novak, J. Chromatogr. 175,261 (1979). [7] A. H. Miguel and D. F. S. Natush, Anal. Chem. 47, 1705
(1975). [8] T. L. C De Souza and S. P. Bhatia, Anal. Chem. 48, 2235
(1976). [9] A. Raymond and G. Guiochon, Analusis 2,357 (1973).
[10] M. McKelvey and H. E. Hoelscher, Anal. Chem. 29, 123 (1957).
[11] R. K. Stevens, A. E. O'Keefe and A. C Ortman, Environ. Sci. Technol. 3,652 (1969).
[12] T. lbusuki. F. Toyokawa and K. tmagami, Bull. Chem. Soc. Jpn. 52, 2105 (1979).
[13] J. Godin and C. Boudene, Anal. Chim. Acta 96, 221 (1978). [ 14] A.E. O'Kee[e and A. C Ortman, Anal. Chem. 38, 760 (1966).
Received: May 18, 1981 Accepted: May 25, 1981 C
Chromatographia Vol. 14 No. 12, December 1981 Short Commun ica t i on 713
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