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A Mass Spectrometer Inlet System For Sampling Gases Dissolved In Liquid Phases

4128 words - 17 pages

A Mass Spectrometer Inlet System for Sampling Gases Dissolved
in Liquid Phases'
From RIAS, Baltimore, Maryland
Received November 23, 1962
.4 mass spectrometer inlet system has been devised which permits continuous sampling of gases dissolved in liquid phases. The principle is that of a semipermeable membrane which allows the dissolved gases, but not the liquid phases, to enter the mass spectrometer. The instrument permits rapid time response and high sensitivity. The application of this system to the study of reaction kinetics is described.
Biological systems consume or produce a variety of ...view middle of the document...

ific Re- search (AF49 (638)-947).
tions. Obviously, measurement of pressure changes cannot separate the reactions for individual study.
A unique instrument, the mass spectrom- eter, exists which can analyze gas mixtures both qualitatively and quantitatively. In addition, the mass spectrometer can de- termine the isotopic composition of the gases in the mixture. The latter ability enables it to study the reactions which result in simultaneous production and consumption of the same gas. Since the instrument is designed for continuous analysis and re- sponds rapidly to concentrat'ion changes, it is well suited for kinetic analysis.
A disadvantage to kinetic studies of biological systems with t,he convernional mass spectrometer method is, however, the fact that samples must be admitted to the instrument as a gas. If the spectrometer samples the gas phase above a liquid in which the reaction is occurring, the limita- tions of response time which apply to a manometer also apply to it. In addition, certain kinetic studies require knowledge of isotope ratios at the reaction site. Measure- ments in the gas phase are necessarily remote from this site and hence the isotope ratios determined may be greatly in error due to the diffusion barrier presented by the gas- liquid interface.
To overcome these difficulties an inlet 160

system has been devised which permits the mass spectrometer to sample the gases dissolved in a liquid phase and eliminates the gas-liquid interface. The reaction system then contains no gas phase. An ancillary advantage of this system is a reduction of the amount of material and isotopes re- quired for analysis and an increase of sensitivity because of the absence of a gas phase which (especially for poorly soluble gases) "dilutes" the effects. The inlet func- tions through a semipermeable membrane which allows the dissolved gases to enter the instrument but retains the liquid phase.
The process of gas permeation through a plastic membrane involves several physical phenomena, at least a sequence of solution, and diffusion and re-evaporation of the permeating gas. The rate of gas transfer across the film is directly proportional to the pressure gradient across the film and inversely proportional to the film thickness. Thus the rate of transfer follows this ex- pression :
Rate of gas
transfer = D ' A ' Ap PXAXAp
1 Xk=
P = permeability constant A = area of film D = diffusion coefficient 1 = thickness of film k = solubility constant Ap = difference in partial pressure across
the film.
In the applications described here, the partial pressure on the mass spectrometer side of the membrane is very low (- lop5 mm. Hg), and therefore the rate of molecules entering the instrument is simply propor- tional to the external concentration. Since the spectrometer signal (a current) is also proportional to the rate of molecules en- tering the instrument, an empirical constant can be...

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