Membrane inlet mass spectrometers

Of the isotopes of hydrogen, deuterium can be detected by mass spectrometric analysis of masses 2, 3 and 4 nsing a membrane-inlet mass spectrometer (Fig. 5.4). 

Kana, T. M., Darkangelo, C., Hunt, M. D., Oldham, J. B., Bennett, G. E., and CornweU, J. C. (1994). Membrane inlet mass spectrometer for rapid high-precision determination of N2, O2 and Ar in environmental water samples. Anal. Chem. 66, 4166—4170. 

Ovemey, F.L., Enke, C.G. (1996) A mathematical study of sample modulation at a membrane inlet mass spectrometer—potential application in analysis of mixtures. Journal of the American Society for Mass Spectrometry, 7, 93-100. 

The newest approach to measuring denitrification uses a mass spectrometer that can rapidly measure the N2/Ar ratio in water using membrane inlet mass spectrometry (MIMS, Kana et al., 1994, 1998). In this approach, sediment cores are incubated under continuous flow conditions. The N2/Ar ratio is then measured at the inlet and outlet of the core over time. Advantages of the technique are the short incubation times needed (usually less than 12 h), and the high throughput and small sample size ( 5 ml) of MIMS analysis. 

Riter et al. applied Membrane Inlet Mass Spectrometry (MIMS) coupled to a miniature mass spectrometer equipped with a cylindrical ion trap (CIT) analyzer to monitor the flavor components directly from human breath [6], Johnson et al. measured ethanol concentrations on-line in fermentation broths from a 9000-L fermentation reactor for a period of four days [7], However, data reported in the above papers referred to experiments lasting no more than a few days. 

The resulting spectra from El usually contain a number of fragments, providing extensive structural information about the analyte. A disadvantage of the observed fragmentation is eventually occurring isobaric overlay from different compounds in the analysis of sample mixtures, which often requires a separation step prior to the MS analysis. For this purpose the coupling of a GC with the ion source of the mass spectrometer via capillary inlet is a well established technique. Volatiles can be selectively introduced into El mass spectrometers via pervaporation membranes. The principle and application of this technique, called membrane introduction (MI) MS was recently reviewed [45]. The accuracy of intensity ratio measurements by El MS is about 0.1 -0.5% [4,8]. 

Dosage of analytes into a mass spectrometer Membrane inlet 1 Mass Spectrometry -MMS)... 

A schemahc diagram of the DEMS apparatus is shown in Fig. 5. The electrochemistry compartment corrsists of a circular block of passivated htanirrm (a) that rests above a stainless-steel support (1) cormected to the mass spectrometer. The space between the cell body and the snpport is a Teflon membrane (j) embedded on a steel mesh (k) the membrane is 75 pm thick, has 50% porosity and pore width of 0.02 pm. The single-crystal disk (h) is the working electrode its face is in contact with the electrolyte solution and separated from the cell body by another Teflon membrane (i) that functions as a spacer to form a ca 100-pm thick electrolyte layer (j). Stop-flow or continnons-flow electrolysis can be performed with this arrangement. For the latter, flow rates have to be minimal, ca 1 pL/s, to allow ample time (ca 2 s) for the electrogenerated products to diffuse to the upper Teflon membrane. Two capillaries positioned at opposite sides of the cell body (b, e) serve as electrolyte inlet and outlet as well as connection ports to the reference (f) and two auxiliary Pt-wire electrodes (d, f). 

One of the instruments obtained by ERDA will be evaluated at ORNL to assess its potential as a monitor for hazardous by-products from alternative energy sources. Investigations to determine the desirability of a membrane inlet system for concentrating organic vapors are planned. The feasibility of using a portable gas chromatograph with the portable mass spectrometer, when a complex mixture analysis is required, is also being studied. 

Another MS technique used in connection to pyrolysis is MIMS (membrane introduction mass spectrometry). MIMS is in fact a special inlet for the mass spectrometer, where a membrane (usually silicone, non-polar) lets only certain molecule types enter the Ionization chamber of the MS. This allows, for example, direct analysis of certain volatile organic compounds from air. The system makes possible the coupling of atmospheric pyrolysis to a mass spectrometer [61a] allowing direct sampling of the pyrolysate. Other parts of the mass spectrometer do not need to be changed when using MIMS. 

To drive the volatile products directly from the electrode surface via a suitable inlet system into the vacuum chamber of the mass spectrometer, a plain gas-permeable membrane (e.g. PTFE or polyethylene) on which the electrode material is deposited by sputtering or vacuum evaporation is usually used. Alternatively, the material can be deposited onto a metal gauze that is fixed close to the gas-permeable membrane. DBMS cell constructions such as that shown in Big. 12.39 have been described that allow the use of massive electrodes, which are more easily available. 

As shown in the right side of Fig. 9, a quadrupole mass spectrometer, MSG 300, with a gas-tight ion source, secondary electron multiplier, direction detector, and a turbo-molecular pump (TURBOVAC 150) is equipped with a membrane inlet (all from Nippon Shinku, Tokyo). The resolution scale is 300. Mass spectrometry can also be used for the measurement of dissolved gases in a liquid phase using a steam sterilizable membrane probe. Recently, the application of the mass spectrometer to fermentation processes has increased markedly. 

AMETEK s ProMaxion process mass spectrometer can monitor up to 32 components. Automated sample switching at the multiport allows unattended analysis of process and calibration gases. Different calibration and analysis methods can be assigned to each sample port. A membrane inlet system is incorporated for ambient gas sampling. The available mass range is 1-200 amu. The detection range is 10 ppm to 100 % with a Faraday cup detector lower LODs are possible with an electron multiplier. 

Jones and Yang used the principle of a silicone-rubber membrane separator as proposed for GC-MS for the development of a three-stage molecular separator to couple a mass spectrometer with a liquid chromatograph (Fig. B). The effluent from the chromatograph is led via a 0.025 on I.D. stainless-steel tube to the inlet of a flash evaporator, which consists of a 15-cni section of a 0.025-cm I.D. stainless-steel tubing, heated by an aluminium block. The vaporized stream expands into a 4-cm long section of 0.16-cm I.D. tubing and passes over the first membrane surface. The vapour not transmitted fay the meinbrane (a circular disc... 

Riter, L.S. Laughlin, B.C. Nikolaev, E.N. Cooks, R.G. Direct analysis of volatile organic compounds in human breath using a miniaturized cylindrical ion trap mass spectrometer with a membrane inlet. Rapid Commun. Mass Spectrom. 2002, 16, 2370-2373. 

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