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A tandem mass spectrometer designed especially for process control can be simplified in several ways by comparison with those instruments used for analysis. A low-cost membrane-inlet triple quadrupole mass spectrometer is described that has been designed especially for process control together with examples of continuous monitoring of different organic compounds in mixtures.Membrane-inlet mass spectrometry is a rapidly developing technique for the continuous monitoring of volatile organic compounds dissolved in water. The technique uses a silicone membrane to separate the reaction solution from the vacuum of the mass spectrometer. Gases and volatile organic compounds can penetrate the membrane and are monitored continuously. With a quadrupole mass spectrometer and electron ionization, detection limits for volatile organic compounds in the range of ppb to ppm can be obtained, depending on volatility, hydrophobicity and polarity.',' The response time has two components, one due to the membrane and one due to interactions between sample molecules and surfaces in the vacuum. With the membrane inlet mounted inside3x4 or in the immediate vicinity' of a closed ion-source, response times are of the order of a few minutes, depending on the membrane temperature and molecular size. Large hydrophobic molecules like alkanes and esters can interact with the membrane, with fast response times as a result (< 1 min).The concentration of different compounds in simple mixtures can be monitored selectively with a quadrupole mass spectrometer and electron ionization (EI). This is not always possible in complex mixtures e.g., cultures of micro-organisms. Recently chemical ionization (CI) and tandem mass spectrometry (MS/MS) have been used in combination with a membrane inlet for selective and continuous reaction m~n i t o r i n g .~,~. ' With a tandem mass spectrometer, selectivity can be obtained either directly by monitoring of daughter ions, or indirectly by searching for characteristic fragments in the mass spectrum.A triple quadrupole mass spectrometer designed especially for process control, can be simplified in several respects as compared to instruments designed for analytical use. In process control, the concentrations of known compounds are monitored continuously as a function of time. Databases or fast switching between different operation modes are not needed. Since a membrane inlet limits the mass range to approximately 200 u, low resolution (MIAM= 200) mass filters are sufficient. In co-operation with VG Quadrupoles (Altrincham, UK) we have designed and built a low-cost triple quadrupole mass spectrometer based upon standard units from residual gas analysers Author to whom correspondence should be addressed. EXPERIMENTAL The instrumentOur triple quadrupole mass spectrometer is similar in principle to the early instrument of Yost and Enke.' It has a VG 20-253T dual CUE1 source and uses 125 mm VG SX200 quadrupole rods. The detector is a dual Faraday-cup/channel electron multiplier. The ion source and the quadrupoles are e...

A membrane inlet tandem mass spectrometer is used for easy identification of volatile metabolites in microbial cultures. Filtrates of the cultures are without further purification placed into a measuring cell and introduced via a membrane inlet into a triple-quadrupole mass spectrometer. The samples are analysed with electron impact and chemical ionization mass spectrometry and tandem mass spectrometry. An extra dimension of separation is obtained by utilizing differences in response time due to differences in transport rates through the membrane. The following examples are used to demonstrate the technique. (i) Filtrates from cultures of three parasitic protozoa (trichomonads) ; two different strains of Trichomonus vuginufis isolated from humans and from Tritrichomonus foetus isolated from cattle were analysed. Indole was found in all samples; dimethyl disulphide was also present in cultures of the two organisms isolated from humans. (ii) The yeast Brettanomyces was grown in the presence of coumaric acid, ferulic acid, vanillic acid or syringic acid and the filtrate analysed for volatile products. In all cases ethyl acetate was found; coumaric acid biotransformation also gave 4-ethylpheno1, whereas ferulic acid also gave 4-ethyl-Zmetboxyphenol.