1998

Ketola, Raimo

Membrane inlet mass spectrometry (MIMS) is an established technique for the analysis of volatile organic compounds in aqueous solutions and in air. A thin membrane is the only interface between a liquid or gaseous sample at atmospheric pressure and the vacuum of a mass spectrometer. Since its introduction about 35 years ago MIMS has been applied mainly in biochemistry and environmental analysis. In this work the applicability of MIMS in the analysis of volatile organic compounds (VOCs) in water samples was investigated, and improved MIMS methods for the analysis of VOCs in air samples were constructed. The possibilities of MIMS for the analysis of polar and/or semivolatile compounds in aqueous samples were enhanced with novel techniques. It was demonstrated that the MIMS method is comparable with static headspace gas chromatography and purge&trap gas chromatography-mass spectrometry methods in the analysis of VOCs in water samples. The MIMS method was also shown to be very suitable for on-site measurement of water samples in a mobile laboratory. A membrane inlet mass spectrometric method was developed for the analysis of volatile organic compounds, especially volatile sulfur compounds, in air samples. The method is very sensitive, i.e. detection limits are at sub or low µg/m3 levels, and also very rapid: it is possible to analyze even 50 to 100 samples in one hour with a thin polydimethylsiloxane membrane because response times are only a few seconds. When MIMS is combined with a temperature-programmed desorption (TPD) technique it is possible to achieve separation of compounds prior to mass spectrometric detection, still conserving a rapid analysis time per sample, 6-10 minutes, and low detection limits. For the analysis of semivolatile and/or polar compounds in aqueous samples two trap&release (T&R) techniques were developed. In these techniques semivolatile compounds accumulated into a membrane are desorbed by heat from a filament and then analyzed by a mass spectrometer. In the traditional T&R-method a silicone membrane is used together with electron ionization, whereas in the desorption chemical ionization (DCI) method a hydrophilic membrane is used to allow a solvent chemical ionization with water as a reagent gas. With these techniques it is possible to measure e.g. caffeine and dicarboxylic acids directly from water samples.