Enhancing the performance of membrane introduction mass spectrometry by organic carrier and liquid chromatographic separation

State-of-the-art of gas chromatography-based methods for analysis of anthropogenic volatile organic compounds in estuarine waters, illustrated with the river Scheldt as an example

This review focuses on a number of key procedural steps in the analysis of volatile organic compounds (VOCs) in estuarine waters. The most critical step, from an analytical point of view, is sample preparation. So far, only purge-and-trap and, to some extent, membrane inlet mass spectrometry have successfully been applied in estuarine monitoring of VOCs. The advantages and disadvantages of both techniques are discussed and novel developments are reviewed. Other key elements in VOC analysis and assessment include quality assurance (QA), quality control (QC) and statistical data analysis. This paper gives a brief overview of QA/QC measures of interest in the estuarine monitoring exercise, and provides guidelines for adequate statistical treatment of environmental data. Finally, field measurements of VOCs in estuarine waters are reviewed. Concentrations are reported, and distribution patterns, sources and time-trends are discussed. In addition to data, results of a 3-year monitoring survey (May 1998-November 2000) in the Scheldt estuary are presented.

In-membrane preconcentration/membrane inlet mass spectrometry of volatile and semivolatile organic compounds

The on-line determination of volatile and semivolatile organic compounds (SVOCs) is reported using membrane inlet mass spectrometry with in-membrane preconcentration (IMP-MIMS). Semivolatile organic compounds in aqueous samples are preconcentrated in a flow-through silicone hollow-fiber membrane inlet held in a GC oven. The sample stream is replaced with air, and the SVOCs are thermally desorbed into the mass spectrometer by rapid heating of the membrane. The method is evaluated for the on-line determination of 4-fluorobenzoic acid, 3,5-difluorobenzoic acid, 2-chlorophenol, p-tert-butylphenol, and dimethyl sulfoxide (DMSO) in water. The selectivity of the IMP-MIMS technique for SVOCs in the presence of VOCs is demonstrated. Cryotrapping and a rapid gas chromatographic separation step were added between the membrane and the mass spectrometer ion source for the determination of SVOCs in complex mixtures. The procedure is demonstrated for the determination of dimethyl sulfoxide (DMSO) in equine urine, using internal standardization with DMSO-d6. Full-scan electron ionization (EI) mass spectrometric detection showed good linearity (R = 0.998) and RSDs, relative to the internal standard, of 2.2% for desorption only and 4.6% for desorption and cryotrapping.

Membrane introduction mass spectrometry: trends and applications

Recent advances in membrane introduction mass spectrometry (MIMS) are reviewed. On-line monitoring is treated by focusing on critical variables, including the nature and dimensions of the membrane, and the analyte vapor pressure, diffusivity, and solubility in the membrane barrier. Sample introduction by MIMS is applied in (i) on-line monitoring of chemical and biological reactors, (ii) analysis of volatile organic compounds in environmental matrices, including air, water and soil, and (iii) in more fundamental studies, such as measurements of thermochemical properties, reaction mechanisms, and kinetics. New semipermeable membranes are discussed, including those consisting of thin polymers, low vapor pressure liquids, and zeolites. These membranes have been used to monitor polar compounds, selectively differentiate compounds through affinity-binding, and provide isomer differentiation based on molecular size. Measurements at high spatial resolution, for example, using silicone-capped hypodermic needle inlets, are also covered, as is electrically driven sampling through microporous membranes. Other variations on the basic MIMS experiment include analyte preconcentration through cryotrapping (CT-MIMS) or trapping in the membrane (trap-and-release), as well as differential thermal release methods and reverse phase (i.e., organic solvent) MIMS. Method limitations center on semivolatile compounds and complex mixture analysis, and novel solutions are discussed. Semivolatile compounds have been monitored with thermally assisted desorption, ultrathin membranes and derivatization techniques. Taking advantage of the differences in time of membrane permeation, mixtures of structurally similar compounds have been differentiated by using sample modulation techniques and by temperature-programmed desorption from a membrane interface. Selective ionization techniques that increase instrument sensitivity towards polar compounds are also described, and comparisons are made with other direct sampling (nonchromatographic) methods that are useful in mixture analysis.

Selective determination of a group of organic compounds in complex sample matrixes by LC/MIMS with on-line immunoaffinity extraction

An on-line immunoaffinity extraction with liquid chromatography/membrane introduction mass spectrometry (IAE/LC/MIMS) method for the determination of BTEX compounds in complex sample matrixes is described. This method uses an immunoaffinity column (1 mm i.d. x 20 mm) for on-line sample cleanup and enrichment, a 5-micron C18 trapping column (2 mm i.d. x 20 mm) for analyte focusing, a 3-micron C18 analytical column (3.2 mm i.d. x 100 mm) for separation, and a membrane introduction mass spectrometer for quantitation. The immunoaffinity column was evaluated in terms of binding capacity, recovery, and enrichment factor. The method was optimized for the determination of BTEX compounds in a mixture of 30 volatile organic compounds, which showed no matrix interference and a dramatic improvement of the detection limit over that of the LC/MIMS method (up to 474-fold). This method was also used for the determination of BTEX compounds in several gasoline-contaminated water samples, and the results were compared with the EPA reference methods.