Temperature-programmed desorption for membrane inlet mass spectrometry

Abstract

We present a novel technique for analyzing volatile organic compounds in air samples using a solid adsorbent together with temperature-programmed desorption and subsequent detection by membrane inlet mass spectrometry (TPD-MIMS). The new system has the advantage of a fast separation of compounds prior to the detection by MIMS. The gaseous sample is simply adsorbed on the adsorbent, which is then rapidly heated from 30°C to 250°C at a rate of 50°C/min. Trapped organic compounds are released from the adsorbent into a helium stream at different temperatures depending on the strength of the interaction between the individual compound and the adsorbent. The helium stream carries the desorbed compounds to a membrane inlet (90°C) equipped with a thin (25 mm) silicone membrane. The thin membrane and the high temperature of the membrane inlet allows most volatile compounds to diffuse through the membrane into the mass spectrometer in a few seconds. In this fashion we could completely separate many similar volatile compounds, for example toluene from xylene and trichloroethene from tetrachloroethene. Typical detection limits were at low or sub-nanogram levels, the dynamic range was 3 orders of magnitude, and the analysis time for a mixture was about 3-4 minutes. # 1998 John Wiley & Sons, Ltd. Received 23 March 1998; Revised 17 April 1998; Accepted 20 April 1998 Membrane inlet mass spectrometry (MIMS) is an analytical method that allows detection of volatile organic compounds in water and air at low parts-per-billion and parts-per-trillion levels. The main advantages of the technique are the lack of pretreatment of samples before analysis, fast response times and on-line monitoring capabilities of water and air streams in biochemical and environmental systems. The most recent developments and applications of MIMS have been reviewed. [1][2][3] Many new MIMS techniques have evolved from standard MIMS with the purpose of expanding the capabilities of MIMS with respect to the variety of compounds and samples that can be analyzed. For example Lauritsen et al. 4 used porous membranes and solvent chemical ionization for the determination of organic compounds in organic matrices, and Yakovlev et al.5 used a strong electric field to drag ions from a liquid sample though a porous membrane into the mass spectrometer. Improved sensitivity has been examined through enrichment devices such as a jet separator, 6 a cold trap, 7 and solid adsorbents, 8,9 all of which were mounted between the membrane and the ion source. In affinity MIMS a chemically modified cellulose membrane was used to selectively adsorb aldehydes from a complex mixture.10 Recently, Lauritsen et al. 11,12 developed trapand-release MIMS, where less volatile (boiling point above 250°C) and relatively polar compounds were trapped inside the membrane itself, before they were thermally released into the ion source of the mass spectrometer.A major drawback of the MIMS system as compared to traditional analytical techniques like GC/MS and LC/MS is a lack of selectivity....

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“…We explore this concept in this paper and term it single-sided membrane introduction mass spectrometry (SS-MIMS). The single-sided membrane inlet combines the features of trap and release MIMS [14][15][16][17] with surface partitioning. Note that the single-sided membrane introduction technique is based on similar physical principles to solid phase extraction (SPE) and solid phase microextraction (SPME), a commonly utilized offline adsorption/desorption technique for extraction, preconcentration, and introduction of samples into gas chromatographs.…”

Fast and direct screening of solid materials for their potential liberation of hydrophobic organic compounds using hot cell membrane inlet mass spectrometry

“…In earlier work36 on the analysis of complex mixtures of VOCs, we demonstrated that temperature programming of a sorbent trap in front of the membrane could be used to create a fast chromatographic separation of the compounds prior to MIMS analysis. It is likely that something similar can be done with the analysis of solid materials in a hot cell whose temperature is ramped relatively quickly.…”

Mobile mass spectrometry for water quality monitoring of organic species present in nuclear waste ponds

“…For better identication of mixed substances in water, a temperature-programmed desorption (TPD) unit may be used. It has been reported that TPD-MIMS can completely separate a number of similar VOCs in air, 14 however there are currently no published results available for TPD-MIMS in water. Table 1 presents a summary of all the organic species from the NIST library that were tested in water with our mobile mass spectrometer.…”