Electron capture ionization

Abstract: LC-MS has become an invaluable technique for trace analysis of polar compounds in aqueous samples of the environment and in water treatment. LC-MS is of particular importance due to the impetus it has provided for research into the occurrence and fate of polar contaminants, and of their even more polar transformation products. Mass spectrometric detection and identification is most widely used in combination with sample preconcentration, chromatographic separation and atmospheric pressure ionization (API). The focus of the first part of this review is directed particularly toward instruments and method development with respect to their applications for detecting emerging contaminants, microorganisms and humic substances (HS). The current status and future perspectives of 1) mass analyzers, 2) ionization techniques to interface liquid chromatography (LC) with mass spectrometry (MS), 3) methods for preconcentration and separation with respect to their application for water analysis are discussed and examples of applications are given. Quadrupole and ion trap mass analyzers with electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) are already applied in routine analysis. Time-of-flight (TOF) mass spectrometers are of particular interest for accurate mass measurements for identification of unknowns. For non-polar compounds, different ionization approaches have been described, such as atmospheric pressure photoionization (APPI), electrochemistry with ESI, or electron capture ionization with APCI. In sample preconcentration and separation, solid phase extraction (SPE) with different non-selective sorbent materials and HPLC on reversed-phase materials (RP-HPLC) play the dominant role. In addition, various on-line and miniaturized approaches for sample extraction and sample introduction into the MS have been used. Ion chromatography (IC), size-exclusion chromatography (SEC), and capillary electrophoresis (CE) are alternative separation techniques. Furthermore, the issues of compound identification, matrix effects on quantitation, development of mass spectral libraries and the topic of connecting analysis and toxicity bioassays are addressed.

Abstract: Structures of fatty acids present at very low quantities in mycobacteria are difficult to determine. A commonly used strategy is to introduce heteroatoms into functional groups by chemical means before subjecting them to gas chromatography/tandem mass spectrometry (GC/MS/MS) analysis. Routinely used methods give very low abundance diagnostic ions leading to ambiguities in structural conclusions. GC/MS/MS associated with electron capture ionization of pentafluorobenzyl esters was used to study very complex mixtures of fatty acids from Mycobacterium fallax and M. aurum. The charge-remote fragmentation of fatty acid carboxylate anions was used for structure determination at the nanogram level of a large number of unsaturated, branched, and cyclopropane-containing fatty acids. Some of them have not been observed previously in these Mycobacteria. On the basis of these studies, biosynthetic pathways of unsaturated, branched, and cyclopropane-containing fatty acid are proposed.

Abstract: Arachidonic acid can be enzymatically oxidized at the terminal methyl group by the cytochrome P450 system found in several tissues and cells, including the human polymorphonuclear leukocyte. The omega-hydroxy metabolite, 20- hydroxy-5,8,11,14-eicosatetraenoic acid (20-HETE) has recently been found to have interesting and diverse biological activities. Accurate measurement of quantities of this metabolite using physical chemical methods has not been previously described, but is necessary to assess biosynthesis of this eicosanoid from endogenous arachidonic acid. A procedure is described to quantitate 20-HETE produced by the human polymorphonuclear leukocyte following physiological stimulation using (18O2)carboxy-20-HETE as internal standard. Since the human neutrophil produces relatively small amounts of this eicosanoid, such a study required substantial sensitivity in the quantitative assay. Following stimulation of the neutrophil, cell extracts and supernatants were purified by reverse-phase high-performance liquid chromatography, catalytically reduced then derivatized to the pentafluorobenzyl ester, trimethylsilyl ethers before electron capture ionization gas chromatography/mass spectrometry. Using selected ion monitoring, the amount of 20-HETE present in a biological extract could be detected when as little as 60 pg per sample were available. Following stimulation of the human neutrophil with formyl-methionyl-leucyl-phenylalanine (0.1 microM), platelet activating factor (0.1 microM) as well as with the calcium ionophore A23187 (2 microM), 20-HETE was generated from endogenous arachidonate in concentrations of 1.2, 1.3 and 5.7 pg per 10(6) cells, respectively.