A review of total reflection x‐ray fluorescence (TXRF) as an effective excitation mode for energy‐dispersive x‐ray spectral analysis is presented. The instrumental conditions of excitation under grazing incidence (ψ<0.1°) are emphasized and the analytical features of powerful detection and simple and reliable quantification are characterized. The applicability of TXRF to environmental analyses is illustrated by some typical examples. The analysis of pure water samples leads to detection limits at the ppt (ng/l) level. A special matrix separation is only needed for river, sea and waste waters. The analysis of air dust is directly possible with a sampling volume of 1 m3 and a sampling time of 1 h. Organ tissue can be analysed down to the lower ppm range after freeze‐cutting of μm thick sections. Plant material has to be pulverized and digested prior to analysis, e.g. with nitric acid. In combination with a chromatographic separation, speciation is made possible for small 0.5 ml fractions, e.g. for vegetable foodstuffs. For all these applications a multi‐element determination can be carried out, for about 20–25 elements simultaneously. Simple and reliable quantification is effected by internal standardization. The reliability of the method has been proved by intercomparison tests. Second‐generation instruments that are compact and user‐friendly are now commercially available.

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The periplasmic sulfite dehydrogenase of Paracoccus pantotrophus GB17 was purified to homogeneity by a four-step procedure from cells grown lithoautotrophically with thiosulfate. The molecular mass of native sulfite dehydrogenase was 190 kDa as determined by native gradient PAGE. SDS-PAGE showed sulfite dehydrogenase to comprise two subunits with molecular masses of 47 kDa and 50 kDa, suggesting an alpha2beta2 structure. The N-terminal amino acid sequence and immunochemical analysis using SoxC-specific antibodies identified the 47-kDa protein as the soxC gene product. SoxD-specific antibodies identified the 50-kDa protein as SoxD. Based on the molecular masses deduced from the nucleotide sequence for mature SoxC (43,442 Da) and SoxD (37,637 Da) sulfite dehydrogenase contained 1.30 mol molybdenum/mol alpha2beta2 sulfite dehydrogenase. The iron content was 3.17 mol/mol alpha2beta2 sulfite dehydrogenase, and 3.53 mol heme/mol alpha2beta2 sulfite dehydrogenase was determined by pyridine hemochrome analysis. These data are consistent with the two heme-binding domains (CxxCH), characteristic for c-type cytochromes, deduced from the soxD nucleotide sequence. Electrospray ionization revealed two masses for SoxC of 43,503 and 43,897 Da. The difference in molecular mass was attributed to the molybdenum cofactor of SoxC. For SoxD a mass of 38,815 Da was determined; this accounted for the polypeptide and two covalently bound hemes. Reconstitution of the catalytic activity of sulfite dehydrogenase required additional fractions; these eluted from Q Sepharose at 0.05, 0.25, and 0.30 M NaCl. The K(m) of sulfite dehydrogenase for sulfite was 7.0 microM and for cytochrome c 19 microM. Sulfite dehydrogenase activity was inhibited by sulfate and phosphate. The structural and catalytic properties make sulfite dehydrogenase from P. denitrificans GB17 distinct from sulfite oxidases of other prokaryotic or eukaryotic sources.