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Nitrogen microwave-induced plasma mass spectrometry was studied for its applicability to the isotope dilution analysis of selenium in biological samples. Spectroscopic interference by calcium, which is present in high concentrations in biological samples, was investigated. No detectable background spectrum was observed for the major selenium isotopes of 78Se and 80Se. No detectable interferences by sodium, potassium, calcium and phosphorus on the isotope ratio 80Se/78Se were observed up to concentration of 200 mg/ml. The method was applied to the analysis of selenium in biological reference materials of marine organisms. The results showed good agreement between the certified and found values.

Substoichiometric isotope dilution mass spectrometry (SIDMS) as a new analytical method has been proposed for the accurate determination of iron. Iron(III) was extracted with a substoichiometric amount of 4-isopropyltropolone (Hipt) in xylene or heptane in the presence of 3,5-dichlorophenol (DCP) as a synergist and then subjected to the isotope measurement (54Fe) with amicrowave-induced plasma mass spectrometer. For more precise determination, 59Co was used as an internal standard. In this method, the correction of the mass discrimination of isotope measurement is not required and the use of expensive enriched stable isotopes can be avoided. KeywordsSubstoichiometry, isotope dilution, mass spectrometry, solvent extractionSubstoichiometry is a unique analytical method and has been developed by a combination with nuclear methods such as the radioisotope dilution and radioactivation analysis.' Most recently, we have proposed a new analytical principle based on the substoichiometry and stable isotope dilution mass spectrometry (IDMS), i.e., substoichiometric isotope dilution mass spectrometry (SIDMS).2In this paper, the substoichiometric separation of trace Iron(III) was studied by the synergistic extraction system of 4-isopropyltropolone (Hipt) and 3,5-dichlorophenol (DCP) and combined with the microwave-induced plasma mass spectrometry (MIP-MS) by optimizing the operating conditions. Experimental Reagents and apparatusHipt and DCP were purified by vacuum sublimation. Tns(acetylacetonato)cobalt(III) (Co(acac)3) was used as obtained. A 56Fe enriched Fe203 was obtained from Ork Ridge National Lab.A mass spectrometer was a Hitachi P-6000. The operating conditions studied were as follows; microwave power, 1.1-1.3 kW; plasma gas, 13 1 min' nitrogen; carrier gas, 1.1-1.41 mini' nitrogen and 120-200 ml mind oxygen; peak jump mode at m/z 54 and 59; dwell time, l ms; integrated time, 3 s. ProcedureAn aqueous phase containing various amounts of iron(III) was shaken for 1 h with an organic phase containing a substoichiometric amount of Hipt and an excess of DCP. The organic phase obtained was appropriately diluted with xylene (if necessary, Co(acac)s as an internal standard was dissolved) and subjected to the MIP-MS. Results and Discussion Substoichiometric extractionIn the preliminary experiment, it was found that Hipt exhibited the surprisingly high extractability in the presence of DCP; 4x106 M iron(III) was quantitatively extracted with 1x103 M Hipt and 5x102 M DCP from a 1 M HCl solution into heptane. From the equilibrium study, the extraction was expressed by the following equation;Fe3~ + 3 Hiptorg + n DCP0rg Fe(ipt)3nDCPorg + 3 W. The synergistic enhancement of the extraction of iron(III) was ascribed to the hydrogen bond association between Fe(ipt)3 and DCP in the organic phase.The extraction of iron(III) with a substoichiometric amount of Hipt in heptane or xylene was done at pH 2.5 in the presence of varying amounts of DCP; the initial concentrations of iron(III) and Hipt were 1x104 -M respectively, and the concentr...

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