CALCULATION METHODS FOR THE DETERMINATION OF ISOTOPE RATIOS OF TRANSIENT SIGNALS OF VOLATILE ORGANOMETALLIC COMPOUNDS

摘要

Volatile organometal(loid)s are covalent hydrides and/or methylated or alkylated metal(loid) compounds, such as stannane SnH_(4), dimethylmercury (CH_(3))_(2)Hg, and trimethylstibine (CH_(3))_(3)Sb. They have been detected in landfill and sewage gases, in both cases contributing to the biogeochemical cycle of metal(loid)s. The majority of elements possess more than one stable isotope. While isotopes closely resemble one another in their behaviour, there are small differences in mass which can lead to partial fractionation in both kinetically and thermodynamically controlled systems. Natural fractionation can take place through isotope exchange reactions, physical processes or kinetic reactions. Fractionation is determined by the (DELTA)m/m ratio, which is higher for lighter elements (e.g. C, O, N, S), giving rise to the well established use of the isotope ratios of such elements, to study biological processes and environmental pathways. Despite the low (DELTA)m/m ratio for elements heavier than sulfur, which prompts the expectation of only very small fractionation, the search for such variations has now become one of the new frontiers in isotope geochemistry. In particular, Fe, Cu and Zn isotope fractionation has been studied on terrestrial basalts in order to evaluate the importance of biogeochemical processes, especially in early earth history. Fractionation of Fe isotopes may result from kinetic isotope effects, such as transport and metabolic reactions carried out by proteins (enzymes), or equilibrium isotope effects, for example during equilibration of dissolved Fe species having different ligands or oxidation states. Se isotope fractionation has been successfully used as an indicator for sources and cycling of selenium in water, oil refinery wastewater, sediment digests and extracts in the northern reach of San Francisco Bay. In addition to the recent developments in isotopic measurements of Fe, Cu, Zn and Se, studies of Sb isotope variations in natural systems have been carried out. Preliminary results on Sb isotopic composition of hydrothermal zinc and iron sulfides show a variation of more than 1.5(per thousand). In the case of volatile organometal(loid) compounds, isotope ratio measurements might reveal whether these compounds were anthrophogenically or naturally introduced, chemically or biologically transformed in waste treatment environments and whether or not microbial transformation can introduce isotopic fractionation of heavy elements such as antimony or tin. Most of these studies consider the isotope ratios of the bulk sample, whereas we have focused on single metal species, generated by a number of different steps, which are probably enzymatically catalysed in nature. GC-ICP-MS is the method of choice for the separation and identification of volatile organometallic compounds such as (CH_(3))_(3)Sb. The resulting transient signals, of about 4 seconds, are detected isotope-specifically by ICP-MS. For isotope ratio analysis a high precision is required in order to be able to distinguish between the natural isotope ratio and the possibly changed isotopic ratio. The performance of a Multicollector-ICP-MS and a Time of flight ICP-MS system, coupled to GC, for isotope ratio measurement, and the observed shift of the retention times of the two compounds, (CH_(3))_(3)Sb and (CH_(3))_(3)Sb, eluting from the GC column, will be discussed. A routine peak integration method will be given.