Inorganic germanium in aqueous solution is determined by a combination of isotope dilution, hydride generation, and inductively-coupled plasma mass spectrometry (ID-HG-ICP-MS). Samples (5-20 ml) are spiked with an enriched stable isotope (Ge-70). Germanic acid is reduced by sodium borohydride to germane (GeH4), stripped in He gas and collected on a liquid nitrogen trap. The hydrides are subsequently evaporated from the cold trap and introduced into the ICP torch. The ion counts at m/z=70 and 74 are monitored during the transient hydride peak. Absolute detection limits are about 400 fg. The method has an overall precision of better than 2% RSD and a dynamic range of two orders of magnitude. The method was applied to seawater samples and rock standards. Results are in excellent agreement with those determined previously by other methods. As an alternative method to isotope dilution, single ion monitoring (m/z=74; SIM-HG-ICP-MS) with trimethylgermanium added as an internal standard can be used to improve absolute detection limits (200 fg) while still maintaining a precision of +/-2 to 4%.
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