Inorganic germanium in aqueous solution is determined by a combination
of isotope dilution, hydride generation, and inductively-coupled plas
ma mass spectrometry (ID-HG-ICP-MS). Samples (5-20 ml) are spiked with
an enriched stable isotope (Ge-70). Germanic acid is reduced by sodiu
m borohydride to germane (GeH4), stripped in He gas and collected on a
liquid nitrogen trap. The hydrides are subsequently evaporated from t
he 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 de
tection limits are about 400 fg. The method has an overall precision o
f better than 2% RSD and a dynamic range of two orders of magnitude. T
he method was applied to seawater samples and rock standards. Results
are in excellent agreement with those determined previously by other m
ethods. As an alternative method to isotope dilution, single ion monit
oring (m/z=74; SIM-HG-ICP-MS) with trimethylgermanium added as an inte
rnal standard can be used to improve absolute detection limits (200 fg
) while still maintaining a precision of +/-2 to 4%.