Precise Analysis of Gallium Isotopic Composition by MC-ICP-MS.

Though an isotope approach could be beneficial for better understanding the biogeochemical cycle of gallium (Ga), an analogue of the monoisotopic element aluminum (Al), the geochemistry of Ga isotopes has not been widely elaborated. We developed a two-step method for purifying Ga from geological (biological) samples for precise measurement of Ga isotope ratio using multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). Ga was thoroughly separated from other matrix elements using two chromatographic columns loaded with AG 1-X4 and Ln-spec resin, respectively. The separation method was carefully calibrated using both synthetic and natural samples and validated by assessing the extraction yield (99.8 ± 0.8%, 2SD, n = 23) and the reproducibility (2SD uncertainty better than 0.05‰, n = 116) of the measured isotopic ratio (expressed as δ71Ga). The validation of the whole protocol, together with instrumental analysis, was confirmed by the investigation of the matrix effect, the result of a standard addition experiment, and the comparison of Ga isotope measurement on two mass spectrometers-Nu Plasma II and Neptune Plus. Although the measurements using the sample-standard bracketing (SSB) correction method on both instruments resulted in identical δ71Ga values for reference materials, the modified empirical external normalization (MEEN) method gave relatively better precision compared to SSB on Neptune. Our preliminary results showed large variation of δ71Ga (up to 1.83‰) for 10 standards, with higher values in industrially produced materials, implying potential application of Ga isotopes.