DETERMINATION OF HYDROGEN IN SILICATES BY SECONDARY-ION MASS-SPECTROMETRY

A quantitative procedure for the determination of hydrogen in silicate s was developed for incorporation in the general procedure used with t he Cameca IMS 4f ion microprobe for light and trace elements. The proc edure employs a static O-16(-) primary beam and the 'energy filtering' technique, Moreover, it allows the analysis of samples placed on sepa rate mounts with respect to standards and, in particular, of mineral g rains directly in thin sections of rock, Factors influencing the hydro gen background were studied in order to ascertain the appropriate anal ytical conditions and to develop a specific procedure for the determin ation of low hydrogen contents, A low and reproducible background was obtained with simple heating and de-gassing of the sample. The backgro und value was equivalent to 0.007-0.015% H2O and its fluctuation over a 1 week working session indicated that the detection limit was approx imate to 0.015% H2O, The results obtained confirm that the use of high -energy ions reduces the influence of the matrix composition on the io nization of hydrogen (relative to that of Si); effects related to the structure of the sample appear to be negligible, Residual matrix compo sition effects are related, to a first approximation, to the Si conten t of the sample. As such effects could not be completely removed, two separate ion yields were used for samples with SiO2 contents in the ra nge 27-51% and for rhyolites (approximate to 72% SiO2), The reproducib ility of the measurements was typically approximate to 3% and the accu racy for the former group of samples was 15% for H2O contents >0.1% an d 30% for H2O contents of approximate to 0.05%.