SELECTIVE APPLICATION OF CHEMICAL SEPARATIONS TO ISOTOPE-DILUTION INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRIC ANALYSES OF STANDARD REFERENCE MATERIALS

Isotope dilution has been used with ICP-MS to certify trace and ultratrace elements in both leaf and soil standard reference materials (SRMs). Mass spectrometric interferences from the sample matrices prevented the direct determination of Cd, Ag, Mo, and Ni in these materials. The ICP-MS was used in a semiquantitative survey mode to identify interferences and to develop customized separations. The fact that isotope dilution analysis requires neither 100% recovery nor absolute isolation of the element(s) of interest results in considerable flexibility in the design of separations. Separated analytical samples were introduced into the ICP-MS, and high accuracy quantitative results were based on precise isotope ratio measurements made in the peak jump mode. Four transition metals, Cd, Ag, Mo, and Ni, in the nanogram to microgram per gram range were determined in five different SRMs with an average bias uncertainty and imprecision of less than 1%.