Combining Isotope Dilution and Standard Addition—Elemental Analysis in Complex Samples

A new method combining isotope dilution mass spectrometry (IDMS) and standard addition has been developed to determine the mass fractions w of different elements in complex matrices: (a) silicon in aqueous tetramethylammonium hydroxide (TMAH), (b) sulfur in biodiesel fuel, and (c) iron bound to transferrin in human serum. All measurements were carried out using inductively coupled plasma mass spectrometry (ICP–MS). The method requires the gravimetric preparation of several blends (b(i))—each consisting of roughly the same masses (m(x,i)) of the sample solution (x) and m(y,i) of a spike solution (y) plus different masses (m(z,i)) of a reference solution (z). Only these masses and the isotope ratios (R(b,i)) in the blends and reference and spike solutions have to be measured. The derivation of the underlying equations based on linear regression is presented and compared to a related concept reported by Pagliano and Meija. The uncertainties achievable, e.g., in the case of the Si blank in extremely pure TMAH of u(rel) (w(Si)) = 90% (linear regression method, this work) and u(rel) (w(Si)) = 150% (the method reported by Pagliano and Meija) seem to suggest better applicability of the new method in practical use due to the higher robustness of regression analysis.