A theoretical study of the use of internal standards for precise isotopic analysis by the surface ionization technique: Part I - General first-order algebraic solutions

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Abstract
Mass discrimination is a variable and unpredictable source of systematic error in mass spectrometric analysis using surface ionization techniques. A method of eliminating it through an internal standard technique is investigated. To simplify the algebra a method of describing mass discrimination has been developed (equations (11)) which is symmetrical with respect to all isotopes in a sample. A set of equations for mixtures of samples with different isotopic compositions is set up and solved for elements with three or four isotopes, four isotopes giving the simplest solutions. The solutions can be applied to quantitative analysis by the isotopic dilution technique, to the measurement of absolute isotopic abundances and to the measurement of relative abundances of samples of polyisotopic elements which have undergone isotopic fractionation. To make the most of these proposals the precision of measurement of ion current ratios from thermal ionization sources must be improved.