Optimizing analysis of samples with complicated composition by inductively coupled plasma mass spectrometry

Mass spectrometry is one of the most current and evolving methods in analytical chemistry. One of the main problems of this method is spectral interference, which leads to a sharp increase in the detection limits for some elements and decrease of analysis accuracy. The most significant cause of spectral interference is molecular (polyatomic) ions, which are associations of argon ions and their impurities, as well as interactions of the solvent components and matrix with each other, which can show a mass that coincides with the mass of the isotope being identified. The possibility of formation of a large number ofpolymolecules and the necessity of accounting for it in analysis are presented in this work, using thermodynamic modeling. As model samples for the experiment, standard samples of black shale were used that were created by the Irkutsk Institute of Geochemistry named after A.P. Vinogradov, and modeling was carried out using the program HSC Chemistry. Thermodynamic calculations were carried out on Al and Ba, for example, because these elements can influence identifying rare and rare-earth elements. Temperatures in the calculations varied from 6000 K to 9000 K in increments of500 K. One of the methods to account for and eliminate spectral interference is deriving individual equations for mathematical correction. These equations involve introducing two types of coefficients, theoretically and practically based. The first type of coefficients is derived during the transition from one isotope to another by the ratio of their natural prevalence. The second type can be obtained by analyzing pure solutions of the elements studied. On the basis of mathematical calculations that account for the prevalence of isotopes and coefficients that were obtained from the experimental results for each isotope, the individual equations for mathematical correction of interference were derived. The general form of the equations can be presented as: Meⁿ=Meⁿ measured - akArMe^m - bkoMf - vkarMeP - dkwMe^s - ekNMe^t where Me^m, Me^l, Me^p, Me^s, Me^t are isotopes of interference elements; kAr, ko, kCl, kN, kH are coefficients that account for contribution of ions with argon, oxygen, chlorine, nitrogen, and hydrogen respectively, and a, b, c, d, e are coefficients that account for the natural prevalence of isotopes. In conclusion, 32 equations were derived for the following isotopes: Sc⁴⁵, Rb⁸⁵, Sr⁸⁸, Y⁸⁹, Zr⁹⁰, Nb⁹³, Cs¹³³, Ba¹³⁷, La¹³⁹, Ce¹⁴⁰, Pr¹⁴¹, Nd¹⁴⁵, Sm^147,149, Eu ¹⁵¹, Gd^158,160, Tb¹⁵⁹, Dy^161,163, Ho¹⁶⁵, Er^{166, 167}, Tm¹⁶⁹, Yb^171,172, Lu¹⁷⁵, Hf¹⁷⁸, Ta¹⁸¹, Pb²⁰⁸, Th²³², and U²³⁸.

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