MOLECULAR ION SPECTRA IN GLOW DISCHARGE MASS SPECTROMETRY OF TOXIC AND PHYSIOLOGICALLY ACTIVE ELEMENTS IN HYDROXYAPATITE 

Abstract

Mass spectra of molecular ions in cathode sputtering of hydroxyapatite within argon glow discharges were studied experimentally and by mathematical simulation. The study was aimed at developing a highly sensitive technique for determination of toxic and physiologically active elements in hydroxyapatite, used for medical purposes, by glow discharge mass spectrometry. Mass spectra were simulated employing the method developed previously for calculation of molecular ion concentrations in glow discharge plasma and computer program for its implementation. Effective equilibrium constants were refined for formation–dissociation reactions of molecular ions that occur in cathode sputtering of hydroxyapatite on a tantalum substrate. Comparison between the experimental and calculated mass spectra indicated that the model was correct. The study revealed molecular interferences in the mass region from ¹⁹F to ²³⁸U that were adequately separated from the isotopes under study and reduced the analysis detection limit. Isotopes with the least impact from molecular interferences were selected, and the resolving power needed to ensure a detection limit of around 1 ppm for monoisotopic elements was calculated. To maintain a sufficiently high ionic current for nonconductive matrix isotopes (⁴⁴Ca, ³¹P), a previously improved design of the analytical cell was used with high-purity tantalum as a substrate. Most of the studied elements can be determined with ppm-ppb sensitivity employing mass spectrometers with high resolution (≥ 9000) at half the peak height.