Characterization of a hydrogen flame as an ion source for mass spectrometry

Abstract

A commercial inductively coupled plasma (ICP) mass spectrometer was modified to employ an air-hydrogen flame in place of the ICP as an ion source. A liquid nitrogen trap was placed in the vacuum line to remove water. A very simple intrinsic mass spectral background was obtained with the hydrogen flame ionization mass spectrometry (FIMS). Molecular ions such as K(H₂O)⁺, Na(H₂O)⁺, Ca(H₂O)⁺ and CaOH(H₂O)_x⁺ (x=0-2) were observed when solutions containing Na, K or Ca were aspirated. Although the presence of the molecular ions complicated the mass spectra, it also provided a wider choice of analytical masses for an analyte. Isotope ratio measurements of Ca were made with both Ca⁺ and CaOH⁺ species at masses 40, 44, 57 and 61. Better isotope ratio precision was obtained at CaOH⁺ masses relative to those for Ca⁺ because the sensitivity was about 10 times higher. Isotope ratio measurement of K was made at masses 39 and 41. A ratio precision of about 0.2 and 0.5% was obtained for K and Ca, respectively. The results suggest that the FIMS is suitable for the isotope ratio measurement of K and Ca in simple matrices, and that the air-hydrogen flame is a more desirable ion source than an air-acetylene flame for FIMS.

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