Enhancement of laser desorption ionization mass spectrometric signals of cesium iodide by elemental sulfur

Rationale: The utility of elemental sulfur as a matrix for inorganic salts such as CsI, AgI, and KI was investigated because the conventional matrices deployed to generate gaseous ions from organic compounds, upon irradiation with a laser beam, are not suitable for inorganic salts. Methods: Sulfur and inorganic salts were admixed and irradiated with a 337-nm UV laser. Laser desorption ionization (LDI) mass spectra were recorded in both positive and negative ion mode on a time-of-flight mass spectrometer. Results: The positive ion laser desorption ionization mass spectrum of CsI showed peaks at m/z 133, 393, etc. for [(CsI)_nCs]⁺ ions. Similarly, negative ion spectra showed peaks at m/z 387, 647, etc. for [(CsI)_nI]^- ions. However, for n >2 ion clusters, the intensities of peaks were negligibly small in both ionization modes. In contrast, spectra recorded from CsI admixed with elemental sulfur showed peaks up to n = 13 for (CsI) _nCs⁺, and n = 9 for (CsI)_nI^-. A similar enhancement of ion abundances by sulfur was observed for the cluster ions generated from KI and AgI. Conclusions: The dramatic increase in intensities of the higher-mass CsI cluster peaks suggests that sulfur acts as a laser-absorbing matrix for inorganic salts far superior to conventional matrices such as 2,5-dihydroxybenzoic acid and α-cyano-4-hydroxycinnamic acid.