Secondary ion mass spectrometry of aromatic compounds in acidic mixtures

Secondary ion mass spectrometry (SIMS) is an analytical method that can be used to obtain mass spectra of involatile/thermally labile compounds. The sensitivity of SIMS for organic compounds is enhanced if samples are first dissolved in an appropriate involatile liquid (or so-called matrix).

The use of acidic liquids and solids to improve the sensitivity of SIMS for aromatic compounds is investigated. SIMS spectra are obtained from substituted aromatic com pounds dissolved in concentrated sulfuric acid (98 wt. %). Detection limits for some substituted aromatic compounds are estimated to be 10^-12 moles. Simultaneously abundant molecular ions and protonated molecules characteristic of dissolved aromatics are observed for several aromatic compound classes including acids, aldehydes, ketones, nitriles, and nitrogen heterocycles. The molecular ion to protonated molecule intensity ratio observed in SIMS spectra of substituted aromatic/sulfuric acid solutions is characteristic of the dissolved analyte. Secondary ions observed from substituted aromatic/sulfuric acid solutions are consistent with known condensed phase chemistry of such solutions.

The utility of sulfuric acid as a SIMS matrix for detection of pterins, a group of biologically significant compounds, is demonstrated. Application of LSIMS using sulfuric acid for detection of aromatics adsorbed on substrates is also investigated.

Use of acidic metal chlorides for enhanced secondary ion emission is also studied. Metal chlorides used as solid matrices or as additives to liquid matrices do not improve the sensitivity of SIMS for aromatic compounds significantly.