The site of protonation for gas-phase aniline has been debated for many years, with many research groups contributing experimental and computational evidence for either the amino-protonated or the para-carbon-protonated tautomer as the gas-phase global minimum structure. Here, we employ differential mobility spectrometry (DMS) and mass spectrometry (MS) to separate and characterize the amino-protonated (N-protonated) and para-carbon-protonated ( p-protonated) tautomers of aniline. We demonstrate that upon electrospray ionization (ESI), aniline is protonated predominantly at the amino position. Similar analyses are conducted on another three isotopically labeled forms of aniline to confirm our structural assignments. We observe a significant reduction of the relative population of the p-protonated tautomer when a protic ESI solvent is employed (methanol/water) compared to when an aprotic solvent (acetonitrile) is employed. We also observe conversion of the p-protonated species into the N-protonated species upon clustering with protic solvent vapor post-DMS selection-a finding supported by previous experimental data acquired using DMS-MS.

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High-energy-resolution fluorescence-detected (HERFD) X-ray absorption near-edge spectroscopy (XANES) is a spectroscopic method that allows for increased spectral feature resolution, and greater selectivity to decrease complex matrix effects compared with conventional XANES. XANES is an ideal tool for speciation of elements in solid-phase environmental samples. Accurate speciation of As in mine waste materials is important for understanding the mobility and toxicity of As in near-surface environments. In this study, linear combination fitting (LCF) was performed on synthetic spectra generated from mixtures of eight measured reference compounds for both HERFD-XANES and transmission-detected XANES to evaluate the improvement in quantitative speciation with HERFD-XANES spectra. The reference compounds arsenolite (As2O3), orpiment (As2S3), getchellite (AsSbS3), arsenopyrite (FeAsS), kaňkite (FeAsO4·3.5H2O), scorodite (FeAsO4·2H2O), sodium arsenate (Na3AsO4), and realgar (As4S4) were selected for their importance in mine waste systems. Statistical methods of principal component analysis and target transformation were employed to determine whether HERFD improves identification of the components in a dataset of mixtures of reference compounds. LCF was performed on HERFD- and total fluorescence yield (TFY)-XANES spectra collected from mine waste samples. Arsenopyrite, arsenolite, orpiment, and sodium arsenate were more accurately identified in the synthetic HERFD-XANES spectra compared with the transmission-XANES spectra. In mine waste samples containing arsenopyrite and either scorodite or kaňkite, LCF with HERFD-XANES measurements resulted in fits with smaller R-factors than concurrently collected TFY measurements. The improved accuracy of HERFD-XANES analysis may provide enhanced delineation of As phases controlling biogeochemical reactions in mine wastes, contaminated soils, and remediation systems.

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