Abstract
Large polarizable n-alkanes (approximately C18 and larger), alcohols, and other nonpolar compounds can be detected as negative ions when sample solutions are injected directly into the sampling orifice of the atmospheric pressure interface of the time-of-flight mass spectrometer with the direct analysis in real time (DART) ion source operating in negative-ion mode. The mass spectra are dominated by peaks corresponding to [M + O2]‾•. No fragmentation is observed, making this a very soft ionization technique for samples that are otherwise difficult to analyze by DART. Detection limits for cholesterol were determined to be in the low nanogram range.
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Acknowledgment
The authors thank Dr. Warren Samms of the Harris County Institute for Forensic Sciences for calling attention to the possibility of negative ion formation from alkanes by DART, and Hannah Moore of Keel University, whose research with insect puparial cases during a visit to the authors’ laboratory led us to refine the adduct formation methodology. The authors also acknowledge Professor John Bartmess for helpful discussions about gas-phase ion energetics and anion formation mechanisms.
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Cody, R.B., Dane, A.J. Soft Ionization of Saturated Hydrocarbons, Alcohols and Nonpolar Compounds by Negative-Ion Direct Analysis in Real-Time Mass Spectrometry. J. Am. Soc. Mass Spectrom. 24, 329–334 (2013). https://doi.org/10.1007/s13361-012-0569-6
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DOI: https://doi.org/10.1007/s13361-012-0569-6