# The sensitivity of benzene cluster cation chemical ionization mass spectrometry to select biogenic terpenes

Benzene cluster cations are a sensitive and selective reagent ion for chemical ionization of select biogenic volatile organic compounds. We have previously reported the sensitivity of a field deployable chemical ionization time-of-flight mass spectrometer (CI-ToFMS), using benzene cluster cation ion chemistry, for detection of dimethyl sulfide, isoprene and inline-formulaα-pinene. Here, we present laboratory measurements of the sensitivity of the same instrument to a series of terpenes, including isoprene, inline-formulaα-pinene, inline-formulaβ-pinene, inline-formulaD-limonene, ocimene, inline-formulaβ-myrcene, farnesene, inline-formulaα-humulene, inline-formulaβ-caryophyllene, and isolongifolene at atmospherically relevant mixing ratios (< 100 pptv). In addition, we determine the dependence of CI-ToFMS sensitivity on the reagent ion neutral delivery concentration and water vapor concentration. We show that isoprene is primarily detected as an adduct (inline-formulaC5H8inline-formula⋅inline-formula $M10inlinescrollmathmlchem{\mathrm{normal C}}_{normal 6}{\mathrm{normal H}}_{normal 6}^{+}$ 29pt15ptsvg-formulamathimg62059ce22eb19c20ee9054786d9fbec6 amt-11-3251-2018-ie00001.svg29pt15ptamt-11-3251-2018-ie00001.png ) with a sensitivity ranging between 4 and 10 ncps pptinline-formula−1, which depends strongly on the reagent ion precursor concentration, de-clustering voltages, and specific humidity (SH). Monoterpenes are detected primarily as the molecular ion (inline-formula $M12inlinescrollmathmlchem{\mathrm{normal C}}_{normal 10}{\mathrm{normal H}}_{normal 16}^{+}$ 36pt15ptsvg-formulamathimg2127466291b6c844b71ae48fc799cd30 amt-11-3251-2018-ie00002.svg36pt15ptamt-11-3251-2018-ie00002.png ) with an average sensitivity, across the five measured compounds, of 14 inline-formula± 3 ncps pptinline-formula−1 for SH between 7 and 14 g kginline-formula−1, typical of the boreal forest during summer. Sesquiterpenes are detected primarily as the molecular ion (inline-formula $M16inlinescrollmathmlchem{\mathrm{normal C}}_{normal 15}{\mathrm{normal H}}_{normal 24}^{+}$ 36pt15ptsvg-formulamathimg12d026e499e50888a639f782e5b105b9 amt-11-3251-2018-ie00003.svg36pt15ptamt-11-3251-2018-ie00003.png ) with an average sensitivity, across the four measured compounds, of 9.6 inline-formula± 2.3 ncps pptinline-formula−1, that is also independent of specific humidity. Comparable sensitivities across broad classes of terpenes (e.g., monoterpenes and sesquiterpenes), coupled to the limited dependence on specific humidity, suggest that benzene cluster cation CI-ToFMS is suitable for field studies of biosphere–atmosphere interactions.

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Lavi, Avi / Vermeuel, Michael P. / Novak, Gordon A. / et al: The sensitivity of benzene cluster cation chemical ionization mass spectrometry to select biogenic terpenes. 2018. Copernicus Publications.

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