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 alpha pinene. Here, we present laboratory measurements of the sensitivity of the same instrument to a series of terpenes, including isoprene, &alpha;-pinene, &beta;-pinene, D-limonene, ocimene, &beta;-myrcene, farnesene, &alpha;-humulene, &beta;-caryophyllene and isolongifolene at atmospherically relevant mixing ratios (<&thinsp;100&thinsp;pptv). In addition, we determine the dependence of CI-ToFMS sensitivity on the reagent ion neutral delivery concentration, the instrument electric field strength and water vapor concentration. We show that isoprene is primarily detected as an adduct (C<sub>5</sub>H<sub>8</sub>∙C<sub>6</sub>H<sub>6</sub><sup>+</sup>) with a sensitivity ranging between 4&ndash;10&thinsp;ncps&thinsp;ppt<sup>&minus;1</sup>, that depends strongly on the reagent ion precursor concentration, de-clustering voltages, and specific humidity (SH). Monoterpenes are detected primarily as the molecular ion (C<sub>10</sub>H<sub>16</sub><sup>+</sup>) with an average sensitivity, across the five measured compounds, of 14&thinsp;±&thinsp;3&thinsp;ncps&thinsp;ppt<sup>&minus;1</sup> for SH between 7 and 14&thinsp;g&thinsp;kg<sup>&minus;1</sup>, typical of the boreal forest during summer. Sesquiterpenes are detected primarily as the molecular ion (C<sub>15</sub>H<sub>24</sub><sup>+</sup>) with an average sensitivity, across the four measured compounds, of 9.6&thinsp;±&thinsp;2.3&thinsp;ncps&thinsp;ppt<sup>&minus;1</sup> 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, suggests that benzene cluster cation CI-ToFMS is suitable for field studies of biosphere-atmosphere interactions.
Lavi, A., Vermeuel, M. P., Novak, G. A., & Bertram, T. H. (2017). The sensitivity of benzene cluster cation chemical ionization mass spectrometry to select biogenic terpenes. Atmospheric Measurement Techniques Discussions, 1–30. https://doi.org/10.5194/amt-2017-408