Temperature dependent sensitivity of iodide chemical ionization mass spectrometers

Abstract. Iodide chemical ionization mass spectrometry (CIMS) is a common analytical tool used in both laboratory and field experiments to measure a large suite of atmospherically relevant compounds. Here, we describe a systematic ion molecule reactor (IMR) temperature dependence of iodide CIMS analyte sensitivity for a wide range of analytes in laboratory experiments. Weakly bound iodide clusters such as HCl, HONO, HCOOH, HCN, phenol, 2-nitrophenol and acyl peroxy nitrate (PAN) detected via the peroxy radical cluster, all exhibit strong IMR temperature dependence of sensitivity ranging from -3.4 to 5.9 % °C^-1 (from 37 to 47 °C). Strongly bound iodide clusters such as Br₂, N₂O₅, ClNO₂, and PAN detected via the carboxylate anion all exhibit little to no IMR temperature dependence ranging from 0.2 to -0.9 % °C^-1 (from 37 to 47 °C). The IMR temperature relationships of weakly bound clusters provide an estimate of net reaction enthalpy and comparison with database values, indicating these clusters are in thermal equilibrium. Ground site HCOOH data collected in the summer of 2021 in Pasadena, CA are corrected showing a reversal in the diel cycle, emphasizing the importance of this correction (35 ± 6 % during the day, -26 ± 2 % at night). Finally, we recommend two approaches to minimizing this effect in the field, namely heating or cooling the IMR; the latter has the added benefit of improving absolute sensitivity and reducing drift in harsh field environments.