Temperature-dependent sensitivity of iodide chemical ionization mass spectrometers

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 peroxynitrate (PAN) detected via the peroxy radical cluster, all exhibit strong IMR temperature dependence of sensitivity ranging from -3.4 % degrees C-1 to 5.9 % degrees C-1 (from 37 to 47 degrees C). Strongly bound iodide clusters, such as Br-2, N2O5, ClNO2, and PAN detected via the carboxylate anion, all exhibit little to no IMR temperature dependence ranging from 0.2 % degrees C-1 to -0.9 % degrees C-1 (from 37 to 47 degrees C). The IMR temperature relationships of weakly bound clusters provide an estimate of net reaction enthalpy, and comparison with database values indicates that these clusters are in thermal equilibrium. Ground site HCOOH data collected in the summer of 2021 in Pasadena (CA) are corrected and show a reversal in the did cycle, emphasizing the importance of this correction (35 +/- 6 % during the day, -26 +/- 2 % at night). Finally, we recommend two approaches to minimize this effect in the field, namely heating or cooling the IMR; the latter technique has the added benefit of improving absolute sensitivity.