Modeling the Detection of Organic and Inorganic Compounds Using Iodide-Based Chemical Ionization

Abstract

Iodide-based chemical ionization mass spectrometry (CIMS) has been used to detect and measure concentrations of several atmospherically relevant organic and inorganic compounds. The significant electronegativity of iodide and the strong acidity of hydroiodic acid makes electron transfer and proton abstraction essentially negligible, and the soft nature of the adduct formation ionization technique reduces the chances of sample fragmentation. In addition, iodide has a large negative mass defect, which, when combined with the high resolving power of a high resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS), provides good selectivity. In this work, we use quantum chemical methods to calculate the binding energies, enthalpies and free energies for clusters of an iodide ion with a number of atmospherically relevant organic and inorganic compounds. Systematic configurational sampling of the free molecules and clusters was carried out at the B3LYP/6-31G* level, followed by subsequent calculations at the PBE/SDD and DLPNO-CCSD(T)/def2-QZVPP//PBE/aug-cc-pVTZ-PP levels. The binding energies, enthalpies, and free energies thus obtained were then compared to the iodide-based University of Washington HR-ToF-CIMS (UW-CIMS) instrument sensitivities for these molecules. We observed a reasonably linear relationship between the cluster binding enthalpies and logarithmic instrument sensitivities already at the PBE/SDD level, which indicates that relatively simple quantum chemical methods can predict the sensitivity of an iodide-based CIMS instrument toward most molecules. However, higher level calculations were needed to treat some outlier molecules, most notably oxalic acid and methylerythritol. Our calculations also corroborated the recent experimental findings that the molecules that the UW-CIMS detects at maximum sensitivity usually have binding enthalpies to iodide which are higher than about 26 kcal/mol, depending slightly on the level of theory.

An electrospray chemical ionization source for real-time measurement of atmospheric organic and inorganic vapors

“…Thus, Na + is generally more sensitive to less oxidized species than I − and NO − 3 , and most of products observed only in the Na + mode show very low oxygen contents (n O ≤ 3). As many of these species have signal intensities larger than 1000 cps, their absence in I − and NO − 3 modes suggests that I − and NO − 3 are extremely insensitive to these least oxidized species, in agreement with the observations in previous studies (Lee et al, 2014;Hyttinen et al, 2015;Iyer et al, 2016). In contrast, the more oxidized products observed in both modes of comparison show a wide range of R values (e.g., R ≤ 1 or R ≥ 1, corresponding to the blue markers having sizes smaller or larger than that of pinic acid).…”

“…It is clear that, with the increase in electrical field strength, the cluster signals for products having higher oxygen contents generally decay more slowly than those having lower oxygen contents. This is consistent with the fact that I − and NO − 3 ions generally bind more strongly to compounds containing more hydroxy or hydroperoxy moieties (Lee et al, 2014;Hyttinen et al, 2015;Iyer et al, 2016). We note that the trends of decay for C 10 H 16 O 2−8 iodide clusters are in excellent agreement with previous measurements using a low-pressure iodideadduct HR-ToF-CIMS (Lopez-Hilfiker et al, 2016a).…”

“…The iodide-based CIMS has been widely used to detect atmospheric inorganic and organic compounds in previous studies (Huey et al, 1995;Kercher et al, 2009;Lee et al, 2014Lee et al, , 2016Brophy and Farmer, 2015;Lopez-Hilfiker et al, 2016b), though almost exclusively at low pressure (20-80 mbar) as opposed to the atmospheric pressure (1013 mbar) implementation used here. The sensitivity of iodide-based CIMS to a given compound mainly depends on the polarity and hydrogen binding energy of a compound to the I − ion (Lee et al, 2014;Iyer et al, 2016). In the atmospheric pressure ESCIMS, the ion molecule reaction time (a few milliseconds) is set by the electric field, and is up to a factor of 30 or more less than those (30-120 ms) in low-pressure CIMS instruments (Bertram et al, 2011;Lee et al, 2014, Lopez-Hilfiker et al, 2016a.…”

“…The presence of water vapor can affect sensitivities, either by competing for I − ions, thus lowering the sensitivity, or by accommodating excess energy from the collision to stabilize the iodide-molecule clusters, thereby increasing the sensitivity (Lee et al, 2014;Iyer et al, 2016). Water vapor may also affect sensitivities by changing the size distribution of reagent ion clusters and thus their residence time (ionmolecule reaction time) in the IMR.…”

“…As such, certain reagent ions such as metal cations (e.g., Li + , Na + , and K + ) and NH + 4 , which are commonly used for detection of atmospheric organic compounds in offline techniques like electrospray ionization (ESI)-MS (Nizkorodov et al, 2011;Laskin et al, 2012;Witkowski and Gierczak, 2013), have remained largely unavailable for CIMS (Fujii et al, 2001). Compared to I − , NO − 3 , and acetate, which are generally more sensitive to more oxygenated organic compounds than to less oxygenated ones (Aljawhary et al, 2013;Lee et al, 2014;Hyttinen et al, 2015;Iyer et al, 2016;Berndt et al, 2016), these metal cations are expected to be able to sensitively detect both less oxygenated (e.g., compounds containing only carbonyl groups) and highly oxygenated multi-functional organic species (Gao et al, 2010;Nguyen et al, 2010;Nizkorodov et al, 2011;Laskin et al, 2012;Witkowski and Gierczak, 2013;Zhao et al, 2015Zhao et al, , 2016Tu et al, 2016;Zhang et al, 2017), and to form more strongly bound ion adducts. In addition, at present most CIMS techniques use a radioactive ion source such as 210 Po to produce the reagent ions, although more recently some utilize X-ray radiation, electrical discharge (Hirokawa et al, 2009;Yuan et al, 2016), or electron impact (Inomata and Hirokawa, 2017).…”