Voltage scanning to calibrate Multi-Reagent Chemical Ionization Mass Spectrometers (MR-CIMS): from signal to sensitivity

Chemical ionization mass spectrometry (CIMS) offers high time-resolution measurements for diverse compounds, but atmospheric quantification remains challenging. Here, we combine a recently published method for determining the collision limit using a single reagent ion with a voltage scanning approach for assessing the relative sensitivities of diverse adduct ions. We used voltage scanning in a Multi-Reagent Chemical Ionization Mass Spectrometer (MR-CIMS) to assess ion–molecule adduct strength. The sensitivities to most detectable compounds were calculated based on this relationship using a collision-limit sensitivity of 13.87 ± 0.69 ncps pptv−1 determined for α-pinene using the benzene channel. Following previously published work, the collision limit sensitivity of the other reagent ions used was assumed to be equal to that of the benzene channel and was further examined using the binding energy and measured sensitivity of nitrophenol in the bromide channel. Calibration of 13 molecules, including nitric acid, formic acid, and oxygenated VOCs, was performed to obtain a universal relationship between the sensitivities and the voltage at which the adduct signal halves (dV50). Quantification uncertainties stayed below 20 % for compounds with sensitivities above 5.69 and 5.30 ncps pptv−1 in bromide and iodide channels, respectively. High-level quantum chemical calculations indicated that the detected compounds predominantly form hydrogen-bonded clusters with bromide and iodide. In a large photochemical chamber, estimated sensitivities of more than 260 compounds were determined. Based on this, we achieved their quantification with multiple negative reagent ions. The quantification was validated by comparing nitrous acid concentrations measured using MR-CIMS with those obtained from a calibrated iterative cavity-enhanced differential optical absorption spectroscopy (ICAD) (R2 = 0.891, slope = 1.24). Six of the organic compounds were taken as examples to show the results of this method for a daytime oxidation chamber experiment. The measurement uncertainties for these pptv-level compounds ranged from 8.9 % to 39.0 %, depending on their sensitivities and concentrations. Further theoretical and experimental investigations showed that halogen compounds can form intermolecular halogen bonds with strength comparable to their intramolecular bonds, preventing this approach from being applied to determine their sensitivities. This work highlights that voltage scanning is a useful approach to the determination ion–molecule adduct sensitivity in CIMS.