A chemical ionization mass spectrometry (CIMS) method utilizing a reagent gas mixture of O<sub>2</sub>, CO<sub>2</sub>, and CH<sub>3</sub>I in N<sub>2</sub> is described and optimized for quantitative gas-phase measurements of hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>), methyl peroxide (CH<sub>3</sub>OOH), formic acid (HCOOH), and the sum of acetic acid (CH<sub>3</sub>COOH) and hydroxyacetaldehyde (HOCH<sub>2</sub>CHO; also known as glycolaldehyde). The instrumentation and methodology were designed for airborne in situ field measurements. The CIMS quantification of formic acid, acetic acid, and hydroxyacetaldehyde used I- cluster formation to produce and detect the ion clusters I (HCOOH), I (CH<sub>3</sub>COOH), and I (HOCH<sub>2</sub>CHO) respectively. Although hydrogen peroxide and methyl peroxide also form cluster ions with I-, the focus here is on the organic acids. Acetic acid and hydroxyacetaldehyde were found to yield equivalent CIMS responses. They are exact isobaric compounds and indistinguishable in the CIMS used. Consequently, their combined signal is referred to as "the acetic acid equivalent sum." Within the resolution of the quadrupole used in the CIMS (1&thinsp;m/z), ethanol and 1- and 2-propanol were potential isobaric interferences to the measurement of formic acid and the acetic acid equivalent sum, respectively. The CIMS response to ethanol was 3.3&thinsp;% that of formic acid and the response to either 1- or 2-propanol was 1&thinsp;% of the acetic acid response; therefore, the alcohols were not considered to be significant interferences to acetic acid or the acetic acid equivalent sum. The multi-reagent ion system was successfully deployed during the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) in 2014. The combination of FRAPPÉ and laboratory calibrations allowed for the post-mission quantification of formic acid and the acetic acid equivalent sum observed during the Deep Convective Clouds and Chemistry Experiment in 2012.
Treadaway, V., Heikes, B. G., McNeill, A. S., Silwal, I. K. C., & O’Sullivan, D. W. (2018). Measurement of formic acid, acetic acid and hydroxyacetaldehyde, hydrogen peroxide, and methyl peroxide in air by chemical ionization mass spectrometry: Airborne method development. Atmospheric Measurement Techniques, 11(4), 1901–1920. https://doi.org/10.5194/amt-11-1901-2018