A dopant-assisted iodide-adduct chemical ionization time-of-flight mass spectrometer based on VUV lamp photoionization for atmospheric low-molecular-weight organic acids analysis

Abstract

Formic and acetic acids are the most abundant gaseous organic acids and play the key role in the atmospheric chemistry. In iodine-adduct chemical ionization mass spectrometry (CIMS), the low utilization efficiency of methyl iodide and humidity interference are two major issues of the vacuum ultraviolet (VUV) lamp initiated CIMS for on-line gaseous formic and acetic acids analysis. In this work, we present a new CIMS based on VUV lamp, and the ion-molecular reactor is separated into photoionization and chemical ionization zones by a reducer electrode. Acetone was added to the photoionization zone, and the VUV photoionization acetone provided low-energy electrons for methyl iodide to generate I−, and the addition of acetone reduced the amount of methyl iodide by 2/3. In the chemical ionization zone, a headspace vial containing ultrapure water was added for humidity calibration, and the vial changes the sensitivity as a function of humidity from ambiguity to well linear correlation (R2 > 0.95). With humidity calibration, the CIMS can quantitatively measure formic and acetic acids in the humidity range of 0%-88% RH. In this mode, limits of detection of 10 and 50 pptv are obtained for formic and acetic acids, respectively. And the relative standard deviation (RSD) of quantitation stability for 6 days were less than 10.5%. This CIMS was successfully used to determine the formic and acetic acids in the underground parking and ambient environment of the Shandong University campus (Qingdao, China). In addition, we developed a simple model based formic acid concentration to assess vehicular emissions.