Cl2− chemical ionization mass spectrometry (Cl2-CIMS) for the measurement of acyl peroxy radicals†

Organic peroxy radicals (RO₂) are produced in the atmosphere by oxidation of volatile organic compounds (VOCs) and, in some cases, VOC photolysis. However, photolytic sources of RO₂ are often poorly understood, in part due to challenges in directly detecting RO₂ in both ambient and laboratory settings. We investigated Cl₂⁻ as a chemical ionization mass spectrometry reagent ion (Cl₂-CIMS) for measuring and speciating RO₂ in a laboratory setting. Cl₂-CIMS was more sensitive to the acetyl peroxy radical (CH₃C(O)O₂; 2.30 ± 0.04 ncps/ppt) than iodide CIMS (I-CIMS; 1.54 ± 0.03 ncps/ppt), but high backgrounds in our setup resulted in a slightly higher detection limit of 5 ppt (1 second integration) for Cl₂-CIMS than I-CIMS (2 ppt). We demonstrate the application of Cl₂-CIMS by quantifying the quantum yields of two radical products, CH₃C(O) and C₂H₅C(O), from methyl ethyl ketone photolysis at 254 nm. We identified O₂⁻ and Cl⁻ as possible secondary reagent ions that created unintended product ions in our experiments and thus could complicate the interpretation of Cl₂-CIMS mass spectra for complex atmospheric samples. While several strategies may minimize these effects, Cl₂-CIMS is suitable for measuring RO₂ in controlled laboratory experiments.