Molecular Oxygen (O₂) Artifacts in Tandem Mass Spectra

Peak annotation plays an important role in mass spectral evaluation of the NIST 2023 tandem mass spectral library. While most fragment ions are formed by neutral losses, there are peaks that represent adduct ions from these fragments. Previously, we have reported two main types of addition reactions in the collision cell, namely addition of H₂O and N₂. Here we report a different reaction in the collision cell, with addition of O₂ leading to a small peak that could only be assigned to a peroxyl radical ion. For example, some protonated iodoaromatics lose an iodine atom to form a radical cation [M+H-I]^+•, which reacts with O₂ to generate a peroxyl radical ion [M+H-I+O₂]^+•. Higher concentrations of O₂ result in higher peroxyl radical peaks, which become dominant while the precursor ions are consumed, as examined by five compounds under different concentrations of O₂. The correlation of the peroxyl radical peak intensities to the concentration of O₂ provides a tool to estimate trace amounts of O₂ within the instrument. In the NIST 2023 tandem mass spectral library, the peaks for [M+H-X+O₂]^+· are most abundant in numbers and in intensity for X = NO₂ or I, are much less abundant for X = Br, and are rare for X = Cl. Other leaving groups in this library are SO₃H, SO₂NH₂, CSNH₂, CO₂C₆F₅, SO₂CH₃, and COCH₃. The O₂ addition reaction is also observed with negative ions in this library. While adducts of H₂O and N₂ often constitute major peaks, the peaks of the peroxyl radicals under standard conditions are mostly very small and may be mistaken for noise, but their correct annotation improves the quality of the spectra and is important when comparing spectra from different instruments or conditions.