Interpretation of α-Pinene Mass Spectra in APCI-Like Ambient Mass Spectrometry Using GC-Coupled Atmospheric Pressure Corona Discharge Ionization

Ambient mass spectrometry (AMS) enables real-time analysis without sample preparation, yet atmospheric pressure corona discharge ionization (APCI)-like ion sources can oxidize analytes in-source, obscuring spectral interpretation. We investigated α-pinene using atmospheric pressure corona discharge ionization mass spectrometry (APCDI-MS; one of APCI-like AMS) and, when needed, coupled gas chromatography (GC) to separate pre-existing oxidation products from species formed inside the ion source. By comparing adduct formation and product-ion patterns, we show that oxygenated ions from primary (pre-source) and secondary (in-source) oxidation display similar ion formation behavior, notably ready NH₄ ⁺-adduct formation, whereas fragment ions rarely form NH₄ ⁺ adducts. GC-based characterization of in-source oxidation provides features that assist interpretation of APCI-like AMS spectra acquired without chromatography.