Proton transfer reaction mass spectrometry (PTR-MS) was used to detect n-alkanes that generally have a lower proton affinity than water and therefore proton transfer (PT) by reaction with H₃O⁺ is not an effective mechanism for their detection. In this study, we developed a method using a conventional PTR-MS to detect n-alkanes by optimizing ion source and drift tube conditions to vary the relative amounts of different primary ions (H₃O⁺, O₂⁺, NO⁺) in the reaction chamber (drift tube).