Ultrasensitive detection of volatile aldehydes with chemi-ionization-coupled time-of-flight mass spectrometry

The chemi-ionization reaction is a high-efficiency pathway to produce molecular ions in plasma, however, it has rarely been applied in mass spectrometry to directly produce analyte ions. In this study, a novel chemi-ionization technique for mass spectrometry was applied for the direct and ultrasensitive detection of gaseous aldehydes. The ionization technique was enacted by a recently observed chemi-ionization reaction: the efficient proton transfer from H₂O to oxygenated compounds was stimulated by vacuum ultraviolet (VUV)-excited CH₂Cl₂. By analyzing a series of aliphatic aldehydes (C2-C5) and benzaldehyde with different proton affinities (PAs) and polarities, the ionization features of the new ionization method were investigated for the first time. The chemi-ionization of aldehydes presented soft ionization characteristics with fragmentation patterns analogous to that of VUV photoionization. The method showed ultrahigh sensitivities toward aldehydes (up to 1108 ± 6 counts pptv^-1 for benzaldehyde in 10 s acquisition time). The corresponding 3σ limits of detection (LODs) achieved 0.30-0.69 pptv, which are equivalent of 1.35-1.92 ng m^-3, for the compounds investigated. The humidity experiments revealed that the moisture in the sample gas had an evident impact on the detection efficiency of the analyte and the influence was PA dependent. In addition, the applicability of this ionization mode was further tested by analysis of aldehydes in cigarette smoke. This study provides a promising ionization method for greatly improving the current on-line detection sensitivity of volatile aldehydes.