Detection of Gas-Phase Formaldehyde via C₂H₅O⁺ Produced by VUV Illumination and Enhanced by Doping Propionaldehyde

Formaldehyde (CH₂O) is one of the main indoor organic pollutants due to its carcinogenicity and teratogenicity. However, CH₂O has been an undetectable species for common VUV lamp-based photoionization mass spectrometry (PI-MS) due to its high ionization energy. In this article, we report a mass spectrometric technique developed for detecting gas-phase CH₂O by monitoring C₂H₅O⁺ (<i>m</i>/<i>z</i> 45) formed in VUV illumination and enhanced by doping propionaldehyde. The unique mass peak at <i>m</i>/<i>z</i> 45 was observed by VUV illuminating ppm-level CH₂O in ∼1300 Pa of nitrogen or synthetic air. The <i>m</i>/<i>z</i> 45 ion was assigned to be C₂H₅O⁺ via verification with a high-resolution mass spectrometric measurement and a deuteron transfer experiment. The formation process of the C₂H₅O⁺ ion observed in the experiment is similar to that of C₂H₅O⁺ formed in the combustion of hydrocarbons, discussed in the paper. Doping propionaldehyde in the carrier gas remarkably enhanced the signal intensity of C₂H₅O⁺ by nearly 13 times. The detection sensitivities of CH₂O via C₂H₅O⁺ reached 46.0 counts ppb^-1, and the LOD of the method was 4.3 ppb, enhanced by doping propionaldehyde. The influence of the humidity on the detection could be alleviated with desiccants, and the recovery with CaCl₂·2H₂O as the desiccant was achieved at 30% of the dry condition under 100% RH. This experiment demonstrated that the <i>m</i>/<i>z</i> 45 ions can be used as the characteristic ion for detecting CH₂O.