Photoionization/Photoinduced Chemical Ionization Source Based on Radio Frequency Amplitude Modulation on an Ion Funnel

Photoionization (PI) technology has been widely used for the analysis of volatile organic compounds (VOCs) due to its nondestructive nature and high sensitivity. However, the application of photoionization sources is constrained by inefficient ion transmission under high-pressure conditions and relatively low photon energy. In this study, an ion-funnel-integrated photoionization (IFPI) source was developed. Following a systematic investigation of the effects of radio frequency (RF) amplitude on the ion transmission efficiency and ionization processes, a dual-mode ionization strategy based on RF amplitude (<i>V</i>_<i>pp</i>) modulation was proposed. In the low-amplitude mode (<i>V</i>_<i>pp</i> = 40 V), the focusing performance of the ion funnel enhanced the sensitivity of single-photon ionization (SPI) by over 10-fold. In the high-amplitude mode (<i>V</i>_<i>pp</i> = 80 V), high-sensitivity detection of compounds with higher ionization energies (IEs) was achieved via the charge transfer reaction of O₂⁺ produced by photoelectron ionization (PEI), overcoming the limitation of photon energy. After comprehensive optimization of ion source pressure, RF amplitude, and direct current (DC) voltage difference of the ion funnel, the limits of detection (LODs) of 11 VOCs using time-of-flight mass spectrometry combined with the IFPI source (IFPI-TOFMS) ranged from 2.3 to 32.5 parts per trillion by volume (pptv). Finally, the practical utility of the IFPI-TOFMS in the field of environmental health and public safety was demonstrated through 90-h continuous monitoring of laboratory air and spatial distribution analysis of trichloromethane near an indoor swimming pool.