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Direct and rapid analysis of glyoxal by soft ionization mass spectrometry remains a great challenge due to its low ionization efficiency in existing soft ionization techniques, such as proton transfer reaction (PTR) and photoionization (PI). In this work, we developed a new VUV lamp-based cluster-mediated CH 2 Br 2 + chemical ionization (CMCI) source for time-of-flight mass spectrometry (TOFMS), which was accomplished by employing photoionization-generated CH 2 Br 2 + as the reactant ion and co-sampling of glyoxal with high-concentration ethanol (EtOH). The signal intensity of glyoxal could be enhanced by more than 2 orders of magnitude by generating protonated cluster ion [Glx·EtOH·H] + . Density function theory (DFT) calculations was performed to obtain the most stable structure of neutral glyoxal-ethanol cluster and confirm that the ionization energy (IE) of glyoxal-ethanol cluster was significantly lower than that of glyoxal and CH 2 Br 2 molecules, which makes it possible for effective ionization of glyoxal. The ionization efficiency of glyoxal could be dramatically enhanced via ion-molecule reaction between CH 2 Br 2 + and glyoxal-ethanol cluster, as larger ionization cross section of glyoxal-ethanol cluster than glyoxal molecule might be achieved. The cluster-mediated signal enhanced effect was also verified by using other alcohols, such as methanol and isopropanol. Consequently, the limit of quantitation (LOQ, S/N = 10) down to 0.17 ppbv for gas-phase glyoxal was achieved. The analytical capacity of this system was demonstrated by trace analysis of glyoxal in food contact papers, exhibiting new insights and wide potentials of chemical ionization and photoionization mass spectrometry for VOCs measurement with higher sensitivity and wider detectable sample range., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)