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However, VUV light irradiation of the metal surfaces within the transmission chamber induces photoelectron emission; excessive background gas ions are generated via photoelectron ionization under the radio frequency (RF) electric field, which affects the quantitative performance and repeatability of PI-MS, especially for deuterium lamps with a higher photon flux. In this study, a size- and position-adjustable baffle electrode was integrated into the ionization chamber to reduce the light leakage into the transmission chamber. For a 3 mm-diameter baffle positioned 5.4 mm from the skimmer, the intensity of background gas ions generated in the transmission chamber decreased to 1\/506, while the intensity of the trichloroethylene ion was maintained at half that without the baffle. The intensities of chlorinated hydrocarbon ions were enhanced by 9-fold compared with the SPI mode when an RF voltage was further applied to the baffle electrode, under which the dominant ionization process was Penning ionization (PenI). Surprisingly, the intensities of chlorinated hydrocarbon ions did not decrease but increased by up to 126% compared to those without the baffle. Simulation of the electric field distribution and electron kinetic energy highlights that the precise overlap of the ion generation region and efficient transmission region plays a pivotal role in the regulation of spatially confined PenI toward sensitivity improvement. This approach has achieved limits of detection down to sub-pptv, even for molecules with ionization energies higher than the photon energy.<\/jats:p>","DOI":"10.1021\/acs.analchem.6c01680","type":"journal-article","created":{"date-parts":[[2026,7,16]],"date-time":"2026-07-16T01:53:46Z","timestamp":1784166826000},"page":"21157-21164","source":"Crossref","is-referenced-by-count":0,"title":["Spatially Confined\nPhotoinduced Penning Ionization\nfor Highly Sensitive Mass Spectrometry"],"prefix":"10.1021","volume":"98","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-4431-3207","authenticated-orcid":true,"given":"Yi","family":"Yu","sequence":"first","affiliation":[{"name":"Chinese Academy of Sciences , , ,","place":["Dalian, People\u2019s Republic of China, 116023"]},{"name":"Dalian Key Laboratory for Online Analytical Instrumentation , ,","place":["Dalian, People\u2019s Republic of China, 116023"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1400-3902","authenticated-orcid":true,"given":"Hang","family":"Li","sequence":"additional","affiliation":[{"name":"University of Chinese Academy of Sciences , , ,","place":["Shanghai, People\u2019s Republic of China, 200032"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Zhenyuan","family":"Zhang","sequence":"additional","affiliation":[{"name":"Chinese Academy of Sciences , , ,","place":["Dalian, People\u2019s Republic of China, 116023"]},{"name":"Dalian Key Laboratory for Online Analytical Instrumentation , ,","place":["Dalian, People\u2019s Republic of China, 116023"]},{"name":"University of Chinese Academy of Sciences , ,","place":["Beijing, People\u2019s Republic of China, 100049"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Zhihao","family":"Zhang","sequence":"additional","affiliation":[{"name":"Chinese Academy of Sciences , , ,","place":["Dalian, People\u2019s Republic of China, 116023"]},{"name":"Dalian Key Laboratory for Online Analytical Instrumentation , ,","place":["Dalian, People\u2019s Republic of China, 116023"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Tianhao","family":"Huang","sequence":"additional","affiliation":[{"name":"Chinese Academy of Sciences , , ,","place":["Dalian, People\u2019s Republic of China, 116023"]},{"name":"Dalian Key Laboratory for Online Analytical Instrumentation , ,","place":["Dalian, People\u2019s Republic of China, 116023"]},{"name":"University of Chinese Academy of Sciences , ,","place":["Beijing, People\u2019s Republic of China, 100049"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Wei","family":"Huang","sequence":"additional","affiliation":[{"name":"Chinese Academy of Sciences , , ,","place":["Dalian, People\u2019s Republic of China, 116023"]},{"name":"Dalian Key Laboratory for Online Analytical Instrumentation , ,","place":["Dalian, People\u2019s Republic of China, 116023"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Junyu","family":"Yang","sequence":"additional","affiliation":[{"name":"Chinese Academy of Sciences , , ,","place":["Dalian, People\u2019s Republic of China, 116023"]},{"name":"Dalian Key Laboratory for Online Analytical Instrumentation , ,","place":["Dalian, People\u2019s Republic of China, 116023"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8485-8893","authenticated-orcid":true,"given":"Ling","family":"Jiang","sequence":"additional","affiliation":[{"name":"Chinese Academy of Sciences , , ,","place":["Dalian, People\u2019s Republic of China, 116023"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2696-3521","authenticated-orcid":true,"given":"Lei","family":"Hua","sequence":"additional","affiliation":[{"name":"Chinese Academy of Sciences , , ,","place":["Dalian, People\u2019s Republic of China, 116023"]},{"name":"Dalian Key Laboratory for Online Analytical Instrumentation , ,","place":["Dalian, People\u2019s Republic of China, 116023"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0007-8203-0025","authenticated-orcid":true,"given":"Jichun","family":"Jiang","sequence":"additional","affiliation":[{"name":"Chinese Academy of Sciences , , ,","place":["Dalian, People\u2019s Republic of China, 116023"]},{"name":"Dalian Key Laboratory for Online Analytical Instrumentation , ,","place":["Dalian, People\u2019s Republic of China, 116023"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6658-4745","authenticated-orcid":true,"given":"Haiyang","family":"Li","sequence":"additional","affiliation":[{"name":"Chinese Academy of Sciences , , ,","place":["Dalian, People\u2019s Republic of China, 116023"]},{"name":"Dalian Key Laboratory for Online Analytical Instrumentation 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Chem."},{"key":"2026072808065998300_cit2","doi-asserted-by":"publisher","first-page":"8756","DOI":"10.1021\/acs.analchem.8b02043","article-title":"Microprobe\nfor the Thermal Analysis of Crude Oil Coupled to Photoionization Fourier\nTransform Mass Spectrometry","volume":"90","author":"Kostyukevich","year":"2018","journal-title":"Anal. Chem."},{"key":"2026072808065998300_cit3","doi-asserted-by":"publisher","first-page":"e202202165","DOI":"10.1002\/anie.202202165","article-title":"Single-Photon-Induced\nPost-Ionization to Boost Ion Yields in MALDI Mass Spectrometry Imaging","volume":"61","author":"Bookmeyer","year":"2022","journal-title":"Angew. Chem., Int. Ed."},{"key":"2026072808065998300_cit4","doi-asserted-by":"publisher","DOI":"10.1016\/j.trac.2022.116542","article-title":"Vacuum\nUltraviolet Photoionization On-Line Mass Spectrometry: Instrumentation\nDevelopments and Applications","volume":"149","author":"Dang","year":"2022","journal-title":"TrAC, Trends\nAnal. Chem."},{"key":"2026072808065998300_cit5","doi-asserted-by":"publisher","first-page":"134151","DOI":"10.1016\/j.jhazmat.2024.134151","article-title":"Ultra-Sensitive Analysis of Exhaled Biomarkers in Ozone-Exposed\nMice via PAI-TOFMS Assisted with Machine Learning Algorithms","volume":"470","author":"Yang","year":"2024","journal-title":"J. Hazard. Mater."},{"key":"2026072808065998300_cit6","doi-asserted-by":"publisher","first-page":"4473","DOI":"10.1021\/acs.analchem.4c05873","article-title":"Increased Sensitivity\nin VOC Detection\nby Using a Novel Photoinduced Multiple Ionization Mass Spectrometry","volume":"97","author":"Bao","year":"2025","journal-title":"Anal. Chem."},{"key":"2026072808065998300_cit7","doi-asserted-by":"publisher","first-page":"3979","DOI":"10.1021\/acs.analchem.4c00605","article-title":"Improving the Sensitivity and Linear\nRange of Photoionization Ion Mobility Spectrometry via Confining the\nIon Recombination and Space Charge Effects Assisted by Theoretical\nModeling","volume":"96","author":"Xu","year":"2024","journal-title":"Anal. Chem."},{"key":"2026072808065998300_cit8","doi-asserted-by":"publisher","first-page":"5028","DOI":"10.1021\/acs.analchem.6b00428","article-title":"Photoionization-Generated\nDibromomethane\nCation Chemical Ionization Source for Time-of-Flight Mass Spectrometry\nand Its Application on Sensitive Detection of Volatile Sulfur Compounds","volume":"88","author":"Jiang","year":"2016","journal-title":"Anal. Chem."},{"key":"2026072808065998300_cit9","doi-asserted-by":"publisher","first-page":"2079","DOI":"10.1021\/acs.analchem.2c05014","article-title":"Field-Gradient-Focusing\nIon Guide for Enhanced Transfer Efficiency of Low-Mass Ions","volume":"95","author":"Du","year":"2023","journal-title":"Anal. Chem."},{"key":"2026072808065998300_cit10","doi-asserted-by":"publisher","first-page":"1070","DOI":"10.1021\/acs.analchem.4c03255","article-title":"Twisted Dipole Ion Guide (TDIG) for\nFlexible Ion Transfer in Atmospheric Pressure Ionization Mass Spectrometry","volume":"97","author":"He","year":"2025","journal-title":"Anal. Chem."},{"key":"2026072808065998300_cit11","doi-asserted-by":"publisher","first-page":"1612","DOI":"10.1021\/acs.analchem.4c04414","article-title":"Double-Helix Electrode Ion Funnel:\nA New Ion Funnel Design with an Extended Mass Range","volume":"97","author":"Wu","year":"2025","journal-title":"Anal. Chem."},{"key":"2026072808065998300_cit12","doi-asserted-by":"publisher","first-page":"6791","DOI":"10.1021\/ac8007187","article-title":"Photoelectron\nEmission as an Alternative\nElectron Impact Ionization Source for Ion Trap Mass Spectrometry","volume":"80","author":"Gamez","year":"2008","journal-title":"Anal. Chem."},{"key":"2026072808065998300_cit13","doi-asserted-by":"publisher","first-page":"4637","DOI":"10.5194\/amt-8-4637-2015","article-title":"Real-Time Monitoring of Trace-Level\nVOCs by an Ultrasensitive Lamp-Based VUV Photoionization Mass Spectrometer","volume":"8","author":"Sun","year":"2015","journal-title":"Atmos. Meas. Tech."},{"key":"2026072808065998300_cit14","doi-asserted-by":"publisher","first-page":"9047","DOI":"10.1021\/acs.analchem.6b01707","article-title":"High-Pressure Photon Ionization Source for TOFMS and Its Application\nfor Online Breath Analysis","volume":"88","author":"Wang","year":"2016","journal-title":"Anal. Chem."},{"key":"2026072808065998300_cit15","doi-asserted-by":"publisher","first-page":"1159","DOI":"10.5194\/amt-15-1159-2022","article-title":"A Versatile\nVacuum Ultraviolet Ion Source for Reduced Pressure Bipolar Chemical\nIonization Mass Spectrometry","volume":"15","author":"Breitenlechner","year":"2022","journal-title":"Atmos. Meas. Tech."},{"key":"2026072808065998300_cit16","doi-asserted-by":"publisher","first-page":"17166","DOI":"10.1021\/acs.analchem.3c04650","article-title":"Hydrogen-Assisted Photoionization\nand Its Use in Promoting Mass Spectrometry Analysis of VOCs","volume":"95","author":"Zhu","year":"2023","journal-title":"Anal. Chem."},{"key":"2026072808065998300_cit17","doi-asserted-by":"publisher","first-page":"19576","DOI":"10.1021\/acs.analchem.4c04404","article-title":"Investigating the\nKinetics of Heterogeneous Lipid Ozonolysis\nby an Online Photoionization High-Resolution Mass Spectrometry Technique","volume":"96","author":"Liu","year":"2024","journal-title":"Anal. Chem."},{"key":"2026072808065998300_cit18","doi-asserted-by":"publisher","first-page":"5489","DOI":"10.1021\/acs.analchem.3c05605","article-title":"Development of Transmission Ambient\nPressure Laser Desorption Ionization\/Postphotoionization\nMass Spectrometry Imaging","volume":"96","author":"Qi","year":"2024","journal-title":"Anal. Chem."},{"key":"2026072808065998300_cit19","doi-asserted-by":"publisher","first-page":"1827","DOI":"10.1021\/jasms.5c00189","article-title":"Photoionization\/Photoinduced\nChemical Ionization Source Based on\nRadio Frequency Amplitude Modulation on an Ion Funnel","volume":"36","author":"Fan","year":"2025","journal-title":"J. Am. Soc. Mass Spectrom."},{"key":"2026072808065998300_cit20","doi-asserted-by":"publisher","first-page":"3137","DOI":"10.1039\/D1AN00114K","article-title":"Hyper-Fast\nGas Chromatography and Single-Photon Ionisation\nTime-of-Flight Mass Spectrometry with Integrated Electrical Modulator-Based\nSampling for Headspace and Online VOC Analyses","volume":"146","author":"Gehm","year":"2021","journal-title":"Analyst"},{"key":"2026072808065998300_cit21","doi-asserted-by":"publisher","first-page":"106224","DOI":"10.1016\/j.jaap.2023.106224","article-title":"Chemical Elucidation\nof Recycled Plastic Pyrolysis Oils by Means of GCxGC-PI-TOF-MS and\nGC-VUV","volume":"176","author":"Lazzari","year":"2023","journal-title":"J. Anal. Appl. Pyrolysis"},{"key":"2026072808065998300_cit22","doi-asserted-by":"publisher","first-page":"3352","DOI":"10.1021\/acs.analchem.5c07498","article-title":"Pressure-Tunable Oxygen\nAttachment Dissociation\nin a Deuterium Lamp-Based Photoionization Source for Isomer Differentiation","volume":"98","author":"Fan","year":"2026","journal-title":"Anal. Chem."},{"key":"2026072808065998300_cit23","doi-asserted-by":"publisher","first-page":"1265","DOI":"10.1021\/acs.jpclett.2c03572","article-title":"Ionization of Dichloromethane\nby a Vacuum Ultraviolet Krypton Lamp:\nCompetition Between Photoinduced Ion-Pair and Photodissociation-Assisted\nPhotoionization","volume":"14","author":"Yu","year":"2023","journal-title":"J. Phys. Chem. Lett."},{"key":"2026072808065998300_cit24","doi-asserted-by":"publisher","first-page":"5686","DOI":"10.1021\/acs.analchem.4c00686","article-title":"Pressure-Driven\nSwitching of Photoelectron Impact Ionization-Chemical Ionization\/Penning\nIonization in Vacuum Ultraviolet Photoionization Mass Spectrometry","volume":"96","author":"Yu","year":"2024","journal-title":"Anal. Chem."},{"key":"2026072808065998300_cit25","doi-asserted-by":"publisher","first-page":"3430","DOI":"10.1021\/acs.est.1c05952","article-title":"Geobacter sp. Strain IAE Dihaloeliminates 1,1,2-Trichloroethane and\n1,2-Dichloroethane","volume":"56","author":"Jiang","year":"2022","journal-title":"Environ. Sci. Technol."},{"key":"2026072808065998300_cit26","doi-asserted-by":"publisher","first-page":"16531","DOI":"10.1021\/acs.analchem.3c02365","article-title":"Novel MCP-Windowed EUV Light Source\nand Its Mass Spectrometric\nApplication for Detecting Chlorinated Methanes","volume":"95","author":"Cheng","year":"2023","journal-title":"Anal. Chem."},{"key":"2026072808065998300_cit27","doi-asserted-by":"publisher","first-page":"11833","DOI":"10.1021\/acs.est.4c02756","article-title":"Single-Atom Iron Can\nSteer Atomic Hydrogen toward Selective Reductive Dechlorination: Implications\nfor Remediation of Chlorinated Solvents-Impacted Groundwater","volume":"58","author":"Liang","year":"2024","journal-title":"Environ. Sci. Technol."},{"key":"2026072808065998300_cit28","doi-asserted-by":"publisher","first-page":"142461","DOI":"10.1016\/j.jclepro.2024.142461","article-title":"Optimal Conditions for Nano-Emulsified\nVegetable Oil Synthesis for the Biostimulation of 1,1,2-Trichloroethane\nand Vinyl Chloride Contaminated Groundwater in Bioreactors","volume":"456","author":"Zhao","year":"2024","journal-title":"J. Cleaner Prod."},{"key":"2026072808065998300_cit29","doi-asserted-by":"publisher","DOI":"10.1002\/rcm.9354","article-title":"Where Have\nAll the Ions Gone, Long Time Passing? Tandem Quadrupole Mass Spectrometers\nwith Atmospheric Pressure Ionization Sensitivity Gains Since the Mid-1970s.\nA Perspective","volume":"39","author":"Covey","year":"2022","journal-title":"Rapid Commun. Mass Spectrom."},{"key":"2026072808065998300_cit30","doi-asserted-by":"crossref","DOI":"10.1007\/978-3-642-61247-3","volume-title":"Gas Discharge Physics","author":"Raizer","year":"1991"}],"container-title":["Analytical Chemistry"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/pubs.acs.org\/doi\/pdf\/10.1021\/acs.analchem.6c01680","content-type":"application\/pdf","content-version":"vor","intended-application":"unspecified"},{"URL":"https:\/\/pubs.acs.org\/ancham\/article-pdf\/98\/29\/21157\/65967884\/acs.analchem.6c01680.pdf","content-type":"application\/pdf","content-version":"vor","intended-application":"syndication"},{"URL":"https:\/\/pubs.acs.org\/ancham\/article-pdf\/98\/29\/21157\/65967884\/acs.analchem.6c01680.pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,7,28]],"date-time":"2026-07-28T12:21:51Z","timestamp":1785241311000},"score":1,"resource":{"primary":{"URL":"https:\/\/pubs.acs.org\/ancham\/article\/98\/29\/21157\/5211378\/Spatially-Confined-Photoinduced-Penning-Ionization"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,7,15]]},"references-count":30,"journal-issue":{"issue":"29","published-online":{"date-parts":[[2026,7,15]]},"published-print":{"date-parts":[[2026,7,28]]}},"URL":"https:\/\/doi.org\/10.1021\/acs.analchem.6c01680","relation":{},"ISSN":["0003-2700","1520-6882"],"issn-type":[{"value":"0003-2700","type":"print"},{"value":"1520-6882","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,7,15]]}}}