A vacuum ultraviolet ion source (VUV-IS) for iodide–chemical ionization mass spectrometry: a substitute for radioactive ion sources

• Published in: Atmospheric measurement techniques Vol. 13; no. 7; pp. 3683 - 3696
• Main Authors: Ji, Yi, Huey, L. Gregory, Tanner, David J, Lee, Young Ro, Veres, Patrick R, Neuman, J. Andrew, Wang, Yuhang, Wang, Xinming
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 09.07.2020; Copernicus Publications
• Subjects: Acids; Air sampling; Benzene; Calibration; Cations; Chlorine; Detection limits; Electron tubes; Formic acid; Gases; Iodides; Ion sources; Ionization; Ionization potentials; Ions; Krypton; Laboratories; Light; Mass spectra; Mass spectrometers; Mass spectrometry; Mass spectroscopy; Nitryl chlorides; Photoelectrons; Photoionization; Quadrupoles; Reagents; Scientific imaging; Sensitivity; Spectrometers; Substitutes; Ultraviolet radiation; Vacuum; Wavelengths
• ISSN: 1867-8548; 1867-1381; 1867-8548
• DOI: 10.5194/amt-13-3683-2020

A new ion source (IS) utilizing vacuum ultraviolet (VUV) light is developed and characterized for use with iodide–chemical ionization mass spectrometers (I−-CIMS). The VUV-IS utilizes a compact krypton lamp that emits light at two wavelengths corresponding to energies of ∼10.030 and 10.641 eV. The VUV light photoionizes either methyl iodide (ionization potential, IP = 9.54 ± 0.02 eV) or benzene (IP = 9.24378 ± 0.00007 eV) to form cations and photoelectrons. The electrons react with methyl iodide to form I−, which serves as the reagent ion for the CIMS. The VUV-IS is characterized by measuring the sensitivity of a quadrupole CIMS (Q-CIMS) to formic acid, molecular chlorine, and nitryl chloride under a variety of flow and pressure conditions. The sensitivity of the Q-CIMS, with the VUV-IS, reached up to ∼700 Hz pptv−1, with detection limits of less than 1 pptv for a 1 min integration period. The reliability of the Q-CIMS with a VUV-IS is demonstrated with data from a month-long ground-based field campaign. The VUV-IS is further tested by operation on a high-resolution time-of-flight CIMS (TOF-CIMS). Sensitivities greater than 25 Hz pptv−1 were obtained for formic acid and molecular chlorine, which were similar to that obtained with a radioactive source. In addition, the mass spectra from sampling ambient air was cleaner with the VUV-IS on the TOF-CIMS compared to measurements using a radioactive source. These results demonstrate that the VUV lamp is a viable substitute for radioactive ion sources on I−-CIMS systems for most applications. In addition, initial tests demonstrate that the VUV-IS can be extended to other reagent ions by the use of VUV absorbers with low IPs to serve as a source of photoelectrons for high IP electron attachers, such as SF6-.