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Published November 2009 | metadata_only
Journal Article

Ionization Mechanism of the Ambient Pressure Pyroelectric Ion Source (APPIS) and Its Applications to Chemical Nerve Agent Detection


We present studies of the ionization mechanism operative in the ambient pressure pyroelectric ionization source (APPIS), along with applications that include detection of simulants for chemical nerve agents. It is found that ionization by APPIS occurs in the gas-phase. As the crystal is thermally cycled over a narrow temperature range, electrical discharges near the surface of the crystal produce energetic species which, through reactions with atmospheric molecules, result in reactant ions such as protonated water clusters or clusters of hydroxide and water. Reactant ions can be observed directly in the mass spectrometer. These go on to react with trace neutrals via proton transfer reactions to produce the ions observed in mass spectra, which are usually singly protonated or deprotonated species. Further implicating gas-phase ionization, observed product distributions are highly dependent on the composition of ambient gases, especially the concentration of water vapor and oxygen surrounding the source. For example, basic species such as triethylamine are observed as singly protonated cations at a water partial pressure of 10 torr. At a water pressure of 4 torr, reactive oxygen species are formed and lead to observation of protonated amine oxides. The ability of the APPIS source to detect basic molecules with high proton affinities makes it highly suited for the detection of chemical nerve agents. We demonstrate this application using simulants corresponding to VX and GA (Tabun). With the present source configuration pyridine is detected readily at a concentration of 4 ppm, indicating ultimate sensitivity in the high ppb range.

Additional Information

© 2009 American Society for Mass Spectrometry Published by Elsevier B.V. Received 27 August 2008; revised 14 July 2009; accepted 15 July 2009. Available online 18 July 2009. The authors acknowledge funding from the National Science Foundation under grant CHE-0416381 as well funding from the Beckman Institute at Caltech. ELN acknowledges a Graduate Research Fellowship in Analytical Chemistry from Pfizer Analytical Research and Development (Groton, CT), as well as many useful discussions with Luther Beegle (Jet Propulsion Laboratory, Pasadena, CA), Jim Weiss (Ionfinity LLC, Walnut, CA), and Professor G. A. Eiceman (New Mexico State University, Las Cruces, NM).

Additional details

August 19, 2023
August 19, 2023