Photoemission ambient pressure ionization (PAPI) with an ultraviolet light emitting diode and detection of organic compounds

Abstract

The development of compact, rugged and low-power ion sources is critical for the further advancement of handheld mass analyzers. Further, there is a need to replace the common (63)Ni source used at atmospheric pressure with a non-radioactive substitute. We present here a description of a light emitting diode (LED) photoemission ionization source for use in mass spectrometry for the detection of volatile organic compounds. This technique relies upon the generation of photoelectrons from a low-work function metal via low-energy ultraviolet (UV) light (280 or 240 nm) generated by a single LED in air at atmospheric pressure. These low-energy photoelectrons result in either direct electron capture by the analyte or chemical ionization. Currently, only negative ions are demonstrated due to operation at atmospheric pressure. Ion generation occurs without use of high electric fields such as those found in corona discharge or electrospray ionization. This source is effective for measuring organic vapors from gases, liquids and surface residues via atmospheric pressure chemical ionization, initiated by photoemission off a conductive surface. Several classes of organic vapors are analyzed and found to be effectively detected, including compounds that ionize via electron attachment, dissociative electron capture, proton abstraction, adduct formation and replacement ionization.

Dopant-Assisted Negative Photoionization Ion Mobility Spectrometry for Sensitive Detection of Explosives

“…This phenomenon was also noted by Ewing et al in recent investigation of photoemission ambient pressure ionization applied in mass spectrometry. 47 Besides the turn-on rapid decay of reactant ion intensity, the reproducibility of the RIP intensity is poor between different run times. One possible explanation for the decay is the contamination (oxidation or coating) of the photocathode surface upon exposure to UV light, which may inhibit electron emission.…”

The role of ion‐bound cluster formation in negative ion mass spectrometry

“…This was caused by the significantly higher water mixing ratios present in laboratory air and should be even higher under CID‐free conditions. Other X – or [X(H 2 O) n ] – clusters, e.g., X = OH, CO 2 , reported for measurements of negative ions were rarely observed. Only [M+NO 2 ] – ( m/z 199) was present with a relative abundance of 0.03%.…”

“…However, for this cluster type also no mechanism leading to bare analyte ions has been proposed. Other background ions have been reported, for example O 4 – , CO 3 – , CO 4 – , NO 2 – , or [NO 2 (H 2 O) n ] – ,, but as far as we are aware no mechanisms for analyte ionization other than reactions (2)–(6) have been proposed.…”

“…In air the formation of superoxide (O 2 – ) as a strong gas‐phase base is generally accepted as the first step of the ionization mechanism directly following thermal electron generation . Observations of [O 2 (H 2 O) n ] – clusters in AP‐NI‐MS have frequently been reported, but no rationale on the ionization mechanisms has been provided that takes the role of the clusters into account …”

“…[9][10][11] Observations of [O 2 (H 2 O) n ]clusters in AP-NI-MS have frequently been reported, but no rationale on the ionization mechanisms has been provided that takes the role of the clusters into account. [9,[12][13][14][15] With superoxide as the reagent ion analyte ions may be formed in two ways. The first is direct electron transfer to analytes which exhibit higher electron affinities than oxygen (EA 0.43 eV [16] ) leading to radical anions (M -); the second is proton transfer from the analyte to O 2 if the gas-phase basicity of the superoxide is higher than that of the deprotonated analyte ([M-H] -).…”