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

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.