title: Negative ion-atmospheric pressure photoionization: electron capture, dissociative electron capture, proton transfer, and anion attachment.
abstract: To better guide the development of liquid chromatography/electron capture-atmospheric pressure photoionization-mass spectrometry (LC/EC-APPI-MS) in analysis of low polarity compounds, the ionization mechanism of 19 compounds was studied using dopant assisted negative ion-APPI. Four ionization mechanisms, i.e., EC, dissociative EC, proton transfer, and anion attachment, were identified as being responsible for the ionization of the studied compounds. The mechanisms were found to sometimes compete with each other, resulting in multiple ionization products from the same molecule. However, dissociative EC and proton transfer could also combine to generate the same [M - H](-) ions. Experimental evidence suggests that O(2)(-*), which was directly observed in the APPI source, plays a key role in the formation of [M - H](-) ions by way of proton transfer. Introduction of anions more basic than O(2)(-*), i.e., C(6)H(5)CH(2)(-), into the APPI source, via addition of di-tert-butyl peroxide in the solvent and/or dopant, i.e., toluene, enhanced the deprotonation ability of negative ion-APPI. Although the use of halogenated solvents could hinder efficient EC, dissociative EC, and proton transfer of negative ion-APPI due to their EC ability, the subsequently generated halide anions promoted halide attachment to compounds that otherwise could not be efficiently ionized. With the four available ionization mechanisms, it becomes obvious that negative ion-APPI is capable of ionizing a wider range of compounds than negative ion chemical ionization (NICI), negative ion-atmospheric pressure chemical ionization (negative ion-APCI) or negative ion-electrospray ionization (negative ion-ESI).
authors: Mass Spectrometry Center, Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600, USA. song@ion.chem.utk.edu
authors: Liguo
authors: L
authors: Song
authors: Amber D
authors: AD
authors: Wellman
authors: Huifang
authors: H
authors: Yao
authors: John E
authors: JE
authors: Bartmess
references: Anal Bioanal Chem. 2006 Jan;384(1):85-99
references: 16456932
references: Rapid Commun Mass Spectrom. 2007;21(8):1343-51
references: 17348085
references: Anal Chem. 2000 Aug 1;72(15):3653-9
references: 10952556
references: J Mass Spectrom. 2003 Oct;38(10):1113-5
references: 14595862
references: Talanta. 2006 Sep 15;70(2):455-9
references: 18970792
references: J Forensic Sci. 2001 Jan;46(1):6-11
references: 11210925
references: Analyst. 2003 Nov;128(11):1365-72
references: 14700231
references: Mass Spectrom Rev. 2003 Sep-Oct;22(5):318-31
references: 12949917
references: J Am Soc Mass Spectrom. 2004 Feb;15(2):203-11
references: 14766288
references: Steroids. 2005 Jan;70(1):1-11
references: 15610891
references: Anal Chem. 1999 Oct 1;71(19):4119-24
references: 21662844
references: J Chromatogr A. 2005 Dec 16;1099(1-2):127-35
references: 16213509
references: Curr Med Chem. 2004 Jun;11(12):1585-94
references: 15180565
references: Anal Chem. 2002 Nov 1;74(21):5470-9
references: 12433075
references: J Am Chem Soc. 2001 Jun 13;123(23):5518-26
references: 11389634
references: Anal Sci. 2003 Jun;19(6):941-3
references: 12834240
references: Rapid Commun Mass Spectrom. 2006;20(19):2906-14
references: 16941723
references: Analyst. 2002 Aug;127(8):1027-30
references: 12195941
references: Rapid Commun Mass Spectrom. 2003;17(21):2423-9
references: 14587089
references: J Chromatogr A. 2003 Jun 6;1000(1-2):549-65
references: 12877188
references: Anal Chem. 2004 May 15;76(10):2842-51
references: 15144196
references: Anal Chem. 2000 Jul 15;72(14):3007-13
references: 10939360