Review of Volatile Perfluorocarboxylic Acids as Ion Pair Reagents in LC: Part II - - Chromatography Online
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Review of Volatile Perfluorocarboxylic Acids as Ion Pair Reagents in LC: Part II
Oct 1, 2009
By: Wayne O. Aruda, Kenneth O. Aruda
LCGC North America

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Ionization suppression is a complex phenomenon associated with atmospheric pressure-based ionization techniques such as electrospray ionization (ESI) that affects mass spectrometry (MS) detection (1–4). The differences in ionization response in ESI have been commonly attributed to deprotonation, increase in surface tension, increase in droplet size, presence of nonvolatile components, and in complex samples, large amounts of competing species that take some of the available ions. Here we summarize, in chronological order, some observations on ionization suppression with these reagents. Approximately one third of the reviewed publications mention ionization suppression, and of these, about 55% observed significant ionization suppression, while approximately 45% reported no significant ionization suppression. For ease of comparison, Tables I and II list some key parameters of the experimental and instrument setup of these investigators, but note that differences in sample matrix also could affect ionization suppression.


Table I: Publications reporting significant ionization suppression
Roughly 70% of the publications that reported significant ionization suppression used HFBA, three used NFPA, and one each used TDFHA and PFPA. Among the publications that reported no significant ionization suppression, only 43% used HFBA; 36% used PDFOA or TDFHA; two used NFPA; and one used PFPA. Amino acids and aminoglycosides accounted for a total of 28% of the significant ionization suppression group, versus 50% of the nonsignificant ionization suppression group. Proteins and peptides accounted for 28% of both groups. Diverse analytes accounted for 44% and 21% of the significant ionization suppression and nonsignificant ionization suppression groups, respectively.


Table II: Publications reporting minimal ionization suppression
Keever and colleagues (15) noted that HFBA produced some signal suppression, but postcolumn addition of propionic acid enhanced response 10-fold for a cephalosporin antibiotic. Venkateshwaran and colleagues (25) noted that peptides and proteins are very susceptible to denaturation at extremes of pH, which leads to unfolding of the biomolecule and a change in the ESI charge state intensities towards lower m/z values due to an increase in the number of exposed amino acids, which increases the number of ionizable sites in the molecule. Chaimbault and colleagues (26), in a study of underivatized amino acids, using 0.25–5 mM PDFOA, reported that an increase in the surfactant concentration increased the intensity of the positive ion MS signal; that is, no suppression, but ionization enhancement. Watt and colleagues (16) observed that buffered 1 mM HFBA (pH 7.4) gave lower sensitivity, possibly due to reduced protonation of epibatidine. Petritis and colleagues (10) observed intense background signals at m/z 102 with 2 mM NFPA.

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Source: LCGC North America, 10/1/2009
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