Generation of gas‐phase sodiated arenes such as [(Na₃(C₆H₄)⁺] from benzene dicarboxylate salts

Abstract

Upon collision-induced activation, gaseous sodium adducts generated by electrospray ionization of disodium salts of 1,2- 1,3-, and 1,4-benzene dicarboxylic acids (m/z 233) undergo an unprecedented expulsion of CO(2) by a rearrangement process to produce an ion of m/z 189 in which all three sodium atoms are retained. When isolated in a collision cell of a tandem-in-space mass spectrometer, and subjected to collision-induced dissociation (CID), only the m/z 189 ions derived from the meta and para isomers underwent a further CO(2) loss to produce a peak at m/z 145 for a sodiated arene of formula (Na(3)C(6)H(4))(+). This previously unreported m/z 145 ion, which is useful to differentiate meta and para benzene dicarboxylates from their ortho isomer, is in fact the sodium adduct of phenelenedisodium. Moreover, the m/z 189 ion from all three isomers readily expelled a sodium radical to produce a peak at m/z 166 for a radical cation [(*C(6)H(4)CO(2)Na(2))(+)], which then eliminated CO(2) to produce a peak at m/z 122 for the distonic cation (*C(6)H(4)Na(2))(+).

Structural characterization of product ions by electrospray ionization and quadrupole time‐of‐flight mass spectrometry to support regulatory analysis of veterinary drug residues in foods. Part 2: Benzimidazoles, nitromidazoles, phenothiazines, and mectins

“…Lehner et al also observed the Na‐adduct product ion in an ivermectin spectrum, but its structure was not proposed . Sodiated OE structures are very uncommon in ESI, and our literature search only found a report describing that the benzene carboxylate sodium salt can form a cation radical by elimination of Na • . However, there is no evidence in the mass spectrum of ivermectin for the addition of more than one sodium to justify the ejection of Na • .…”

Gas‐phase fragmentation of metal adducts of alkali‐metal oxalate salts

“…For example, we reported previously that the alkali adducts of the disodium salts of phthalic acids undergo specific fragmentation reactions in which the charge‐imparting sodium atom is often retained . In a subsequent survey of other dicarboxylic acid compounds, we noted that metal adducts of oxalate salts fragment in a similar manner.…”

“…However, for liquid chromatography mass spectrometry (LC‐MS), the acidic modifiers that are added to the mobile phase for better chromatography of acidic analytes cause a high degree of matrix interference and suppress the ionization to a certain extent in the negative mode. Although many applications of negative‐mode LC‐MS have been reported recently, the majority of the procedures for quantitative profiling of acidic compounds and their salts have been conducted traditionally by positive‐mode mass spectrometry . The collision‐induced fragmentation spectra of salt‐metal adducts are often different from those of the corresponding proton adducts, providing additional structural information.…”

“…For example, CID mass spectra of metal‐cationized peptides, nucleic acids and lipids are known to provide structural information complementary to that obtained from the spectra of their protonated counterparts . However, very few detailed investigations have been undertaken on the fragmentation of cationic adducts of low‐molecular‐weight salt molecules . Often, CID spectra of cationized small molecules are not very informative because most spectra show only a single product ion peak for the ejected metal cation.…”

“…Often, CID spectra of cationized small molecules are not very informative because most spectra show only a single product ion peak for the ejected metal cation. However, during our explorations on fragmentation mechanisms of low‐molecular‐weight carboxylic acids, we noted that when a metal atom binds in a multi‐dentate fashion to several heteroatoms simultaneously, the product ions retain the metal atom instead of ejecting it …”