Formation and decompositions of chloride adduct ions, [M + Cl]⁻, in negative ion electrospray ionization mass spectrometry

Abstract

The ability to promote chloride-attachment ions of the form [M + Cl]⁻ in negative ion electrospray ionization mass spectrometry (ESI-MS) has been developed using chlorinated solvents such as chloroform and carbon tetrachloride. This approach expands the current capabilities of negative ion ESI-MS by enabling detection of analytes that lack acidic sites and thus exhibit weak [M − H]⁻ signals. In contrast to the remote-site collision-induced dissociation (CID) often observed in positive ion ESI-MS/MS for alkali metal cation adducts, the decomposition of chloride adducts usually proceeds via competitive dissociations to form Cl⁻, which is not structurally informative, or [M − H]⁻. The latter can provide structural information via consecutive decompositions. For compounds having higher gas-phase acidities than HCl, a low CID collision energy can promote the formation of [M − H]⁻, whereas for the majority of compounds with lower gas phase acidities than HCl, higher collision energies generally improve the relative yield of [M − H]⁻. Because chloride attachment occurs primarily at electrophilic hydrogens, the daughter ion ratio, Cl⁻/[M − H]⁻, depends primarily upon the difference in gas phase acidity between the analyte molecule and HCl. At higher collision energies, entropic factors take on increased importance in determining the product ratio. The difference between the ΔS ⁰ terms for formation of Cl⁻ and formation of [M − H]− has been estimated for a series of substituted phenols and a series of acetic acid analogs. Finally, a novel neutral loss of CH₃Cl from glycerophosphocholine and from ganglioside GM3 methyl ester is reported.