Hydroxycarbonyl anion (m/z 45), a diagnostic marker for alpha-hydroxy carboxylic acids

Collision-induced dissociation mass spectra of anions derived from alpha-hydroxy carboxylic acids (AHAs) show a diagnostic peak at m/z 45. Product ion spectra recorded from this m/z 45 ion confirm that it represents the hydroxycarbonyl anion [DIAGRAM: SEE TEXT], and not the formate anion [DIAGRAM: SEE TEXT] as sometimes described in the literature. For example, the formate anion is not only defiant to further fragmentation but is also unreactive toward CO2. In contrast, the hydroxycarbonyl anion easily fragments to produce a peak at m/z 17 for the hydroxyl anion, and also readily reacts with CO2 to produce a peak at m/z 61 for the bicarbonate anion. The hydrogen atom in the hydroxycarbonyl anion and that in the formate anion are not mobile within the skeletal framework of the ions, since the two ions did not manifest any interconversion under the conditions and time scales of our mass spectrometric experiments. The other significant product ion peak in the spectra of deprotonated AHAs represents a 46-Da loss. MS/MS data from appropriately deuteriated compounds confirmed that one hydrogen atom from the C-2 position, and the other from the hydroxy group are specifically removed for this loss of elements of formic acid. Moreover, the two oxygen atoms eliminated for the HCOOH loss originate exclusively from the carboxylate group.