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

Upon collisional activation, gaseous metal adducts of lithium, sodium and potassium oxalate salts undergo an expulsion of CO₂, followed by an ejection of CO to generate a product ion that retains all three metals atoms of the precursor. Spectra recorded even at very low collision energies (2 eV) showed peaks for a 44-Da neutral fragment loss. Density functional theory calculations predicted that the ejection of CO₂ requires less energy than an expulsion of a Na⁺ and that the [Na₃CO ₂]⁺ product ion formed in this way bears a planar geometry. Furthermore, spectra of [Na₃C₂O ₄]⁺ and [³⁹K₃C₂O ₄]⁺ recorded at higher collision energies showed additional peaks at m/z 90 and m/z 122 for the radical cations [Na ₂CO₂]^+• and [K₂CO ₂]^+•, respectively, which represented a loss of an M^• from the precursor ions. Moreover, [Na₃CO ₂]⁺, [³⁹K₃CO₂] ⁺ and [Li₃CO₂]⁺ ions also undergo a CO loss to form [M₃O]⁺. Furthermore, product-ion spectra for [Na₃C₂O₄]⁺ and [ ³⁹K₃C₂O₄]⁺ recorded at low collision energies showed an unexpected peak at m/z 63 for [Na ₂OH]⁺ and m/z 95 for [³⁹K₂OH] ⁺, respectively. An additional peak observed at m/z 65 for [Na ₂¹⁸OH] ⁺ in the spectrum recorded for [Na ₃C₂O₄]⁺, after the addition of some H₂¹⁸O to the collision gas, confirmed that the [Na ₂OH] ⁺ ion is formed by an ion-molecule reaction with residual water in the collision cell.