Complex formation of lactic acid by atmospheric pressure chemical ionization

Oligomers and polymers of lactic acid are generally synthesized through condensation reactions by dehydration at high emperature under catalysis. In the present work, ionization behaviors of lactic acid produced by atmospheric pressure chemical ionization were investigated. Influence of the sample concentration, the heating zone temperature, and the source fragmentor voltage on kinds and abundances of the product ions was examined. Complex formation of the product ions with neutral species was also investigated. Not only lactate, [M–H]⁻ and its complexes but also ions of condended species, [nM–(n-1)H₂O–H]⁻ with n = 2–5, and their complexes were observed. The condensation reactions occurred in an aerosol state generated in the heating zone for evaporation. By increasing the concentration of lactic acid, abundances of the product ions were increased and the increase of larger ones was noticeable. By increasing the heating zone temperature, abundances of the product ions were decreased and the decrease of larger ones was remarkable. By increasing the source fragmentor voltage, abundances of small product ions were increased and those of the complexes, [nM–(n-2)H₂O–H]⁻ with n = 2–5, were significantly decreased. Complex formation of lactate with the neutral condensed species was more favorable than that of the condensed oligomer ions with a neutral lactic acid. The experimental results were explained by energies and structures of the product ions and neutral species obtained by theoretical calculations.