Identification and quantification of new isomers of isopropyl-malic acid in wine by LC-IT and LC-Q-Orbitrap

Organic acids of wine comprise one of the main taste groups, that of sourness. Two isomeric compounds with molecular weight of 176 u have been identified in wine using two LC-MS systems, LC-IT (ion trap) and LC-Q-Orbitrap. The two isomers are organic acids, 3-isopropylmalic acid (3-IPMA) never identified in wines, and 2-isopropylmalic acid (2-IPMA), never quantified in wines. After the definitive identification against the authentic standards, an analytical method for their determination in wines was optimised and validated using the LC-IT platform. Linearity was verified in the range 5-320 mg L-1 (correlation coefficients higher than 0.9914) and the recoveries obtained spiking the samples at two fortification levels were higher than 86.7%, with RSDs (n = 9) lower than 15.1%. Finally, the two compounds were quantified in ten red and white Italian wines, and average concentrations were determined at 1.78 mg L-1 (0.56-4.13) and 23.0 mg L-1 (6.7-41.6) of 3-IPMA and 2-IPMA, respectively.