Background: Fatty acids and other metabolites containing a carboxyl group are of high interest
in biomedicine because of their major role in many metabolic pathways and, particularly in the
case of oxidised fatty acids, their high biological activity. Tagging carboxylic acid compounds with a
permanent positive charge such as a quaternary ammonium compound could increase the LC-MS detection
sensitivity and selectivity. This paper describes a new and novel strategy for analysing carboxylcontaining
compounds in biological samples by ESI-MS through coupling to choline.
Methods: Coupling of carboxylic acid derivatives in biological samples was performed by coupling to
2-Fluoro-1, 3 dimethyl –pyridinium (FDMP). The variation in the fatty acid profile of five different
plasma samples was studied and was illustrated by using principal components analysis (PCA) to group
the samples. Orthogonal partial least squares discriminant analysis (OPLS-DA) modelling was then
applied to identify the fatty acids that were responsible for the variation.
Results: The test results showed that choline coupling reactions were successful in detecting fatty acids,
oxidised fatty acids and other compounds containing carboxylic acid groups in biological samples. The
PCA results showed loadings of different fatty acids according to the plasma sample allowing identification
of the fatty acids responsible for the observed variation.
Conclusion: A new and easy tagging method was developed to detect carboxylic acids in plasma samples.
The method proved to be precise and reproducible and can quantify fatty acid compounds to 50