Despite recent advances in hyphenated systems involving chromatographic and spectrometric techniques, qualitative analysis of complex mixtures with several class of compounds and in large range of concentrations remains a challenge. Retention indices and mass spectral data can be consistently obtained for volatile or volatilizable compounds by GC-MS. However, similar efficiency has not been obtained in cases where LC is the separation technique of choice. The lack of commercial spectral databases and a series of reference compounds with consolidated use in LC make difficult the use of this technique in routine qualitative analysis. Several series of homologous have been proposed for this purpose, each one with a major shortcoming: n-alkyl phenyl ketones (more polar compound of the series is very hydrophobic), alkyl 4-hydroxybenzoates (tR of the minor compound still high) and nitroalkanes (poor UV chromophores to UV detection and poor linear correlation between tR and Cn) (1, 2, 3).

In this context, was evaluated the behavior of alkyl gallates homologous in reverse phase chromatography using linear gradients, in order to use them as reference compounds for achieving LC retention index. These compounds are stable, possess good properties to UV and ESI-MS. Additionally, the minor compound (Gallic acid, n = 0) elutes near the dead time in the used conditions (ACN 5%) while the higher homologue (dodecyl gallate, n = 12), elutes near in the final condition of gradient (100% ACN). Was obtained excellent linearity (r2> 0.99) for the correlation between retention time of the homologous and the number of carbons of the alkyl chain. Examples of IR to different class of natural products will be showed.