Analysis of amino acids and related compounds in fruits by CE-LIF and LC-FL

Amino acids are among the most important compounds present in fruits, both for their nutritional value and for the development of aroma: for example, grape amino acids are among the precursors of volatile substances that confer to wines their peculiar bouquet. Moreover, some of them have antioxidant properties by themselves or can be metabolised to health-promoting compounds, either in the fruits or by the human body after intake: tryptophan is a source of serotonin and melatonin, while phenylalanine and tyrosine are precursors of catecholamines and hormones. Of course, even within the same species, the peculiar variety, the region of cultivation and the climate can cause wide differences in the content of amino acids and their derivatives. Thus, their analysis could be a valuable tool in determining the exact origin and/or variety of the fruits, and consequently possibly discovering frauds or certifying authenticity: this is especially true when dealing with transformed products, such as juices, jams and wine. Given these premises, two analytical methods have been developed and compared for the analysis of amino acids and related compounds in red fruits, grape, wines and other beverages of different kind and origin. The two methods are based on different, orthogonal separative techniques (CE vs. LC) and exploit different chemical characteristics for analyte detection (fluorescence of derivatised analytes vs. native fluorescence). In this way, an independent evaluation of each method can be carried out, and different and possibly complementary information can be obtained from the analysis of each sample. The first method is based on CE-LIF (laser-induced fluorescence). Most amino acids do not possess strong chromophores and are thus difficult to quantify by spectrometric means of detection; for this reason, the analytes were derivatised with 5-(4,6-dichloro-triazinyl)aminofluorescein, a fluorescein analogue conferring strong emission intensity to the amino acid derivatives, which is further enhanced by the high energy intensity of the laser source. The second method is based on LC-FL and exploits the native fluorescence of some key amino acids and related compounds to quantify them with good sensitivity and selectivity. The sample pre-treatment is carried out by solid phase extraction (SPE) and analyte separation is achieved on a reversed phase column. The two methods are currently being applied to the analysis of commercial red fruits, derived foodstuffs and wines; both seem to be suitable for the determination of the relevant analytes in these matrices.