Capillary electrophoresis/visible-LED induced fluorescence of tryptophan: What's new?

Tryptophane (Trp) labelled by 3-(4-carboxybenzoyl)-2-quinolinecarboxaldehyde (CBQCA) is very difficult to identify using CE and fluorescence detection (480 nm). Why in this article some mass spectrometry experiments show that Trp is really labelled by CBQCA as Leucine (Leu)? If the maximum of UV absorption (lambda(max)) is the same between Leu-CBQCA and Trp-CBQCA, the molar extinction coefficient is around 2 fold higher for Trp-CBQCA. The fluorescence of the Leu-CBQCA derivative is 50 times more important than for Trp-CBQCA. The addition of 7.5 mM of beta-cyclodextrin (beta-CD) was found to be a good mean to improve 2.1 fold the sensitivity of the Trp-CBQCA fluorescence. Using a buffer containing SDS and beta-CD in CE, a LOD of 0.7 mu M of L-Trp can be reached and the ratio of the intensities between Leu, Isoleucine, Valine, Trp is 100, 21, 15, 1. Negative (ESIMS)-M-/ and MS/MS of the labeled amino acids show that a loss of the carboxylate function takes place. In the presence of two enantiomers of Trp-CBQCA, we have shown that this decarboxylation is not due to the derivatization process in the solution but rather occurs in the source of the mass spectrometer.