Laser-based non-fluorescence detection techniques for liquid separation systems

Over the last two decades, the possibility to use lasers for detection purposes in column liquid chromatography (LC) and capillary electrophoresis (CE) received much attention in the analytical chemistry literature. Most attention has been devoted to laser-induced fluorescence. The present review covers developments on non-fluorescence techniques for LC and CE. The techniques considered are thermal lens spectrometry, photoacoustic detection, refractive index detection including refractive index backscattering, Raman spectroscopy and degenerate four-wave mixing (a special mode of transientholographic spectroscopy). The paper starts with an outline of the characteristics of lasers; it ends with an overall evaluation and a discussion of the perspectives of the techniques dealt with.

Methylated Nepsilon-dansyl-L-lysine as a fluorogenic reagent for the chiral separation of carboxylic acids

Dansyl amino acids having a free amino group and an asymmetric carbon atom were examined as a labeling reagent for chiral compounds containing carboxylic moieties to realize enantiomeric separation as well as fluorimetric determination. We tested dansyllysine by reacting it with (+)- and (-)-ibuprofen as a model carboxylic enantiomer. As the intramolecular carboxylic moiety of the dansyl amino acid interfered in the condensation reaction to form an amide bond with the carboxylic acid, the moiety was masked by methylation with trimethylsilyldiazomethane before the reaction. These derivatives were reacted with (+)- and (-)-ibuprofen and better enantiomeric resolution was achieved with methylated dansyllysine on a reversed-phase column. The derivatisation reaction was facilitated by the use of catalysts that are commonly employed in peptides synthesis. The reaction was completed within 5 min at room temperature when diethyl phosphorocyanidate was used. Due to the dansyl moiety, methylated dansyl-lysine enables a sensitive determination of ibuprofen with a fluorescence detector, in addition to the capability of enantiomer resolution. In tests, the detection limits for (+) and (-)-ibuprofen were 4 pmol per injection (S/N=3) at an excitation wavelength of 340 nm and an emission wavelength of 523 nm. Linear responses for the determination of (+) and (-)-ibuprofen in human urine were also demonstrated (r > or = 0.998) in the range from 10 to 1000 ng/ml. The precision and accuracy for urine samples spiked with (+)- and (-)-ibuprofen at 10, 100 and 1000 ng/ml were <10.1 and <14.6% (n=4), respectively.

Determination of carboxylic acid salts in pharmaceuticals by high-performance liquid chromatography after pre-column fluorogenic labelling

2-Bromoacetyl-6-methoxynaphthalene (Br-AMN) was used as a fluorogenic labelling reagent for high-performance liquid chromatographic (HPLC) analyses of pyroglutamic acid, 4-hydroxybutyric acid and thioctic acid in pharmaceutical formulations. The reaction was carried out in aqueous medium in the presence of a cationic surfactant to obtain the direct derivatization of the salts of 4-hydroxybutyric acid and thioctic acid or in acetonitrile for pyroglutamic acid. The resulting naphthacylesters were then chromatographed under reversed-phase (C-18) conditions and detected fluorimetrically (lambda exc, 300 nm; lambda em, 460 nm). The method proved to be suitable for the sensitive and selective analysis of commercial formulations of the cited acidic drugs.

Fluorogenic reactions for biomedical chromatography

A number of fluorogenic reactions, which have been used for HPLC detection systems by means of pre- and/or postcolumn derivatization, are surveyed with respect to both sensitivity and selectivity for the determination of biomedically important substances. For the derivatization of the substances, two types of fluorogenic reactions, fluorescence-generating and fluorescence-tagging, have been studied. The former are usable in most instances for both pre- and postcolumn derivatization methods, and the latter only for precolumn derivatization methods. HPLC methods utilizing the fluorogenic reactions allow analytes to be detected at picomole-subfemtomole levels. In the fluorescence-generating reactions, several fluorogenic reagents possessing two or more reactive sites in the molecule, which show molecular recognition for a variety of analytes, permit facile and reproducible detection in HPLC because there are fewer interferences from biological matrices.