Evaporative light scattering detection for in-line monitoring of stopped-flow liquid chromatography-nuclear magnetic resonance analysis of compounds with weak or no chromophore groups

Optimal Conditions for the Direct RP-HPLC Determination of Underivatized Amino Acids with Online Multiple Detection

The combined use of a dual-UV detector as well as a fluorimetric and a multielectrode electrochemical detector (equipped with a dual electrode, consisting of a conventional size 3 mm diameter glassy carbon electrode (GCE) and of a pair of 30 μm thick carbon microfibers) is proposed for the detection of the following 15 underivatized amino acids: L-histidine (His), L-cysteine (Cys), creatine (Crn), S-methyl-L-cysteine (Me-Cys), DL-homocysteine (Hcy), L-methionine (Met), beta-(3,4-dihydroxyphenyl)-L-alanine (DOPA), L-tyrosine (Tyr), DL-m-tyrosine (m-Tyr), L-a-methyl-DOPA (Me-DOPA), L-phenylalanine (Phe), DL-alpha-methyltyrosine (Me-Tyr), 5-hydroxy-tryptophan (5-HTP), 3-nitro-L-tyrosine (NO₂Tyr), and L-tryptophan (Trp), as well as of 2 dipeptides L-cystathionine (Cysta) and L-carnosine (Car), and of creatinine (Cre). A multilinear solvent (acetonitrile) gradient elution program, determined by a simple optimization algorithm, is required for the efficient reversed-phase separation of the above mixture of 18 solutes within 27 min at a flow rate of 1.0 mL/min and at 25 °C.

Optimal conditions for the direct RP-HPLC determination of underivatized amino acids with online multiple detection

The combined use of a dual-UV detector as well as a fluorimetric and a multielectrode electrochemical detector (equipped with a dual electrode, consisting of a conventional size 3 mm diameter glassy carbon electrode (GCE) and of a pair of 30 μm thick carbon microfibers) is proposed for the detection of the following 15 underivatized amino acids: L: -histidine (his), L: -cysteine (cys), creatine (crn), S-methyl-L: -cysteine (me-cys), DL: -homocysteine (hcy), L: -methionine (met), beta-(3,4-dihydroxyphenyl)-L: -alanine (dopa), L: -tyrosine (tyr), DL: -m-tyrosine (m-tyr), L: -a-methyl-dopa (me-dopa), L: -phenylanine (phe), DL: -alpha-methyltyrosine (me-tyr), 5-hydroxy-tryptophan (5htp), 3-nitro-L: -tyrosine (NO(2)Tyr) and L: -tryptophan (trp), as well as of two dipeptides: L: -cystathionine (cysta), L: -carnosine (car), and of creatinine (cre). A multilinear solvent (acetonitrile) gradient elution program, determined by a simple optimization algorithm, is required for the efficient reversed phase separation of the above mixture of 18 solutes within 27 min at a flow rate of 1.0 mL/min and at 25°C.

Direct RP-HPLC determination of underivatized amino acids with online dual UV absorbance, fluorescence, and multiple electrochemical detection

The combined use of a dual-UV detector, a fluorimetric one and of a multiple electrochemical (EC) detector equipped with a dual electrode, consisting of a conventional size 3 mm diameter glassy carbon electrode (GCE) and of a pair of 30 mum thick carbon microfibers, is proposed for the determination of 15 amino acids, two dipeptides and creatinine. This online coupling of the above detection modes could partially replace amino acid analysis by derivatization methods, since it solves problems concerning the direct detection of selected underivatized amino acids. Additionally, it was proved that the use of multiple-detection allows positive peak identification in a single chromatographic run, yields more information for free amino acids and solves in some cases the problem of chromatographic resolution. In order to optimize the detection conditions of the underivatized amino acids and related compounds by different detectors, their detection characteristics were determined by adequate preliminary experiments. The electro-oxidation characteristics of the underivatized compounds of interest were determined by hydrodynamic voltammetry using a flow cell with a macrodisc GCE and by ex-situ voltammetry using both a GCE of conventional size and a carbon fiber disk microelectrode. Important practical advantages of microfiber and microdisk electrodes with respect to macroelectrodes were demonstrated.

HPLC determinations of enantiomeric ratios

Evaporative light-scattering detection (ELSD) is widely applied in the HPLC analysis of organic compounds lacking a UV chromophore. However, this detection method is generally unsuitable for determination of enantiomeric ratios (er). The er calculated from a UV trace and an ELS trace of the same compound differs significantly. Because of the nonlinear concentration response of the ELS detector, a compound with an er of 95:5 appears to be enantiomerically pure by ELS detection. It is possible to obtain a calibration curve and to calculate a correction factor, but this procedure is time consuming and therefore not very practical for routine analyses. In contrast, a charged aerosol detector allows a very accurate determination of the enantiomeric ratios. Like the ELS detection, the CA detection is independent of the chromophore properties of the substrate. Therefore, we recommend the use of CA instead of ELS detection for determination of the enantiomeric ratios of non-UV active compounds.

The simultaneous determination of 1-aminocyclopropane-1-carboxylic acid and cyclopropane-1,1-dicarboxylic acid in Lycopersicum esculentum by high-performance liquid chromatography--electrospray tandem mass spectrometry

Varying concentrations of cyclopropane-1,1-dicarboxylic acid (CDA), an inhibitor of 1-aminocyclopropane-1-carboxylic acid oxidase, added to the solid culture medium of tomato nodal shoot segments resulted in a reduction in the level of endogenous ethylene according to the concentration of inhibitor applied. Following treatment with inhibitor, plants were homogenised and the concentrations of CDA and of 1-aminocyclopropane-1-carboxylic acid (ACC) were measured simultaneously in the resulting juice using an HPLC-ESI/MS-MS method. The levels of CDA and ACC measured in the plant tissues were associated with the concentration of inhibitor added to the solid medium. The HPLC-ESI/MS-MS method described produced limits of detection of 0.8 pmol for ACC and of 4 pmol for CDA.