Fluorescence-labeling reaction of vitamin D metabolites and analogs with fluorescent 1,2,4-triazoline-3,5-dione (DMEQ-TAD)

Comparing derivatization reagents for quantitative LC-MS/MS analysis of a variety of vitamin D metabolites

The present study systematically compares the sensitivity and selectivity of the analysis of multiple vitamin D metabolites after chemical derivatization using different reagents for liquid chromatography-tandem mass spectrometry (LC-MS/MS). Generally, chemical derivatization is applied to vitamin D metabolites to increase the ionization efficiency, which is particularly important for very low abundant metabolites. Derivatization can also improve the selectivity of the LC separation. A wide variety of derivatization reagents has been reported in recent years, but information on their relative performance and applicability to different vitamin D metabolites is, unfortunately, not available in the literature. To fill this gap, we investigated vitamin D₃, 3β-25-hydroxyvitamin D₃ (3β-25(OH)D₃), 3α-25-hydroxyvitamin D₃ (3α-25(OH)D₃), 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃), and 24,25-dihydroxyvitamin D₃ (24,25(OH)₂D₃) and compared response factors and selectivity after derivatizing with several important reagents, including four dienophile reagents (4-phenyl-1,2,4-triazoline-3,5-dione (PTAD), 4-[2-(6,7-dimethoxy-4-methyl-3-oxo-3,4-dihydroquinoxalinyl)ethyl]-1,2,4-triazoline-3,5-dione (DMEQ-TAD), Amplifex, 2-nitrosopyridine (PyrNO)) as well as two reagents targeting hydroxyl groups: isonicotinoyl chloride (INC) and 2-fluoro-1-methylpyridinium-p-toluenesulfonate (FMP-TS). In addition, a combination of dienophiles and hydroxyl group reagents was examined. For LC separations, reversed-phase C-18 and mixed-mode pentafluorophenyl HPLC columns using different compositions of the mobile phase were compared. With respect to detection sensitivity, the optimum derivatization reagent for the profiling of multiple metabolites was Amplifex. Nevertheless, FMP-TS, INC, PTAD, or PTAD combined with an acetylation reaction showed very good performance for selected metabolites. These reagent combinations provided signal enhancements on the order of 3- to 295-fold depending on the compound. Chromatographic separation of the dihydroxylated vitamin D₃ species was readily achieved using any of the derivatization reactions, while for 25(OH)D₃ epimers, only PyrNO, FMP, INC, and PTAD combined with acetylation enabled complete separation. In conclusion, we believe this study can serve as a useful reference for vitamin D laboratories, to help analytical and clinical scientists decide which derivatization reagent to choose for their application.

Analysis of vitamin D metabolic markers by mass spectrometry: Recent progress regarding the "gold standard" method and integration into clinical practice

Liquid chromatography/tandem mass spectrometry is firmly established today as the gold standard technique for analysis of vitamin D, both for vitamin D status assessments as well as for measuring complex and intricate vitamin D metabolic fingerprints. While the actual mass spectrometry technology has seen only incremental performance increases in recent years, there have been major, very impactful changes in the front- and back-end of MS-based vitamin D assays; for example, the extension to new types of biological sample matrices analyzed for an increasing number of different vitamin D metabolites, novel sample preparation techniques, new powerful chemical derivatization reagents, as well the continued integration of high resolution mass spectrometers into clinical laboratories, replacing established triple-quadrupole instruments. At the same time, the sustainability of mass spectrometry operation in the vitamin D field is now firmly established through proven analytical harmonization and standardization programs. The present review summarizes the most important of these recent developments.

Fluorimetric assay of 1 alpha,25-dihydroxyvitamin D3 in human plasma

The first fluorimetric method for assaying plasma 1 alpha,25-dihydroxyvitamin D3 [1,25-(OH)2D3] is described. Lipid extracts from human plasma were successively purified by a Bond Elut NH2 column and a normal-phase HPLC column, and the 1,25-(OH)2D3 fractions obtained were fluorescence-labelled with 4-[2-(6,7-dimethoxy-4-methtyl-3-oxo-3,4-dihydroquinoxalyl)et hyl]-1, 2,4-triazoline-3,5-dione (DMEQ-TAD). The fluorescent adducts of 1,25-(OH)2D3 were analyzed by HPLC on a reversed-phase column, after extensive elimination of the degradation products of excess reagents, by a Bond Elut PSA column followed by a normal-phase HPLC column. The intra- and inter-assay coefficients of variation were within 10% in both assays. The analytical recoveries of standard 1,25-(OH)2D3 added to plasma were quantitative. The present fluorimetric assay was compared with the established radioreceptor assay (RRA) used in measuring plasma 1,25-(OH)2D3 concentration. The regression analysis of the data afforded a relationship y = 1.049x - 2.657 and a correlation coefficient (r) of 0.900.