Quantification of vitamin D2, vitamin D3, and the monohydroxy and diihydroxy metabolites of vitamin D2 and vitamin D3, can comprise labeling analytes with mass spectrometry (MS) tagging reagents and performing LC-MSMS analysis of the labeled analytes. The labeled analytes can include a labeled standard and can have distinct retention times on a reversed phase column, as well as distinct masses. Under high energy collisions, reporter groups can be generated. The intensity or the peak area detected for each reporter group can be used for quantitation. In some embodiments, a one-step tagging reagent is used that is a dienophile-containing, labeled Diels Alder reagent.
