1. A method for quantitating a vitamin D analyte in a sample, the vitamin D analyte comprising one or more of vitamin D2, vitamin D3, a metabolite of vitamin D2, and a metabolite of vitamin D3, the method comprising: treating a vitamin D analyte with a dienophile-containing one-step tagging reagent to form a labeled Diels-Alder adduct; and analyzing the labeled adduct using mass spectrometry.