1. A method for determining an amount of one or more vitamin D metabolites in a sample by mass spectrometry, the method comprising:

(i) subjecting the sample and an internal standard to a Cookson-type derivatizing reagent under conditions sufficient to generate

one or more Cookson-type vitamin D metabolite derivatives and one or more Cookson-type internal standard derivatives, wherein

the internal standard comprises at least one of 25OHD2-[6, 19, 19]-2H3, 25OHD2-[26, 26, 26, 27, 27, 27]-2H6, 25OHD3[6, 19, 19]-2H3, 25OHD3-[26, 26, 26, 27, 27, 27]-2H6, 1?,25(OH)2D3-[6, 19, 19]-2H3, 1?,25(OH)2D2-[26, 26, 26, 27, 27, 27]-2H6, 1?,25(OH)2D3-[6, 19, 19]-2H3 and 1?,25(OH)2D2-[6, 26, 26, 27, 27, 27]-2H6;

(ii) subjecting the one or more Cookson-type vitamin D metabolite derivatives and the one or more Cookson-type internal standard

derivatives to turbulent flow liquid chromatography (TFLC);

(iii) ionizing the one or more Cookson-type vitamin D metabolite derivatives and the one or more Cookson-type internal standard

derivative to generate respectively one or more Cookson-type vitamin D metabolite derivative ions and one or more Cookson-type

internal standard derivative ions detectable by mass spectrometry;

(iv) determining an amount of the one or more of the Cookson-type vitamin D metabolite derivative ions and an amount of the

one or more Cookson-type internal standard derivative ions by mass spectrometry; and

(v) relating the amounts of the Cookson-type vitamin D metabolite derivative ions and the one or more Cookson-type internal

standard derivative ions determined in (iv) to the amount of a vitamin D metabolite in the sample.