USA) and a 320-Triple Quad LC–MS2 (Palo Alto, CA, USA). Elution of
compounds was monitored spectrophotometrically at 250 and
270 nm and by APCI+-MS, as reported (Wittig et al., 2013). The vitamin
D isomers were identified according to retention time, UV and
APCI-MS spectra and, wherever possible, by comparisons with
authentic standards.