Читать книгу Vitamin D in Clinical Medicine - Группа авторов - Страница 31

Vitamin D is seldom measured in the blood clinically, but methods for its analysis have been developed for the food industry. Vitamin D is the most lipophilic of the compounds we will consider. Relative to 25(OH)D it has reduced affinity for DBP, and is cleared within hours from the blood, presumably deposited into fat tissues. Typical methods include saponification of the sample with organic extraction and high performance liquid chromatography (HPLC) to separate D₂ and D₃. These peaks are detected and quantitated by UV absorption with a diode array detector (LC-UV) or mass spectrometry (LC-MS) [36]. The latter detection system is more specific and less sensitive to interfering substances, but ionization of vitamin D required for MS is limited thus affecting precision. This plus lack of a universal standard has resulted in wide variation between laboratories in measuring vitamin D. More recently, however, the situation has improved with the use of a triple quadrupole tandem mass spectrometer equipped with an atmospheric pressure photo ionization source that enhances the ionization of D versus earlier chemical methods (atmospheric pressure chemical ionization or APCI and electrospray ionization [ESI]) [37]. This method was used to measure D₂, D₃, 25(OH)D₂, and 25(OH)D₃ in the same sample with limits of quantitation (LOQ) for each D of 2 ng/mL and for each 25(OH)D of 1 ng/mL [38].