I. Introduction
Vitamin D is a group of fat soluble vitamins, chemically related to secosteroids, which promotes intestinal absorption of calcium and phosphate. Vitamin D insufficiency plays a significant role in occurrence of musculoskeletal diseases such as osteoporosis and osteomalacia. Moreover, as discovered newly, higher normal blood level of vitamin D can decrease prevalence of conditions such as cancer, diabetes, cardiovascular and autoimmune diseases [1]–[4]. Hence there is an ever growing request for easy, handy and sensitive methods for determination of vitamin D metabolites in medical and nutritional research.