Description Nicotinic acid (pyridine-3-carboxylic acid) is also known as niacin or vitamin B3. It is a water-soluble vitamin found in some vegetable and animal sources. Niacin is also a precursor for the synthesis of NAD and NADP.Nicotinic acid-(ring-d4) can be used as an internal standard for the quantification of nicotinic acid in the biological sample by gas chromatography-mass spectrometry (GC-MS). It can be also used as a precursor for the preparation ofN′-nitrosonornicotine-2,4,5,6-d4derivative.