CHAPTER 19:Isotope Dilution Mass Spectrometry for Niacin in Food

Niacin in foods is usually determined using either microbiological methods or liquid chromatography with ultraviolet (UV) detection. Isotope dilution mass spectrometry (IDMS) used with liquid chromatography is also an option for determining niacin and has certain advantages over the more common approaches, such as in selectivity and savings in labour and time of analysis. Isotopically labelled versions of both nicotinic acid and niacinamide are commercially available at a reasonable cost, lowering one of the common barriers to adoption of isotope dilution methods. The method reviewed here is based on acid digestion, solid-phase extraction with a strong cation exchange column and reversed phase chromatography with a C18 column. Ionization is by positive ion electrospray. Analysis in selected ion recording mode is subject to interference problems similar to those found in determinations of niacin using liquid chromatography with UV detection, but the additional selectivity of multiple reaction monitoring (MRM) mode virtually eliminates interference problems. Results are shown for five different food matrices and for appropriate reference materials. Included are milk samples with niacin levels near 1 ppm. The method exhibits good accuracy, based on levels obtained for the reference materials, and relative standard deviations in the range of 0.5% to 5%.