Niacin labelled samples

The method described here makes use of stable isotope dilution mass spectrometry (IDMS) for quantitative analysis of niacin. Isotopically labelled versions of both nicotinic acid and niacinamide are commercially available at a reasonable cost. The use of an isotopically labelled internal standard has distinct advantages in quantitative analysis, as it can correct for analyte losses and makes possible high levels of accuracy and precision (Fassett and Paulson 1989). Sample digestion and clean-up is based on a previously published LC-UV method (LaCroix et al. 1999, 2002 LaCroix and Wolf 2001). This chapter expands on a previous report of LC-IDMS analysis of niacin (Goldschmidt and Wolf 2007), with material from that report used with permission of the publisher. 

Precision obtained for the samples in Table 19.2, expressed as relative standard deviations (RSD), ranges from 0.5% to 2.7%. Mean levels of niacin obtained for the commercial flour and cereal samples are higher than the expected values, but this is not surprising. Due to labelling requirements for fortified foods (Bender et al. 1998), such foods are often fortified in excess of the stated amounts. Results for the flour (RM 8437) and cereal (RM25C) reference... 

Table 19.2 Results of niacin determinations for food samples. Niacin determinations by LC-IDMS are compared to expected values for six grain-derived food materials. Expected levels for the commercial flour and cereal samples (Brands A, B, C, and D) are according to the package labelling, although it should be noted that actual vitamin levels of vitamin fortified foods are often higher than the stated amounts. The determined niacin levels for these samples, listed in the third column with their 95% confidence limits, are from 30% to 70% higher than the claimed levels. Determined niacin levels for the two reference materials (RM) are in agreement with the expected levels. Data are from Goldschmidt and Wolf (2007), with permission from the publisher.

The added, isotopically labelled nicotinic acid and the nicotinic acid deriving from the endogenous niacin of the food sample have virtually identical physical and chemical properties and, by virtue of the digestion step, are assumed to be in a state of equivalence in regard to their chemical and physical environment thereafter in the analysis. 

---