Robert J. Goldschmidt

Simultaneous determination of water-soluble vitamins in SRM 1849 Infant/Adult Nutritional Formula powder by liquid chromatography–isotope dilution mass spectrometry

AbstractAssessing dietary intake of vitamins from all sources, including foods, dietary supplements, and fortified foods, would be aided considerably by having analytical methodologies that are capable of simultaneous determination of several vitamins. Vitamins naturally present in foods may occur in different chemical forms, with levels ranging over several orders of magnitude. Vitamins in dietary supplements and fortified foods, however, are typically added in a single chemical form, and matrix issues are usually not as complex. These sources should thus be relatively amenable to approaches that aim for simultaneous determination of multiple vitamins. Our recent work has focused on development of liquid chromatography (LC)–UV/fluorescence and LC–tandem mass spectrometry methods for the simultaneous determination of water-soluble vitamins (thiamine, niacin, pyridoxine, pantothenic acid, folic acid, biotin, and riboflavin) in dietary supplement tablets and fortified foods, such as formula powders and breakfast cereals. As part of the validation of our methods and collaboration in characterization of a new NIST SRM 1849 Infant/Adult Nutritional Formula powder, we report data on SRM 1849 using isotope dilution mass spectrometric methods. Use of available NIST Standard Reference Materials® as test matrices in our method development and validation gives a benchmark for future application of these methods. We compare three chromatographic approaches and provide data on stability of vitamin standard solutions for LC-based multiple vitamin determinations. FigureExtracted ion chromatograms of seven vitamins using RP chromatography treatment

Determination of selenomethionine in yeast using CNBr derivatization and species specific isotope dilution GC ICP-MS and GC-MS

A comparison of several published and in-house methods for quantitation of selenomethionine (SeMet) in yeast was undertaken using species specific isotope dilution (ID) with a 74Se enriched SeMet. An in-house method was based on digestion of samples by refluxing for 16 h with 4 M methanesulfonic acid, derivatization of SeMet with cyanogen bromide and extraction into chloroform for determination by GC ICP-MS. Concentrations of 3434 ± 19 and 3419 ± 15 μg g−1 (one standard deviation, n = 6) with relative standard deviations of 0.55% and 0.42% for SeMet were obtained in yeast based on measurement of 78Se/74Se and 82Se/74Se ratios, respectively, in agreement with a value of 3417 ± 27 μg g−1 (one standard deviation, n = 6) obtained using ID GC-MS detection based on the ratio of 106/100 of SeCN+ ion. The SeMet accounts for 67% of the total Se in the sample. A method detection limit (three standard deviations) of 0.9 μg g−1 was estimated for SeMet based on a 0.25 g subsample. Significantly lower concentrations of 2220 ± 7 and 2215 ± 9 μg g−1 (one standard deviation, n = 4) with RSDs of 0.30 and 0.41% were obtained by ID GC ICP-MS using 78Se/74Se and 82Se/74Se ratios, respectively, following digestion with 2% SnCl2 in 0.1 M HCl and reaction with CNBr to form volatile CH3SeCN. Similar SeMet concentrations of 3415 ± 200 and 3447 ± 198 μg g−1 (one standard deviation, n = 6) and significantly degraded precisions of 5.86 and 5.74%, respectively, were obtained in yeast using 78Se/74Se and 82Se/74Se ratios, respectively, following digestion with 4 M methanesulfonic acid and derivatization with methyl chloroformate.

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%.