Updated estimates of the selenomethionine content of NIST wheat reference materials by GC–IDMS

Updated estimates of the selenomethionine content of four NIST wheat reference materials have been obtained by use of a revised gas chromatography-stable-isotope dilution mass spectrometric method. The revised method makes use of digestion with methanesulfonic acid, which enables more complete recovery of endogenous selenomethionine than was previously achieved by overnight denaturing treatment in 0.1 mol L(-1) HCl. The NIST wheat reference materials each contain approximately 55% of their total Se content as selenomethionine. Information about forms of Se in reference materials adds value to these materials in Se speciation studies. Estimates of selenomethionine content are also provided for other wheat samples, including several grown under conditions of exposure to high Se levels. These samples also contain approximately 55% of their total Se content as selenomethionine. The consistent level of 55% of total selenium occurring in the form of selenomethionine when the total selenium content varies by a factor of 500 is suggestive of an active mechanism of incorporation of selenium into wheat grain. Figure Selenomethionine content of wheat samples.

LC/UV/MS-MRM for the simultaneous determination of water-soluble vitamins in multi-vitamin dietary supplements

The purpose of this study was to optimize chromatographic and detection conditions for the simultaneous determination of water-soluble vitamins in multi-vitamin dietary supplements using a single chromatographic run. An approach using liquid chromatography with diode array and/or mass spectrometry for quantitation of seven B-complex vitamins [thiamine (B(1)), riboflavin (B(2)), nicotinamide (B(3)), pyridoxine (B(6)) pyridoxine, biotin, pantothenic acid, and folic acid] in multi-vitamin/multi-mineral daily supplements is described. This approach utilizes a reversed phase C18 column (4 mum; i.d.: 250x2.0 mm) with a gradient mobile elution profile, performed at a flow rate of 0.25 ml/min. After a 5-min isocratic elution at 100% A (0.1% formic acid in water), a linear gradient to 50% A and 50% B (0.1% formic acid in acetonitrile) at 15 min and then to 5% A and 95% B at 17 min was employed. Detection was performed with a photodiode array detector (DAD) in sequence with a triple-quad mass spectrometer in the multiple reaction mode (MS-MRM). Although good chromatographic separation of ascorbic acid was also obtained in extracts from multi-vitamin/multi-mineral supplements, the ascorbic acid could not be quantified properly due to rapid oxidation catalyzed by the minerals. This method was initially applied to determine water-soluble vitamins in representative multi-vitamin/multi-mineral tablets following the extraction of ground samples with a phosphate buffer (10 mM, pH 2.5). For multi-vitamin supplement tablets, this approach does not require any sample clean-up/pre-concentration steps except for centrifugation and filtration of the extract.

Progress in development of an integrated dietary supplement ingredient database at the NIH Office of Dietary Supplements

Several activities of the Office of Dietary Supplements (ODS) at the National Institutes of Health involve enhancement of dietary supplement databases. These include an initiative with US Department of Agriculture to develop an analytically substantiated dietary supplement ingredient database (DSID) and collaboration with the National Center for Health Statistics to enhance the dietary supplement label database in the National Health and Nutrition Examination Survey (NHANES). The many challenges that must be dealt with in developing an analytically supported DSID include categorizing product types in the database, identifying nutrients, and other components of public health interest in these products and prioritizing which will be entered in the database first. Additional tasks include developing methods and reference materials for quantifying the constituents, finding qualified laboratories to measure the constituents, developing appropriate sample handling procedures, and finally developing representative sampling plans. Developing the NHANES dietary supplement label database has other challenges such as collecting information on dietary supplement use from NHANES respondents, constant updating and refining of information obtained, developing default values that can be used if the respondent cannot supply the exact supplement or strength that was consumed, and developing a publicly available label database. Federal partners and the research community are assisting in making an analytically supported dietary supplement database a reality.

Selenomethionine contents of NIST wheat reference materials

Values of the total selenium and selenomethionine (Semet) content of four wheat-based reference materials have been obtained by gas chromatography-stable isotope dilution mass spectrometry methods. The total Se method is an established one, and the results obtained with it are consistent with previously-assigned values. The Semet method (previously reported by our laboratory) is based on reaction with CNBr. Our data indicate that the four wheat samples (wheat gluten, durum wheat, hard red spring wheat, and soft winter wheat), though having a 30-fold range in total Se content, all have about 45% of their total Se values in the form of selenomethionine. Investigation of the CNBr-based method suggests that additional experiments are needed to verify that all selenomethionine in the wheat samples is accounted for, but also indicates that the values obtained are within 15% of the true values. As the form in which Se occurs in foods and dietary supplements is important from a nutritional perspective, adding information about Se speciation to total Se values in appropriate reference materials makes these materials more valuable in relevant analytical work.

Determination of total fat in milk- and soy-based infant formula powder by supercritical fluid extraction

Commercially available simple benchtop systems using CO2 supercritical fluid extraction (SFE) eliminate expensive organic solvent disposal problems and offer potential to meet a demand for rapid, accurate high-volume gravimetric determinations of total fat content of infant formula powders. A Data Quality Objectives (DQOs) approach was used to evaluate the performance characteristics of instrumental SFE extraction for determination of total gravimetric fat in infant formula. The established DQOs included the following: ACCURACY: Correct values were obtained for a suitable reference material, SRM 1846 Infant Formula [National Institute of Standards and Technology (NIST), Gaithersburg, MD]. RUGGEDNESS: Variables were defined as (1) extraction time (35 min optimum); (2) ratio of sample size to diatomaceous earth support material (1 g sample/2 g support); (3) ratio of distilled water to alcohol (50% isopropanol optimum for both milk- and soy-based infant formula samples); (4) extraction flow rate was 3-3.5 mL/min optimum. PRECISION: Relative standard deviations of multiple determinations fell within the Horwitz limits of acceptability of < or = 2.8% at the level of analyte determined (0.34-2.5% obtained). SCOPE OF APPLICABILITY: Includes milk- and soy-based infant formula powders. Research data were obtained by use of a commercially available fat analyzer. Samples of the SRM, 2 commercial milk-based and 3 commercial soy-based infant formula products were distributed to 2 additional collaborating laboratories. Very good agreement was obtained among the submitting and collaborating laboratories for these samples. The use of clearly defined DQOs to establish method performance characteristics, along with the commercially available reference material, provided the mechanism for verification and validation of analytical methodology.

Methylseleno-amino acid content of food materials by stable isotope dilution mass spectrometry

Selenium, an important dietary nutrient, is found in many foods. Selenium occurs in various chemical forms including in amino acids with methylselenium functional groups, such as selenomethionine (Semet) and Se-(methyl)selenocystine (Metsecys). We developed a procedure for determining methylselenium in foods such as wheat, a significant dietary source of selenium in the United States. This method is based upon the reaction of cyanogen bromide (CNBr) to cleave the CH3Se-functional group of Semet and Metsecys to form the volatile compound, CH3SeCN. Addition of stable isotope (74Se) enriched selenomethionine to an analytical sample allows direct determination of naturally occurring protein bound Semet by gas chromatography/mass spectrometry (GC/MS), without a protein digestion step, using highly precise stable isotope dilution techniques. We found that a wheat gluten reference material (NIST RM 1818) contains 64% methylselenium of its assigned value of 2.58 micrograms Setotal/g. and that commercial selenium yeast tablets contained 73% of total selenium as methylselenium [147 +/- 10 micrograms Semetse/g (n = 9)]. These two materials would be good candidates for further study and characterization as reference materials for determining this important food component.

Determination of niacin in infant formula by solid-phase extraction/liquid chromatography: peer-verified method performance-interlaboratory validation

This paper reports the results of the interlaboratory peer validation study of AOAC Peer-Verified Method (PVM) 1:2,000 for the determination of niacin in infant formula by solid-phase extraction/liquid chromatography. We have used a Data Quality Objectives (DQO) approach to address not only method variability and robustness but also accuracy of data through the use of an appropriate reference material in conjunction with the interlaboratory validation study. Our DQO included the following: (1) statistical agreement of analytical results and quantitative recovery between 2 collaborating laboratories; (2) the repeatability relative standard deviation (RSDr) values and the HORRAT (Horwitz ratio) obtained (1.07), which satisfied the criteria of the Horwitz "limits of acceptability" at the analyte level present; (3) validation of lack of interference; and (4) accuracy agreement within assigned values for a certified reference material. National Institute of Standards and Technology Standard Reference Material (NIST SRM) 1846 Infant Formula, with a certified value of 63.3 +/- 7.6 microg/g for niacin content, was used as a test material for collaborative study and accuracy assessment. Niacin values obtained by the originating laboratory were 59.7 +/- 4.0 microg/g (95% confidence interval [CI] = 1.4 microg/g with a relative standard deviation [RSD] of 6.7%) and by the peer laboratory were 56.6 +/- 6.6 microg/g (95% CI = 4.1 microg/g, with an RSD of 11.7%). Statistical evaluation using the means equivalence test showed that nicotinic acid values obtained by the peer laboratory were equivalent to those values obtained by the originating laboratory. Linear calibration curves and quantitative recovery were obtained. Integration of the PVM process with a readily available certified reference material gives the user confidence in the accuracy of the data generated by the method through traceability to the reference material used.

Matching reference materials with AOAC International methods of analysis

Proper implementation and use of validated analytical methodology with use of appropriate reference materials (RM) is a preferred means of helping to ensure equivalent analytical method performance in diverse laboratories. Choice of an appropriate RM that not only matches the analyte and matrix of the required determination, but also has been demonstrated to be within the applicability of a specific analytical method, are key factors. In response to numerous requests since its founding in 1993, the Technical Division on Reference Materials (TDRM), AOAC International is implementing a program for recognizing the matching of specific reference materials to specific AOAC methods of analysis. This recognition is accomplished by means of a thorough peer-reviewed selection system, under the auspices of the AOAC official methods board and the executive committee of the TDRM. Potential RM/method matching (RM/MM) proposals will be submitted to an RM/MM committee. After technical review of the suitability of the proposed RM by the RM/MM committee, acceptable matches are recommended for review by the current AOAC process responsible for review and recognition of new methods and modifications to existing AOAC methods of analysis. Several trial matches have been used to develop and test this system. The end product of this effort will ultimately be made available as either a stand-alone document, a section of the AOAC Official Methods of Analysis, or a site within the AOAC web site listing recognized matches.

Selenomethionine content of candidate reference materials

Selenium has been identified as an antioxidant of importance in the diet. Accurate determination of its chemical forms depends on the availability of suitable reference materials (RMs). Two candidate reference materials for determination of selenomethionine (Semet) in food-related materials, a standard wheat gluten sample (NIST RM 8418 Wheat Gluten) and a commercial selenium enriched yeast, have been examined by use of a gas chromatography-isotope dilution mass spectrometry (IDMS) procedure, after treatment of the matrix with 0.1 mol L(-1) hydrochloric acid containing stannous chloride, addition of CNBr, and extraction with chloroform. This procedure results in cleavage of the CH3Se group to form volatile CH3SeCN. Addition of isotopically enriched 74Semet to an analytical sample enables estimation of the naturally occurring protein-bound 80Semet by IDMS without a protein-digestion process. We found that the Wheat Gluten RM contains a significant amount of Semet as a portion of its assigned value of 2.58 microg Se(total g(-1). Commercial selenium yeast tablets are labeled as containing an elevated level of "organic selenium", usually as Semet. The sample we investigated contained 210 microg Se(total) g(-1) sample as determined separately by IDMS, measuring elemental selenium after digestion. 73% of this total (153 +/- 21 microg Se(semet) g(-1); n = 23) was present as Semet. Thus, these two materials contain significant amounts of their total selenium content as Semet and would be good candidates for further study and characterization as reference materials for determining this important food component. The CNBr reaction used will also enable the determination of Se-(methyl)selenocysteine, the biological role of which is of recent interest. In addition to matrix RMs for Semet, it is important to have standard materials of the pure substance. We have examined a sample of a candidate standard material of selenomethionine being prepared by the USP. It was confirmed that this material is pure selenomethionine.

Determination of niacin in infant formula by solid-phase extraction and anion-exchange liquid chromatography

A peer-verified, solid-phase extraction (SPE)/anion exchange liquid chromatographic method is presented for the determination of niacin in milk-based and soy-based infant formula. Analysis is in 3 steps: test sample digestion, extraction/cleanup, and liquid chromatography (LC). Digestion uses a standard AOAC digestion procedure that involves autoclaving at 121 degrees C for 45 min in (1 + 1) H2SO4 to free endogenous niacin from protein and to convert added niacinamide to niacin. The digest solution is adjusted to pH 6.5 with 7.5M NaOH. Acidification to pH <1.0 with (1 + 1) H2SO4 precipitates the protein. The clarified solution is then filtered, and the filtrate is brought to volume. SPE of niacin is accomplished by passing an aliquot of the digest solution through an aromatic sulfonic acid-SPE (ArSCX-SPE) column. After the column is washed with methanol and water to remove extraneous material, the niacin is eluted with 0.25M sodium acetate/acetic acid buffer at pH 5.6. An anion-exchange polystyrene-divinylbenzene column with 0.1 M sodium acetate/acetic acid buffer at pH 4.0 is used for LC. Niacin is determined by UV detection at 260 nm. A standard curve is prepared by passing known amounts of niacin through the ArSCX-SPE columns used for niacin extraction. The following values for x and relative standard deviation (RSD) were obtained for National Institute of Standards and Technology Standard Reference Material (NIST SRM) 1846 Infant Formula with a certified value for niacin of 63.3 +/- 7.6 microg/g: Submitting laboratory.-- x = 59.7 +/- 4.0 microg/g; RSD = >6.7%; confidence interval (CI) = +/- 1.4 microg/g; n = 27. Peer laboratory.--x = 56.6 +/- 6.6 microg/g; RSD = >11.7%; CI =+/- 4.1 microg/g; n = 8.

Determination of niacin in infant formula and wheat flour by anion-exchange liquid chromatography with solid-phase extraction cleanup

Niacin content must be included on food labels of infant formula products and bakery products containing enriched flour. Liquid chromatographic (LC) determination of niacin in complex food matrixes is complicated by the presence of endogenous compounds that absorb at the commonly used wave-length of 260 nm. Also, the presence of particulate matter in the standard sulfuric acid extraction procedure results in reduced life of LC columns and precolumns. A simple, rapid, solid-phase extraction (SPE) procedure for separation and cleanup of niacin from a complex food matrix digest has been developed. By using a vacuum manifold with the SPE column system, multiple samples can be processed quickly and efficiently for LC analysis, compared with gravimetric column cleanup. Sulfuric acid sample digest is passed over an aromatic sulfonic acid cation-exchange (ArSCX-SPE) or a sulfonated Florisil SPE column. Niacin is eluted with 0.25M sodium acetate-acetic acid, pH 5.6 buffer in vacuo. LC chromatograms of the resulting eluate are free of interference from other components absorbing at 260 nm at the retention time of niacin. Validation of the method was obtained from agreement of analytical results on available reference materials. For both SPE methods, values for niacin in SRM 1846 Infant Formula (milk-based powder) were within uncertainty ranges of the certified value. Use of several calibration procedures (the LC computer program, a peak area response graphic standard curve, or the method of standard additions) with both SPE procedures resulted in niacin values for 3 RM-Wheat Flours (not certified for niacin) in agreement (90-105%) with their respective values reported in the literature. Several commercial wheat flours showed a broad 260 nm interference, resulting in high niacin values. Niacin recoveries from spiked soy-based liquid infant formulas ranged from 95-107% with the ArSCX-SPE column. Calibration curves of niacin were linear up to 400 micrograms/mL, with a detection limit of 0.2 microgram/mL.