Effects of structural damping on acoustic scattering by flexible plates.

We investigate the effects of structural damping on the interaction of a turbulent eddy with flexible plates with respect to the efficiency of aerodynamic noise generation. Potential benefits are studied using a model based on a point-reacting compliant semi-infinite plate on a spring-damper foundation. This scattering problem is solved using the Wiener-Hopf technique. We compare results for semi-infinite compliant plates with finite ones. In both cases, plate vibration lead to reductions of sound radiation, especially at resonance; damping tends to reduce such acoustic benefits. We also present a formulation that considers the effect of structural damping on the acoustic properties of finite elastic plates. Numerical results are obtained by applying a boundary element method to solve the Helmholtz equation subject to the boundary conditions imposed by the plate vibration. Under specific conditions, such as high fluid loading factor and low bending-wave Mach number, the acoustic power scattered by an edge tends to be smaller than that which propagates over the plate as bending waves. Results show that structural damping attenuates these waves and may modify the far-field acoustic pressure, mostly by reducing the scattered sound at structural resonances. All models show that large damping coefficients lead to locally over-damped responses. There is thus an ideal range of structural damping to reduce both plate vibration and acoustic scattering.

Numerical solution of acoustic scattering by finite perforated elastic plates.

We present a numerical method to compute the acoustic field scattered by finite perforated elastic plates. A boundary element method is developed to solve the Helmholtz equation subjected to boundary conditions related to the plate vibration. These boundary conditions are recast in terms of the vibration modes of the plate and its porosity, which enables a direct solution procedure. A parametric study is performed for a two-dimensional problem whereby a cantilevered perforated elastic plate scatters sound from a point quadrupole near the free edge. Both elasticity and porosity tend to diminish the scattered sound, in agreement with previous work considering semi-infinite plates. Finite elastic plates are shown to reduce acoustic scattering when excited at high Helmholtz numbers k0 based on the plate length. However, at low k0, finite elastic plates produce only modest reductions or, in cases related to structural resonance, an increase to the scattered sound level relative to the rigid case. Porosity, on the other hand, is shown to be more effective in reducing the radiated sound for low k0. The combined beneficial effects of elasticity and porosity are shown to be effective in reducing the scattered sound for a broader range of k0 for perforated elastic plates.

Certification of vitamins and carotenoids in SRM 3280 multivitamin/multielement tablets.

A new multivitamin/multielement dietary supplement Standard Reference Material (SRM) has been issued by the National Institute of Standards and Technology (NIST), with certified and reference concentration values for 13 vitamins, 24 elements, and 2 carotenoids. The constituents have been measured by multiple analytical methods with data contributed by NIST and by collaborating laboratories. This effort included the first use of isotope dilution mass spectrometry for value assignment of both fat-soluble vitamins (FSVs) and water-soluble vitamins (WSVs). Excellent agreement was obtained among the methods, with relative expanded uncertainties for the certified concentration values typically ranging from <2% to 15% for vitamins.

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.

Certification of nutrients in Standard Reference Material 1846: infant formula.

In 1996, the National Institute of Standards and Technology (NIST) released Standard Reference Material 1846 (Infant Formula), which can be used as a control material for assigning values to in-house control materials and for validating analytical methods for measurement of proximates, vitamins, and minerals in infant formula and similar matrixes. The SRM was manufactured by preparing a spray-dried formula base containing fat, protein, carbohydrates, and minerals and then combining that formula base with a dry-blend vitamin premix that supplied the vitamins. The Certificate of Analysis for SRM 1846 provides assigned values for concentrations of proximates (fat, protein, etc.), vitamins, and minerals for which product labeling is required by the Infant Formula Act of 1980 and by the Nutrition Labeling and Education Act of 1990. These assigned values were based on agreement of measurements by NIST and/or collaborating laboratories. Certified values are provided for vitamins A (trans), E, C, B2, and B6 and niacin. Noncertified values are provided for solids, ash, fat, nitrogen, protein, carbohydrate, calories, vitamin D, delta-tocopherol, gamma-tocopherol, vitamin B1, vitamin B12, folic acid, pantothenic acid, biotin, choline, inositol, calcium, phosphorus, magnesium, iron, zinc, copper, sodium, potassium, and chloride. Information values are provided for iodine, manganese, selenium, and vitamin K.

Utilization of chromatographic and spectroscopic techniques to study the oxidation kinetics of selenomethionine.

Ion-exchange LC and spectroscopic supporting techniques have been successfully used to study the kinetics and mechanism of oxidation reactions of selenomethionine (SeMet). Oxidation of selenomethionine with both cyanogen bromide (CNBr) and hydrogen peroxide (H(2)O(2)) proceeds through a stable intermediate which undergoes cyclization and C-Se bond cleavage to form 2-amino-4-butyrolactone. This stable intermediate was identified by IR spectroscopy as methionine dihydroxy selenide. The CH(3)-Se moiety of SeMet formed methyl selenic acid upon reaction with H(2)O(2) and methyl selenocyanate (CH(3)SeCN), characterized by GC-MS, for the reaction with CNBr. Both reactions were of apparent first order with respect to the concentration of SeMet. A rate constant (k(1))of 4.0x10(-3) s(-1) for the reaction of SeMet with HO and 4.0x10(-3) s(-1) for the reaction with CNBr were determined at a temperature of 22 degrees C. Oxidation of methionine (Met) gives disparate kinetics and oxidation products from SeMet. Thus the differential rate method can be utilized to quantitatively separate SeMet in biological samples in the presence of much higher concentrations of Met.

Synthesis of a selenomethionine peptide and a preliminary study of transport into Escherichia coli monitored by high-performance liquid chromatography.

The tripeptide Gly-SeMet-Gly has been synthesized by a combination of solution and solid-phase methods. Increase in weight of the resin was very nearly theoretical, and purification was straightforward. Its absorption was compared to that of the corresponding peptide, Gly-Met-Gly, in E. coli using HPLC ion-exchange separation and fluorometric determination of the disappearance of peptides in the culture medium and the appearance of methionine and selenomethionine in the same culture medium. As E. coli are not known to possess extracellular peptidases, and in fact have been shown to possess transport systems for peptides, this absorption is interpreted as transport of the peptide through the cell wall and membrane into the cytoplasm, endohydrolysis of the peptide, and efflux of the peptides' amino acids. Uptake of both peptides was approximately equal, but was slowed when both peptides were present simultaneously.

Optimization of an Escherichia coli formate dehydrogenase assay for selenium compounds.

A microbiological assay to detect different chemical compounds of selenium for potential future use in the study of the distribution of these chemical forms in foods is being developed. This assay is based on the detection, by infrared analysis, of CO2 in a culture of Escherichia coli when the bacteria are grown in the presence of various selenium compounds. The CO2 production is the result of selenium-dependent formate dehydrogenase activity, which catalyzes oxidation of formic acid produced during glucose metabolism. Smooth response curves were generated over several orders of magnitude for selenocystine, selenite, and selenomethionine. The assay detects selenium concentrations (above background) as low as 1.5 nM for selenocystine and selenite and 4 nM for selenomethionine in minimal medium. Detection of selenomethionine was enhanced (to a sensitivity of 1.5 nM) by the addition of methionine to minimal medium and was enhanced even further (to a sensitivity of 0.8 nM) by the addition of a defined mixture of amino acids. Selenomethionine could be assayed in the presence of an amino acid concentration which is proportional to the amino acid/elemental selenium ratio found in a wheat gluten reference material (NIST SRM 8418). This implies that the assay can detect selenium compounds in a variety of foods at low concentrations, avoiding the background CO2 production caused by high concentrations of non-selenium-containing amino acids. The observation that methionine enhanced selenomethionine availability for formate dehydrogenase synthesis supports studies in animals demonstrating that methionine controls selenomethionine incorporation into selenoenzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Potential application of LASER/microbe bioassay technology for determining water-soluble vitamins in foods.

A microbiological technique was developed for quantitating niacin by determining microbial growth rates in response to the amount of vitamin available. Unlike the current official AOAC method, the new procedure for niacin measured the growth rates during the early exponential growth phase rather than during the stationary phase. Lactobacillus plantarum was used to determine niacin to a lower limit of 100 pg/mL. The assay time was approximately 6 h, compared with 16-24 h for the current AOAC method. The extent of microbial growth was determined by differential light scattering of a LASER beam. Multiple photodetectors were integrated with a computer system to collect and analyze the data. The use of differential light scattering to determine 8 water-soluble vitamins under stationary phase conditions demonstrated the potential application of the new technology for microorganisms and foods.

Selenium content of a core group of foods based on a critical evaluation of published analytical data.

References published since 1960 that report analyses of selenium in foods were collected and evaluated according to criteria in five categories: number of samples, analytical method, sample handling, sampling plan, and analytical quality control. Data were grouped by food item and rated according to the criteria that had been developed specifically for evaluating the quality of selenium data. Ratings assigned to the data from each study yielded a Quality Index, indicating which data would be included in the calculation of the mean selenium value for each food item. The Quality Indexes for acceptable studies were summed to determine a Confidence Code, intended to indicate the relative degree of confidence the user can have in each mean selenium value. The selection of selenium core foods was based on their selenium concentration and frequency of consumption. Foods were ranked by multiplying selenium concentration by the amount consumed by the 36,255 individuals who provided 3-day dietary intake data in the U.S. Department of Agriculture's 1977-78 Nationwide Food Consumption Survey. Mean, minimum, and maximum selenium values, Confidence Codes, ranks, and references have been compiled for 114 food items. The five most highly ranked food aggregates (beef, white bread, pork, chicken, and eggs) provided half of the selenium accounted for in the diets of the survey respondents.

Preparation of a mixed human diet material for the determination of nutrient elements, selected toxic elements and organic nutrients: a preliminary report.

Using 201 foods from the United States Food and Drug Administration's Total Diet Study (FDA TDS), a mixed diet composite (USDIET-I) was prepared to represent the intake of 25-30-year-old males in the United States. Proximate analyses, phytate determination, and assays for nutrient elements and selected toxic elements, as well as organic nutrients were carried out on this composite. As part of a quality control exercise for a coordinated research program, atomic absorption spectrophotometry, inductively coupled atomic emission spectrometry, colorimetry and neutron activation analysis were used to determine up to 30 elements in this diet material. A comparison of the daily intakes of As, Ca, Cd, Cu, Fe, Hg, K, Mg, Mn, Na, P, Se and Zn from the composite USDIET-I shows excellent to good agreement with FDA TDS values calculated from results of single food analyses. These USDIET-I results demonstrate the feasibility of the mixed diet concept as a viable approach for a reliable assessment of daily intakes, especially for a number of elements such as Cd, Cr, Hg and Mo that occur at low concentrations in individual food products. Simultaneously, stability of some organic nutrients during storage was also investigated. Initial findings suggest that this program may also be useful in the development of reference materials for organic nutrients, for which there is a great need. These aspects are discussed.

Selenium in self-selected diets of Maryland residents.

The mean selenium level in six self-selected, daily diets chosen by each of 22 free-living Maryland residents was 81 +/- 41 microgram. per day. Diets that were higher in selenium also were significantly higher in copper, iron, magnesium, and manganese and slightly higher in zinc. However, in general, the levels of these minerals were low, probably because of low consumption of grains, legumes, and nuts. Diets higher in selenium contained more fish and less processed meat than other diets.

Zinc and copper in self-selected diets.

Levels of zinc and copper in the self-selected diets of twenty-two men and women were determined by atomic absorption spectrophotometry. Sixty-eight per cent of the subjects consumed less than two-thirds of the recommended allowance for zinc (15 mg). Eighty-one per cent consumed less than two-thirds of the suggested level of copper (2.0 mg). Average daily intake of protein exceeded the recommended allowance. However, the mean zinc:calorie ratio for those subjects was only 4.19 mg. per 1,000 kcal. Using the recommended allowances for zinc and for energy, the zinc:calorie ratio would be 6.5 mg. per 1,000 kcal. For copper, the suggested nutrient density is 0.85 mg. per 1,000 kcal vs. the 0.50 mg. per 1,000 kcal observed in this study. These metallo:calorie ratios indicate that the nutrient density of these two trace elements in the diet is low and that diets supplying adequate amounts of energy and protein do not guarantee adequate levels of zinc and copper.