Characterization of NIST food-matrix standard reference materials for their vitamin C content

The vitamin C concentrations in three food-matrix Standard Reference Materials (SRMs) from the National Institute of Standards and Technology (NIST) have been determined by liquid chromatography (LC) with absorbance detection. These materials (SRM 1549a Whole Milk Powder, SRM 1849a Infant/Adult Nutritional Formula, and SRM 3233 Fortified Breakfast Cereal) have been characterized to support analytical measurements made by food processors that are required to provide information about their products’ vitamin C content on the labels of products distributed in the United States. The SRMs are primarily intended for use in validating analytical methods for the determination of selected vitamins, elements, fatty acids, and other nutrients in these materials and in similar matrixes. They can also be used for quality assurance in the characterization of test samples or in-house control materials, and for establishing measurement traceability. Within-day precision of the LC method used to measure vitamin C in the food-matrix SRMs characterized in this study ranged from 2.7 % to 6.5 %.     

Iodine in different food articles and standard reference materials

Summary The greater part of essential iodine enters living organisms via the food chain. Nevertheless, quantitative data on its concentration in diets, food articles and also in available SRMs are very poor and scarce. This and WHO recommendations on daily allowances of iodine for man via food articles caused an added demand for accurate and reliable determination of iodine in these samples. From this point of view the purpose of the present was to analyse and to establish the concentration levels of total iodine in some food articles, diets, SRMs and candidate reference materials by the use of rapid radiochemical separation, developed in our laboratory. The results were checked by the analysis of SRMs with available certified values for iodine and good agreement is evident.     

Comparison of NAA methods to determine medium-lived radionuclides in NIST soil standard reference materials

In order to determine the elemental concentrations of three new soil standard reference materials SRMs 2709, 2710 and 2711 from the National Institute of Standards and Technology (NIST), a comparative study of different medium-lived neutron activation analysis methods was carefully performed. Three irradiation conditions (1-hour thermal, 1-hour epithermal and 5-minute epithermal) and two counting modes (normal and Compton suppression) have been evaluated for following ten elements: As, Au, Cd, Ga, K, La, Mo, Sb, Sm, and W. The results show that the method of 5-minute epithermal and a 1-day decay is the optimum way to analyze Ga, while the addition of the Compton suppression is very beneficial for the determination of K. Using the Compton suppression system in conjunction with the 1-hour epithermal and a 1-week decay, is ideal to determine Au, Cd, Mo and W, while routine 1-hour thermal and a 1-week decay, is adequate to determine As, Sb, Sm and La in concentrations found in soil.     

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.     

Isotope-ratio measurements of lead in NIST standard reference materials by multiple-collector inductively coupled plasma mass spectrometry

The capability of a second-generation Nu Instruments multiple collector inductively coupled plasma mass spectrometer (MC-ICP-MS) has been evaluated for precise and accurate isotope-ratio determinations of lead. Essentially the mass spectrometer is a double-focusing instrument of Nier-Johnson analyzer geometry equipped with a newly designed variable-dispersion ion optical device, enabling the measured ion beams to be focused into a fixed array of Faraday collectors and an ion-counting assembly. NIST SRM Pb 981, 982, and 983 isotopic standards were used. Addition of thallium to the lead standards and subsequent simultaneous measurement of the thallium and lead isotopes enabled correction for mass discrimination, by use of the exponential correction law and 205Tl/203Tl = 2.3875. Six measurements of SRM Pb-982 furnished the results 206Pb/204Pb = 36.7326(68), 207Pb/204Pb = 17.1543(30), 208Pb/204Pb = 36.7249(69), 207Pb/206Pb = 0.46700(1), and 208Pb/206Pb = 0.99979(2); the NIST-certified values were 36.738(37), 17.159(25), 36.744(50), 0.46707(20), and 1.00016(36), respectively. Direct isotope lead analysis in silicates can be performed without any chemical separation. NIST SRM 610 glass was dissolved and introduced into the MC-ICP-MS by means of a micro concentric nebulizer. The ratios observed were in excellent agreement with previously reported data obtained by TIMS and laser ablation MC-ICP-MS, despite the high Ca/Pb concentration ratio (200/1) and the presence of many other elements at levels comparable with that of lead. Approximately 0.2 microg lead are sufficient for isotope analysis with ratio uncertainties between 240 and 530 ppm.     

Results of the ASTM Nuclear Methods intercomparison on NIST apple and peack leaves standard reference materials

The ASTM Task Group on Nuclear Methods of Chemical Analysis (E10.05.12) has conducted a trace element intercomparison among some of its members over the past two years. Eight non-NIST laboratories submitted data using nuclear techniques, with a total of 111 values for the apple leaves and 116 values for the peach leaves, on 46 and 50 elements, respectively. This intercomparison provided a unique opportunity for the analytical laboratories, because the analytical values submitted could be later compared to the NIST certified values. For the seven elements which were certified by NIST and had three or more intercomparison values, the results showed that: 1) 61% of all 56 intercomparison values submitted had results whose stated uncertainty overlapped the uncertainty limits of the NIST certified values, and 2) less than 6% of the intercomparison values had means which fell outside ±20% of the NIST values. In general, the intercomparison values submitted showed excellent agreement with the NIST values. However, many reported uncertainties accompanying intercomparison values appeared overly optimistic.     

NIST reference materials to support accuracy in drug testing

Substance abuse is a major problem worldwide. There is considerable emphasis placed upon testing individuals for evidence of use of controlled substances. Because the consequences of a positive test can be quite severe, laboratories conducting such tests must rigorously follow a carefully designed quality assurance program. Such a QA program should include use of reference materials to assure that the methods used to detect and quantify drugs are providing accurate results. The National Institute of Standards and Technology (NIST) supports accuracy in drugs of abuse testing by providing Standard Reference Materials (SRMs) with certified concentrations of drugs of abuse in urine- and hair-based reference materials. NIST, working in collaboration with the College of American Pathologists (CAP), has developed urine-based SRMs for marijuana metabolite, cocaine metabolite, morphine and codeine, and morphine glucuronide and CAP Reference Materials for amphetamines and phencyclidine. Certification measurements performed at NIST involve two independent methods for each analyte, one of which always uses GC/MS with the other usually being an LC method with either MS or UV detection. Work has recently been completed on a seven component drug in urine SRM. In addition NIST conducts research in the analysis of hair for drugs of abuse. To assist laboratories testing hair for that purpose, NIST has developed two drugs in hair reference materials.College of American Pathologists Research Associate at NIST     

Certification of a new NIST fly ash standard reference material

In June 1993 the National Institute of Standards and Technology (NIST) released the third version of Constituent Elements in Coal Fly Ash Standard Reference Material (SRM 1633b). This material is intended for quality assurance purposes in evaluating the analytical methods used for the determination of constituent elements in coal fly ash or in materials with similar matrices. It has been certified for 23 major, minor and trace elements using ten different analytical techniques. For an element to be certified in a NIST SRM, its concentration is usually determined by at least two independent analytical techniques. The concentrations of additional 24 elements are provided for information only purposes in the new fly ash. Current plans are to certify the concentrations of a number of rare earths upon completion of additional analytical work now in progress. Homogeneity testing was accomplished using instrumental neutron activation analysis and X-ray fluorescence. This presentation summarizes the preparation of this new material and the analytical results used for certification.     

The NIST natural-matrix radionuclide standard reference material program for ocean studies

In 1977, the Low-level Working Group of the International Committee on Radionuclide Metrology met in Boston, MA (USA) to define the characteristics of a new set of environmental radioactivity reference materials. These reference materials were to provide the radiochemist with the same analytical challenges faced when assaying environmental samples. It was decided that radionuclide bearing natural materials should be collected from sites where there had been sufficient time for natural processes to redistribute the various chemically different species of the radionuclides. Over the succeeding years, the National Institute of Standards and Technology (NIST), in cooperation with other highly experienced laboratories, certified and issued a number of these as low-level radioactivity Standard Reference Materials (SRMs) for fission and activation product and actinide concentrations. The experience of certifying these SRMs has given NIST the opportunity to compare radioanalytical methods and learn of their limitations. NIST convened an international workshop in 1994 to define the natural-matrix radionuclide SRM needs for ocean studies. The highest priorities proposed at the workshop were for sediment, shellfish, seaweed, fish flesh and water matrix SRMs certified for mBq per sample concentrations of ⁹⁰ Sr, ¹³⁷ Cs and ²³⁹ Pu + ²⁴⁰ Pu. The most recent low-level environmental radionuclide SRM issued by NIST, Ocean Sediment (SRM 4357) has certified and uncertified values for the following 22 radionuclides: ⁴⁰ K, ⁹⁰ Sr, ¹²⁹ I, ¹³⁷ Cs, ¹⁵⁵ Eu, ²¹⁰ Pb, ²¹⁰ Po, ²¹² Pb, ²¹⁴ Bi, ²²⁶ Ra, ²²⁸ Ra, ²²⁸ Th, ²³⁰ Th, ²³² Th, ²³⁴ U, ²³⁵ U, ²³⁷ Np, ²³⁸ U, ²³⁸ Pu, ²³⁹ Pu + ²⁴⁰ Pu, and ²⁴¹ Am. The uncertainties for a number of the certified radionuclides are non-symmetrical and relatively large because of the non-normal distribution of reported values. NIST is continuing its efforts to provide the ocean studies community with additional natural matrix radionuclide SRMs. The freeze-dried shellfish flesh matrix has been prepared and recently sent to participating laboratories for analysis and we anticipate receiving radioanalytical results in 2000. The research and development work at NIST produce well characterized SRMs that provide the world's environment-studies community with an important foundation component for radionuclide metrology.     

A review of the DNA standard reference materials developed by the National Institute of Standards and Technology

The Standard Reference Materials Program at the US National Institute of Standards and Technology (NIST) has three human DNA standard reference materials (SRM 2390, SRM 2391a, and SRM 2392) currently available¹ (Orders and requests for information concerning these SRMs should be directed to the Standard Reference Materials Program, National Institute of Standards and Technology, 100 Bureau Drive, Stop 2321, Gaithersburg, MD 20899-2321, Telephone (301) 975-6776, FAX: (301) 948-3730.) [1, 2]. Both the DNA profiling SRM 2390 and the polymerase chain reaction (PCR)-based DNA profiling SRM 2391a are intended for use in forensic and paternity identifications, for instructional law enforcement, or for non-clinical research purposes and are not intended for clinical diagnostics. The mitochondrial DNA (mtDNA) SRM 2392 is to provide standardization and quality control when performing PCR and sequencing any segment or the entire 16,569 base pairs that comprise human mitochondrial DNA. SRM 2392 is designed for use by the forensic, medical, and toxicological communities for human identification, disease diagnosis or mutation detection.     

A review of the DNA standard reference materials developed by the National Institute of Standards and Technology

The Standard Reference Materials Program at the US National Institute of Standards and Technology (NIST) has three human DNA standard reference materials (SRM 2390, SRM 2391a, and SRM 2392) currently available [1, 2]. Both the DNA profiling SRM 2390 and the polymerase chain reaction (PCR)-based DNA profiling SRM 2391a are intended for use in forensic and paternity identifications, for instructional law enforcement, or for non-clinical research purposes and are not intended for clinical diagnostics. The mitochondrial DNA (mtDNA) SRM 2392 is to provide standardization and quality control when performing PCR and sequencing any segment or the entire 16,569 base pairs that comprise human mitochondrial DNA. SRM 2392 is designed for use by the forensic, medical, and toxicological communities for human identification, disease diagnosis or mutation detection.     

Determination of arsenic in food and dietary supplement standard reference materials by neutron activation analysis

Arsenic was measured in food and dietary supplement standard reference materials by neutron activation analysis for the purpose of assigning certified or reference As mass fractions and to assess material homogeneity. Instrumental neutron activation analysis was used to value assign As in candidate SRM 3532 Calcium Dietary Supplement and candidate SRM 3262 Hypericum perforatum (St. John’s Wort) Aerial Parts down to about 100 μg/kg. Values were also determined for two additional candidate St. John’s Wort SRMs with As mass fractions <100 μg/kg. The presence of significant amounts of ²⁴Na and ⁸²Br limited the reproducibility of the method below 100 μg/kg. For measurement of lower As mass fractions, a radiochemical neutron activation analysis method with extraction of As³⁺ into diethyl-dithiocarbamate in chloroform and detection limits down to 0.1 μg/kg. As was used to value-assign As mass fractions for SRM 3280 Multivitamin/Multielement Tablets and for candidate SRM 3233 Fortified Breakfast Cereal, and at <10 μg/kg in candidate SRM 1845a Whole Egg Powder.     

中红外光谱透射比标准物质研制

采用物化性质稳定的单晶硅作为基质,以直流磁控溅射方式轻蒸镀不同厚度的镍、铬混合膜层,实现了中红外波段(500~4000 cm^–1)不同透射比标准物质的研制。研制的中红外光谱透射比标准物质其透射比标称值分别为10%和50%,扩展不确定度优于0.008(k=2)。该标准物质具有量值稳定、光谱平坦等特点,可用于各类傅里叶变换红外光谱设备的透射比校准。     

Temperature standard reference materials for thermal analysis

The feasibility of the use of potassium nitrate and potassium perchlorate as temperature standards in Differential scanning calorimetry has been studied. The solid-state phase transition temperatures of KNO₃ and KClO₄ were determined by means of DSC. The metrological properties of these salts as calibration materials were examined. The reliability of KNO₃ and KClO₄ calibrations was investigated by twofold determination of the bismuth melting temperature after the apparatus had been calibrated with indium and lead, and with KNO₃ and KCl_O4. Conclusions were drawn concerning the suitability of these salts for use as DSC temperature calibrants.

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Zusammenfassung Es wurde die Möglichkeit der Verwendung von Kaliumnitrat und Kaliumperchlorat als Temperaturstandard für DSC untersucht. Die Bestimmung der Temperaturen für Feststoff-Phasenumwandlungen von Kaliumnitrat und Kaliumperchlorat wurde mittels DSC erreicht. Weiterhin wurden die meßtechnischen Eigenschaften der untersuchten Salze (KNO₃ und KClO₄) als Bezugssubstanzen geprüft. Die Zuverlässigkeit der Kalibrierung mittels KNO₃ und KClO₄ wurde durch eine zweifache Bestimmung der Schmelztemperatur von Wismut überprüft: einmal nach der Kalibrierung des Gerätes mit Indium und Blei und andererseits nach der Kalibrierung mit Kaliumnitrat und Kaliumperchlorat. Angesichts der Resultate kann man feststellen, daß die untersuchten Salze zur Temperaturkalibrierung in der DSC geeignet sind.

     

Long-term stability of hydrocarbons in NIST gas standard reference material (SRM) 1800

A gas standard reference material (SRM) containing fifteen hydrocarbons in nitrogen at a nominal 5 nmol mol(-1) was issued in 1993. The certification period for SRM 1800 was assigned as 2 years, because of limited stability data. Over a period of 10 years reanalysis of the lot standard (a sample chosen from the SRM lot to which all other lot samples are compared), SRM samples remaining in stock for sale, and SRMs returned to the National Institute of Standards and Technology (NIST) for recertification, were compared with primary standards to assess the stability of the hydrocarbons. New primary standards were periodically introduced into the original primary standard suite to assess the stability and consistency of the primary standards. Over this ten-year period 11 SRM 1800 samples were reanalyzed, resulting in 210 amount-of-substance fraction (concentration) determinations performed for quality-assurance purposes. Of these measurements 209 (99.5%) agreed within the original 95% confidence interval of the +/-4% expanded uncertainty, demonstrating the stability of the standards. There was also agreement to within +/-2% of the original concentration for 204 (97%) of the measurements. This is well within the original +/-4% expanded uncertainty assigned to the hydrocarbon concentrations at the approximate 95% confidence interval demonstrating stability. These results will enable the expiry date to be increased for future restock issues of SRM 1800.     

Global perspectives for secondary reference materials for analysis of food and related biological samples

Primary, secondary and tertiary reference materials (RM) play an important role in quality controls of analytical measurements. Logistics of preparation and proper use of primary and secondary RMs are presented. Tertiary (i.e. in-house) control materials are useful as substitutes in the absence of recognized primary or secondary RMs. The lack of interdisciplinary interaction during development of RMs (e.g. in specific areas such as foods), has an important impact on limiting the usefulness of certain types of RMs. The abundance of RMs in some countries and regions appears to have little effect on the existing paucity in RMs in other regions, and the underlying causes are outlined. The ability of a laboratory to produce good quality in-house RMs traceable to recognized primary or secondary RMs is a direct measure of its quest for reliable analytical data. Therefore many laboratories should be encouraged to engage in secondary and tertiary RM activities designed to answer specific measurement problems. In this context, assistance (e.g. practical training opportunities) in identifying simple methods of analyses for their efficacy in determining specific analytes is a source of help that can be extended to countries experiencing limitations in laboratory instrumentation.     

Particle size determination of some IAEA and NIST environmental and biological reference materials

Particle size distributions in fifteen International Atomic Energy Agency (IAEA) and sixteen National Institute of Standards and Technology (NIST) reference materials (RMs) were measured with the basic aim to investigate the potential of these materials to be used as reference or quality control materials in analyses where a small sample mass is required (< 100 mg). Most of the investigated materials are commercially available environmental or biological natural matrix RMs with certified values for trace elements, radionuclides, or organometallic compounds. The laser diffraction technique was used in all measurements. From the point of particle size distribution, materials IAEA-390 (Algae, a set of three materials), IAEA-396m (Urban Dust, 3x air jet milled), NIST-SRM 1515 (Apple Leaves), NIST-SRM 1547 (Peach Leaves), NIST-SRM 1566a (Oyster Tissue), NIST-SRM 1570a (Spinach), NIST-SRM 1573a (Tomato Leaves), and NIST-SRM 1648 (Urban Particulate) can be considered appropriate for small sample mass analysis. However, additional analytical tests are needed to confirm the appropriate homogeneous distribution of chemical composition, the level of heterogeneity for individual elements, at this sample mass level.     

Value assignment of nutrient concentrations in five standard reference materials and six reference materials

A number of food-matrix reference materials (RMs) are available from the National Institute of Standards and Technology (NIST) and from Agriculture Canada through NIST. Most of these materials were originally value-assigned for their elemental composition (major, minor, and trace elements), but no additional nutritional information was provided. Two of the materials were certified for selected organic constituents. Ten of these materials (Standard Reference Material [SRM] 1,563 Cholesterol and Fat-Soluble Vitamins in Coconut Oil [Natural and Fortified], SRM 1,566b Oyster Tissue, SRM 1,570a Spinach Leaves, SRM 1,974a Organics in Mussel Tissue (Mytilus edulis), RM 8,415 Whole Egg Powder, RM 8,418 Wheat Gluten, RM 8,432 Corn Starch, RM 8,433 Corn Bran, RM 8,435 Whole Milk Powder, and RM 8,436 Durum Wheat Flour) were recently distributed by NIST to 4 laboratories with expertise in food analysis for the measurement of proximates (solids, fat, protein, etc.), calories, and total dietary fiber, as appropriate. SRM 1846 Infant Formula was distributed as a quality control sample for the proximates and for analysis for individual fatty acids. Two of the materials (Whole Egg Powder and Whole Milk Powder) were distributed in an earlier interlaboratory comparison exercise in which they were analyzed for several vitamins. Value assignment of analyte concentrations in these 11 SRMs and RMs, based on analyses by the collaborating laboratories, is described in this paper. These materials are intended primarily for validation of analytical methods for the measurement of nutrients in foods of similar composition (based on AOAC INTERNATIONAL's fat-protein-carbohydrate triangle). They may also be used as "primary control materials" in the value assignment of in-house control materials of similar composition. The addition of proximate information for 10 existing reference materials means that RMs are now available from NIST with assigned values for proximates in 6 of the 9 sectors of the AOAC triangle. Five of these materials have values assigned for total dietary fiber-the first such information provided for materials available from NIST.