Thomas W. Vetter

Development of a Standard Reference Material for Metabolomics Research

The National Institute of Standards and Technology (NIST), in collaboration with the National Institutes of Health (NIH), has developed a Standard Reference Material (SRM) to support technology development in metabolomics research. SRM 1950 Metabolites in Human Plasma is intended to have metabolite concentrations that are representative of those found in adult human plasma. The plasma used in the preparation of SRM 1950 was collected from both male and female donors, and donor ethnicity targets were selected based upon the ethnic makeup of the U.S. population. Metabolomics research is diverse in terms of both instrumentation and scientific goals. This SRM was designed to apply broadly to the field, not toward specific applications. Therefore, concentrations of approximately 100 analytes, including amino acids, fatty acids, trace elements, vitamins, hormones, selenoproteins, clinical markers, and perfluorinated compounds (PFCs), were determined. Value assignment measurements were performed by NIST and the Centers for Disease Control and Prevention (CDC). SRM 1950 is the first reference material developed specifically for metabolomics research.

Comparison of reflux and microwave oven digestion for the determination of arsenic and selenium in sludge reference material using flow injection hydride generation and atomic absorption spectrometry

A microwave oven digestion procedure was developed for rapid dissolution of sludge samples (NIST SRM 2781) for the determination of As and Se. Microwave oven digestion with HNO3 and H2SO4 provides results comparable to those obtained by reflux column digestion using HNO3, H2SO4 and HClO4. The experimental details for sample preparation and the flow injection hydride generation atomic absorption spectrometric method are discussed. The effects of matrix and various acid concentrations on the extraction and absorbance were also studied. The proposed method has detection limits of 0.15 and 0.17 ng ml–1 for As and Se, respectively.

Recognizing and overcoming analytical error in the use of ICP-MS for the determination of cadmium in breakfast cereal and dietary supplements.

The potential effect of spectral interference on the accurate measurement of the cadmium (Cd) mass fraction in fortified breakfast cereal and a variety of dietary supplement materials using inductively coupled plasma quadrupole mass spectrometry was studied. The materials were two new standard reference materials (SRMs)—SRM 3233 Fortified Breakfast Cereal and SRM 3532 Calcium Dietary Supplement—as well as several existing materials—SRM 3258 Bitter Orange Fruit, SRM 3259 Bitter Orange Extract, SRM 3260 Bitter Orange-containing Solid Oral Dosage Form, and SRM 3280 Multivitamin/Multielement Tablets. Samples were prepared for analysis using the method of isotope dilution and measured using various operating and sample introduction configurations including standard mode, collision cell with kinetic energy discrimination mode, and standard mode with sample introduction via a desolvating nebulizer system. Three isotope pairs, 112Cd/111Cd, 113Cd/111Cd, and 114Cd/111Cd, were measured. Cadmium mass fraction results for the unseparated samples of each material, measured using the three instrument configurations and isotope pairs, were compared to the results obtained after the matrix was removed via chemical separation using anion exchange chromatography. In four of the six materials studied, measurements using the standard mode with sample introduction via the desolvating nebulizer gave results for the unseparated samples quantified with the 112Cd/111Cd isotope pair that showed a positive bias relative to the matrix-separated samples, which indicated a persistent inference at m/z 112 with this configuration. Use of the standard mode, without the desolvating nebulizer, also gave results that showed a positive bias for the unseparated samples quantified with the 112Cd/111Cd isotope pair in three of the materials studied. Collision cell/kinetic energy discrimination mode, however, was very effective for reducing spectral interference for Cd in all of the materials and isotope pairs studied, except in the multivitamin/multielement matrix (SRM 3280) where the large corrections for known isobaric interferences or unidentified interferences compromised the accuracy. For SRM 3280, matrix separation provided the best method to achieve accurate measurement of Cd.

Comparison of clinical methods with isotope dilution inductively coupled plasma mass spectrometry for the new standard reference material 955c lead in caprine blood

The National Institute of Standards and Technology (NIST) has developed Standard Reference Material (SRM) 955c, a new caprine-based, four-level blood standard with certified blood lead levels (BLLs) ranging from 0.4 µg/dL (0.02 µmol/L) Pb to 45 µg/dL (2.2 µmol/L) Pb. Certified values are based on ID-ICP-MS. Strict control and accurate measurement of the procedure blank were necessary to minimize uncertainty for the lowest level (Level 1) and obtain a relative expanded uncertainty (k = 2) of 2.6%. Level 1 is intended to represent a baseline BLL and provides a means to define detection levels and validate methods developed to measure lead at background environmental levels in blood. Level 2 is near 10 µg/dL (0.48 µmol/L), the current threshold defined by the U.S. Centers for Disease Control and Prevention (CDC) for public health action and clinical follow up. The standard has been developed in collaboration with the Wadsworth Center, New York State Department of Health, which provided measurements based on GFAAS and ICP-MS. Results from these clinical methods are statistically compared to the Isotope Dilution (ID) values. The Level 1 SRM 955c standard is below the detection limit of the GFAAS method, but comparison of Level 1 results for the ICP-MS method with the ID-ICP-MS values shows no evidence of statistically significant disagreement, suggesting that the ICP-MS method appears capable of measuring BLLs at background concentrations. Comparison of both GFAAS and ICP-MS results with the ID ICP-MS values for the Level 2 through Level 4 SRM 955c standards similarly shows no statistically significant disagreement between methods at these elevated blood Pb levels. In addition to the SRM 955c data, long-term method performance data are presented for SRM 955b Lead in Bovine Blood and for SRM 966 Toxic Elements in Bovine Blood. The clinical methods performed well within CLIA guidelines for these materials; however, a small negative bias for the ICP-MS value relative to the certified value for SRM 966 requires further investigation.

Using instrumental techniques to increase the accuracy of the gravimetric determination of sulfate

A gravimetric method for the determination of sulfate in a sulfate solution standard by the precipitation of barium sulfate is coupled with the instrumental determination of trace sulfate and precipitate contaminants to improve the accuracy and precision of the analysis. Sulfate in a solution of potassium sulfate is separated by a reverse precipitation with barium chloride in very dilute hydrochloric acid. Coulometry, ICP-MS, and flame atomic emission spectrometry (FAES) are used to quantify the level of the contamination in the barium sulfate precipitate and the solubility loss of sulfate in the filtrate, from which correction factors are calculated. Coprecipitating contaminants contribute about 0.3% to the total precipitate mass, while the analyte lost to the filtrate contributes 0.4%. Despite the poorer precision and accuracy of instrumental methods, the over-all precision and accuracy of the sulfate determination is actually improved, since the instrumental methods are used to determine only a very small part of the analyte. The expanded uncertainty (k= 2) of the method is below 0.2% relative to the precipitate mass.

Determination of arsenic, selenium and mercury in an estuarine sediment standard reference material using flow injection and atomic absorption spectrometry

A flow-injection analysis atomic absorption spectrometric (FIA-AAS) method was developed for the determination of trace amounts of arsenic, selenium and mercury in a proposed estuarine sediment standard reference material (SRM 1646a). The samples were prepared in two manners: a) A wet digestion procedure with HNO3, H2SO4, and HClO4 using a reflux column and b) A microwave-oven digestion procedure utilizing HNO3, H2SO4, and HCl for As and Se, and HNO3 for Hg. Microwave-oven digestion provides results comparable to those found by reflux column digestion and reduces the sample preparation time by a factor of 10. The proposed method employing the microwave-oven digestion procedure coupled with FIA-AAS for As and Se, and FIA-CVAAS for Hg, has detection limits of 0.15 ng As/ml, O.17 ng Se/ml and 0.15 ng Hg/ml.

Application of Microwave-Induced Combustion and Isotope Dilution Strategies for Quantification of Sulfur in Coals via Sector-Field Inductively Coupled Plasma Mass Spectrometry

In recent years, microwave-induced combustion (MIC) has proved to be a robust sample preparation technique for difficult-to-digest samples containing high carbon content, especially for determination of halogens and sulfur. National Institute of Standards and Technology (NIST) has applied the MIC methodology in combination with isotope dilution analysis for sulfur determinations, representing the first-reported combination of this robust sample preparation methodology and high-accuracy quantification approach. Medium-resolution mode sector-field inductively coupled plasma mass spectrometry was invoked to avoid spectral interferences on the sulfur isotopes. The sample preparation and instrumental analysis scheme was used for the value assignment of total sulfur in Standard Reference Material (SRM) 2682c Subbituminous Coal (nominal mass fraction 0.5% sulfur). A description of the analytical procedures required is provided, along with metrological results, including an estimation of the overall method uncertainty (<1.5% relative expanded uncertainty) calculated using the IDMS measurement function and a Kragten spreadsheet approach.

Determination of sodium in blood serum by inductively coupled plasma mass spectrometry

Owing to its importance as a serum electrolyte, sodium is determined routinely in clinical laboratories. In the United States, the accuracy of these measurements is assessed through a number of proficiency testing programs. Clinical measurements are supported by the National Institute of Standards and Technology through the production of serum matrix Standard Reference Materials® (909 and 956 series). For reference material certification, a gravimetric primary method has traditionally been used, but this method is time consuming. In this work, an alternative method has been developed in which sodium is determined by inductively coupled plasma mass spectrometry (ICP-MS) using a procedure, which is a cross between internal standardization and isotope dilution analysis. Diluted serum is spiked with 26Mg enriched isotope and the isotope ratio 23Na/26Mg measured in analog detection mode. The ratios are standardized by measuring a sodium primary standard (SRM 919a) spiked with 26Mg. As a relatively high concentration of sodium is measured, the sodium background from the ICP-MS instrument is comparatively small. The method has been successfully applied to the determination of sodium in SRM 909b (Human Serum), SRM 956a (Electrolytes in Frozen Human Serum) and serum pools from the College of American Pathologists.