Achieving 0.2% relative expanded uncertainty in ion chromatography analysis using a high-performance methodology.

A high-performance (HP) technique that was originally developed for inductively coupled plasma optical emission spectrometry (ICP-OES) has been successfully translated to ion chromatography (IC) to enable analyses with extremely low uncertainty. As an example application of the HP-IC methodology, analyses of several National Institute of Standards and Technology (NIST) Standard Reference Materials (SRMs) in the SRM 3180 series of anion standard solutions are reported. The relative expanded uncertainty values expressed at 95% confidence for these analyses range from 0.087% to 0.27% and average 0.18%. Strong correlation between analyte and internal standard anion peak heights or peak areas, as well as the use of a unique drift-correction approach, is shown to be important for attaining such low uncertainty.

Improving the high-performance inductively coupled plasma optical emission spectrometry methodology through exact matching.

Exact matching is investigated as a means of improving high-performance inductively coupled plasma optical emission spectrometry (HP-ICP-OES), a technique developed at the National Institute of Standards and Technology (NIST) to enable elemental determinations with relative expanded uncertainty of approximately 0.2% expressed at 95% confidence. "Exact matching" refers to the very careful matching of analyte mass fractions, internal standard mass fractions, and matrix compositions among the calibration and unknown sample solutions prepared for an analysis. Computer spreadsheet modeling results and laboratory data involving 16 pairs of analyte and internal standard wavelengths show that exact matching of analyte and internal standard mass fractions mitigates imprecision and bias that can be caused by even subtle nonlinearity in the ICP-OES instrument response. Laboratory experiments also demonstrate matrix effects caused by small variations in acid composition and by mass fractions of Na less than 4 mg kg(-1), emphasizing the need for exact matching of matrix compositions. HP-ICP-OES analyses performed at NIST with and without exact matching illustrate that exact matching enables relative expanded uncertainties to be halved to approximately 0.1%.

Certification of beryllium mass fraction in SRM 1877 Beryllium Oxide Powder using high-performance inductively coupled plasma optical emission spectrometry with exact matching.

High-performance inductively coupled plasma optical emission spectrometry (HP-ICP-OES) was used to certify the Be mass fraction in National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 1877 Beryllium Oxide Powder. The certified value and expanded uncertainty expressed at a 95% confidence level is (0.3576 +/- 0.0024) g/g. To obtain best results, the Be mass fractions, Mn (internal standard) mass fractions, and matrix compositions of the calibration solutions were carefully matched to those of the sample solutions for each individual HP-ICP-OES analysis. This "exact matching" approach was used because experience at NIST has shown that it often affords improved accuracy and precision in HP-ICP-OES analysis. NIST has never published these observations. Due to the toxicity of BeO and the difficulty of containing the very fine powder material, sets of solutions for HP-ICP-OES analysis were prepared by laboratories collaborating with NIST who have the experience and equipment needed to work with the material safely. Each laboratory utilized a unique digestion protocol(s). After preparing the sets of solutions, the collaborating laboratories shipped them to NIST for HP-ICP-OES analysis. NIST provided the collaborating laboratories with solution preparation kits and spreadsheets to help establish traceability of the HP-ICP-OES results to the International System of Units (SI) and to allow exact matching to be accomplished. The agreement observed among the four individual Be mass fraction values determined from the sets of solutions prepared by the collaborating laboratories was 0.074% relative (1s of mean). The excellent agreement provides a measure of confidence in the robustness of each of the digestion procedures, as well as in the certified Be mass fraction value. The analytical benefits of using exact matching for this particular certification were investigated. Results show that exactly matching the matrix compositions of the standards to the samples for each HP-ICP-OES analysis was critical to obtaining the excellent agreement observed among the individual Be mass fraction values and also helped to minimize bias and uncertainty in the certified value. Unlike previous NIST studies, exactly matching the Be and Mn mass fractions of the standards to the samples for this particular certification appears to have had little effect on the data.

Preparation and characterization of a suite of ephedra-containing standard reference materials.

The National Institute of Standards and Technology, the U.S. Food and Drug Administration, Center for Drug Evaluation and Research and Center for Food Safety and Applied Nutrition, and the National Institutes of Health, Office of Dietary Supplements, are collaborating to produce a series of Standard Reference Materials (SRMs) for dietary supplements. A suite of ephedra materials is the first in the series, and this paper describes the acquisition, preparation, and value assignment of these materials: SRMs 3240 Ephedra sinica Stapf Aerial Parts, 3241 E. sinica Stapf Native Extract, 3242 E. sinica Stapf Commercial Extract, 3243 Ephedra-Containing Solid Oral Dosage Form, and 3244 Ephedra-Containing Protein Powder. Values are assigned for ephedrine alkaloids and toxic elements in all 5 materials. Values are assigned for other analytes (e.g., caffeine, nutrient elements, proximates, etc.) in some of the materials, as appropriate. Materials in this suite of SRMs are intended for use as primary control materials when values are assigned to in-house (secondary) control materials and for validation of analytical methods for the measurement of alkaloids, toxic elements, and, in the case of SRM 3244, nutrients in similar materials.

Evaluation of Serum Volume Losses During Long-Term Storage.

Aliquots of serum collected in a large case-control study of cervical cancer were stored at -70°C for up to 4 years during implementation of the study. When 500 µL serum aliquots were thawed in preparation for carotenoid and vitamin A assays, volumes were noticeably variable and fell below 500 µL in the majority of the samples. We were concerned about evaporation/sublimation during storage of the samples because loss of water would concentrate the analytes of interest. We evaluated the use of density and sodium ion concentration measurements to confirm its occurrence. We found that serum density was an unreliable indicator of extent of volume loss since the anticipated increases in density due to evaporation were of the same magnitude as inter-individual variation in serum density. In contrast, Na+ concentration is tightly regulated and would rise if water had been lost from the samples. In a representative sample of serum aliquots from the case-control study, 24 of 25 vials contained less than 500 µL of serum. The mean sodium ion concentration (138.1 ± 3.6 mmol/L) was within the normal range for human serum of 136-145 mmol/L, and no correlation was observed between serum volume and Na+ concentration. These results strongly suggest that the observed low volumes were not due to evaporative losses. Instead, the variably low volumes of serum aliquots were probably due to pipetting errors in the initial aliquotting resulting from the use of air-displacement pipettes.

Effect of valence state on ICP-OES value assignment of SRM 3103a arsenic spectrometric solution.

The certification of Standard Reference Material (SRM is a registered trademark of NIST) 3103a As Spectrometric Solution is based on the gravimetric preparation value that is verified by inductively coupled plasma optical emission spectrometry (ICP-OES) measurements. A disagreement between the gravimetric and the spectrometric values for a batch of As calibration solutions led to the discovery that the solutions contained a mixture of trivalent and pentavalent As species and that the pentavalent species was approximately 8% more sensitive than the trivalent species with ICP-OES determination. The kinetics of the reaction between As metal and nitric acid were studied, and the results were applied to develop a procedure that would consistently produce single-species pentavalent As standards, which eliminates As speciation as a source of measurement bias in the SRM certification process.

Determination of cyanide in blood by isotope-dilution gas chromatography-mass spectrometry.

BACKGROUND: Cyanide (CN) is a lethal toxin. Quantification in blood is necessary to indicate exposure from many sources, including food, combustion byproducts, and terrorist activity. We describe an automated procedure based on isotope-dilution gas chromatography-mass spectrometry (ID GC/MS) for the accurate and rapid determination of CN in whole blood. METHODS: A known amount of isotopically labeled potassium cyanide (K13C15N) was added to 0.5 g of whole blood in a headspace vial. Hydrogen cyanide was generated through the addition of phosphoric acid, and after a 5-min incubation, 0.5 mL of the headspace was injected into the GC/MS at an oven temperature of -15 degrees C. The peak areas from the sample, 1H12C14N+, at m/z 27, and the internal standard, 1H13C15N+, at m/z 29, were measured, and the CN concentration was quantified by ID. The analysis time was 15 min for a single injection. RESULTS: We demonstrated method accuracy by measuring the CN content of unfrozen whole blood samples fortified with a known amount of CN. Intermediate precision was demonstrated by periodic analyses over a 14-month span. Relative expanded uncertainties based on a 95% level of confidence with a coverage factor of 2 at CN concentrations of 0.06, 0.6, and 1.5 microg/g were 8.3%, 5.4%, and 5.3%, respectively. The mean deviation from the known value for all concentrations was <4%. CONCLUSION: The automated ID GC/MS method can accurately and rapidly quantify nanogram per gram to microgram per gram concentrations of CN in blood.