Vitamin C is a necessary micronutrient that is involved in many biological processes. In preserved human plasma and serum, vitamin C is most meaningfully analyzed as total ascorbic acid (TAA). From 1993 through 2015, the National Institute of Standards and Technology (NIST) coordinated 40 interlaboratory studies (ILS) devoted to improving the between-participant comparability of TAA measurements. The results from these ILS demonstrate that the concentration of TAA ([TAA]) is stable for at least 20 years in serum diluted 1 + 1 (volume fraction) with 10% mass concentration aqueous metaphosphoric acid (MPA) and stored at −80 °C. The between-participant relative reproducibility precision, expressed as a coefficient of variation (CV), improved from over 16% to under 9% over the course of the studies. Normalization of test samples (i.e., ex post-facto recalibrating the as-submitted results) using participant-prepared serum-free calibration solutions did not improve reproducibility. Normalization to one control sample having a similar serum-based matrix as the test samples improved the CV to 7%; normalization to two such controls reduced the CV to 4%. Multicenter studies that require the highest degree of measurement comparability among the participants should consider calibrating with materials that have a serum-based matrix as similar as possible to that of the samples of interest.

The National Institute of Standards and Technology coordinates the Micronutrients Measurement Quality Assurance Program (MMQAP) for laboratories that measure fat-and water-soluble vitamins and carotenoids in human serum and plasma.This report describes

The National Institute of Standards and Technology coordinates the Micronutrients Measurement Quality Assurance Program (MMQAP) for laboratories that measure fatand water-soluble vitamins and carotenoids in human serum and plasma. This report describes the design of and results for the Winter, Spring and Fall 1998 MMQAP measurement comparability improvement studies: 1) Round Robin XLII Fat-Soluble Vitamins and Carotenoids in Human Serum, 2) Round Robin XLIII FatSoluble Vitamins and Carotenoids in Human Serum, 3) Round Robin XLIV Fat-Soluble Vitamins and Carotenoids in Human Serum, and 4) Round Robin 11 Ascorbic Acid in Human Serum. The materials for Round Robin XLII were shipped to participants in February 1998; participants were requested to provide their measurement results by April 3, 1998. The materials for Round Robin XLIII were shipped to participants in May 1998; participants were requested to provide their measurement results by June 22, 1998. The materials for Round Robin XLIV were shipped to participants in August 1998; participants were requested to provide their measurement results by October 9, 1998. The sample materials for Round Robin 11 were distributed in September 1998 with results due by October 20, 1998.

The National Institute of Standards and Technology coordinates the Micronutrients Measurement Quality Assurance Program (MMQAP) for laboratories that measure fat-and water-soluble vitamins and carotenoids in human serum and plasma.This report describes the design of and results for the Winter 2001 MMQAP measurement comparability improvement studies: 1) Round Robin XLIX Fat-Soluble Vitamins and Carotenoids in Human Serum and 2) Round Robin 14 Total Ascorbic Acid in Human Serum.The materials

The National Institute of Standards and Technology coordinates the Micronutrients Measurement Quality Assurance Program (MMQAP) for laboratories that measure fat-and water-soluble vitamins and carotenoids in human serum and plasma.This report describes

Using an automated oven evaporation technique combined with the coulometric Karl Fischer method, the mass fraction of water has been measured in cement, coal, and refined oil samples. The accuracy of this method was established by using SRM 2890, water-saturated 1-octanol that was added to white oil. The samples were analyzed for total reactive Karl Fischer reagent (KFR) material, for interfering materials, and for material that does not react with the aldehyde–ketone KFR. All of the samples yielded volatile material that reacted with the standard KFR. None of the samples contained significant masses of material that reacted with iodine. The cement and coal SRMs contained no material that reacted with methanol and very little material that did not volatilize at 107°C. The refined oils contained some material that was volatile at 107°C and some at 160°C. However, none of this material reacted with the aldehyde–ketone reagent. These results show that the material in the solid samples is water and that the material in the refined oils is a material other than water which reacts with methanol to form water.

The measurement of the amount of water in oils is of significant economic importance to the industrial community, particularly to the electric power and crude oil industries. The amount of water in transformer oils is critical to their normal function and the amount of water in crude oils affects the cost of the crude oil at the well head, the pipeline, and the refinery. Water in oil Certified Reference Materials (CRM) are essential for the accurate calibration of instruments that are used by these industries. Three NIST Standard Reference Materials (SRMs) have been prepared for this purpose. The water in these oils has been measured by both coulometric and volumetric Karl Fischer methods. The compounds (such as sulfur compounds) that interfere with the Karl Fischer reaction (interfering substances) and inflate the values for water by also reacting with iodine have been measured coulometrically. The measured water content of Reference Material (RM) 8506a Transformer Oil is 12.1±1.9 mg kg−1 (plus an additional 6.2±0.9 mg kg−1 of interfering substances). The measured water content of SRM 2722 Sweet Crude Oil, is 99±6 mg kg−1 (plus an additional 5±2 mg kg−1 of interfering substances). The measured water content of SRM 2721 Sour Crude Oil, is 134±18 mg kg−1 plus an additional 807±43 mg kg−1 of interfering substances. Interlaboratory studies conducted with these oil samples (using SRM 2890, water saturated 1-octanol, as a calibrant) are reported. Some of the possible sources of bias in these measurements were identified, These include: improperly calibrated instruments, inability to measure the calibrant accurately, Karl Fischer reagent selection, and volatilization of the interfering substances in SRM 2721.

The precision and bias of the coulometric Karl Fischer ASTM method D1533–00 have been assessed in a collaborative ASTM round robin program for a group of 34 laboratories. The test materials used in this study included water saturated 1-octanol (WSO), water saturated 1-butanol (WSB), and a series of new and used transformer oil samples. Fundamental systematic biases have been demonstrated in the accuracy of the measurement of water in the WSO, WSB, and transformer oil samples. The systematic bias in the measurement of the WSO and WSB standards indicates that for some laboratories either the instruments were not accurate or the quantity of the standard was not measured accurately. A second type of systematic bias consisted of measurement errors associated with the selection of the Karl Fischer solvent that was used with each instrument, and this was superimposed upon the error in the measurement of the water in the standards. Using the statistical calculation method ASTM D 6300 the repeatability and reproducibility for water in transformer oil were found to be 7 mg/kg and 14 mg/kg respectively. The method detection limit of water was 8 mg/kg oil. The method bias was estimated based on the National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 2890, WSO, since no suitable reference material for water in transformer oil was available for this study.