From 1984 to 2017, the National Institute of Standards and Technology (NIST) Micronutrients Measurement Quality Assurance Program (MMQAP) coordinated 79 Round Robin (RR) interlaboratory studies designed to improve measurement comparability among laboratories measuring fat soluble vitamins and carotenoids in human serum and plasma. There were 22 participants in the initial study; participation increased to 58 in 1997 and then declined in stages to 29 by 2017. A total of 138 academic, commercial, governmental, or non-governmental organizations actively participated in at least one RR; two laboratories participated in 77 of the 79 RRs. A total of 350 human serum or plasma samples were distributed in the RRs, typically three to five per RR. One hundred thirty one (131) of these samples were unique materials. While the initial RRs focused on just retinol (vitamin A), ? tocopherol (vitamin E), and ? carotene (provitamin A) over the life of the program 57 vitamin related measurands were reported at least once. Fifteen (15) measurands were reported sufficiently often to enable analysis of measurement performance over time: total retinol, retinyl palmitate, ? tocopherol, ? plus ? tocopherol, total ? carotene, trans ? carotene, total cis ? carotene, total ? carotene, total lycopene, trans lycopene, total ? cryptoxanthin, total lutein, total zeaxanthin, total lutein plus zeaxanthin, and coenzyme Q10. In addition to documenting the number and nature of the MMQAP's participants, measurands, materials, and reported measurements, this report explores the evolution of among-participant concordance and within-participant apparent precision as functions of calendar date documents and the relationships among results reported for materials delivered to participants both as liquid-frozen and lyophilized samples.

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coordinated the Micronutrients Measurement Quality Assurance Program (MMQAP) for laboratories that measure fat-soluble vitamins and carotenoids in human serum and plasma.This report describes the design of and results for the Spring and Fall 1987 MMQAP measurement comparability improvement studies: 1) Round Robin IX Fat-Soluble Vitamins and Carotenoids in Human Serum and 2) Round Robin XI Fat-Soluble Vitamins and Carotenoids in Human Serum.The first participant results for Round Robin IX were received April 22, 1987; the last results were received June 23, 1987.The first participant results for Round Robin XI were received July 8, 1987; the last results were received September 4, 1987.The analytes in Round Robin X were selenium and zinc and are not discussed in this report.

The NIST/NCI Micronutrient Measurement Quality Assurance Program has conducted 33 interlaboratory comparison exercises for fat-soluble vitamin-related compounds in human sera over the past 12 years, Periodic reanalysis of lyophilized serum samples prepared from more than 70 different sera has enabled estimation of the short- and long-term measurement characteristics. Median- and interquartile-range-based statistics adequately estimate the distribution of results from laboratories that are in analytical control from total distributions that include a significant minority of outlier data, Short-term interlaboratory reproducibility standard deviations (SDs) are predictable functions of analyte concentration, with an asymptotic limit at low analyte concentration and a linear relationship at high concentrations, Long-term trends in the interlaboratory reproducibility can be estimated by standardizing the short-term SD at the observed analyte concentration to an expected SD at a given physiologically significant analyte concentration. The ''average'' laboratory's same-day analytical repeatability SD is about one-third of the estimated interlaboratory reproducibility; repeatability for longer periods between analyses is, on average, no better than the reproducibility. While a few exceptional laboratories have maintained excellent repeatability over the entire decade, long-term study measurements generated within a single laboratory are not generally more internally consistent than results from multiple laboratories, Enhanced and more consistently implemented intralaboratory quality control and quality assurance methods are required to further improve and maintain interlaboratory measurement comparability.

We examine the difference between a linear and a non-linear model for the calculation of analyte concentration by interpolation between standards in isotope dilution/mass spectrometry. Equations are developed for calculating this difference for various increments of the bracketing intensity ratios and for various compositions of the sample; graphs are also presented to depict this difference for these various conditions. As an illustration, the result for a series of measurements on urea are presented and discussed.

AbstractA new monoxanthen‒9‒yl derivative of urea has been synthesized and the structure of this product (N‒9 H‒xanthen‒9‒ylurea) and that of the previously known N,N′‒di‒9 H‒xanthen‒9‒ylurea have been proved by 15N NMR and other spectroscopic techniques. A series of 13C and 15N labeled urea derivatives has been prepared and the utility of their 13C and 15N chemical shifts and coupling constants in the structural analysis of urea derivatives has been investigated.

Cholesterol-d7 is addedto serum, with the weight ratio of cholesterol-d7 to total serum cholesterol kept near to 1:1. The esters are hydrolyzed and the cholesterol is separated and converted into the trimethylsilyl ether de- rivative for measurement by combined gas chromatog- raphy/mass spectrometry. The intensity ratio of the mo- lecular ions at m/z 465 and 458 is measured for each sample and for two calibration mixtures, according to a prescribed bracketing protocol. A weight ratio for the sample is obtained by linear interpolation of the ion-in- tensity ratios, and the total cholesterol is then calcu- lated. The method was applied four times over several weeks to each of five serum pools. Statistical analysis involving consideration of both replication error and variability be- tween weeks gave a coefficient of variation for a single measurement of 0.36%. The absence of interferences in the method was demonstratedby measurements at several other masses. As a participant in a Study Group organized under the Committee on Standards of the American Association for Clinical Chemistry (AACC), whose objective was to develop a reference method of known accuracy for total serum cho- lesterol, our laboratory was to provide a definitive method, i.e., a method with which the accuracy of the reference method could be established by intermethod comparison (1,2). The Study Group, chaired by Dr. C. R. Cooper (Center for Disease Control Atlanta, GA 30333), suggested as a desirable accuracy goal for the definitive method a coefficient of variation (CV) no greater than ±0.5% and a total uncertainty no greater than 1% for serum pools having total cholesterol concentrations ranging from 3.5 to 8.5 mmolfL (about 1.35 to 3.75 g/L).