March 7, 2011
by Catherine A. Rimmer, Ph.D., and Melissa M. Phillips, Ph.D.
One of the most significant questions in the dietary supplement analytical laboratory is How do I know my method is working? While linearity and repeatability (precision) tests can provide valuable information about the performance of analytical methods and spike recovery studies may indicate the presence of unresolved peaks, it can be difficult to evaluate how well an analytical method works with a complex sample. Analyte stability, extraction efficiency and availability of calibration materials and appropriate analytical controls all contribute to the difficulty. The National Institute of Standards and Technology (NIST), in collaboration with the National Institutes of Health (NIH) Office of Dietary Supplements (ODS), has developed several tools to help laboratories in the dietary supplement and food industries improve measurement capabilities. The tools include calibration solution Standard Reference Materials (SRMs), natural-matrix SRMs, and a Dietary Supplement Laboratory Quality Assurance Program.
Calibration Solution SRMs: Good-quality calibration materials are expensive and difficult to qualify; however, without them, it is impossible to make good measurements. NIST has a number of dietary supplement calibration solutions available or in preparation, including organic acids, catechins, naphthodianthrones and isoflavones. Prior to the production of the SRM solutions, the purity of the crystalline materials (including residual solvents and moisture analysis) is carefully determined. Next, the solution is prepared gravimetrically, ampouled in amber vials blanketed with inert gas and stored under conditions set to maximize stability. Finally, the concentration of the analytes in solution is measured relative to independently prepared calibration solutions with either internal or external standard approaches. If the results of the gravimetric solution preparation and instrumental determination agree, a statistical analysis is performed, and a certificate of analysis (C of A) is prepared with certified values and uncertainties. Solution SRMs are intended for use in method development, for quality control (QC) checks, and as calibration standards. (Note: a solution cannot be used as a QC check and a calibration standard at the same time.)
Natural-Matrix SRMs: Natural-matrix SRMs for dietary supplement analysis have been designed in suites of materials. The SRMs are not meant to represent the ideal dietary supplement product; instead, they are meant to encompass the range of analytical challenges. For example, an upcoming soy suite includes soy protein isolate, soy protein concentrate, soy-containing tablets, soy milk and soy flour. The different processing of each of the materials results in different analyte levels and ratios, as well as different extraction issues and interfering chromatographic peaks. As with solution SRMs, natural-matrix SRMs require at least two independent methods for the certification of each analyte. The methods are designed to be as different as possible, from the sample extraction/preparation steps through the instrumental method (e.g., changing chromatographic selectivity). The data from the different methods (sometimes including data from collaborating laboratories) are then used to determine the amount of analyte in the matrix and the associated uncertainty. Natural-matrix SRMs are intended for use in method development, method validation and as quality control checks. In general, they should not be used for calibration because the uncertainties associated with their assigned values are typically larger than those associated with a calibration solution. The contribution from these larger uncertainties would then have to be included in the measurement.
Dietary Supplement Laboratory Quality Assurance Program (DSQAP): In 2007, NIST established the NIST/NIH DSQAP, which is a free, anonymous interlaboratory comparison program. Twice a year, three to six sets of samples are sent to participants for measurement of analytes such as toxic elements, nutritional elements, fat- and water-soluble vitamins, fatty acids and botanical compounds. After the data are returned, a report containing consensus and target ranges (where appropriate) as well as coded individual participant data and recommendations for improvement is prepared and distributed to participants. The reporting style is similar to that seen in proficiency testing programs; however, pass/fail scores are not issued. In addition to assessing individual laboratory performance, problem areas can be identified through careful selection of samples. The difficulty extracting phytosterols from Serenoa repens fruit and the importance of selecting high-quality niacinamide standards are two issues that have been identified in previous exercises. In addition to providing reports, the DSQAP team holds workshops to help analysts improve their skills and to help the community identify and rectify measurement problems.
Through the use of SRMs and the DSQAP, it is possible for dietary supplement and food laboratories to evaluate method performance. Matrix-based reference materials allow for additional method assessment from sample extraction through instrumental analysis. If an analyst obtains the SRMs certified values using a given method, the method is likely to perform well for similar sample types. If the analyst does not obtain the certified values when analyzing the SRM, there is a problem with some part of the method, and it may possible to determine the source of the error. Participation in interlaboratory comparison programs such as the DSQAP can be used to demonstrate the use of appropriate analytical methods to investigators, to exchange methodological information and advice, and ultimately to improve analytical performance.
Catherine A. Rimmer, Ph.D., and Melissa M. Phillips, Ph.D., work with the National Institute of Standards and Technology (NIST), focusing on the development and certification of food and dietary supplement Standard Reference Materials (SRMs) and the Dietary Supplement Quality Assurance Program (DSQAP). For more information about SRMs, visit NIST.gov/SRM/Index.cfm ; for more on the DSQAP, visit NIST.gov/mml/Analytical/DSQAProgram.cfm or e-mail [email protected] .
You May Also Like