Standard reference material requirements

Laser based mass spectrometric methods, such as laser ionization (LIMS) and laser ablation in combination with inductively coupled plasma mass spectrometry (LA-ICP-MS) are powerful analytical techniques for survey analysis of solid substances. To realize the analytical performances methods for the direct trace analysis of synthetic and natural crystals modification of a traditional analytical technique was necessary and suitable standard reference materials (SRM) were required. Recent developments allowed extending the range of analytical applications of LIMS and LA-ICP-MS will be presented and discussed. For example ... 

Requirements for standards used In macro- and microspectrofluorometry differ, depending on whether they are used for Instrument calibration, standardization, or assessment of method accuracy. Specific examples are given of standards for quantum yield, number of quanta, and decay time, and for calibration of Instrument parameters. Including wavelength, spectral responslvlty (determining correction factors for luminescence spectra), stability, and linearity. Differences In requirements for macro- and micro-standards are considered, and specific materials used for each are compared. Pure compounds and matrix-matched standards are listed for standardization and assessment of method accuracy, and existing Standard Reference Materials are discussed. 

Conventional XRF analysis uses calibration by regression, which is quite feasible for known matrices. Both single and multi-element standards are in use, prepared for example by vacuum evaporation of elements or compounds on a thin Mylar film. Comparing the X-ray intensities of the sample with those of a standard, allows quantitative analysis. Depending on the degree of similarity between sample and standard, a small or large correction for matrix effects is required. Calibration standards and samples must be carefully prepared standards must be checked frequently because of polymer degradation from continued exposure to X-rays. For trace-element determination, a standard very close in composition to the sample is required. This may be a certified reference material or a sample analysed by a primary technique (e.g. NAA). Standard reference material for rubber samples is not commercially available. Use can also be made of an internal standard,... 

The establishment of performance criteria for a given tumor marker test is not a simple process because accuracy and precision are unique for each type of analyte and its application. Establishing methodological limits for accuracy, precision, sensitivity, and specificity often requires standard reference materials, quality control materials, comparative studies, and actual clinical specimens. Accuracy and precision must be measured over the analyte reportable range for which the device is intended to be used. Sensitivity and specificity must be considered with respect to the intended clinical use of the device. Also, the indications for use should be carefully considered in the design of the study protocol. The indications for class II should be to monitor residual tumor after surgery (or radiation), the recurrence of tumor, or response to therapy. A 510(k) must provide clear evidence that the device is accurate, safe, effective, and substantially equivalent to a device legally marketed in the United States. 

In order to measure the exact amount of a specific protein (analyte) by IHC signal intensity, a critical requirement is the availability of a standard reference material (present in a known amount by weight) that can be used to calibrate the assay (IHC stain). It is then possible to determine the amount of test analyte (protein) by a translation process from the intensity of IHC signals. In this respect it is helpful to consider the IHC stain as a tissue based ELISA assay (Enzyme Linked ImmunoSorbent Assay), noting that ELISA is used in the clinical laboratory as a standard quantitative method for measuring protein by weight in fluids, by reference to a calibrating reference standard. 

List all of the reagents, materials, blanks, QC samples, standards and Certified Reference Materials required for the analytical process, numbered for later reference. List the following ... 

While the broad mission of the National Bureau of Standards was concerned with standard reference materials, Dr. Isbell centered the work of his laboratory on his long interest in the carbohydrates and on the use of physical methods in their characterization. Infrared spectroscopy had shown promise in providing structural and conformational information on carbohydrates and their derivatives, and Isbell invited Tipson to conduct detailed infrared studies on the extensive collection of carbohydrate samples maintained by Isbell. The series of publications that rapidly resulted furnished a basis for assigning conformations to pyranoid sugars and their derivatives. Although this work was later to be overshadowed by application of the much more powerful technique of nuclear magnetic resonance spectroscopy, the Isbell— Tipson work helped to define the molecular shapes involved and the terminology required for their description. 

Inorganic mass spectrometry requires the development of suitable reference materials, such as matrix matched standard reference materials for trace, surface (including depth profiling and microlocal) analysis and/or the creation of matrix independent calibration procedures. The development of species specific standards will be intensified for speciation studies in the future. 

Furthermore, isotope standard reference materials are required in different chemical forms for a multitude of elements in order to calibrate and evaluate isotope ratio measurements by mass spectrometry. In addition, reliable analytical data required in science and routine work will only be obtained by improving the quality assurance of the measurement procedures and also using interlaboratory comparisons of analytical results in round robin tests. 

Work by Catanzaro et al. in 1968 (6) led to a new analytical procedure permitting the measurement of isotopic ratios to about 0.05% (95% L.E.) this resulted in the availability of three standard reference materials, so that results could be placed on an absolute basis. This procedure, still the most precise and accurate one available, requires about 1 mg of lead for an analysis. A second procedure (7) has been developed which utilizes silica gel as an ionization enhancer. This method permits the measurement of isotopic ratios to about 0.1% (95% L.E.), but it requires only 0.1 /xg of lead per analysis. In addition, the instrumentation and data handling have been vastly improved so that many samples can be studied quickly and conveniently. 

Specifications could include required accuracy and precision, reagent purity, tolerances for apparatus, the use of Standard Reference Materials, and acceptable values for blanks. Standard Reference Materials (Box 3-1) contain certified levels of analyte in realistic materials that you might be analyzing, such as blood or coal or metal alloys. Your analytical method should produce an answer acceptably close to the certified level or there is something wrong with the accuracy of your method. 

It is important that there is a low level of doubt about the identity of the compound being measured and a high level of certainty that the quantity determined is a true reflection of the amount actually present. To achieve this, confirmatory techniques usually employ complex separation procedures to isolate the compound of interest and require calibration procedures that involve adding known amounts of the compound to uncontaminated specimens of the material under analysis. As a final check on the validity of the method it is recommended that the method should be evaluated in a collaborative trial in a minimum of three independent laboratories against a standard reference material. 

Marvin et al. [14] compared sonication with Soxhlet for the extraction of PAHs from sediments, and from an urban dust standard reference material (SRM 1649). The sonication method required less than 5 g of sample. The amount of organic materials extracted by sonication with two solvents was 2.53 + 0.10% of the sediment samples (w/w), while 2.41 + 0.14% was extracted by Soxhlet. Sequential sonicaion with two solvents was much faster (45 minutes) than Soxhlet (2 days), with practically the same extraction efficiency. The variation of PAH extracted by sonication from the urban dust SRM was within 15%. 

If the calculated value of t (without regard to sign) exceeds a certain critical value (defined by the required confidence limit and the number of degrees of freedom) then the null hypothesis is rejected. For example, a method for determining lead by atomic absorption returned the following values for a standard reference material containing 38% Pb 38.9, 37.4, and 37.1%... 

Procedures for the analysis of some organochlorines in environmental samples (e.g., chlorinated dioxins - EPA Method 1613) [68] also require the use of surrogate standards (usually 13C-labeled) and certified or standard reference materials (CRMs or SRMs). At present there are no stable isotope labeled reference materials and no reference materials have yet been certified for PCA... 

The dependence of molecular weight results on these parameters means that great care must be taken to obtain accurate initial calibration constants. Furthermore, standard reference materials are required to check system operation at regular intervals. 

There are no comprehensive data files for CD spectra for standard reference materials (SRM) that compare with the exhaustive libraries which have been compiled for absorbance data in the electronic and vibrational spectroscopy ranges. Analysts are required to create their own CD spectral files using SRM prepared by the usual purveyors of fine chemicals. A significant problem with an SRM is that although it might meet the industry specifications for chemical purity, its enantiomeric purity is open to question. The few cases in which absolute enantiomeric purity might be assured involve natural products whose syntheses are under total enzymatic control. To prove 100% enantiomeric purity is beyond current capabilities. The problem is compounded even more with the risk that the material might racemize after its extraction from its natural environment. Therefore, it is not possible to assume absolute enantiomeric purity with firm conviction. 

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