Accuracy evaluation reference material

Spiking is the most common method to evaluate accuracy because reference materials are not usually available find a second analytical method may not be readily available. Spiking ensures that the matrix remains nearly constant. 

Consider the situation when the accuracy of a new analytical method is evaluated by analyzing a standard reference material with a known )J,. A sample of the standard is analyzed, and the sample s mean is determined. The null hypothesis is that the sample s mean is equal to p. 

The more classical approach to assess the presence of marine biotoxins in seafood is the in vivo mouse bioassay. It is based on the administration of suspicious extracted shellfish samples to mice, the evaluation of the lethal dose and the toxicity calculation according to reference dose response curves, established with reference material. It provides an indication about the overall toxicity of the sample, as it is not able to differentiate among individual toxins. This is a laborious and time-consuming procedure the accuracy is poor, it is nonspecific and generally not acceptably robust. Moreover, the mouse bioassay suffers from ethical implications and it is in conflict with the EU Directive 86/609 on the Protection of Laboratory Animals. Despite the drawbacks, this bioassay is still the method of reference for almost all types of marine toxins, and is the official method for PSP toxins. 

A QC assessment was done for all samples. De-ionized water field blanks were collected on seven different days to evaluate process contamination. Site duplicates were taken at eight sites to evaluate repeatability and site variation. Instrumental precision was constrained by analysis of laboratory duplicate solutions, and is typically less than 5%. Finally, standard reference material (SRM) water standards were analyzed with sample batches, to assess instrumental accuracy. 

Some examples include evaluation of uncertainty components associated with published values (i.e., the analyst did not measure them), uncertainties in a certificate of a certified reference material, manufacturer s statements about the accuracy of an instrument, or perhaps even personal experience. The latter could be viewed as an opportunity for anyone to just make up an uncertainty, but experience does count for something, and it is indeed usually better than nothing. Leaving out a component because of lack of exact knowledge immediately underestimates the uncertainty. 

The levels of accuracy and precision determine the quality of a measurement. The data are as good as random numbers if these parameters are not specified. Accuracy is determined by analyzing samples of known concentration (evaluation samples) and comparing the measured values to the known. Standard reference materials are available from regulatory agencies and commercial vendors. A standard of known concentration may also be made up in the laboratory to serve as an evaluation sample. 

Definitions 2 and 3 allow an evolution in the different techniques and methods as definitive methods for the same analyte (Leijnse, 1982). Indeed, even though systematic errors were investigated during the initial research work, later technical advances may uncover errors that were undetected during the original measurements. The end use and end purposes of the definitive method include the evaluation of the accuracy of reference methods and its application to the quantitation of analytes in certified reference materials present in a biological matrix. 

Several types of bias are common in analytical methodology, including laboratory bias and method bias. Laboratory bias can occur in specific laboratories, due to an uncalibrated balance or contaminated water supply, for example. This source of bias is discovered when results of interlaboratory studies are compared and statistically evaluated. Method bias is not readily distinguishable between laboratories following a standard procedure, but can be identified when reference materials are used to compare the accuracy of different methods. The use of interlaboratory studies and reference materials allows experimentalists to evaluate the accuracy of their analysis. 

NBS (12) maintains a comprehensive program on standard reference materials (SRMs) SRMs are well-characterized, homogeneous, stable materials or simple artifacts with specific properties measured and certified by NBS. They are widely used in a variety of measurement applications, including the evaluation of the accuracy of test methods, improvement of measurement compatability among different laboratories, and establishment of measurement traceability to NBS. The Bureau currently has more than 1000 different SRMs available. ... 

As the accuracy of nuclear material accountability and safeguards measurements improves, it is appropriate to examine whether the reference materials used for the calibration of such measurements and the physical constants used in their evaluation provide with sufficient accuracy the expected link between field measurements and the SI system. 

SRMs supplied by NIST make the verification of commercial calibration sera possible if referee methods of analysis are available for use in conjunction with the SRMs, so the first important issue here is how to achieve traceability of the certified value of the clinical reference materials to the SI unit. Several traceability schemes for reference materials in analytical measurements have been recommended [4, 5], Closely connected is the problem of the evaluation of the uncertainty of the clinical reference material. In this respect, a major aspect is the significance of this uncertainty in the medical treatment or diagnosis, as it is widely recognized that it is rather difficult to show clearly and unambiguously a limit of accuracy required for a clinical result. 

The variation of sensitivity between different sensors was also checked. Calibration curves with five different sensors were performed. A Relative Standard Deviation of 13, 13 and 42% of calibration slopes (sensitivity) were obtained for Cu, Pb and Cd respectively. These variations should have limited consequence on bias and precision when the standard addition method is used. However, for Cd, variations in the limit of quantification between two electrodes could be expected. Finally, the accuracy of the method was evaluated by the measurement of a SWIFT reference material used during the 2nd SWIFT-WFD Proficiency Testing exercise (Table 4.2.2). The reference value was chosen as the consensus value of the selected data population obtained after excluding the outliers. The performances of the device were estimated according to the Z-score (Z) calculation. Based on this score, results obtained with the SPEs/PalmSens method were consistent with those obtained by all methods for Pb and Cu ( Z < 2) while the result was less satisfactory for Cd (2 < Z < 3). 

For the analysis of antibiotic residues in foods, there is a limited number of methods that have been evaluated by collaborative study and a lack of certified reference materials (see other chapters on specific methods of analysis and quality assurance for additional details). Hence, the assessment of accuracy is most frequently based on analytical recovery from fortified materials when developing and validating these methods. 

Blind traceable reference material To evaluate the accuracy of the analyst. Sufficient spike is added for precise measurement. 

Method performance in air analysis involves terms such as accuracy, storage stability, capacity, sampling rate, recovery, and sensitivity. To evaluate the performance of a developed method, certified reference materials for particulate matter, such as urban dust SRM 1649a particulate matter from NIST (Gaithersburg, MD, USA) can be purchased. In addition, a standard reference material has been recently developed for the determination of organic compounds in house dust the SRM 2585 is intended for using in method validation for the analysis of PAHs, PCBs, chlorinated pesticides, and PBDEs (Poster et al. 2007). 

With the relative analysis techniques described above, accuracy is best evaluated by reference to external standards. Inter-laboratory comparisons have a role to play in multilaboratory projects, but the basic tool is the certified reference material. These are natural materials which have been homogenized and analyzed by a range of laboratories, and have agreed, or certified , concentrations for some of the elements they contain. 

A flow injection method based on the catalytic action of iodide on the colour-fading reaction of the FeSCN2+ complex was proposed and applied in order to determine iodine in milk. At pH 5.0, temperature 32°C and measurements at 460 nm, the decrease in absorbancy of Fe -SCN (0.10 and 0.0020 mol /I) in the presence of N02" (0.3 mol/ 1) is proportional to the concentration of iodide, with a linear response up to 100.0 pg/1. The detection limit was determined as 0.99 pg/1 and the system handles 48 samples per hour. Organic matter was destroyed by means of a dry procedure carried out under alkaline conditions. Alternatively, the use of a Schoninger combustion after the milk dehydration was evaluated. The residue was taken up in 0.12 mol/1 KOH solubilization. For typical samples, recoveries varied from 94.5 to 105%, based on the amounts of both organic matter destroyed. The accuracy of the method was established by using a certified reference material (IAEA A-11, milk powder) and a manual method. The proposed flow injection method is now applied as an indicator of milk quality on the Brazilian market (de Araujo Nogueira et al., 1998). 

The FIA method was applied to the determination of iron (II) and total iron in water samples and ore samples. In order to evaluate the accuracy of the proposed method, the determination of total iron in a standard reference material (Zn/ Al/Cu 43XZ3F) and metal alloy sample was carried out. The analytical results obtained by the propxjsed method are in good agreement with the certified values as is shown in Table 2. 

Before results from a new calibration are accepted, it is essential that the accuracy over the full calibration range is validated independently. Although the intensity response of a WD spectrometer is expected to be linear over six orders of magnitude (subject to corrections for counter dead-time effects), the accuracy of the calibration function is as sensitive as that for any other instrumental technique to the effects of bias, which are likely to be most significant in the analysis of samples at the extremes (low or high) of the calibration range. Discrepancies of this nature are sometimes caused by the use of inaccurate values in reference samples. These discrepancies can only be overcome satisfactorily by a critical evaluation of all calibration data. If the analysis of independently characterized reference materials cannot be used in this evaluation (because, for example, these samples have had to be used as primary calibration samples) then it is possible, though not entirely satisfactory, to evaluate the self-consistency of calibration data in order to identify discrepant points. 

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