Accuracy evaluation standard reference materials

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. 

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. 

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. 

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. ... 

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). 

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. 

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. 

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). 

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. 

Aggarwal et al. recently implemented methods for the determination of isotope ratios and concentrations of different elements in biological specimens. Thermally stable and volatile chelates were identified for a number of trace elements Ni [43], Cr [45], Pt [46], Cu [33], Se [47], Co [48], Cd [49], Pb [44], Hg [50], and Te [51]. Memory effect was evaluated and found to be insignificant. Precision and accuracy in the measurement of isotope ratios was evaluated. Finally, the isotope dilution GC-MS method was validated by analyzing NIST reference materials (e.g., urine, semm), by comparing the results with ETAAS and by using the standard addition approach. 

For a number of materials it is vital that propaty values are provided which have a validated accuracy and are identified as standard reference data (SRD), preferably with international approval and recognition. The prq>aration of such internationally approved data standards is a timeconsuming and delicate activity that requires a critical evaluation of all the available measurements with a detailed assessment of the accuracy of each individual datum reported. For these reasons, only results obtained in instruments characterized by high quality and a complete working equation based upon a sound theory can be employed for the establishment of standard reference data. Whenever possible the results of measurements made with different experimental techniques should be included. 

Careful consideration should be given in determining the necessity to include, and subsequently select criteria, for any required system suitability test which demonstrates the analytical technology will operate within the bovmdaries to provide valid analytical results. This may either include the use of reference materials or standards, as described above, which evaluate performance at the time of analysis. For nonchromatographic methods, this might include determining the accuracy of a wavelength determination, or an assessment of the reproducibility of the response to a test material. 

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