Reproducibility, precision and accuracy

To determine the precision of the PHSS response, 5 to 10 successive 2 xL injections of a lOmM anoxic stock were made into a 2mL anoxic chamber at pH 7.3 and 37°C, 

FIGURE 8.7 PHSS calibration. A representative PHSS calibration regression shows linearity of the signal current from 0 to 1200 nM H2S. Inset An expanded segment of the calibration trace shows the PHSS detection limit near 10 nM H2S with a sensor that exhibited low background current at high amplifier gain (after [41]). 

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Currently, nutrient analytical methods development often utilizes the method of standard additions as an intrinsic aspect of the development process. Essentially, the analyte to be measured exists in the matrix to which an identical known pure standard is added. The spiked and non-spiked matrix is extracted and analysed for the nutrient of interest. By spiking at increasing levels the researcher can establish, to some degree of certainty, the recovery and linearity of the standard additions. One can also evaluate data to determine reproducibility, precision, and accuracy. Unfortunately, the method of standard additions does not allow the evaluation of the method at nutrient concentrations less than 100 % of the endogenous level. 

The possibility of preconcentration of selenium (IV) by coprecipitation with iron (III) hydroxide and lanthanum (III) hydroxide with subsequent determination by flame atomic absorption spectroscopy has been investigated also. The effect of nature and concentration of collector and interfering ions on precision accuracy and reproducibility of analytical signal A has been studied. Application of FefOH) as copreconcentrant leads to small relative error (less than 5%). S, is 0.1-0.2 for 5-100 p.g Se in the sample. Concentration factor is 6. The effect of concentration of hydrochloric acid on precision and accuracy of AAS determination of Se has been studied. The best results were obtained with HCl (1 1). 

Molecular modeling is an indispensable tool in the determination of macromolecular structures from NMR data and in the interpretation of the data. Thus, state-of-the-art molecular dynamics simulations can reproduce relaxation data well [9,96] and supply a model of the motion in atomic detail. Qualitative aspects of correlated backbone motions can be understood from NMR structure ensembles [63]. Additional data, in particular residual dipolar couplings, improve the precision and accuracy of NMR structures qualitatively [12]. 

Thin-layer chromatography does not provide quantitative information of the highest precision and accuracy. Linear relationships between the mass of a substance and the logarithm or square-root of the spot area can sometimes be established under very closely controlled conditions. The optical absorbance of a spot determined by reflectance measurements can be similarly related to mass, or the substances can be scraped from the plate and dissolved in a suitable solvent for a spectrometric determination. The main difficulties with area and density measurements lie in defining the boundaries of spots and controlling chromogenic reactions in a reproducible manner. Relative precision can be as good as 1-2% but is more usually 5-10%. 

FIGURE 1.45 Recovery, LOQ, LOD, and precision and accuracy data from Millipore SPE versus PPT comparison experiments.164 (Reproduced with permission from Millipore.)... 

Precision and accuracy Quantitative analysis by NMR is very precise with relative standard deviations for independent measurements usually much lower than 5%. The largest errors in NMR measurements are likely due to sample preparation, not the NMR method itself. If a good set of standards is available and all NMR measurements for the test and standard samples are performed under the same acquisition conditions, the quantitative results can be readily reproduced on different instruments operated by different analysts at different times. Therefore, good intermediate precision can also be achieved. An accurate quantitative NMR assay will require accurately prepared standards. The accuracy of an NMR assay can be assessed, for example, by measuring an independently prepared standard or an accurate reference sample with the assay. In many cases, a spike recovery experiment can also be used to demonstrate the accuracy of an NMR assay. 

Whereas precision (Section 6.5) measures the reproducibility of data from replicate analyses, the accuracy (Section 6.4) of a test estimates how accurate the data are, that is, how close the data would represent probable true values or how accurate the analytical procedure is to giving results that may be close to true values. Precision and accuracy are both measured on one or more samples selected at random for analysis from a given batch of samples. The precision of analysis is usually determined by running duplicate or replicate tests on one of the samples in a given batch of samples. It is expressed statistically as standard deviation, relative standard deviation (RSD), coefficient of variance (CV), standard error of the mean (M), and relative percent difference (RPD). 

Precision and Accuracy. Statistical requirements for precision and accuracy have been established to ensure that the method is reproducible and free from bias. 

What factors can be used to predetermine the quality and utility of a method An analyst must consider the following questions Do I need a proximate analytical method that will determine all the protein, or carbohydrate, or lipid, or nucleic acid in a biological material Or do I need to determine one specific chemical compound among the thousands of compounds found in a food Do I need to determine one or more physical properties of a food How do I obtain a representative sample What size sample should I collect How do I store my samples until analysis What is the precision (reproducibility) and accuracy of the method or what other compounds and conditions could interfere with the analysis How do I determine whether the results are correct, as well as the precision and accuracy of a method How do I know that my standard curves are correct What blanks, controls and internal standards must be used How do I convert instrumental values (such as absorbance) to molar concentrations How many times should I repeat the analysis And how do I report my results with appropriate standard deviation and to the correct number of significant digits Is a rate of change method (i.e., velocity as in enzymatic assays) or a static method (independent of time) needed ... 

Between-operator variation Repeat of the precision and accuracy studies within the same laboratory using the same instrument but different analysts to challenge the reproducibility of the method. 

The specification of repeatability and reproducibility intervals, without specification of a statistical confidence level, weakens the precision and accuracy... 

Finally, how do precision and accuracy compare Precision is a determination of the reproducibility of a measurement. It tells you how closely several measurements agree with one another. Several measurements of the same quantity showing high precision will cluster together with little or no variation in value however, if the measurements show a wide variation, the precision is low. Random errors are errors which lead to differences in successive values of a measurement and affect precision some values will be off in one direction or another. One can estimate the precision for a set of values for a given quantity as follows estimate = A/2, where A is the difference between the highest and lowest values. 

In most cases, absolute retention times are used for the identification. Modem GCs have high precision and accuracy of retention times within 0.05 %, or better can be obtained in subsequent runs during the same day. When the analysis is repeated by another instrument, the deviation may be of several percent. The reproducibility of absolute retention times is strongly dependent on the proper adjustment of all chromatographic parameters. In addition, the column properties are not exactly the same even when similar columns from the same manufacturer are used. On the other hand, absolute retention times are the most useful identification parameters if a few chemicals are to be monitored and the background is low. This method requires frequent calibration because even small changes in chromatographic conditions will influence the absolute retention times. 

When one wants to measure low-level radionuclides with good precision and accuracy, one has to adjust the counter settings properly and to select the most suitable scintillators and vials to ensure the highest sensitivity and reproducibility. For these purposes, the Figure of Merit is adopted as a quantitative criterion ... 

Before any discussion of the precision and accuracy of activation analysis, it is important that the distinction between them should be appreciated. Precision is an index of the reproducibility of repeated individual analyses, whereas accuracy is a measure of the closeness of the mean value to the truth. 

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