Accuracy, analytical

Analytical accuracy (sample size and handling). It is impossible to be dogmatic concerning the absolute accuracy of isotopic measurement obtainable 

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In practical terms, we can usually develop satisfactory calibrations with training set concentrations, as determined by some referee method, that are accurate to 5% mean relative error. Fortunately, when working with typical industrial applications and within a reasonable budget, it is usually possible to achieve at least this level of accuracy. But there is no need to stop there. We will usually realize significant benefits such as improved analytical accuracy, robustness, and ease of calibration if we can reduce the errors in the training set concentrations to 2% or 3%. The benefits are such that it is usually worthwhile to shoot for this level of accuracy whenever it can be reasonably achieved. 

Gemand, W., Steckenreuter, K., and Wieland, G., Greater Analytical Accuracy through Gravimetric Determination of Quantity, Fresenius Z. Anal. Chem. 334, 1989, 534-539. 

This development has taken place remarkably quickly over the past thirty years. In that period the first attempts to arrive at a proven analytical accuracy were made. Those who led the move were often considered as people with hobbies, obsessive even. Some eye-opening publications demonstrated clearly that generating analytical results could be compared with the generation of numbers in a lottery (G. Tolg), but were received skeptically by the scientific establishment. Even as recently as the late 1970 s even the most highly respected universities still had to be made aware that a result was not necessarily an accurate result. 

Analytical standards are prepared for two purposes for fortifying control matrices to determine the analytical accuracy and for calibrating the response of the analyte in the electrochemical detector. 

Analytical standards are prepared for two purposes for fortifying control matrices to determine the analytical accuracy and for calibrating the response of the analyte in the mass spectrometer detector. The purity of all standards must be verified before preparation of the stock solutions. All standards should be refrigerated (2-10 °C) in clean amber-glass bottles with foil/Tefion-lined screw-caps. The absolute volume of the standard solutions may be varied at the discretion of the analyst, as long as the correct proportions of the solute and solvent are maintained. Calibrate the analytical balance before weighing any analytical standard material for this method. 

In order to estimate the analytical accuracy of the method with a given set of water samples, a certain number of control water samples should be fortified with a known amount of each herbicide. Control water samples are fortified at different analyte levels across the range of anticipated concentrations. For example, 0.010g of analyte is necessary for a 0.05 agL fortification of a 200-mL sample. This would be accomplished by adding 1.0 mL of a 0.010 o.gmL solution to the sample. The deuterated standards are not incorporated in the fortification solutions but may be added to all control and fortified samples for internal correction of recovery. The following solutions are used to fortify control water samples ... 

Analytical accuracy. The mixture of all deuterium-labeled internal standards is added to each water sample before extraction. This does not prevent the loss of the unlabeled herbicides from the sample in subsequent processing steps, but a proportional loss of the deuterated internal standard precludes the need to correct for recovery. Although referring to recovery in this type of analysis is inappropriate, the accuracy of this method should be monitored. 

The estimated analytical accuracy of the method can be obtained from the mean of the accuracies of each individual fortification using the following equation ... 

There should be nearly equal numbers of fortifications at each level, so the estimated analytical accuracy will not be disproportionately weighted. 

There should be nearly equal numbers of fortifications at each level, so the estimated analytical accuracy will not be disproportionately weighted. If a control water sample to be fortified is found to contain a significant concentration of any of the six metabolites, then this concentration is subtracted from the amount found in the fortified control sample in order to calculate the accuracy for the sample. This is done for those samples that have been found to contain low concentrations with respect to the fortification level. 

The spatial resolution in quantitative analysis is defined by how large a particle must be to obtain the required analytical accuracy, and this depends upon the spatial distribution of X-ray production in the analysed region. The volume under the incident electron beam which emits characteristic X-rays for analysis is known as the interaction volume. The shape of the interaction volume depends on the energy of the incident electrons and the atomic number of the specimen, it is roughly spherical, as shown in Figure 5.7, with the lateral spread of the electron beam increasing with the depth of penetration. 

Limitations in Analytical Accuracy Part 1 - Horwitz s Trumpet... 

Two technical papers recognized as significant early contributions in the discussion of the limitations of analytical accuracy and uncertainty include those by Horwitz of the U.S. FDA [1, 2], For this next series of articles we will be discussing both the topic and the approaches to this topic taken by the classic papers just referenced. The determination and understanding of analytical error is often approached using interlaboratory collaborative studies. In this book we have previously delved into that subject in Chapters 34-39. 

Limitations in Analytical Accuracy Part 2 - Theories to Describe the Limits in Analytical Accuracy... 

Workman, J. and Mark, H., Chemometrics in Spectroscopy Limitations in Analytical Accuracy - Part 1 Horwitz s Trumpet, Spectroscopy 21(9), 18-24 (2006). 

Limitations in Analytical Accuracy Part 3 - Comparing Test Results for Analytical Uncertainty... 

Sections on matrix algebra, analytic geometry, experimental design, instrument and system calibration, noise, derivatives and their use in data analysis, linearity and nonlinearity are described. Collaborative laboratory studies, using ANOVA, testing for systematic error, ranking tests for collaborative studies, and efficient comparison of two analytical methods are included. Discussion on topics such as the limitations in analytical accuracy and brief introductions to the statistics of spectral searches and the chemometrics of imaging spectroscopy are included. 

As Hem (1985) notes, a chemical analysis with concentrations reported to two or three, and sometimes four or five, significant figures can be misleadingly authoritative. Analytical accuracy and precision are generally in the range of 2 to 10%, but depend on the technique used, the skill of the analyst, and on whether or not the constituent was present near the detection limit of the analytical method. The third digit in a reported concentration is seldom meaningful, and confidence should not necessarily be placed on the second. 

Several physical parameters may interfere with analytical accuracy. High sampling flow rates and high temperature and humidity may cause decreased adsorption of 1,4-dichlorobenzene vapor on the solid sorbent (APHA 1995a). Interference by other VOCs with similar retention times may be resolved by using different GC column materials and temperatures or be using MS techniques. 

The selectivity (or specificity ratio) is useful for defining the magnitude of an analytical interference for real situations. Photon ratios serve only to demonstrate the demands upon the spectrometer. The selectivity ratio is the concentration of interfer-ent that causes a unit concentration error in the analyte. If the selectivity ratio of 2000 (defined as adequate by industry)(41) is used, the apparent lead concentration in the bone ash will be 250 ppm. A calcium/lead selectivity ratio of 5,000,000 is required to achieve an analytical accuracy of 10 per cent for one ppm lead in bone ash. (The authors are aware of a lead analysis for bone ash containing approximately 30 ppm lead that was reported by an ICP laboratory to contain approximately 550 ppm lead.) In this instance the selectivity ratio was only 1 x 103. 

Accuracy, Precision, and Detection Limits — Analytical cost, accuracy, precision, and detection limits are the four main evaluation criteria for selecting an analytical method. Detection limit information will not be givdn here, as it is easily obtained from the literature or from instrument manufacturers. The analysis of NBS orchard leaves and bovine liver is often used to demonstrate the accuracy and precision of ICP analysis. We feel that the analysis of United States Geological Survey (USGS) and Canadian Centre for Mineral Energy Technology (CANMET) standard rocks, is a more rigorous test of ICP analytical accuracy because of the resistance to sample... 

On the basis of the above discussion, if a relative precision of 1% in age is required, the application of the U- Pb geochronometer to ages younger than 47 Ma (if instrumental analytical accuracy allows such determination) would require a careful account of the intermediate species, where 47 M5n" is the time required for 7200 ppm of all to decay and 72/7200 = 0.01. If both and are incorporated with equal activity, then for ages younger than 11 Ma it would be necessary to consider the intermediate species. 

To provide the guideline for validation of the microbiological methods to ensure analytical accuracy and precision and that the methods are suitable for the intended use... 

Accuracy is determined by replicate analysis of samples containing known amounts of the analyte. Accuracy should be measured using a minimum of five determinations per concentration. A minimum of three concentrations in the range of expected concentrations is recommended. The mean value should be within 15% of the actual value except at LLOQ, where it should not deviate by more than 20%. The deviation of the mean from the true value serves as the measure of accuracy. 

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