Accuracy evaluation instrumentation

Note that instruments 1-4 were photometric devices with less then 0.01 absorbance accuracy evaluated against reference neutral filters at 546 nm and A = 1.000, traceable to INM. The bandwidth provided by the interference filters equipping the absorption photometers was within the range of 4-10 nm. Instrument 5 was a 10 nm bandwidth photometric device with less than 1.0% absorbance linearity, evaluated at 405 nm and 500 nm, against liquid absorbance RMs type 16.02 and 16.03 [5], Enzymatic colorimetric methods for determination of glucose and urea were used. The o-cresoftalein colorimetric method was used for calcium determination. 

Eisenhart, C., Realistic Evaluation of the Precision and Accuracy of Instrument Calibration Systems, /. Res. Natl. Bur. Stand. (U.S.) 67C,... 

Due to the limited accuracy of instruments for flow rates measurement, the accuracy of experimental data is limited. An error of about 10 % in the evaluation of pilot plant iso-amylenes conversion has to be considered. 

Part of the planning should include the evaluation of test uncertainty. This evaluation can be limited to a common sense approach based on available instrumentation and the locations relative to the ideal. A more sophisticated study can be made in which instrumentation accuracy and the impact of any inaccuracy on the measured parameters is evaluated. This is a complex task with the need being based on the motivation for the test. If the test is being performed to settle a dispute, a formal understanding of the uncertainty should be developed. Methods for evaluation of test uncertainty are found in ANSI/ASME PTC 19.1 [11]. 

It will be extremely difficult for the typical plant user to determine the level of accuracy of the various instruments that are available for predictive maintenance. Vendor literature and salesmen will assure the potential user that their system is the best, most accurate, etc. The best way to separate fact from fiction is a comparison of the various systems in your plant. Most vendors will provide a system on consignment for periods up to thirty days. This will provide sufficient time for your staff to evaluate each of the potential systems before purchase. 

A fully automated instrumental procedure has been developed for analyzing residual corrosion inhibitors in production waters in the field. The method uses ultraviolet (UV) and fluorescence spectrophotometric techniques to characterize different types of corrosion inhibitors. Laboratory evaluations showed that fluorescence is more suitable for field application because errors from high salinity, contamination, and matrix effect are minimized in fluorescence analysis. Comparison of the automated fluorescence technique with the classic extraction-dye transfer technique showed definite advantages of the former with respect to ease, speed, accuracy, and precision [1658],... 

MS equipment is evaluated on several performance metrics. Mass accuracy, mass resolution, and mass range are standard parameters frequently assessed to determine the suitability of an instrument. Mass accuracy is defined as the extent to which a mass analyzer reflects true m/z values and is measured in atomic mass units (amu), parts per million (ppm), or percent accuracy. 

The relative instrumental sensitivity factors for cobalt and nitrogen were determined by measuring core level (Co 2p and N Is) XPS spectra for a series of pure cobalt amine complexes of established stoichiometry. To evaluate the core level photopeak intensities, peak areas, including shake-up satellite intensity were used. The precision for the measurements of the nitrogen to cobalt atomic ratio is 10% while the accuracy is approximately 15%. Additional details of the XPS measurements are contained in the literature (24,25). 

Corrections for instrumentally-produced mass fractionation that preserve natural mass dependent fractionation can be approached in one of two ways a double-spike method, which allows for rigorous calculation of instrumental mass fractionation (e.g., Dodson 1963 Compston and Oversby 1969 Eugster et al. 1969 Gale 1970 Hamelin et al. 1985 Galer 1999 see section Double-spike analysis ), or an empirical adjustment, based on comparison with isotopic analysis of standards (Dixon et al. 1993 Taylor et al. 1992 1993). The empirical approach assumes that standards and samples fractionate to the same degree during isotopic analysis, requiring carefully controlled analysis conditions. Such approaches are commonly used for Pb isotope work. However, it is important to stress that the precision and accuracy of isotope ratios determined on unknown samples may be very difficult to evaluate because each filament load in a TIMS analysis is different. 

HPLC methods can usually be transferred without many modifications, since most commercially available HPLC instruments behave similarly. This is certainly true when the columns applied have a similar selectivity. One adaptation, sometimes needed, concerns the gradient profiles, because of different instrumental or pump dead-volumes. However, larger differences exist between CE instruments, e.g., in hydrodynamic injection procedures, in minimum capillary lengths, in capillary distances to the detector, in cooling mechanisms, and in the injected sample volumes. This makes CE method transfers more difficult. Since robustness tests are performed to avoid transfer problems, these tests seem even more important for CE method validation, than for HPLC method validation. However, in the literature, a robustness test only rarely is included in the validation process of a CE method, and usually only linearity, precision, accuracy, specificity, range, and/or limits of detection and quantification are evaluated. Robustness tests are described in references 20 and 59-92. Given the instrumental transfer problems for CE methods, a robustness test guaranteeing to some extent a successful transfer should include besides the instrument on which the method was developed at least one alternative instrument. 

This chapter sheds light on the different validation requirements and methods to investigate them. Evaluation of the typical validation characteristics, namely accuracy, precision, specificity, DL, QL, linearity, and range in CE, has been discussed in details. Validation in CE is similar to validation in other separation techniques such as HPEC, but in CE, the capillary surface properties and namely the EOF have to be especially addressed. Eurther, the instrument performance has to be carefully considered during validation and method transfer. Here, the condition of the lamp and the thermostating system is of particular importance. 

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. 

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... 

Neither the effect of the used instrumentation (the pumps, columns and detectors) nor the software applied for the data processing was evaluated from the point of view of accuracy of results. It is evident, that the worldwide standardization of both measurement and data processing are badly needed in SEC. At least the experimental conditions within the same laboratory should be kept constant. This may prove, however, difficult as the detectability of various samples differs substantially and the Cj/Vj is to be adjusted for each kind of sample. 

ICH recommends three approaches to the estimation of QL. The first approach is to evaluate it by visual evaluation and may be used for noninstrumental methods and instrumental methods. QL is determined by the analysis of samples with known concentrations of analyte and by establishing the minimum level at which the analyte can be quantitated with acceptable accuracy and precision. 

Sucrose quantitation has also been performed by colorimetric methods. However, in recent years, automated enzymatic analyzers and instrumental methods (eg, ion chromatography and hplc) have become increasingly popular, as they provide greater sensitivity and accuracy. Near infrared (nir) spectroscopy is currendy under evaluation as a tool for sucrose quantitation in sugar mills and food processing operations. 

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. 

An alternative to quantitative analysis by ICP-MS is semiquantitative analysis, which is generally considered as a rapid multielement survey tool with accuracies in the range 30-50%. Semiquantitative analysis is based on the use of a predefined response table for all the elements and a computer program that can interpret the mass spectrum and correct spectral Interferences. This approach has been successfully applied to different types of samples. The software developed to perform semiquantitative analysis has evolved in parallel with the instrumentation and, today, accuracy values better than 10% have been reported by several authors, even competing with typical ones obtained by quantitative analysis. The development of a semiquantitative procedure for multielemental analysis with ICP-MS requires the evaluation of the molar response curve in the ICP-MS system (variation of sensitivity as a function of the mass of the measured isotope) [17]. Additionally, in the development of a reliable semiquantitative method, some mathematical approaches should be employed in order to estimate the ionisation conditions in the plasma, its use to correct for ionisation degrees and the correction of mass-dependent matrix interferences. 

As an example, a recent intercomparison (37) included three N02 measurement techniques aTDLAS-based system and two chemical-based systems— the photolysis-ozone chemiluminescence system diagramed in Figure 7 and an instrument based on N02 plus luminol chemiluminescence. Above 2 ppbv the three instruments gave similar results, but at sub-ppbv the results from the three techniques became dissimilar. Tests on the prepared mixtures showed that the luminol results were affected by expected interferences from 03 and PAN. No interferences were found in the TDLAS system, but near the detection limit the data analysis procedures calculated levels of N02 that were too high. The outcome of this intercomparison was close to the ideal the sensitivity, specificity, accuracy, and precision of each instrument were objectively analyzed previous data sets taken by different systems can now be reliably evaluated and each investigator was able to perceive areas in which the technique could be improved. 

Circuitry similar to that presented in Figure 8.13b has been used to analyze cells with impedances ranging from 102 to 1011 Q with 1% accuracy and resolution better than 1 part in 104 over a frequency range of 0.005 Hz to 10 kHz [14]. The technique has been especially useful for studies of the reaction kinetics of moderately fast chemical reactions. Kadish et al. [15] used phase-selective techniques to make ac impedance measurements to evaluate reference electrodes for use in nonaqueous solvents. Recent decreases in the cost of integrated function modules such as analog multipliers, oscillators, and phase-locked loops make this type of phase-selective instrumentation more accessible than ever. 

To evaluate the performance of our method, the response linearity of the instrument was tested. We also determined the detection limits, reproducibility and accuracy. 

Event records (the information is recorded contemporaneously with the actions, e.g., electronic data from laboratory instruments or manufacturing control operations) Review records (records that provide evidence that events, information, or data have been evaluated for content, completeness, or accuracy, e.g., reviews required in 21 1.192. Usually these records are approved.)... 

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