Precision, of measurement

The precision of measurement does not appear to be very high. Confidence levels in the precision may be made by use of "Student f" Tables. 

From a series of isotope ratio measurements, the precision of measurement can be assessed statistically, as shown here. Precision reveals the reproducibility of the measurement method, but it does not provide information on the accuracy of the measurement (see also Figures 48.8 and 48.9). 

The precision of measurement is clearly much better than in the series given In Figure 7 since the variation from the mean is now less than 1. 

Now, at high linear velocities, the longitudinal diffusion term will become insignificant and, equally important, the resistance to mass transfer term that incorporates the inverse function of diffusivity will become large, thus improving the precision of measurement. 

Purification of solvents and salts is essential for reliable electrochemical studies and measurements. A water content of 20ppm already corresponds to a 10 3molL solution. This is in the concentration range of dilute solutions used in conductivity studies for the determination of association constants (see Sec.7.3.2). Traces of water may affect chemical equilibria and therefore act on specific conductivities and limiting ion conductivities. For example, addition of 30 ppm water to a 2xl0-4 mol LT1 solution of LiBF4 in THF at 15 °C increases its conductivity by 4.4 percent (precision of measurements about 0.02 percent) 380 ppm water causes an increase by 51.7 percent see Fig. 3 [20J. 

Loops are not calibrated accurately and a loop of nominally 20 p.l is unlikely to have this exact volume. This will not affect either the precision of measurement and, as long as the same loop is used for obtaining the quantitative calibration and for determining the unknowns , the accuracy of measurement. 

Chlorambucil - there is no problem with the quantitation ion (at m/z 254), although the second ion proves to be a little difficult. While the ion at m/z 303 is the obvious choice, this is not very intense and therefore for samples containing small amounts of analyte the precision of measurement of this ion will be reduced and it may not be detectable at all levels at which the quantitation ion is observed. We could possibly consider the (M- -2) ion, as the combination o/m/z 254 (high mass, and therefore reasonable specificity), the presence of one chlorine, and the chromatographic retention time could be considered sufficient for definitive identification in those cases in which the intensity o/m/z 303 is insufficient. 

The assumed residual standard deviation, i.e., the precision of measurement, can be varied to study its effect. 

When 1.00 mole of hydrogen reacts with oxygen, a few nanograms are converted to energy. This amount, which is typical of the mass consumed in conventional chemical reactions, is too small to detect. Thus, within the precision of measurements, mass is conserved in ordinary chemical reactions. 

International Standard ISO 5725, Accuracy (Tmeness and Precision) of Measurement Methods and Results, first edition, 1994-12-15 Part 2, International Standards Organization, Geneva... 

The key factor in voltammetry (and polarography) is that the applied potential is varied over the course of the measurement. The voltammogram, which is a current-applied potential curve, / = /( ), corresponds to a voltage scan over a range that induces oxidation or reduction of the analytes. This plot allows identification and measurement of the concentration of each species. Several metals can be determined. The limiting currents in the redox processes can be used for quantitative analysis this is the basis of voltammetric analysis [489]. The methods are based on the direct proportionality between the current and the concentration of the electroactive species, and exploit the ease and precision of measuring electric currents. Voltammetry is suitable for concentrations at or above ppm level. The sensitivity is often much higher than can be obtained with classical titrations. The sensitivity of voltammetric... 

Accuracy (Trueness and Precision) of Measurement Methods and Results - Part 1. General Principles and Definitions , ISO 5725-1 1994, International Organization for Standardization (ISO), Geneva, Switzerland 1994. 

There are now a number of options open to scientists who need to analyse stable isotopes as part of their research. The choice is dependent on many factors, such as the type of sample, the precision of measurements required, the amount of sample available, and the number of samples to be analysed. Dual inlet IRMS is still the technique of choice those scientists who need to measure a few samples to very great precision. More often, scientists want to determine a trend in conditions over time, or a flux integrated over an area. In these cases, CF-IRMS will be preferred. Continuous-flow IRMS is also preferred when a limited amount of sample material is available, as it is able to analyze far smaller samples (between 10 and several hundred times smaller) than DI-IRMS. 

Flame AAS can be used to measure about 70 elements, with detection limits (in solution) ranging from several ppm down to a few ppb (and these can be enhanced for some elements by using a flameless source). Both sensitivity and detection limits (as defined fully in Section 13.4) are a function of flame temperature and alignment, etc. The precision of measurements (precision meaning reproducibility between repeat measurements) is of the order of 1-2% for flame AA, although it can be reduced to <0.5% with care. The accuracy is a complicated function of flame condition, calibration procedure, matching of standards to sample, etc. 

The infrared technique has been described in numerous publications and recent reviews were published by Davies and Giangiacomo (2000), Ismail et al. (1997) and Wetzel (1998). Very few applications have been described for analysis of additives in food products. One interesting application is for controlling vitamin concentrations in vitamin premixes used for fortification of food products by attenuated total reflectance (ATR) accessory with Fourier transform infrared (FTIR) (Wojciechowski et al., 1998). Four vitamins were analysed - Bi (thiamin), B2 (riboflavin), B6 (vitamin B6 compounds) and Niacin (nicotinic acid) - in about 10 minutes. The partial least squares technique was used for calibration of the equipment. The precision of measurements was in the range 4-8%, similar to those obtained for the four vitamins by the reference HPLC method. 

Seisakusho differential refractometer). For a solution of known concentration, the difference in refractive index, (n - nj>), between the liquid in the prism and that in the solvent bath causes refraction of the light beam, which is observed as a displacement 8 of the image at a distance d from the prism centre. The overall sensitivity depends on the precision of measuring 8 and the refractive index difference is given by... 

Table 7.9 Precision of measurement for CH4/CD4 VPIE s at three temperatures (Calado, J. C. G. et al. J. Phys. Chem. 93, 3355 (1989)) ...

In general, the strengths of the ELISA are its selectivity and specificity, whereas its weaknesses are related to precision of measurement. Each assay varies — depending on the antibody, enzyme, enzyme conjugate, and measurement, as well as on assay format. Each should be validated so that its unique performance characteristics are known. 

It should be clear from the preceding discussions that practical application of equilibrium stable isotope fractionation theory often requires a certain amormt of simplification of complex and poorly studied systems. Given this reality, one should not be surprised to find that theoretically determined equilibrium fractionations rarely achieve accuracies approaching the nominal precisions of measurements made with modem analytical techniques. It should... 

International Organization for Standardization (ISO), Statistical methods for quality control, Vol. 2, 4th Edition, Accuracy (trueness and precision) of measurement methods and results - Part 2 Basic method for the determination of repeatability and reproducibility of a standard measurement method, ISO 1994(E), 5725-2. 

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