A Precision and Accuracy

Quantum-chemical calculations now can provide values of enthalpies of formation with a precision and accuracy comparable with thermochemical values and those calculated from statistical thermodynamics. The basis for these calculations is beyond the scope of this text, but it is interesting to observe some values calculated in this way for comparison with other values in Tables 4.3-4.5. The data in Table 4.6 were obtained by a method called Gaussian-3 (G3) [5]. 

Chlorofluorocarbon (CFC) compounds (Freons ) are important tracers of ocean circulation. Since CFC-11 and CFC-12 were released at different rates, their concentrations as well as their ratios can be used to determine when a water mass left contact with the sea surface over the 50 years since Freons were released into the atmosphere and taken up by the ocean. CFC-113 and carbon tetrachloride are being investigated as additional tracers. Picomole levels of CFCs dissolved in seawater are determined using a gas chromatograph equipped with an electron capture detector. The World Ocean Circulation Experiment (WOCE) Hydrographic Program calls for measurements with a precision and accuracy of 1% and a detection limit of... 

Valentine et al. applied, in principle, the method of Midha and Charette for a quantitative assay of quinidines (quinidine and hydroquinidine) in plasma, using methylene chloride instead of benzene for the extraction of a small volume of plasma after basification. To the extract was added the internal standard, cinchonine evaporation to dryness and reconstituting in a methanolic solution of trimethylanilinium hydroxide followed. An aliquot of this solution was analyzed by GLC via on-column methylation reaction. Evaluation of the method over the range 0.5 - 10 ug/ml in human plasma gave a precision and accuracy overall of 4.5 %... 

Figure 4.15 Electron affinities of charge transfer complex acceptors calculated from C2 = 2.9 versus the current best adiabatic electron affinities. This is a precision and accuracy plot. The zero intercept slope indicates that the same quantities are measured. The compounds are maleic anhydride, tetrachlorophthalic anhydride, benzoquinone, trinitro-flourenone, s-trinitrobenzene, chloranil, tetracyanoquinodimethane, and tetracyanoethylene in order of their electron affinities.

Figure 5.13 A precision and accuracy plot of the atomic electron affinities determined before 1967 versus the current best values. The deviations from the unit slope and zero intercept line result from random and systematic errors.

The values of the atomic electron affinities determined before 1967 are plotted against the current EvV in Figure 5.13. This is a precision and accuracy (or P and A) plot. The plot of the electron affinities of the charge transfer acceptors (Figure 4.15) was also a P and A plot. It is used to concisely illustrate the quality of the experimental data. By comparing the data to a line with unit slope and zero intercept, an immediate picture of the precision (random) errors and accuracy (bias) errors can be visualized and outliers identified. By inspection the electron affinities of O (2 eV), Cl (4 eV), and F (4 eV) are suspected outliers. The higher values deviate... 

Conners Advantage (Frequency Precision). The dispersive instrument depends on calibration (polystyrene at 1601 cm 0, and the ability of gears and levers to move slits and gratings in a reproducable fashion. The FTIR carries its own internal frequency standard, usually a He-Ne gas laser, that serves as the master timing clock, tracking mirror movement and frequency calibration to a precision and accuracy of better than 0.01 wavenumbers (cm ). 

Nuclear quadrupole resonance thermometers [83] can be used between 20 and 400 K. In the ultralow temperature range below 1 K, a direct measurement of temperature is feasible by using the spectrometer to measure the intensity ratio of magnetic resonance lines [84]. A precision and accuracy of 1 mK can be achieved. However, these thermometers are quite expensive and are therefore used most often as transfer standards... 

Isotope-ratio mass spectrometry is used to determine the relative isotopic abundances of major and trace elements. Mass spectrometric methods are able to measure the relative concentrations of both radioactive and stable isotopes with a precision and accuracy of better than 0.001%. To achieve measurements of this quality, isotope-ratio MS relies almost exclusively on magnetic-sector instruments operated in temperature-controlled laboratories (Becker and Dietze 2000). 

In the 1970s (Pella and von Baeckman 1969 von Baeckman 1971), an instrument was developed for the automatic dilution and L-X-ray fluorescence analysis (XRFA) of U and Pu in spent fuel solutions with a precision and accuracy of about 1%. 

Coulometry. Two methods of coulometry are used coulometry at controlled potential and coulometric titrations. The main advantage of the coulometric method is the elimination of the necessity of standardization as the Faraday constant is a standard. In analysis of complicated samples encountered in environmental analysis the coulometric titrations are more advantageous where 100% current efficiency can be more readily attained by suitable choice of the reagent-solvent system. Coulometric titrations are suitable for determining the amount of substance in the range 0.01 to 100 mg (and sometimes below 1 iJg). Under optimum conditions these titrations can be carried out with a precision and accuracy of 0.01%. Automatic coulometric analyzers for the determination of gaseous pollutants (SO2, O3, NO, etc.) have proven to be useful in environmental chemistry. 

This instrument, in combination with LA, permits the screening of suspect uranium samples, providing information on their enrichment (U isotopic analysis), and also on the possible presence or absence of other actinide elements, such as thorium or plutonium. The authors reported that it is feasible to achieve a precision and accuracy of 4% for U and Pb isotopic analysis, without performing any mass bias correction, which, in this context, can be considered fit-for-purpose, as discussed earlier. 

The results of this study are summarized in Table VI-8. The agreement is excellent. At the 30 /g/g level a precision and accuracy of the order of 5 % can be obtained. In 1980 these methods were used to certify nitrogen in three existing reference materials for oxygen in zirconium (41). 

As shown in a certification round organized within BCR, different methods can be used to determine oxygen in nickel with a precision and accuracy better than 10 % at the 10 Mg/g level (242). Table VII-28 summarizes the results obtained for oxygen in BCR-CRM No.99 (certified value ... 

Triton activation analysis has been applied, to demonstrate that charged particle activation analysis can also be used at high concentrations and yields a precision and accuracy comparable to those obtained with more classical techniques. 

In a scenario that occurs often in a protein biochemistry lab setting, a researcher has isolated a protein of unknown function, or he/she has overexpressed a protein in Escherichia coli and wishes to characterize it. A common practice today is to submit a small amount of the protein to a core mass spectrometry laboratory for a molecular weight measurement. Using either ESI or MALDl, a molecular weight with a precision and accuracy of 0.05% or better can be measured. This, of course, depends heavily on the purity of the protein sample, the relative size of the protein, the presence or absence of post-translational modifications (PTM) (e.g., phosphorylation, glycosylation, etc.), the resolution of the mass analyzer, and so on. Primary structure information (i.e., amino acid... 

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