Principles of Analytical Measurement

In the following, the stages of the analytical process will be dealt with in some detail, viz. sampling principles, sample preparation, principles of analytical measurement, and analytical evaluation. Because of their significance, the stages signal generation, calibration, statistical evaluation, and data interpretation will be treated in separate chapters. 

Figure 10.8. Principle of analyte measurement in a flow cell [101].

The principles of quality assurance are commonly related to product and process control in manufacturing. Today the field of application greatly expanded to include environmental protection and quality control within analytical chemistry itself, i.e., the quality assurance of analytical measurements. In any field, features of quality cannot be reproduced with any absolute degree of precision but only within certain limits of tolerance. These depend on the uncertainties of both the process under control and the test procedure and additionally from the expense of testing and controlling that may be economically justifiable. 

Intensified metabolic control, especially in case of diabetes, demands minimal-invasive or non-invasive methods of analytical measurement. For this goal, a method has been developed to measure the blood glucose content in vivo, in direct contact with the skin, by means of diffuse reflection near infrared (NIR) spectroscopy on the basis of multivariate calibration and neural networks (Muller et al. [1997] Fischbacher et al. [1997] Danzer et al. [1998]). Because no patients with any standard blood glucose value are available in principle, a method of indirect calibration has... 

The principle of chemical measurements, as realized in the analytical process (Fig. 2.1), corresponds in its significant steps to the general principle of information processing (Fig. 3.1). 

Measurement uncertainty is increasingly gaining attention, in particular in the framework of accreditation. The new accreditation standard ISO/IEC 17025 [17], which has been in force from December 2002 on, contains clear requirements on the estimation of MU and when and how it should be stated in test reports. ISO/IEC 17025 requires MU to be reported when required by the client and when relevant to the application and interpretation of the measurement results in the framework of certain specifications or decision limits. The MU should be readily available and reported together with the result as X U, where U is the expanded uncertainty [17, 47, 51, 54]. Also Eurachem and CCMAS within the Codex Alimentarius deal with MU as a separate issue [14,18-20]. Some even claim that MU will become the main unifying principle of analytical data quality [37]. 

M17. Mellon, M. G., General principles of absorptimetric measurements. In Analytical Absorption Spectroscopy (M. G. Mellon, ed.). Wiley, New York, 1950. 

In aqueous solutions the principles of analytical chemistry established the protocol to use strong acids (or bases) as titrants they "see" all bases (or acids) in the system, and the result of titration is a "total basicity" (or "acidity") number. If an additional method to measure the concentration of "free" acid at different titration points and under equilibrium conditions were available, then one could use aqueous titration methods to isolate bases (or acids) of different strengths that might be present in the system and evaluate their amount. 

The sensors combine preconcentration of an intermediary product with a bio-catalytic indicator system. Oxygen probes as well as chemically modified electrodes are the base sensors. The principle of the measurement is illustrated in Fig. 4. In the first step of the measurement the reaction of the analyte Si and a saturating concentration of appropriate cosubstrate A proceeds for a certain time during which an intermediate product I is formed by the generator enzyme Ei. The intermediate is accumulated in the enzyme membrane, due to its slow diffusion. When this reaction approaches equilibrium, the second step, the actual measurement, is triggered by injection of an excess of substrate (S2) of the indicator enzyme (E2), which converts the accumulated intermediate under... 

Electroanalytical Methods Many publications described the use of electro-analytical methods for measuring uranium in water and some examples are discussed here. A detailed review article with illustrative tables that summarize the electroanalytical methods for the determination of uranium can be found elsewhere (Shrivastava et al. 2014). The tables in that review article list the method, the principles of the measurement technique, the linear range, limit of detection, tolerance to interferences (where defined), and the field in which the method is applied. 

Schweitzer M, Pohl M, Hanna-Brown M, Nethercote P, Borman P, Hansen G, et al. Implications and opportunities of applying QbD principles to analytical measurements. Pharm Tech 2010 34 52-9. 

One of the main purposes of measuring NIR data is the determination of chemical composition or physical properties in a quantitative way. The principle of the measurement procedure for quantitative analysis is based on recording the NIR spectra of reference samples (the number depending on the number of components or parameters to be determined) of known composition. The levels of the constituents or the physical parameters are determined by independent, conventional analytical or physical methods. Then the set of reference spectra and the independently determined values of the parameters under investigation are used by a selected statistical method to build a calibration. This enables unknown samples to be evaluated with regard to the individual parameters of interest. The accuracy of the NIR technique depends upon the validity of the calibration data set, which must incorporate the entire range of concentrations that will be determined by the instrument. This set must contain samples with varying ratios of each component. NIR calibrations do not typically extrapolate or interpolate well across concentrations. Typical calibration sets include more than... 

Turbidimetry and Nephelometry. In contrast to classical absorbance methods, immunoassay reactions frequently involve agglutination in which the optical scatter signal of the agglutinated particles is measured by turbidimetric or nephelometric means. The principles of light scattering as it relates to analytical methods is discussed in reference 6. 

The abundance of a trace element is often too small to be accurately quantihed using conventional analytical methods such as ion chromatography or mass spectrometry. It is possible, however, to precisely determine very low concentrations of a constituent by measuring its radioactive decay properties. In order to understand how U-Th series radionuclides can provide such low-level tracer information, a brief review of the basic principles of radioactive decay and the application of these radionuclides as geochronological tools is useful. " The U-Th decay series together consist of 36 radionuclides that are isotopes (same atomic number, Z, different atomic mass, M) of 10 distinct elements (Figure 1). Some of these are very short-lived (tj j 1 -nd are thus not directly useful as marine tracers. It is the other radioisotopes with half-lives greater than 1 day that are most useful and are the focus of this chapter. 

Historically, EELS is one of the oldest spectroscopic techniques based ancillary to the transmission electron microscope. In the early 1940s the principle of atomic level excitation for light element detection capability was demonstrated by using EELS to measure C, N, and O. Unfortunately, at that time the instruments were limited by detection capabilities (film) and extremely poor vacuum levels, which caused severe contamination of the specimens. Twenty-five years later the experimental technique was revived with the advent of modern instrumentation. The basis for quantification and its development as an analytical tool followed in the mid 1970s. Recent reviews can be found in the works by Joy, Maher and Silcox " Colliex and the excellent books by Raether and Egerton. ... 

In this chapter we discuss the principles of the Kalman filter with reference to a few examples from analytical chemistry. The discussion is divided into three parts. First, recursive regression is applied to estimate the parameters of a measurement equation without considering a systems equation. In the second part a systems equation is introduced making it necessary to extend the recursive regression to a Kalman filter, and finally the adaptive Kalman filter is discussed. In the concluding section, the features of the Kalman filter are demonstrated on a few applications. 

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