Assurance of Measurement Compatibility

Direct calibration of methods and instrumentation i.e. ensuring that an analytical device is giving a correct reading. For some types of direct soUd sample analysis, sample results can be calibrated using several CRMs with suitable matrices (Kur-furst 1998) see also Section 4.4. 

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Matrix SRM s, Matrix SRM s can be used for a variety of functions. Taylor (71) reviewed the uses of matrix standards, which Include (a) method development and evaluation, (b) establishment of measurement traceability, and (c) assurance of measurement compatibility. During certification of these matrix SRM s, the selectivity of fluorescence... 

The selection of methods used in the determination of persistent organic contaminants depends on the physical and chemical properties of the analytes imder study. Liquid chromatography is often well suited to applications that are not compatible with gas chromatography, and LC is the preferred method for the determination of POPs that are polar, thermally labile, or isomeric. Mass spectrometric detection is essential to achieve sufficient specificity and sensitivity for trace level analysis of complex matrix environmental samples. CRMs and SRMs serve as useful tools in the development and validation of new methods, as control materials in quality assurance, and in instrument calibration. Proper use of reference materials can also support claims of measurement traceability. 

Assessment of the present situation regarding quality of spectrophotometric measurements [5] suggested the need for basic principles to be followed to assure the required traceability. Lately, some efforts have been focused on elaborating traceability schemes for clinical chemistry measurements. The purpose of any traceability scheme is to provide comparability, compatibility and consistency between the huge numbers of chemical measurements needed everyday, universally in clinical laboratories. On the other hand, the quality accredita-... 

Robinson and co-workers have analyzed ESR TTc as a fimction of temperature measured at 9.5 and 16 GHz and as a function of frequency at rt, as shown in Figure 6.34 [164]. They assumed a model that the mechanism for is the modulation of the electron-electron dipolar interaction due to scattering of the spins by acoustic phonons. They also assumed two levels of diffusion (1) almost free rapid motion within a domain of 50 carbons as proposed by the ENDOR analysis in an unoriented C enriched sample [117], which dominates the ESR 77,. at the X-band and (2) slow migration of such a domain which assures compatibility with the motional narrowing of the ESR linewidth. The rapid diffusion within the domain is not of activation type and the diffusion rate increases with decreasing temperature. On the contrary, the migration is of activation type. However, such an analysis of the ENDOR data [117] was pointed out to be incorrect in section 3.2.5 [105]. Then, it is probable that this model will give rise to some inconsistency with other observations as follows. 

While MALDI is a typical off-line ionization technique/interface, one can also implement classical interfaces to carry out off-line (or at-line) analyses of reaction mixtures. Aliquots can be obtained from a reaction mixture (a dynamic sample/matrix) at specific time points, and injected to the ion source of a mass spectrometer for analysis (e.g., [49-54]). In some cases, quenching is conducted [49, 50]. If the transient intermediates (e.g., radicals) are to be detected, it is important to assure that the reaction has not finished at the time of ionization [55]. Short-lived intermediates can be reacted with auxiliary compounds in order to enable subsequent measurements in the methodology referred to as spin-trapping (see, e.g., [56, 57]). The off-line analyses based on aliquoting of dynamic matrices, and subsequent separation, provide temporal resolutions in the order of a few minutes (see, e.g., [58]). Nonetheless, they are uncomplicated, and - in some cases - they may enable offline sample treatment prior to detection by MS. For instance, inorganic ions, present in the reaction mixture, can readily be removed on an exchange resin to render the collected samples compatible with MS [59]. 

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