Sampling methods disadvantages

However, it does have some disadvantages. The / and g histograms obtained from a finite-length simulation contain errors themselves. The histogramming and graphical analysis require additional steps in the calculation, and can introduce new sources of errors [54]. Better alternatives, such as Bennett s method and the overlap sampling method, will be discussed further in this chapter. 

There is also a practical disadvantage of the external validation method. It requires that the reference analytical method be performed on an additional set of samples, namely the external validation samples. Considering the possible high cost of the reference analytical method, and the possibility of requiring a large number of external samples to provide sufficient representation of the calibration samples, this disadvantage can be rather costly. 

Methods are not presented in detail, space considerations alone would not permit this. Instead, the chemist is presented with details of methods available for a variety of types of water samples. Methods are described in broad outline, giving enough information for the chemist to decide whether he or she wishes to refer to the original paper. To this end information is provided on applicability of methods, advantages and disadvantages of one method compared to another, interferences, sensitivity, detection limits and data relevant to accuracy and precision. 

More success was obtained using on-line solid phase extraction before offline analysis by capillary electrophoresis with matrix-assisted laser-desorption ionization (MALDI) MS [18]. Because microdialysis is a method used for analysis of small molecules and peptides, MALDI is not used frequently with microdia lysis sampling. Another disadvantage is that MALDI cannot be used on-line with the separation because it is a vacuum ionization technique. However, if the pep-tide is large enough (0 1000 Da) MALDI can be useful. For the analysis of peptides in dialysate an appropriate separation is important before mass spectrometric detection. In a comparison with direct sampling of dialysate in MALDI, capillary electrophoresis provides the high efficiency separations necessary to resolve all... 

Lower (i.e.,better) detection limits are achieved with the thermal desorption method owing to the higher aliquot transferred to the analytical device. The shortcoming of the charcoal method is the injection of only a very small part (aliquot) of a sample, which can only be partly compensated by sampling a larger air volume. The disadvantage of the thermal desorption technique is that quality assurance requires the collection of at least duplicate samples. Another disadvantage of this method is the need for special equipment and specially trained personnel. 

The major disadvantages of the on-column injection approach are associated with drenching the column inlet directly with the introduced liquids, and a repeated deposition of non-volatile impurities. As a solution to the former problem, the development of immobilized stationary phases appears appropriate. However, removal of non-volatile sample impurities requires more judicious sample clean-up, a potential source of compound losses that may well counterbalance the advantages of the sampling method. While technical improvements are still needed, the on-column injection in capillary GC has undeniable advantages for the analyses of biological materials. 

For more practical purposes, therefore, one should take recourse to metal particles as produced by other means, in particular on supports or in matrices. The advantage is the availability of macroscopic amounts of sample the disadvantage is that interaction with the supporting medium must be assessed. A great variety of synthetic methods exists, of which we can mention only a few. Metal clusters can be produced by aerosol techniques, by vapor deposition, by condensation in rare-gas matrices, by chemical reactions in various supports, e.g. zeolites, SiOi, AI2O3, or polymer matrices. Many different metal-nonmetal composites, such as the ceramic metals (cermets) have been obtained with metal particles with sizes varying from nanometers upward. In alternative approaches, metal particles are stabilized by chemical coordination with ligand molecules, as in metal colloids and metal cluster compounds. 

This sampling method is similar to the sampling from a truck. The same sampling quantities are required and it has the same advantages and disadvantages as the sampling from a truck. One additional disadvantage is that there is the possibility of heap contamination by other materials and hence contamination of the sample received. 

Both alternative sampling methods have the advantages that there is certainty of the location of the material in the pavement, there is minimal risk of segregation and there is no interruption to paving operations. The disadvantages are that there is a possibility of affecting the finished surface and it requires more labour. 

The advantages of this method are that it is straightforward and is not affected by different values for different samples. The disadvantage is that the injected amount of sample must be known accurately. This implies that the injection volume is known accurately. 

One disadvantage of this method is that it is not possible to measure surface-active solutions, since the self-centering of the rotor depends on the surface tension of the sample. Another disadvantage is that different types of rotors for different solution densities have to be used. 

The advantages of the GC method are its simplicity, ease of standardization, and lack of cleanup steps in the sample preparation. Disadvantages are the relatively large amount of sample required and the inability to separate thiamine phosphate esters. 

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