Choosing a method

When presented with an ion analysis problem, the worker may use a logical process of decision making to devise a working method for a new type of determination. The method may be based on one already published in the literature or on a standard method published by organizations such as the EPA, AOAC, or ASTM. Or the method may be entirely new, based on analyzing the problem of sample mixture and matrix. 

There are probably several different methods that could be developed to solve the same problem. The decision to use one method over the other is frequently based on the availability of a certain column or detector. The method chosen may not even be the best available in a perfect world, but it may be the best given financial, time or instrumental constraints. Certain sacrifices may have to be made on sensitivity, accuracy, and the ease of analysis, e.g., the number of dififerent rans needed. It is even possible a decision may have to be made on whether the total goal can be accomplished. 

The chances of success are far greater if the literature is first searched. Good research papers or published methods are based on comprehensive testing of the proposed method. Gaining access to this experience can be quite valuable when faced with a new problem. Several papers may be published on a particular topic that, together, outline the various options available. Even a paper that shows Hm-ited success is useful because it can be compared to more successful methods. 

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AppHciilion s of seim-empirital and ah initio maLhods rrec in iuly follow Lhc course of chemical rcacLioii s that break anti form covalent bonds. Yon must take care in choosing a method for these application s. 

In general, the computation of absolute chemical shifts is a very difficult task. Computing shifts relative to a standard, such as TMS, can be done more accurately. With some of the more approximate methods, it is sometimes more reliable to compare the shifts relative to the other shifts in the compound, rather than relative to a standard compound. It is always advisable to verify at least one representative compound against the experimental spectra when choosing a method. The following rules of thumb can be drawn from a review of the literature ... 

Organic molecule calculations can be done routinely to good accuracy on workstation-class hardware. It is advisable to examine tabulations of results in order to choose a method with acceptable accuracy and computational time for the property of interest. The trend toward having microcomputer versions of computational chemistry codes is making calculations on small organic molecules even more readily accessible. 

The requirements of the analysis determine the best method. In choosing a method, consideration is given to some or all the following design criteria accuracy, precision, sensitivity, selectivity, robustness, ruggedness, scale of operation, analysis time, availability of equipment, and cost. Each of these criteria is considered in more detail in the following sections. 

When one tries to fit a mathematical function to a set of data one has to choose a method or algorithm for achieving this end, and choose a weighting model with which to judge the goodnss of fit. 

Just as there is no prescription in the algorithm that defines the fitness function itself, we are free to choose a method for scaling that seems to be... 

To be able to identify the factors which have to be considered when choosing a method of analysis. 

In Section 4.3, some factors which need to be considered in choosing a method of analysis were discussed in general terms. The next step is to consider the properties of a method that will enable a choice to be made. This is done for a specific case, e.g. the determination of residues of chemicals used in veterinary practice to treat animal diseases and to prevent the development and spread of disease where large numbers of animals are kept in close proximity to each other. Such chemicals may be administered by injection, or orally as a constituent of the feed. Some chemicals are metabolized and excreted while others may be partially retained in edible products such as milk, eggs, meat and offal (liver or kidney). The detection and determination of such residues is a very difficult analytical problem. 

Many of the technical requirements of the Standard are covered in Chapters 4 to 7. The analytical requirements, including choosing a method and method validation, are covered in Chapter 4. The other measurement requirements, such as calibration, traceability and equipment qualification, are dealt with in Chapter 5. Some of the general issues not covered elsewhere are mentioned in the following sections. It has already been mentioned that staff should be trained and proven to be competent to carry out the testing. This applies to permanent and contracted staff. The laboratory should have a job description for all members of staff. There are more stringent requirements on staff who are also able to provide customers with opinions or interpretation of the results. 

Choosing a method to determine isotope effects on rate constants, and selecting a particular set of techniques and instrumentation, will very much depend on the rate and kind of reaction to be studied, (i.e. does the reaction occur in the gas, liquid, or solid phase , is it 1st or 2nd order , fast or slow , very fast or very slow , etc.), as well as on the kind and position of the isotopic label, the level of enrichment (which may vary from trace amounts, through natural abundance, to full isotopic substitution). Also, does the isotopic substitution employ stable isotopes or radioactive ones, etc. With such a variety of possibilities it is useless to attempt to generate methods that apply to all reactions. Instead we will resort to discussing a few examples of commonly encountered strategies used to study kinetic isotope effects. 

How pure must your product be If you require high purity, you could choose a method that involves a volatile compound as starting material because these are generally easier to purify than solids. 

A brief comparison of the different methods is given in Table 2.5. You may notice that the methods that use more information (i.e., the value of the function, not only its sign a pair of values, not only one of them) converge more rapidly. You already know, however, that robustness is decreasing along the same line. Therefore, choosing a method you aught to consider how much is known on the form of the function and the position of its roots. 

In choosing a method for the recovery of solvent operational safety is a primary factor to be taken into account. While the machinery is in operation the greatest danger is associated with the mixture of air and alcohol-ether vapour which flows through the pipelines and appears in various parts of the plant. 

All goods are adequately identihed using a code numbering system (choose a method suitable to your company). 

Taking into account the nature of the analyte to be determined, the starting point consists of choosing a method of ainalysis the spectroscopic method, electrochemical method, separation method, etc. 

In choosing a method, we also consider selectivity and sensitivity. Selectivity (also called specificity) means being able to distinguish analyte from other species in the sample (avoiding interference). Sensitivity is the capability of responding reliably and measurably to changes in analyte concentration. A method must have a detection limit (discussed in Section 5-2) lower than the concentrations to be measured. 

There are many ways of solving the differential-difference equation (2.1) we choose a method that can also be used in more general cases. The essential tool is the probability generating function F(z, t) defined in 1.2 ... 

Owing to small inaccuracies, the lines do not intersect in a single point in XYZ color space. Thus, we have to choose a method to find this intersection. One possibility would be to compute the intersection for all possible combinations between two lines. This gives us a set of intersections (Herrmann 2004)... 

Morris, G. E. (1996) Choosing a method for epitope mapping, in Epitope Mapping Protocols (G.E. Morris, ed.), Humana Press. Totowa, NJ, pp. 1-9. 

So far as Eq. (5.197) can deliver them, quantitatively accurate reaction rates, say to within a factor of 2, require activation energies accurate to within about 2 kJ mol-1. Nevertheless, the equation does provide a simple way of obtaining serviceably good rate constants. The (admittedly small) selection of reactions here shows no bias toward low or high calculated barriers for any of the four methods, and for a particular kind of reaction it is advisable to choose a method based on a comparison of methods with experiment results where this information is available. 

Performing the chemical speciation analysis on the unaltered specimen with SIMS alleviates this uncertainty. However, the results are fundamentally more difficult to interpret, for certain species that are more ambiguous, and for other species with which the technique cannot be made to work. Each situation is obviously quite different and care must be exercised in choosing a method. 

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