Selecting the Method

As discussed in Chapter 1, analysis involves the determination of the composition of a material, i.e. the identification of its constituent parts and in many cases how much of each is present and, sometimes, in what form each is present. This chapter describes the process of selecting a suitable analytical method to carry out such determinations and how to check that the procedure selected is adequate for the job in hand. Before starting work on a sample, it is vital to enquire why the analysis is being done and what will happen to the result(s) and what decisions will be taken based on the constituent parts identified and the numerical values obtained. It is essential that the requirements of the customer, internal or external, are fully understood. Which property needs to be measured (the measurand) For example, is it the total amount of iron in a tablet that is required or the amount extracted into stomach acid simulant  

Quality Assurance in Analytical Chemistry E. Prichard and V. Barwick 2007 LGC Limited 

The purpose of an analysis and the use to which the analytical report or certificate of analysis might be put are numerous. Can you suggest some of these  

Some examples were given in Chapter 1 but hopefully you will have thought of additional ones. You might have suggested any one, or all, of the following  

Chapter 1 dealt with this in some detail. What we now need to know are the criteria to consider in selecting a method fit for our purpose and where such methods may be found. 

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Even after 30 years of research activity in the field, my personal experience encompasses only a fraction of the methodologies included in the book. I have selected the methods and examples on the basis of utility and reliability as judged by frequency of application in the literature or by indication that specific procedures are especially convenient. I will welcome comments from chemists whose experience, good or bad, will help in recognizing the best of the many methods that are available. 

For construction of suitable samples molecular beam epitaxy was selected, the method of choice for the production of complicated epitaxial layer systems with different materials. As substrates Si wafer material (about 20x20 mm-, thickness 1 mm) and SiO, discs (diameter 30 mm, thickness 3 mm) were used. Eight layered structures (one, two and three layers) were built up with Al, Co, and Ni, with an indicated thickness of 70 nm, each. 

Select the method that is optimal in this case (efficiency, economy, etc.). 

There are many other specific techniques applicable to particular situations, and these should often be investigated to select the method for developing the vapor-liquid relationships most reliable for the system. These are often expressed in calculation terms as the effective K for the components, i, of a system. Frequently used methods are Chao-Seader, Peng-Robinson, Renon, Redlich-Kwong, Soave Redlich-Kwong, Wilson. 

The last tvo approaches represent promising beginnings for new methods to characterize stationary phase selectivity. The methods are evolutionary and not fully developed at present. Their future prospects are quite good and should eventually evolve into a standardized protocol for phase characterization. This is urgently required to make both the selection of stationary phases from those currently available and the rationale synthesis of new phases a logical process. 

The plutonium concentration in marine samples is principally due to environmental pollution caused by fallout from nuclear explosions and is generally at very low levels [75]. Environmental samples also contain microtraces of natural a emitters (uranium, thorium, and their decay products) which complicate the plutonium determinations [76]. Methods for the determination of plutonium in marine samples must therefore be very sensitive and selective. The methods reported for the chemical separation of plutonium are based on ion exchange resins [76-80] or liquid-liquid extraction with tertiary amines [81], organophosphorus compounds [82,83], and ketones [84,85]. 

Selectivity The method you choose must be selective enough to measure the analyte of interest in what may be a complicated matrix. Frequently, not one method is selective enough, and a separation technique must be used before the determination step. Selectivity is a continuum from highly selective to completely non-specific for a given analyte. Different degrees of selectivity can be achieved in different ways (Table 21.7). 

At the beginning of the analytical process the analyst has to select the method of analysis. That at least partly determines the sampling strategy, and it completely determines the preparation of the sample. 

Partial hydrolysis of a fully methylated polysaccharide and investigation of the products as just described may provide valuable information, even when the partial hydrolysis is not very selective. The method has been used in studies of the Klebsiella type 38 (Ref. 17) and type 52 (Ref. 28) capsular polysaccharides. In their studies of the Klebsiella type 56 capsular polysaccharide, Choy and Dutton used a modification of this method.29... 

Select the method of calculation for tray efficiency. Two methods are presented the O Connell method and the two-film method. In the programs accompanying this book, you may select the O Connell method by entering either an F for fractionator or an A for absorbers. In 1946, O Connell [4] published curves on log-log plots showing both absorber and fractionator efficiencies vs. equilibrium-viscosity-density factored equations. Separate curves for absorbers and fractionators were given. Such data have been curve-fit using a modified least-squares method in conjunction with a log scale setup. The fit is found to be reasonably close to the O Connell published curves. 

The effectiveness of dilution is highly chemical-specific and must be well conceived and tested. For instance, the improper addition of another material to a spill could cause the rapid evolution of heat, which could accelerate the evaporation rate and make the situation worse. Even with the proper diluent selection, the method and rate of addition are both important for controlling vapor evaporation rates. If water is to be used as a diluent for a material with a high heat of dilution, such as sulfuric acid or oleum, it must be added rapidly and in a large enough quantity to not only effect the dilution, but also serve as a heat sink for the heat of dilution produced. In this way, high evaporation rates for the pool being diluted will be minimized. 

Select the method to be used. For two-phase flow, the friction pressure drop inside a tube can be calculated using the equation [31]... 

In Chapter 4.2.2 we already discussed methods for the theoretical calculation of the detonation velocity and detonation pressure. In this chapter we now want to focus on the experimental determination of the detonation velocity. Bearing in mind that detonation velocities of known high explosives may reach up to 10,000 m s the experimental determination of the detonation velocity is not easily achieved. There are several methods which are suitable to measure the detonation velocity [38], Most of these methods are based on the fact that the detonation process is accompanied by the emission of light (autoluminous process). Depending on the measuring equipment selected, the methods for the detonation velocity determination can be divided into,... 

Laboratory studies developed several methods from which was selected the method of using cheap and available raw materials. 

Pyranonaphthoquinones. The reaction of phthaloylmetal complexes with alkynes to give naphthoquinones (10, 221-222) has been extended to a synthesis of pyrano-naphthoquinones. An intramolecular version of the reaction is used to control the regio-selectivity. The method is outlined in Scheme (1) for a synthesis of nanaomycin (6) and the cw-epimer (7). 

Cyclopropanation of Allylic Alcohols. Simmons-Smith type cyclopropanation of the allylic alcohol 22 in the presence of a catalytic amount of the bis-sulfonamide la leads to formation of the corresponding cyclopropane 23 in high yield and selectivity (eq 6, Table 3). The reaction is rapid (< 1 h) and can be performed at low temperature (either 0 °C or —20 °C). Substrate scope encompasses both di- and tri-substimted allylic alcohols (24 and 26). However, substimtion at the 2 position, as in 28, leads to a drastic decrease in selectivity. The presence of additional oxygenated functionality (30) in the proximity of the alkene also lessens selectivity." The method is limited to the cyclopropanation of allylic alcohols. Other alkene-containing substrates, such as allylic ethers, homo-allylic alcohols and allylic carbamates, do not react with high selectivity. 

Polyelectrolyte complexes are very promising materials for preparing semi-permeable membranes of definite permeability and selectivity The methods of preparation and the properties of membranes made of polyelectrolyte complexes based on strong polyelectrolytes, e.g. poly(sodium sterene sulfonate) and poly(vinylbenzyl-trimethyl ammonium chloride) were described These membranes may be applied for reverse osmosis in the desalting of sea-water, for dialysis and ultrafiltration in purifications and concentration of water solutions containing coUoids or micro-and macroparticles ... 

Another question is how to select the method with the lower cost. The use of hybrid methods in which part of the elements is introduced from metal-organic precursors offers the best compromise (Figure 12.1a). 

The process of solid flow is extremely complex and dependent on many material and process variables. The answer to the question of which is the best method to use is therefore dependent on your process and what you are trying to predict or measure and select the method or combination of methods that best meets your needs. 

Following the choice of grid type, one has to select the approximations to be used in the discretization process. For the finite volume method, one has to select the methods of approximating surface and volume integrals. The choice of method of approximation influences the accuracy and computational costs. The number of nodes involved in approximation controls the memory requirements, speed of the code and difficulty in implementing the method in the computer program. More... 

Sizes of particle are important in selecting the method of separation. Different authors do not necessarily use the same nomenclature in classifying separations in terms of size. A crude relationship of particle size to particle size measurements and methods of SLS and particle size analysis is provided in Fig. 2. ... 

The first task in the analysis or characterisation of any sample is to consider the requirements. Why is the sample being tested and what information is actually required There can be many reasons for carrying out analysis (as discussed at the start of this section) and these reasons can influence the technique used for the analysis, the method selected and the way the test is actually carried out. When considering the requirements and selecting the method it will normally be necessary to take into account a variety of issues including the foUowing ... 

For the assessment of the contribution of the emission categories to the observed NMVOC concentrations the Chemical Mass Balance (CMB) modelling technique, version 8 from United States Environmental Protection Agency (Watson et al., 1998 Watson et al., 2001) was selected. The method uses source specific ratios between the emission rates of certain set of compounds and aims to recognise these fingerprints, or soiuce profiles, in the profile measured at the receptor point. As a result the CMB model delivers contributions from each source type to the total ambient NMVOC and individual hydrocarbon species at receptor points and their uncertainties. 

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