Choice of Analysis Method

Since there are the two techniques available for estimating the change in property that will result at a time or temperature other than those experimentally chosen, the question arises as to which is the better. Both techniques have certain advantages and drawbacks. 

The WLF approach relies on the validity of the time-temperature superposition principle whilst the Arrhenius approach is dependent on the validity of the assumption that increasing temperature merely increases the rates of change and does not introduce new types of change. Neither of these is likely to be completely true in all cases, primarily because there will be different reactions taking place at different temperatures. 

The WTF approach provides a master curve that encapsulates the whole of the accumulated data and therefore potentially provides the greatest information. The Arrhenius approach on the other hand usually disregards the bulk of the data gathered being limited to a specific end point, which may have been arbitrarily chosen. If it is possible to use all the data at each temperature to obtain a reaction rate then this disadvantage of the Arrhenius approach would disappear. 

Although the Arrhenius approach is mathematically simpler, with computer help the WLF approach is practically easier to use because of there being no need to specify a measure of reaction rate nor to make any assumptions when interpolating between points. 

The WLF approach is also more versatile in that it is relatively easy to produce predictions in terms of time to reach an end point and as change in a given time. With the Arrhenius approach this necessitates re-doing the calculation completely with a different measure of reaction rate. 

---

Analytical Methods. Most analytical methods use the oxidizing power of iodine for its deterrnination. The results are generaHy expressed as an equivalent concentration of elemental iodine. The choice of a method for the analysis of iodine depends on the concentration range to be deterrnined. 

Computational fluid dynamics (CFD) is the numerical analysis of systems involving transport processes and solution by computer simulation. An early application of CFD (FLUENT) to predict flow within cooling crystallizers was made by Brown and Boysan (1987). Elementary equations that describe the conservation of mass, momentum and energy for fluid flow or heat transfer are solved for a number of sub regions of the flow field (Versteeg and Malalase-kera, 1995). Various commercial concerns provide ready-to-use CFD codes to perform this task and usually offer a choice of solution methods, model equations (for example turbulence models of turbulent flow) and visualization tools, as reviewed by Zauner (1999) below. 

K. Mehlhose, Explosivstoffe 20 (3-4), 37-70 (1972) CA 78, 113513 (1973) Methods developed for detn of H20 in gun and rocket propints are column distn with n-PrOH and gas-chromatographic analysis of the distillate, corrected for continuous formation of water in decompn reactions and photometric methods, both based on the reaction of CoCl2 with water. Choice of the methods is based on a critical review (117 refs) of the usual methods of water detn by chemical and physical methods. A theoretical and exptl analysis of the new methods was made and their results compared with those of older methods. Application to mono-, di-, and tribasic and to double-base NC and poly (vinyl nitrate) propints is discussed... 

The choice of the method of analysis depends on the question to address. Spectrophotometry is sufficient for total curcuminoid content determination in a turmeric extract. Separation techniques coupled to mass spectrometry detection and MALDl-TOF are highly sensitive techniques that are more adapted to the identification of metabolites in biological fluids such as urine or plasma. ... 

Gardner and Yates [26] developed a method for the determination of total dissolved cadmium and lead in estuarine waters. Factors leading to the choice of a method employing extraction by chelating resin, and analysis by carbon furnace atomic absorption spectrometry, are described. To ensure complete extraction of trace metals, inert complexes with humic-like material are decomposed by ozone [27]. The effect of pH on extraction by and elution from chelating resin is discussed, and details of the method were presented. These workers found that at pH 7 only 1-2 minutes treatment with ozone was needed to completely destroy complexing agents such as EDTA and humic acid in the samples. 

The implications of the analysis have to be considered before taking the sample or devising a sampling scheme. It is the responsibility of the analytical chemist, through discussion with the customer, to establish the real nature of the problem. How much cadmium is there in this sample is not sufficiently specific. You must always ask why the information is required. The answer affects both the sampling plan and the choice of analytical method. These will depend on the acceptable level of uncertainty in the final result. 

Sample size and matrix Your choice of analytical method will also be dependent on the amount of sample you have, especially if the amount is limited and some of the methods under consideration are destructive to the sample. In the Bulging Drum Problem, sample size was not an issue. However, sampling the gas in the drum was challenging, since loss and contamination were quite likely. Getting the samples to the lab presented other challenges. Sample matrix is another important factor in method choice. As you know, some methods and instrumental techniques are not suitable for analysis of solids, without sample preparation. Table 21.8 lists some of the issues that must be considered for different sample matrices. 

The abovementioned data show that the spectral characteristics of C60/PVP complexes vary depending on the PVP molecular mass and fullerene content in the complex. Therefore, the quantitative determination of fullerene concentration in such complexes by measuring their absorbance at 336 nm without extraction (Lyon et al., 2006) can give non-reliable results. For quantitative analysis of fullerene in such complexes we used the heterophase and homophase methods, based on destruction and isolation of pure fullerene C60 (Krakovjak et al., 2006). The choice of the method was determined by fullerene concentration - at concentration less... 

APCI is widely used nowadays in different application fields for low molecular weight analytes. Many of them can either be analyzed with ESI or APCI, and the choice of the method should take into account several aspects, such as the physical-chemical properties of the molecule, the mobile phase composition and the required flow rate, and possible matrix effects. Typical APCI applications are in pharmaceutical, environmental, and food safety analysis. 

Existence and uniqueness of the particular solution of (5.1) for an initial value y° can be shown under very mild assumptions. For example, it is sufficient to assume that the function f is differentiable and its derivatives are bounded. Except for a few simple equations, however, the general solution cannot be obtained by analytical methods and we must seek numerical alternatives. Starting with the known point (tD,y°), all numerical methods generate a sequence (tj y1), (t2,y2),. .., (t. y1), approximating the points of the particular solution through (tQ,y°). The choice of the method is large and we shall be content to outline a few popular types. One of them will deal with stiff differential equations that are very difficult to solve by classical methods. Related topics we discuss are sensitivity analysis and quasi steady state approximation. 

Measure the concentration of analyte in several identical aliquots (portions). The purpose of replicate measurements (repeated measurements) is to assess the variability (uncertainty) in the analysis and to guard against a gross error in the analysis of a single aliquot. The uncertainty of a measurement is as important as the measurement itself, because it tells us how reliable the measurement is. If necessary, use different analytical methods on similar samples to make sure that all methods give the same result and that the choice of analytical method is not biasing the result. You may also wish to construct and analyze several different bulk samples to see what variations arise from your sampling procedure. 

Other types of reactions which have been studied using FABMS include those catalyzed by enzymes. This application is particularly interesting because it represents for the first time a generally useful and molecularly specific probe with which to measure a wide variety of enzyme substrates and products. Two approaches have been successful, one in which the reaction is followed by the removal of aliquots of sample taken at timed intervals with subsequent analysis by FABMS and the other allowing the reaction to take place in a glycerol-water mixture on the probe directly inside the mass spectrometer. The choice of either method depends upon the application. If the prime interest is to analyze a substrate, for example, monitoring the release of amino acids from a polypeptide using an exopeptidase, then direct analysis inside the spectrometer may be preferred. If, on the other hand, the prime interest lies... 

Another factor to consider in the choice of ionization method is the formation of adducts with sodium or other metal ions, in addition to the protonated molecule. Although these may help confirm the molecular mass, they tend to lower the signal-to-noise ratio of the protonated molecule, resulting in higher detection limits in trace analysis and causing problems in quantitative analysis. Sodium... 

As shown, various calibration methods can be applied in chemical analysis. The choice of method depends on the kind of analytical problems and sources of random errors expected in the course of analysis. Nevertheless, it is hard to say that any of the discussed methods is especially adapted to trace analysis. However, because of its specificity, trace analysis does require special attention in the choice of calibration method, as well as special care in realization of the selected method at every step of the calibration procedure. 

The activities of some isotopes, in particular °Sr- °Y, can also be detected by liquid-crystal spectrometry with the use of the Cherenkov phenomenon [10, 11]. The Cherenkov effect is used to determine beta isotopes emitting particles whose iiniax IS above 500 keV [12]. The main advantage of beta activity determination by the Cherenkov effect is the use of analytical preparation used for another chemical analysis (e.g. calculation of recovery). Moreover, the addition of low energy beta or alpha radiation does not disturb the measurement, thereby lowering the cost of analysis. The weakness of this method is the decreased recovery registration and the decline in information about the realistic appearance of the beta spectrum [13]. The determination of beta isotopes in environmental samples is very difficult and requires their chemical isolation. The type of sample and the time of chemical analysis determine the choice of analytical method. Also, the time between contamination and sample collection is important procedures used for samples recently contaminated are different to those used for old samples in which the decay of short-lived radionuclides has aheady taken place [1, 5]. 

It is important to make the distinction between the determination of polymorphic identity and polymorphic purity. The former is essentially a qualitative determination, asking the question, Ts a particular polymorph present in a given sample The latter is a question of quantitative analysis, and it is generally (though not always) assumed that the sample is chemically pure, so the analytical problem to be addressed is the determination of the relative amounts of different polymorphs in the sample. Recalling that different polymorphs are for all intents and purposes different solids, the determination of polymorphic purity is then no different in principle from quantitative determination of the composition of a mixture of solids. Such quantitative determinations comprise one of the traditional activities of analytical chemistry, especially when the materials are different chemical entities. In those cases, a variety of different analytical methods may be employed. In the case of polymorphic mixtures, or the determination of polymorphic purity, the choice of analytical method is considerably more restricted, and X-ray diffraction is one of the most definitive techniques (see e.g. Stowell 2001). 

---