Specialised techniques

Mercury and arsenic and selenium and the other metalloids all show poor sensitivity when determined by conventional flame AAS. Because of this, techniques have been developed for the determination of these elements involving a specialised method of atomisation of the analyte. In this way, the sensitivity of the atomic absorption determination has been increased. These methods are now standard routine techniques in water analysis and most atomic absorption spectrophotometer manufacturers market special accessories for the determination of mercury, arsenic and selenium. 

There are, however, interferences in the method which should be mentioned. 

In addition to analytical interferences, there are two important problems with mercury determinations in water. 

Reagents. Sulphuric acid (18 N), nitric acid (sp. gr. 1.42), potassium permanganate 5% (m/v) solution, potassium persulphate 5% (m/v) solution, sodium chloride 12% (m/v)—hydroxylamine sulphate 12% (m/v) solution. 

Method. Transfer 100 ml of sample to a 300 ml glass flask. Add 5 ml of sulphuric acid and 2.5 ml of nitric acid, mixing after each addition. Add 15 ml of potassium permanganate solution to each sample bottle. Shake and add additional portions of permanganate solution until the purple colour persists for at least 15 min. Add 8 ml of potassium persulphate to each bottle and heat for two hours in a water bath at 95° C. Cool and add 6 ml of sodium chloride—hydroxylamine sulphate solution to reduce the excess of oxidant prior to analysis. Standards and blanks should be treated in the same way as samples. 

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It must be appreciated that one drop, say, of 0 06 ml. weighs 60-76 mg., hence manipulation with quantities of less than 1 ml. presents new problems involving a specialised technique (micro technique), which will not be discussed here. 

However, world production is only about 55 000 tonnes per annum and this is a reflection of the high volume cost, the rather specialised techniques involving lengthy processing times and to a smaller extent the high creep rate under load. 

In addition to the main general methods of analysis outlined above there are also certain specialised techniques which are applied in special circumstances. Among these are X-ray methods, methods based upon the measurement of radioactivity, mass spectrometry, the so-called kinetic methods, and thermal methods. 

SFC-MS appears to be a rather specialised technique which as yet finds limited routine application. 

Ion Chromatography. Ion chromatography is a specialised technique that uses high-performance ion exchange columns. Its major use in the food industry is the analysis of sugars. 

Although the general principles of separation processes are applicable widely across the process industries, more specialised techniques are now being developed. Reference is made in Chapter 13 to the use of supercritical fluids, such as carbon dioxide, for the extraction of components from naturally produced materials in the food industry, and to the applications of aqueous two-phase systems of low interfacial tensions for the separation of the products from bioreactors, many of which will be degraded by the action of harsh organic solvents. In many cases, biochemical separations may involve separation processes of up to ten stages, possibly with each utilising a different technique. Very often, differences in both physical and chemical properties are utilised. Frequently... 

The characterisation techniques discussed in this chapter are of a fairly general nature. Specialised techniques applicable to particular problems are discussed as the questions arise in the text. 

The answer to the second question, why the study of the very fast primary processes , is, first of all, because, like Everest, they are there. We need to study all aspects of photosynthesis and it is the one we ourselves have chosen because it involves specialised techniques where we are best equipped to contribute. 

With most of the implicit methods to be described, however, the solution is found by specialised techniques that make the process efficient, and these will be described in their proper place. 

A survey of the many highly specialised techniques, to be used for the most difficult NMR analyses (e.g. those of high-carbon materials such as coals and asphalts) was given by Snape (1989). It is reproduced here as Table 12.8. 

One of the most critical aspects in the production of fruit and vegetables is harvesting the produce. The potential to spoil the crop and end up getting a lower return is very acute. It is important that growers consider how they will harvest a crop before they grow it. Many crops require specialised techniques to ensure they reach market in the right condition. The most obvious factors are adequate field heat removal, chilled storage and distribution. Shelf life and performance will be severely curtailed if the produce is not subjected to the correct temperature controls. 

As a last resort it is possible to apply neural networks (NN). NN can in principle model surfaces with any complexity. However, the number of experiments required is laige. This, together with the fact that NN is a rather specialised technique, explains that the number of applications in the literature is limited. Examples are to be found in 70-72). In the latter application two variables (pH and modifier content) are investigated for four chlorophenols and the authors found that when 15 to 20 experiments are carried out, better results are obtained with a multi-layer feed-forward NN than when using quadratic or third-order models. Although we believe that for the optimization of separations, NN will prove practical only in few cases, it seems useful to explain the first principles of the methodology here. A simple network is shown in Fig. 6.25. 

This is a specialised technique which has been applied in field emission and field ion microscopy (see Section 2.1.5c). It is achieved by giving the tip a positive potential. Tungsten can then be removed at liquid helium temperatures with an applied field of 5.7 x 10 V.cm Perfectly regular surface structures are exposed containing many different lattice planes. Clean surfaces have been produced on tungsten, nickel, iron, platinum, copper, silicon and germanium. It is potentially applicable to a wide range of materials, but the area of clean surface exposed is only about 10 ° cm . 

In NMR, the sample is usually analysed in solution (solid-state NMR does exist, but this is a specialised technique). Since hydrogen is active in NMR,... 

Ion cyclotron resonance (i.c.r.) spectrometry is a specialised technique of growing importance. It enables the detailed study of ion-molecule reactions (and their mechanisms) in the gas phase, giving both absolute and relative thermochemical quantities and rate constants. 

Two- and three-dimensional or even higher-dimensional NMR spectroscopy is changing from specialised techniques to more commonly used ones. As the complexity of the acquired NMR data increases, the task of analysing these data constantly becomes more and more demanding and new methods are required to facilitate the analysis. With one-dimensional NMR data multivariate data analysis has proven to be a strong tool, but how should one analyse higher-dimensional NMR data in order to extract as much relevant information as possible without having to break data down into smaller dimensions and thus lose the inherent structure A class of... 

To summarise, FAAS is very easy to use. Interferences are known and can be controlled. Extensive application information is also readily available. Its precision makes it an excellent technique for the determination of a number of commonly analysed elements at higher concentration in polluted soil samples. Its main drawback is its speed in relation to multi-element techniques such as ICP-AES and ICP-MS. Where direct-aspiration flame atomic absorption technique does not provide adequate sensitivity, reference is made to specialised techniques (in addition to graphite furnace procedure) such as the gaseous-hydride method for arsenic, antimony and selenium and the cold-vapour technique for mercury. 

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