Preparation of Solid Samples

As noted earlier, some of the steps that precede the insertion of the treated sample into the instrument for measurement (e.g. dissolution, clean-up, preconcentration, individual separation, derivatization) can have a critical influence on accuracy and precision depending on the particular step. All analytical processes include a sample preparation step which is a function of a number of factors such as the physical state of the sample, the nature of the sample matrix and analytes or the type of detector, for example. The first distinction therefore refers to the nature of the sample solid, liquid or gas. Solid samples are the most difficult to process as most analytical instruments cannot handle them. Therefore, the first operation in solid sample preparation involves transferring the target analytes to a liquid phase. This can be carried out in various ways including total dissolution of the test sample or partial dissolution or separation of a portion thereof. The different choices, which can be assisted by ultrasound, are depicted in Fig. 2.2, and discussed in the following sections. 

As defined by lUPAC, dissolution is the process of mixing two phases with the formation of a new homogeneous phase (i.e. the solution) . No distinction is made regarding nature of the phases. 

There are various ways of dissolving a solid, of which suspension in a solvent with agitation is the simplest, but also unfeasible in most cases. On the other hand, digestion 

Although digestion, leaching and slurry preparation refer to three different operations, the distinction is purely theoretical. Thus, the three terms are used interchangeably by many authors, either because they pay little attention to the specific mechanism by which analytes are made ready for determination or because a confluence of processes conceals the precise underlying mechanism. Thus, the following overlapped processes are possible  

Misused terms in relation to ultrasound-assisted solid sample pretreatment 

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Table 3.4 summarises the main characteristics of a variety of sample preparation modes for in-polymer additive analysis. Table 3.5 is a short literature evaluation of various extraction techniques. Majors [91] has recently reviewed the changing role of extraction in preparation of solid samples. Vandenburg and Clifford [4] and others [6,91-95] have reviewed several sample preparation techniques, including polymer dissolution, LSE and SEE, microwave dissolution, ultra-sonication and accelerated solvent extraction. 

Szpunar, J., Schmitt, VO., Donard, O.F.X. and Lobinski, R. (1996) Low-power focussed microwave technology as a new tool for rapid preparation of solid samples for speciation analysis. Trends Anal. Chem., 15, 181-187. 

The Role of Organic Solvents and the Mixture of Methanol-Water in the Preparation of Solid Samples Prior to As Speciation... 

Laser ablation. LA/ICP-MS is highly versatile and can analyze any kind of solid material. Compared with conventional ICP-MS, the main advantages of this technique are the fast preparation of solid samples without time-consuming dissolution processes, the certainty of a quantitative analysis, and the reduction of potential contaminations during sample preparation (Longerich etal., 1993 Jarvis and Williams, 1993). 

Majors, R. E. 1996. The changing role of extraction in preparation of solid samples, LC-GC, 14 88-96. 

To appreciate the different approaches available for the preparation of solid samples for elemental analysis. 

The next method of the preparation of solid samples is called the film technique. In this method, the sample is solved in an appropriate solvent and smeared on a glass plate in the form of a thin film. 

The preparation of solid sample by the film technique is the same as the preparation of liquid samples. In the two cases, there are many special requirements for the solvent used. The most important of them are as follows solvent must be chemically inert to the sample analyzed solvent must be chemically and physically inert to the material of the cuvette... 

The homogeneity of the sample is very important for successful x-ray fluorescence spectrometry. Hence, the preparation of solid samples for this analysis by melting. The samples analyzed by crude oil chemist are, in most cases, liquids or... 

Preparation of Solid Samples for Analysis (Soil, Sediment, etc.).99... 

The methods of preparation of solid samples as XAS standards are also of interest. In the early days of EXAFS analysis the eolleetion of data from structurally characterized model compounds was an essential prerequisite for the determination of empirical phase and amplitude functions that were used to conduct curve-fitting analysis of unknown samples. While modern EXAFS analysis relies upon theoretical phases and amplitudes, as discussed above, standard compounds are still important for eliminating other uncertainties, such as that regarding AEq, as discussed in Seetion 4.2.4. Moreover, our understanding of near-edge spectra is vitally dependent upon standard... 

Integration of sample preparation with the rest of the analysis is essential for full productivity gains to be achieved. This is achieved with some automated sample preparation techniques, e.g., LC columnswitching illustrated above. The trend in automated sample preparation is clear the technique will be online and part of the instrumental analysis. To use this approach, the sample must be Hquid before the analysis can commence. Therefore, more effort needs to be spent investigating how to automate the sample preparation of solid samples. 

Preparation of solid samples for analysis by infrared analysis has tended to be much more labor intensive than analysis of liquids. The traditional methods involve preparation of a KBr pellet or a Nujol mull described in sections 25.5 B and 25.5 C. The use of an attenuated total reflectance accessory with an FT-IR instrument has dramatically improved the preparation time previously required. One simply places a small amount of a solid on the instrument. For these laboratories equipped with this accessory, it is strongly recommended for analysis of both solids and liquids (Section 25.1). 

As discussed in Chapter 2, the rate of the chemical reaction between a gas and a solid surface is frequently dependent on trace impurities, other species absorbed on the solid surface, and even the mechanical history of the surface [1, 2]. For this reason it is necessary to use great care in comparing results of experiments on samples of solid that are apparently chemically identical but have been prepared in different ways. The investigator should therefore aim for consistency in his preparation of solid samples. 

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