Solid sampling strategies

It is important to note that the application of electrochemical methods to the analysis of samples of art objects and archaeological artifacts allows much more than only simple identification of certain constituents advanced methods of speciation may provide information about constituents that are only slightly differing in then-composition, or for which there are only slight differences in the matrices in which the components are embedded. Further, redox speciation—and in the case of solid samples, phase speciation—can be used to derive information on production processes or corrosion (deterioration) of the components in the time that passed since their formation. The second part of this chapter is devoted to illustrating the capabilities of advanced speciation strategies. 

For measurements of isotope ratios or isotope abundances, any of the mass spectrometers discussed in the previous chapters, such as SSMS, LIMS, GDMS56 and LA-ICP-MS,6 are of benefit for the direct isotope analysis of solid samples. SSMS and LIMS are rarely applied in isotope analysis due to their relatively low precision. Several applications of the isotope dilution technique as a calibration strategy in SSMS, mostly on geological samples, are known.57-59 GDMS has been mostly applied in multi-element trace analysis and depth profiling and plays only a minor role... 

For formulated products an essential analysis is the assay for API content. This is usually performed by HPLC, but Raman spectroscopy can offer a quantitative analytical alternative. These applications have been extensively researched and reviewed by Strachan et al. [48] and provide over 30 literature references of where Raman spectroscopy has been used to determine the chemical content and physical form of API in solid dosage formulations. As no sample preparation is required the determination of multiple API forms (e.g. polymorphs, hydrates/solvates and amorphous content) provides a solid state analysis that is not possible by HPLC. However, as previously discussed sampling strategies must be employed to ensure the Raman measurement is representative of the whole sample. A potential solution is to sample the whole of a solid dosage form and not multiple regions of it. As presented in Chap. 3 the emerging technique of transmission Raman provides a method to do just this. With acquisition times in the order of seconds, this approach offers an alternative to HPLC and NIR analyses and is also applicable to tablet and capsule analysis in a PAT environment. 

In Sections IV.C and IV.D we shall see that extended sampling strategies provide a rich variety of ways of tackling the phase coexistence problem, including the distinctive problems arising when one (or both) of the phases is of solid form. 

The objective of improvement schemes is to study and validate each step of different analytical procedures applied by different laboratories in a collaborative manner. Such programmes usually involve groups of 20-50 laboratories. In the best case, each critical step of the procedure should be evaluated in an adapted exercise. The individual steps may be studied with a series of different materials in a stepwise manner. In principle the strategy consists of starting from the simplest matrix, e.g. pure solutions and/or mixtures of compounds in solution, for testing the performance of the detector. The analysis of more complex matrices (e.g. raw extract, purified extract) enables the separation and/or clean-up steps to be tested, whereas solid samples are used to test the entire procedure. Spiked samples can be analysed to evaluate the extraction procedure, within the limits of this evaluation (as commented in Section 2.3.1). Such an approach is actually similar to the steps that should be followed when developing and validating a new method in a laboratory. 

To alleviate these drawbacks, alternative methodologies relying on the continuous provision of fresh extractant volumes to the solid sample under mvestigation have been developed, characterized, and contrasted with the classical end-over-end extraction procedures. The fundamental principles of these novel, dynamic (nonequilibrium) strategies, based primarily on the use of continuous-flow analysis (Ruzicka and Hansen, 1988), flow injection analysis (Ruzicka and Hansen, 1988 Trojanowicz, 2000 Miro and Frenzel, 2004b), or sequential injection analysis (Ruzicka and Marshall, 1990 Lenehan et al., 2002), are described in detail below, and their advantageous features and limitations for fractionation explorations are discussed critically. 

It should be also stressed that online dynamic leaching protocols are currently being employed by several researchers as an alternative to steady-state measurements, since kinetic information from the extraction process is thus obtained. With respect to sampling modalities, the exploitation of slurry sampling strategies as well as the implementation of microcolumn reactors packed with solid materials should be underlined. 

As stated before, the concentration of the element selected as internal reference must be known in advance. There are different possibilities to comply with this requisite (1) previous analysis of the sample, ideally using a fast nondestructive solid sampling technique capable of providing reliable values for elements found at high levels, such as SEM-EDX [62,71,75,76], a strategy that is very common (2) on occasions, the concentration of the main matrix constituent might be known with sufficient accuracy from the stoichiometry of the material or from historical information [48,66] or can be assumed... 

De Schrijver, I., Aramendia, M., Resano, M., Dumoulin, A., Vanhaecke, F (2008) Novel strategies for rapid trace element analysis of polyamide by graphite furnace atomic absorption spectrometry and inductively coupled plasma mass spectrometry. Dissolution in an organic solvent versus direct solid sampling approaches. /. Anal. At. Spectrom., 23, 500-507. 

More recently, an in situ preconcentration strategy to improve the sensitivity of the overall extraction method for solid samples using IL-based surfactants as extracting solvents has been proposed [56]. The method is based on transforming a water-soluble IL-based surfactant into a water-insoluble IL-based surfactant by means of... 

During these preliminary preparation steps, additional modification of the sample such as preconcentration of the analyte or separation from potentially interfering components in the sample can be achieved. Therefore, the sample introduction technique of choice can play an important role in the design of an optimum strategy for analysis. A summary of proven sample introduction techniques for use with ICP-MS is given in Table 5.1. This table shows the types of techniques that are useful for the determination of analyte elements in gas, liquid, or solid samples and mixtures thereof. 

To fulfill the objectives of quantitative trace element determination in solid materials, different calibration strategies have been established to accurately and precisely describe a relationship between the signal response and the elemental concentration by LA-ICP-MS. There are three main criteria that, in principle, have to be met for a successful absolute quantification of trace elements in solid samples. These are ... 

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