Solid sampling techniques Methods

Nondestructive radiation techniques can be used, whereby the sample is probed as it is being produced or delivered. However, the sample material is not always the appropriate shape or size, and therefore has to be cut, melted, pressed or milled. These handling procedures introduce similar problems to those mentioned before, including that of sample homogeneity. This problem arises from the fact that, in practice, only small portions of the material can be irradiated. Typical nondestructive analytical techniques are XRF, NAA and PIXE microdestructive methods are arc and spark source techniques, glow discharge and various laser ablation/desorption-based methods. On the other hand, direct solid sampling techniques are also not without problems. Most suffer from matrix effects. There are several methods in use to correct for or overcome matrix effects ... 

An excellent review of modern sorptive sampling techniques that could be considered for the enrichment of volatiles from mammalian secretions appeared recently [10]. To be on the safe side, more than one sample preparation and sample enrichment method should be used to analyze mammalian secretions. If GC and GC-MS analyses are employed, the results obtained with split/split-less, on-column, SPME and solventless (solid) sample introduction methods [11,12] should be compared. 

The most widely used solid sample extraction method is Soxhiet extraction. This technique was introduced by Franz Von Soxhiet almost a century ago. Soxhiet extraction is described in U.S.-EPA method 3540 as a procedure for extracting nonvolatile and semivolatile organic compounds from solids such as soils, sludges and wastes. ... 

A method applied for placenta tissue and compared with a solid sampling technique has been published by Herber et al. (1985). 

Unlike the hexahalo complexes of the Ad and 5d series, many of which have been extensively investigated in solution, the hexafluoro anions of the first transition series are generally rather unstable towards water, this tendency increasing towards the end of the series and with increase in the oxidation number. Because of this the normal solution techniques are virtually inapplicable, although a study of NiFe in anhydrous HF has been reported (29), and recourse must be made to methods which can utilise solid samples. Such methods include measurements on dry powders between KBr discs (30), which do not appear very successful, and investigations of similarly confined fluorocarbon mulls (31), which seem to be much more satisfactory. However, the one generally available method for such materials is diffuse reflectance spectroscopy, which the authors have adopted for their own studies. 

Other solid sampling techniques While in an article such as this, it is not possible to cover all mid-infra-red sampling techniques two other methods that have been used significantly in analytical spectroscopy applications are photoacoustic spectroscopy and emission spectroscopy. 

Figure 1.25 MALDI mass spectrum of HALS components in PP composite sample containing 1.0 wt% of HALS before UV irradiation (a) solvent extracts from the sample obtained by solution-based preparation method (b) HALS components directly desorbed from the PP composite obtained by the solid sampling technique Reproduced from Taguchi and co-workers, American Chemical Society [93]...

In some cases, it may be convenient to dissolve a solid and present it for analysis as a solution that can be nebulized and sprayed as an aerosol (mixed droplets and vapor) into the plasma flame. This aspect of analysis is partly covered in Part B (Chapter 16), which describes the introduction of solutions. There are vaporization techniques for solutions of solids other than nebulization, but since these require prior evaporation of the solvent, they are covered here. There are also many solid samples that need to be analyzed directly, and this chapter describes commonly used methods to do so. 

Ion chromatography has been successfully applied to the quantitative analysis of ions in many diverse types of industrial and environmental samples. The technique has also been valuable for microelemental analysis, e.g. for the determination of sulphur, chlorine, bromine, phosphorus and iodine as heteroatoms in solid samples. Combustion in a Schoniger oxygen flask (Section 3.31 )is a widely used method of degrading such samples, the products of combustion being absorbed in solution as anionic or cationic forms, and the solution then directly injected into the ion chromatograph. 

For the application of flame spectroscopic methods the sample must be prepared in the form of a suitable solution unless it is already presented in this form exceptionally, solid samples can be handled directly in some of the non-flame techniques (Section 21.6). 

The technique as we have described it works only for polar molecules, because only they can interact with microwave radiation. However, variations of these spectroscopic methods can be used to investigate nonpolar molecules, too. A major limitation of the technique is that only the spectra of simple molecules can be interpreted. For complex molecules, we use solid samples and x-ray diffraction techniques. 

Solid-phase microextraction eliminates many of the drawbacks of other sample preparation techniques, such as headspace, purge and trap, LLE, SPE, or simultaneous distillation/extraction techniques, including excessive preparation time or extravagant use of high-purity organic solvents. SPME ranks amongst other solvent-free sample preparation methods, notably SBSE (Section 3.5.3) and PT (Section 4.2.2) which essentially operate at room temperature, and DHS (Section 4.2.2),... 

Glow discharge is essentially a simple and efficient way to generate atoms. Long known for its ability to convert solid samples into gas-phase atoms, GD techniques provide ground-state atoms for atomic absorption or atomic fluorescence, excited-state atoms for atomic emission, and ionised atoms for MS [158], Commercial instrumentation has been developed for all these methods, except for GD-AFS and pulsed mode GD. 

Different analytical procedures have been developed for direct atomic spectrometry of solids applicable to inorganic and organic materials in the form of powders, granulate, fibres, foils or sheets. For sample introduction without prior dissolution, a sample can also be suspended in a suitable solvent. Slurry techniques have not been used in relation to polymer/additive analysis. The required amount of sample taken for analysis typically ranges from 0.1 to 10 mg for analyte concentrations in the ppm and ppb range. In direct solid sampling method development, the mass of sample to be used is determined by the sensitivity of the available analytical lines. Physical methods are direct and relative instrumental methods, subjected to matrix-dependent physical and nonspectral interferences. Standard reference samples may be used to compensate for systematic errors. The minimum difficulties cause INAA, SNMS, XRF (for thin samples), TXRF and PIXE. 

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