Bulk techniques

CHOICE OF TECHNIQUES FOR THE STUDY OF ARCHAEOMATERIALS 2.1. Bulk Techniques 

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Xps is a surface sensitive technique as opposed to a bulk technique because electrons caimot travel very far in soHds without undergoing energy loss. Thus, even though the incident x-rays penetrate the sample up to relatively large depths, the depth from which the electron information is obtained is limited by the "escape depth" of the photoemitted electrons. This surface sensitivity of xps is quantitatively defined by the inelastic mean free path parameter which is given the symbol X. This parameter is defined to be the distance an electron travels before engaging in an interaction in which it experiences an energy loss. 

Polymers obtained by the bulk technique are usually pure due to the absence of a solvent. The purity of the final polymer depends on the purity of the monomers. Heat and viscosity are not easily controlled, as in other polymerization techniques, due to absence of a solvent, suspension, or emulsion medium. This can be overcome by carrying the reaction to low conversions and strong agitation. Outside cooling can also control the exothermic heat. 

Because neutron and y-radiation are highly penetrating, the method is virtually matrix-independent. NAA is a bulk technique. NAA can also be a selective technique, because appropriate choice of experimental parameters (such as irradiation and decay times), and use of neutrons of varying energies, can discriminate against unwanted elements. 

Conventional fluorination is usually done on 10-20 mg of whole-rock powder or minerals separated from much larger samples the inability to analyze small quantities means that natural heterogeneity cannot be detected by such bulk techniques. Recent advances in the development of laser microprobes, first described by Sharp (1990), have revolutionized mineral analyses. Laser techniques have both the resolution and precision to investigate isotopic zoning within single mineral grains and mineral inter- and overgrowths. 

The bulk technique is used when measurement of concentration profile is not available. In this technique, many grains of similar size and shape are heated to and held at the desired temperature for a given duration. After the experiment, the total mass loss or gain of the component by the grains is measured. From the mass loss or gain, the diffusion coefficient is calculated. To obtain diffusivity from mass loss experiments (most Ar and He diffusivities in minerals are obtained this way), it is necessary to assume that the initial concentration of the diffusion component is uniform. It is also necessary to assume the effective shape of the diffusing grains (cf. Section 3.2.11). 

If it is possible to measure the diffusion profile, the profiling technique is preferred over the bulk technique. The disadvantage of the profiling technique is that it requires high spatial resolution in concentration measurement, as well... 

Bulk techniques still have a place in the search for presolar components. Although they cannot identify the presolar grain directly, they can measure anomalous isotopic compositions, which can then be used as a tracer for separation procedures to identify the carrier. There are several isotopically anomalous components whose carriers have not been identified. For example, an anomalous chromium component enriched in 54Cr appears in acid residues of the most primitive chondrites. The carrier is soluble in hydrochloric acid and goes with the colloidal fraction of the residue, which means it is likely to be submicron in size (Podosck el al., 1997). Measurements of molybdenum and ruthenium in bulk primitive meteorites and leachates from primitive chondrites show isotopic anomalies that can be attributed to the -process on the one hand and to the r- and /7-processes on the other. The s-process anomalies in molybdenum and ruthenium correlate with one another, while the r- and /7-process anomalies do not. The amounts of -process molybdenum and ruthenium are consistent with their being carried in presolar silicon carbide, but they are released from bulk samples with treatments that should not dissolve that mineral. Thus, additional carriers of s-, r-, and/ -process elements are suggested (Dauphas et al., 2002). 

Inductively coupled plasma-mass spectrometry (ICP-MS) has been utilized as a bulk technique for the analysis of obsidian, chert and ceramic compositional analyses 12-14). However, due to the high level of spatial variation of ceramic materials, increased sample preparation is necessary with volatile acids coupled with microwave digestion (MD-ICP-MS) to properly represent the variability of ceramic assemblages IS, 16). Due to the increased sample preparation and exposure to volatile chemicals, researchers have continued to utilize neutron activation analysis (INAA) as the preferred method of chemical characterization of archaeological ceramics (77). 

Perhaps of greater concern than calibration may be the increased sensitivity to surfaces in powder XRD of nanocrystals relative to macroscopic crystalline materials. Although XRD is routinely described as a bulk technique, it is important to recognize that the nanocrystals themselves are up to 30% surfaces, making the XRD experiment of nanocrystals more sensitive to surface effects... 

Figure 2 shows a typical set of kxS curves determined from a suite of silicate standards including olivine, pyroxene, and amphibole which have been characterised by bulk techniques such as electron microprobe and x-ray fluoresence analysis. These minerals were chosen so as to provide a wide concentration range of many elements within the one set of standards and because beam degradation effects were negligible. In a plot of Cx/Cg versus Ix ISi/ kxSi determined from the slope of the best fit line and the intercept should be close to zero, since the (0,0) point itself can be considered a data point. 

Three techniques give access to the environment of nuclei (electronic shells, valency, symmetry, matrix interactions). All of them are bulk techniques but when properly used, are extremely useful for catalysis. 

Raman spectroscopy is a bulk technique, although the depth of the analyzed volume is limited. The information depth and thus the spectra depend on the excitation frequency and the absorption coefficient and crystallinity of the sample (Cardona, 1983). To characterize catalyst surfaces and their interactions with reactants, the spectral contributions from the surface have to be discriminated from those of the catalyst bulk. This complication has to be considered when applying Raman spectroscopy to working catalysts (Banares, 2005). 

Sample purity is a key concern. The NRVS experiment is a bulk technique sampling all Fe nuclei, and impurities that also contain the probe nucleus may confound quantitative data interpretation. Impurities may be introduced during sample preparation or may result from sample instability during measurement. Because of this, care must be taken to ensure purity and reproducibility as judged by Mossbauer (see Mdssbauer Spectroscopy), single-crystal X-ray diffraction, electronic absorption spectroscopy (see Electronic Spectroscopy), Raman spectroscopy or other qnalitative techniqnes. 

The majority of characterization techniques discussed thus far have been surface-related, with some capable of analyzing sub-surface depths through in situ ion etching. This final section will focus briefly on a selection of common bulk techniques that may be used to characterize as-synthesized materials such as polymers, ceramics, etc. More details on these and other techniques not discussed herein may be found in the Further Reading section at the end of this chapter. In particular, these additional resources, as well as countless others online, will highlight solid-state characterization techniques such as ... 

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