Mineral analysis sampling procedures

Before mineral analysis it is usually necessary to treat the sample to ensure that the sample is homogeneous and also to prepare it for the analytical procedure that follows. Various processes may be necessary, but among the most important is sample mineralization, often associated with the need to destroy organic matter present in the sample and always necessary to make the sample soluble. Moreover, the treatment of a sample may entail reduction and homogenization of its size or elimination of interferences. In any case, contamination of the sample or loss of volatile compounds may occur during these steps of the analytical process, affecting the quality of the analytical results. 

In any analytical investigation, the need for care in selecting the site and in taking samples of the soil is important. If the purpose of the heavy mineral analysis is merely to ascertain if the soil is sedentary or transported and to establish the origin of the parent material, it will usually be sufficient to take samples from only the A and C horizons of each profile. However, valuable additional information may be missed by following this procedure, and it is advisable to take samples from each of the morphologically recognizable horizons down to the parent material. Samples of the latter should be chosen to check for either horizontal or vertical variations. 

Microscopy (qv) plays a key role in examining trace evidence owing to the small size of the evidence and a desire to use nondestmctive testing (qv) techniques whenever possible. Polarizing light microscopy (43,44) is a method of choice for crystalline materials. Microscopy and microchemical analysis techniques (45,46) work well on small samples, are relatively nondestmctive, and are fast. Evidence such as sod, minerals, synthetic fibers, explosive debris, foodstuff, cosmetics (qv), and the like, lend themselves to this technique as do comparison microscopy, refractive index, and density comparisons with known specimens. Other microscopic procedures involving infrared, visible, and ultraviolet spectroscopy (qv) also are used to examine many types of trace evidence. 

Most organic substances can be dissolved readily in a suitable organic solvent and some are directly soluble in water or can be dissolved in aqueous solutions of acids (basic materials) or of alkalis (acidic materials). Many inorganic substances can be dissolved directly in water or in dilute acids, but materials such as minerals, refractories, and alloys must usually be treated with a variety of reagents in order to discover a suitable solvent in such cases the preliminary qualitative analysis will have revealed the best procedure to adopt. Each case must be considered on its merits no attempt at generalisation will therefore be made. It is however of value to discuss the experimental technique of the simple process of solution of a sample in water or in acids, and also the method of treatment of insoluble substances. 

Major emphasis in studies of N-nitroso compounds in foods has been placed upon volatile nitrosamines, in part because these compounds are relatively easy to isolate from complex matrices by virtue of their volatility. Procedures utilizing atmospheric pressure or vacuum distillation have been used by most investigators, with variations of the method of Fine e al. (2) being among the most popular. This procedure employs vacuum distillation of a mineral oil suspension of the sample with optional addition of water to improve nitrosamine recovery from low moisture content samples (6) The usual approach to prevention of nitrosamine formation during analysis involves adding sulfamic acid or ascorbate to destroy residual nitrite at an early stage of sample preparation. 

In many cases, there is difficulty in preserving residues in samples after collection and prior to pesticide analysis which coincides with a rapid further degradation and mineralization of the pesticide residues under most environmental conditions. Storage stability studies and studies on the reactivity of sample collection equipment in addition to field quality assurance procedures can help address some of these questions. Concerns are accentuated for compounds that have short half-lives in the environment but still have high acute toxicity. 

SSMS can be classified among the milliprobe techniques (Figure 8.3), i.e. it is a unique link between microprobe techniques and macroanalytical methods that are characterised by poor lateral and in-depth resolutions (as in OES), or that have no lateral resolution whatsoever (as in NAA). Also, the achievable precision and accuracy are poor, because of the irreproducible behaviour of the r.f. spark. Whereas analysis of metals, semiconductors and minerals is relatively simple and the procedures have become standardised, the analysis of nonconducting materials is more complex and generally requires addition of a conducting powder (e.g. graphite) to the sample [359]. Detection limits are affected by the dilution, and trace contamination from the added components is possible. These problems can be overcome by the use of lasers [360]. Coupled with isotope dilution, a precision of 5% can be attained for SSMS. 

Quantitative determination of the major and minor minerals In geological materials Is commonly attempted by x-ray diffraction (XRD) techniques. Mineralogists use a variety of sophisticated and often tedious procedures to obtain semlquantltatlve estimates of the minerals In a solid sample. The mineralogist knows that XRD Intensities depend on the quantity of each mineral component In the sample even through expressions for conversion of signal Intensity to quantitative analysis often are unknown or difficult to determine. Serious difficulties caused by variables such as particle size, crystallinity, and orientation make quantification of many sample types Impractical. Because of a lack of suitable standards, these difficulties are particularly manifest for clay minerals. Nevertheless, XRD remains the most generally used method for quan-... 

The procedures developed for the analysis of the minerals and inorganics were the result of an extensive series of tests to determine the optimum conditions for these specific sample types. This was then combined with general procedures for the analytical equipment. 

Because SEM-AIA is often used to explain behavior under specific processing conditions, samples are prepared in the same size in which they are received. Coal samples with their included mineral matter are prepared for image analysis by mixing samples. of the dry coal with polyethylene powder (as a diluent) and molten carnauba wax in a volume ratio of 1 2 2. Pellets are then cut along the cylindrical axis to expose a vertical cross section of coal and mineral matter and polished using standard petrographic procedures. The surfaces are coated with 150 A of carbon to provide a conductive surface for SEM examination. 

Factor analysis extracts information from the sample data set (e.g., IR spectra) and does not rely on reference minerals. However, because abstract factors have no physical meaning, reference minerals may be needed in target transformations or other procedures to extract mineralogical information. One valuable piece of information obtainable without the use of extraneous data is the number of components required to represent the data within experimental error. Reported applications of factor analysis to mineralogy by FTIR are few (12). However, one commercial laboratory is offering routine FTIR mineral analyses to the petroleum industry, based on related methods (22). 

Metal concentrations are determined using molecular spectrophotometric, atomic spectrometric, and electrochemical techniques. All of these require samples to be homogenous, or at least to contain the smallest possible amounts of organic matter that could interfere with the metal determination by interacting with the metal ions and the analytical reagents. Traditionally, decomposition of the sample in elemental analysis requires it to be mineralized in order to remove the organic content.1 Sample decomposition for total element determination therefore appears to be the recommended procedure on every occasion. 

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