Sampling bulk material

The literature on sampling bulk materials is more specific and more quantitative than the general discussion presented above. Nevertheless, experiments on the sampling and subsampling procedures... 

In contrast, when sampling bulk material, the material cannot generally be viewed as a set of distinct units. For example, we sample liquids from tanks, drums, and pipelines, and particulate solids such as ore, powders, and soil. Individual units cannot be identified for sampling. Rather, we must decide on a sample mass Mg or volume the chemical sample size. Further, we must be concerned about whether to composite samples, and, if so, how much to include in each increment of the composite. An additionaJ complication is the restriction on the sample mass that must be used in a chemical analysis due to the method or instrumentation. In fact, a subsample is usually taken in the lab. 

Figure 1.3 The cone and quarter method of sampling bulk materials.

One fiirther method for obtaining surface sensitivity in diffraction relies on the presence of two-dimensional superlattices on the surface. As we shall see fiirtlrer below, these correspond to periodicities that are different from those present in the bulk material. As a result, additional diffracted beams occur (often called fractional-order beams), which are uniquely created by and therefore sensitive to this kind of surface structure. XRD, in particular, makes frequent use of this property [4]. Transmission electron diffraction (TED) also has used this property, in conjunction with ultrathin samples to minimize bulk contributions [9]. 

The striking size-dependent colours of many nanocrystal samples are one of tlieir most compelling features detailed studies of tlieir optical properties have been among tire most active research areas in nanocrystal science. Evidently, tire optical properties of bulk materials are substantially different from Arose of isolated atoms of tire... 

Whether an adequate sampling of phase space is obtained Whether the system size is large enough to represent the bulk material Whether the errors in calculation have been estimated correctly... 

Smith, R. James, G. V. The Sampling of Bulk Materials. Royal Society of Chemistry London, 1981. 

Several factors affect the bandshapes observed ia drifts of bulk materials, and hence the magnitude of the diffuse reflectance response. Particle size is extremely important, siace as particle size decreases, spectral bandwidths generally decrease. Therefore, it is desirable to uniformly grind the samples to particle sizes of <50 fim. Sample homogeneity is also important as is the need for dilute concentrations ia the aoaabsorbiag matrix. 

Sampling is the operation of removing a portion from a bulk material for analysis in such a way that the portion removed has representative physical and chemical properties of that bulk material. From a statistical point of view, sampling is expected to provide analytical data from which some property of the material may be determined. These data should have known and controlled errors and be produced at low cost. 

Sampling. A sample used for trace or ultratrace analysis should always be representative of the bulk material. The principal considerations are determination of population or the whole from which the sample is to be drawn, procurement of a vaUd gross sample, and reduction of the gross sample to a suitable sample for analysis (15) (see Sampling). 

Instrumental Methods for Bulk Samples. With bulk fiber samples, or samples of materials containing significant amounts of asbestos fibers, a number of other instmmental analytical methods can be used for the identification of asbestos fibers. In principle, any instmmental method that enables the elemental characterization of minerals can be used to identify a particular type of asbestos fiber. Among such methods, x-ray fluorescence (xrf) and x-ray photo-electron spectroscopy (xps) offer convenient identification methods, usually from the ratio of the various metal cations to the siUcon content. The x-ray diffraction technique (xrd) also offers a powerfiil means of identifying the various types of asbestos fibers, as well as the nature of other minerals associated with the fibers (9). 

Variations in measurable properties existing in the bulk material being sampled are the underlying basis for samphng theory. For samples that correctly lead to valid analysis results (of chemical composition, ash, or moisture as examples), a fundamental theoiy of sampling is applied. The fundamental theoiy as developed by Gy (see references) employs descriptive terms reflecting material properties to calculate a minimum quantity to achieve specified sampling error. Estimates of minimum quantity assumes completely mixed material. Each quantity of equal mass withdrawn provides equivalent representation of the bulk. 

Composite Samples Obtained by Multiple Sample Extractions Material flow streams are sampled in practice by combining extractions taken at successive time intei vals into a composite sample. Multiple increment collection to obtain representative composite sampfes for specified bulk-material flows is performed according to a... 

In the early days of TEM, sample preparation was divided into two categories, one for thin films and one for bulk materials. Thin-films, particularly metal layers, were often deposited on substrates and later removed by some sort of technique involving dissolution of the substrate. Bulk materials were cut and polished into thin slabs, which were then either electropolished (metals) or ion-milled (ceramics). The latter technique uses a focused ion beam (typically Ar+) of high-energy, which sputters the surface of the thinned slab. These techniques produce so-called plan-view thin foils. 

The analytical techniques covered in this chapter are typically used to measure trace-level elemental or molecular contaminants or dopants on surfaces, in thin films or bulk materials, or at interfaces. Several are also capable of providing quantitative measurements of major and minor components, though other analytical techniques, such as XRF, RBS, and EPMA, are more commonly used because of their better accuracy and reproducibility. Eight of the analytical techniques covered in this chapter use mass spectrometry to detect the trace-level components, while the ninth uses optical emission. All the techniques are destructive, involving the removal of some material from the sample, but many different methods are employed to remove material and introduce it into the analyzer. 

Relative photoionization cross sections for molecules do not vary gready between each other in this wavelength region, and therefore the peak intensities in the raw data approximately correspond to the relative abundances of the molecular species. Improvement in quantification for both photoionizadon methods is straightforward with calibration. Sampling the majority neutral channel means much less stringent requirements for calibrants than that for direct ion production from surfaces by energetic particles this is especially important for the analysis of surfaces, interfaces, and unknown bulk materials. 

The procedure commonly used to quantify EDX spectra was originally outlined by Castaing [4.109], although for the general situation of investigating bulk materials. To a good approximation it can be assumed that the concentration Csp of an element present in an unknown sample is related to the concentration Cst of the same element in a standard specimen by... 

MDHS 77 Asbestos m bulk materials - Sampling and identification by polarized light microscopy (PLMj... 

Ebbesen[4] was the first to estimate a conductivity of the order of lO fim for the black core bulk material existing in two thirds of tubes and one third of nanoparticles. From this observation, it may naturally be inferred that the carbon arc deposit must contain material that is electrically conducting. An analysis of the temperature dependence of the zero-field resistivity of similar bulk materials[14,15] indicated that the absolute values of the conductivity were very sample dependent. 

Ebbesen and Ajayan [16] measured a conductivity of the order of 10 2 Ocm in the blaek eore bulk material, inferring that the carbon arc deposit contains electrically conducting entities. A subsequent analysis of the temperature dependenee of the eleetrieal resistivity of similar bulk materials [17,18] revealed that the resistivities were strongly sample dependent. 

It must be stressed, however, that the whole object may be the analytical sample, e.g. a specimen of moon-rock. Ideally this sample would be analysed by non-destructive methods. Occasionally the bulk material may be homogeneous (some water samples) and then only one increment may be needed to determine the properties of the bulk. This increment should be of suitable size to provide samples for replicate analyses. 

If the composition of the bulk material to be sampled is unknown, it is sensible practice to perform a preliminary investigation by collecting a number of samples and determining the analyte of interest. 

Mechanical methods also exist for dividing up particulate material into suitably sized samples. Samples obtained by these means are usually representative of the bulk material within limits of less than 1 per cent, and are based upon the requirements established by the British Standards Institution. Sample dividers exist with capacities of up to 10 L and operate either by means of a series of rapidly rotating sample jars under the outlet of a loading funnel, or by a rotary cascade from which the samples are fed into a series of separate compartments. Sample dividers can lead to a great deal of time-saving in laboratories dealing with bulk quantities of powders or minerals. 

Part A, dealing with the Fundamentals of Quantitative Chemical Analysis, has been extended to incorporate sections of basic theory which were originally spread around the body of the text. This has enabled a more logical development of theoretical concepts to be possible. Part B, concerned with errors, statistics, and sampling, has been extensively rewritten to cover modern approaches to sampling as well as the attendant difficulties in obtaining representative samples from bulk materials. The statistics has been restructured to provide a logical, stepwise approach to a subject which many people find difficult. 

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