Heterogeneous samples

In the direct insertion technique, the sample (liquid or powder) is inserted into the plasma in a graphite, tantalum, or tungsten probe. If the sample is a liquid, the probe is raised to a location just below the bottom of the plasma, until it is dry. Then the probe is moved upward into the plasma. Emission intensities must be measured with time resolution because the signal is transient and its time dependence is element dependent, due to selective volatilization of the sample. The intensity-time behavior depends on the sample, probe material, and the shape and location of the probe. The main limitations of this technique are a time-dependent background and sample heterogeneity-limited precision. Currently, no commercial instruments using direct sample insertion are available, although both manual and h ly automated systems have been described. ... 

The nuclear equipment failure rate database has not changed markedly since the RSS and chemical process data contains information for non-chemical process equipment in a more benign environment. Uncertainty in the database results from the statistical sample, heterogeneity, incompleteness, and unrepresentative environment, operation, and maintenance. Some PSA.s use extensive studies of plant-specific data to augment the generic database by Bayesian methods and others do not. No standard guidance is available for when to use which and the improvement in accuracy that is achieved thereby. Improvements in the database and in the treatment of data requires, uhstaiui.il indu.sinal support but it is expensive. 

The value has an assodated uncertainty that generally spedfies a range within which the true value is expected to lie at a level of confidence of approximately 95 % if the sample is homogeneous or the uncertainty represents a prediction interval within which the true values of 95 % of all samples are expected to lie at a stated level of confidence if significant sample heterogeneity is included. 

Ks is expressed in the units of mass and is numerically equal to the sample mass necessary to limit the error due to sample heterogeneity (sampling uncertainty) to I % (with 68 % confidence). 

The reliability of any environmental analytical data depends upon the reliability of sample quality. To generalize from analytical results on a small portion of material to a larger population requires careful planning and execution if bias is to be avoided. This article considers the general problems involved in sampling heterogeneous bulk populations such as soil, air, and natural waters specific details for particular types of materials are not included. These problems include the heterogeneity of most environmental materials the costs in time, manpower, and effort required for collection of real samples and the need to avoid contamination or decomposition of samples after collection. 

An obvious limitation to the use of bulk analysis studies is the direct result of sample heterogeneity. Not only do aerosol samples show wide variability in the physico-chemical characteristics of different particles, but even a single airborne particle may be highly heterogeneous. With regard to the latter, the surface chemical composition of a particle may bear little resemblance to that of its interior (11-14). 

In recent years, microanalytical techniques, which permit relatively precise isotopic determinations on a variety of samples that are orders of magnitude smaller than those used in conventional techniques, have become increasingly important. Different approaches have been used in this connection, which generally reveal greater isotope heterogeneity than conventional analytical approaches. As a rule of thumb the smaller the scale of measurement, the larger the sample heterogeneity. 

Figure 2.6 The figure shows the different types of analyses that can be performed on chemical imaging data. The types of analyses that are performed can be grouped into three categories component abundance estimation, statistical analysis of component distribution, and morphological analysis of discrete particles. All three analyses are used to make inter- and intrasample comparisons, generating abundance and content uniformity estimates, sample heterogeneity and blend uniformity characterization, as well as domain statistics and domain size uniformity data.

Table 2.3 Individual normalized standard deviation values of histogram distributions from PLS score images a measure of sample heterogeneity for individual group A and B samples...

For group B samples, the acetaminophen component is relatively homogeneously distributed, but the aspirin and caffeine components are much more heterogeneously distributed. This correlates with what can be visualized in the image of the samples in Fig. 2.8, but provides quantitative and objective criteria for determining sample heterogeneity. 

The experimental detection and quantitation of surface species on soil particles and other natural colloids is a difficult area of research because of sample heterogeneity, low surface concentrations and the need to investigate solid adsorbents in the presence of liquid water. Unambiguous information about the molecular structure and stability of adsorbed species can be obtained only with in situ surface spectroscopy (see also Chapter 3). Invasive spectroscopic methods that require sample desiccation or high-vacuum techniques (e.g. electron microscopy, X-ray... 

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