Measurement science sample complexity

In measurement science, the desired signal is often very small and may be embedded in highly variable samples. Environmental samples are seldom controlled. They are likely to contain complex mixtures of compounds in a wide variety of matrices that can further complicate the sampling problem. Further-... 

If sample patterns in a large database are each defined by just two values, a two-dimensional plot may reveal clustering that can be detected by the eye (Figure 3.1). However, in science our data often have many more than two dimensions. An analytical database might contain information on the chemical composition of samples of crude oil extracted from different oilfields. Oils are complex mixtures containing hundreds of chemicals at detectable levels thus, tire composition of an oil could not be represented by a point in a space of two dimensions. Instead, a space of several hundred dimensions would be needed. To determine how closely oils in the database resembled one another, we could plot the composition of every oil in this high-dimensional space, and then measure the distance between the points that represent two oils the distance would be a measure of the difference in composition. Similar oils would be "close together" in space,... 

Noninvasive glucose monitoring demands absolute glucose concentration measurements that match results obtained from conventional test strip technology. Absolute concentration measurements are complicated by the complexity of the sample matrix and variations of this matrix between individuals. Physical separations and selective chemical reactions are commonly used in analytical science to improve measurement accuracy. Such steps are not possible in a noninvasive analysis where all the selectivity information must be derived solely from the spectral information collected from the illuminated sample. 

Although composed of weak and overlapping spectral features, near-infrared spectra can be used to extract analytical information from complex sample matrices. Chemical sensing with in-line near-infrared spectroscopy is a general technique that can be used to quantify multiple analytes in complex matrices, often without reagents or sample pretreatment.7-9 Applications are widespread in the food sciences, agricultural industry, petroleum refining, and process analytical chemistry.10-13 These activities demonstrate that near-infrared spectroscopy can provide selective and accurate quantitative measurements both rapidly and nondestructively. 

Even today, the determination of components at a concentration level of 100 ppm, including samples with complex matrices, poses no major problems and is routine in many laboratories. This is mainly due to the rapid development of instrumentation—the science of the construction and use of monitoring and measuring devices. Hence, we can expect the definition of the term trace component to change again soon. 

FIG. 26 Stability at pH 8 of bacteriorhodopsin (BR) and cytochrome b6f com-plexed by short amphiphilic polymers (C8-modified polyacrylic acids). The proteins were kept in a neutral surfactant solution ( , ) as a standard, or in the absence of surfactant and complexed by a polymer of high charge density (o, A), or by a polymer of lower charge density ( , A). All the samples were stored at 4°C between measurements of the residual absorbance at 546 nm (BR) or enzyme activity (b6f). (Reprinted from Ref. 3. Copyright 1996 National Academy of Sciences, U.S.A.)... 

Near-infrared absorption spectroscopy is increasingly used in agriculture, food science, medicine, fife sciences, pharmaceuticals, textiles, general chemicals, polymers, process monitoring, food quality control and in clinical in vivo measurements [215, 216]. The increase in popularity is largely due to the availability of miniaturised NIR-spectrometers by a variety of vendors (e.g. Ocean Optics Inc.). The most promising applications of NIR-absorbance spectroscopy clearly lie in process control, because of the relatively low complexity of the sample in chemical and biochemical processes, e.g. compared to biological tissues. Also in food quality control... 

The materials of interest for earth sciences are generally very complex from the chemical viewpoint, and may sometimes contain most of the elements of the periodic table, with concentrations varying from the trace- to the major-element level and with variable spatial distribution (zoning). As a consequence, their complete structural and chemical characterization is possible only by means of in situ analytical methods that are able to solve specific measurement problems. The starting material is often available in very small amounts (e.g., some crystals) thus, the requirement of minimum (or even null) sample consumption is also important. 

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