Qualitative, chemical measurements

In addition to the direct analysis of a sample for its quantitative and/or qualitative composition, HS-GC can be used for physico-chemical measurements, such as the determination of vapour pressures. 

During the last several years, a number of new instrumental surface techniques have been developed that are quite effective in detecting changes in the surfaces of minerals that have undergone chemically induced or natural geologic alteration. These techniques are quite sensitive (approximately 0.1-0.5% atomic concentration for x-ray photoelectron and Auger spectroscopy, for example), and they make it possible to monitor very small amounts of elements that may be present in the near surface material. Any change in the surface with respect to chemical composition may readily be measured qualitatively... 

Not all chemical measurements are, or should be, traceable to the mole. We have seen instances where the unit of mass was the proper SI unit for a quantitative measurement of a material of unspecified entities. There are chemical measurements that are not, but probably should be, referred, and preferably be traceable, to the SI unit. Color is used either simply as a qualitative attribute not subject to a measurement, or it is measured quantitatively by some spectrometry, where it may inevitably be subject to high uncertainties from both the measurement itself as well as from theory, such as the Lambert-Beer Law, but well understood in relation to SI. 

Analytic surface charge of clay plates known accurately from the chemical formula of the clay surface charge in situ measured qualitatively by neutron diffuse scattering (see Chapter 8)... 

A tumor marker is a substance produced by a tumor or by the host in response to a tumor that is used to differentiate a tumor from normal tissue or to determine the presence of a tumor based on measurements in the blood or secretions. Such substances are found in cells, tissue, or body fluids and are measured qualitatively or quantitatively by chemical, immunological, or molecular biological methods. 

There were many investigations to explain the mechanism of reduction. Since direct chemical measurements are out of question at carrier-free concentrations of Tc (10 M), carrier technetium ( Tc) in hydrochloric acid was used to determine the oxidation state of technetium in diethylene triamine pentaacetate (DTPA) and in citrate solution. Polaro-graphic and iodometric techniques were used to analyze for unreacted stannous ion and to perform direct potentiometric titrations of pertechnetate-99 with stannous chloride (Mtinze 1980 Steigman et al. 1975). No quantitative kinetic studies had been made, but qualitative conclusions have been drawn for the reduction mechanism. Most probably, the first step is the reduction to Tc(V). Reduction to Tc(III) proceeds in two successive complementary reactions, both of which should be rapid in the low concentrations at radiopharmaceutical level ... 

For quantitative analysis, a history of the sample composition will often be known (it is known that blood contains glucose), or else the analyst will have performed a qualitative test prior to performing the more difficult quantitative analysis. Modem chemical measurement systems often exhibit sufficient selectivity that a quantitative measurement can also serve as a qualitative measurement. However, simple qualitative tests are usually more rapid than quantitative procedures. Qualitative analysis is composed of two fields inorganic and organic. The former is usually covered in introductory chemistry courses, whereas the latter is best left until after the student has had a course in organic chemistry. 

The strong similarity in terms between instrumental chemical analysis (qualitative and quantitative measurements) and the field of bioindicators (as a qualitative approach to pollution control) and biomonitors (as a quantitative approach) makes it worthwhile to compare the two techniques. 

Once obtained from the sample and reagent, analytical information must be translated into chemical information (qualitative and quantitative data). For simplification, amounts of species are characterized by Pj and measuring signals by x,. 

Qualitative analysis. This concerns the identification of the analytes present in a sample being subjected to the chemical measurement process. 

Two different terms can be found in the literature when dealing with qualitative analysis although they present slightly different connotations. The word detection is normally used to refer to a chemical measurement process for qualitative purposes, opposed to determination, which is reserved for quantitative analysis. Identification is usually employed for qualitative analysis aimed at recognizing the analyte (or its reaction product) from some chemical or physicochemical properties. As it entails the use of a standard for signal comparison, it is more appropriate as an alternative to qualitative analysis from a metrological point of view. 

The relevance of such errors depends on the particular analytical problem. In general, all positive results from a qualitative test will be systematically confirmed by a conventional chemical measurement process when any error made may have a significant social or economical impact and, therefore, their practical impact in the decision-making is low. However, quality assurance of negative responses is crucial, as their practical effects are more relevant. Indeed, they are especially serious when detecting or identifying a toxic chemical in environmental, food, or clinical samples as no confirmatory step is carried out. 

Qualitative analysis can be classified according to a variety of criteria. One of them considers the analytical technique used and, therefore, qualitative chemical measurement processes can be divided into two main blocks classical and instrumental qualitative analyses. Their main characteristics will be briefly discussed below. 

The chemical measurement process used in classical qualitative analysis can be either a direct or a systematic procedure using sequential separations to indirectly raise sensitivity and selectivity. The quaHtative analysis also differs depending on whether a single analyte, a family of analytes, a small group, or a wide range of groups are to be identified. 

In this test, a metal sample is rotated in the solution. A rotating cylinder is used to simphfy fluid dynamics equations so that corrosion rate can be correlated with shear stress or mass transfer, which in turn can be related to velocity effects in piping and equipment. The same electn> chemical techniques used on static samples are applicable to the rotating cylinder electrode. By coupling the samples to electrochemical measirring equipment, one can measure qualitatively the effects of stepped velocity changes in one experiment. 

Consideration of the energy involved in the process and of the energy of the quantum absorbed by chlorophyll showed that interaction with at least three excited molecules was required for each molecule of CO2 formed [186]. The investigations by Emil Warburg, fostered also by the formulation of the Einstein equivalence law (1905 in the first formulation see Chap. 2), and by his son Otto gave a quantitative framework to photosynthesis substituting well-defined chemical measurements to qualitative observations [188]. 

In order to compare two chemical (or any other) objects, e.g., two molecules, we need a measure. Plenty of similarity measures have been proposed they are listed in Table 6-2. Generally speaking these measures can be divided into two cases one of qualitative characteristics, and the other of quantitative characteristics. Here we consider these two cases. 

Analytical chemistry is often described as the area of chemistry responsible for characterizing the composition of matter, both qualitatively (what is present) and quantitatively (how much is present). This description is misleading. After all, almost all chemists routinely make qualitative or quantitative measurements. The argument has been made that analytical chemistry is not a separate branch of chemistry, but simply the application of chemical knowledge. In fact, you probably have performed quantitative and qualitative analyses in other chemistry courses. For example, many introductory courses in chemistry include qualitative schemes for identifying inorganic ions and quantitative analyses involving titrations. 

In Section lA we indicated that analytical chemistry is more than a collection of qualitative and quantitative methods of analysis. Nevertheless, many problems on which analytical chemists work ultimately involve either a qualitative or quantitative measurement. Other problems may involve characterizing a sample s chemical or physical properties. Finally, many analytical chemists engage in fundamental studies of analytical methods. In this section we briefly discuss each of these four areas of analysis. 

Qualitative and, hopefully, quantitative estimates of how the process result will be measured must be made in advance. The evaluations must allow one to estabhsh the importance of the different steps in a process, such as gas-liquid mass transfer, chemical reac tion rate, or heat transfer. 

At the micro-scale level, there really is no way to measure concentration fluc tuations. Resort must be made to other qualitative interpretation of results for either a process or a chemical reac tion studv. 

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

There are at least four kinds of information available from an Auger spectrum. The simplest and by far most frequently used is qualitative information, indicating which elements are present within the sampling volume of the measurement. Next there is quantitative information, which requires a little more care during acquisition to make it extractable, and a little more effort to extract it, but which tells how much of each of the elements is present. Third, there is chemical information which shows the chemical state in which these elements are present. Last, but by far the least used, there is information on the electronic structure of the material, such as the valance-band density of states that is folded into the line shape of transitions involving valance-band electrons. There are considerations to keep in mind in extracting each of these kinds of information. 

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