Relative quantitation

Chronocoulometric data have been used for accurate determination of the composition of solid phases, as described by Scholz et al. [74-78, 224-228]. It should be noted, however, that coulometric measurements are always more demanding than voltammetric ones, because coulometry offers opportunity for more pronounced systematic errors. However, its use is highly recommended for cases where voltammetry fails [77, 78, 224]. 

When two or more electrochemical species exist in the solid sample, their relative amounts can be estimated from electrochemical data using different procedures. Remarkably, when a mixture of solids is used, the voltammetric response looks like the superposition of the voltammograms of the parent solids. This can be seen in 

In the voltammetry of microparticles, relative quantitations can easily be obtained as follows. Let us first consider a sample containing two electroactive species, A and B. Assuming that under the selected electrochemical conditions (technique, parameters, electrolyte), separated signals are obtained for A and B, the voltanunetric peak currents (or peak areas), IaJe, can be taken as proportional to the number of mols of A and B, xa,Xb, respectively transferred to the electrode. Then, one can write 

This methodology has been applied for determining the relative composition of alloys [225], amalgams [226], and mixed crystals [227], among others [74-78], based on peak current measurements. The essential requisite is that both electroactive components behave independently—i.e., that the components of a mechanical mixture do not influence each other with respect to their thermodynamic activities in electrochemical reactions [77]. In Fig. 4.4, theoretical calibration plots for the absolute peak current, when the amount of mixture is constant for each measurement (left) and for the percentage peak current (right), are shown [77, 228]. 

Peak areas can also be used for relative quantitation procedures. This is illustrated in Fig. 4.7, corresponding to the determination of the tenorite/(azurite + smalt) ratio 

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The concluding chapters, Chapters XVI through XVIII, take up the important subjects of physical and chemical adsorption of vapors and gases, and heterogeneous catalysis. As with the earlier chapters, the approach is relatively quantitative and problem assignments regain importance. 

Williams, T. L. Cahahan, J. H. Monday, S. R. Feng, P. C. Musser, S. M. Relative quantitation of intact proteins of bacterial ceh extracts using coextracted proteins as internal standards. Anal. Chem. 2004, 76,1002-1007. 

Both absolute quantitation and relative quantitation of species in mixtures is of interest in some circumstances. Quantitation in a 5-minute analysis can be achieved by addition of an internal standard, ideally the target microorganism grown in special media to incorporate heavy isotopes92-95 and determination of the relative peak heights of pairs of proteins from the analyte and the standard. Isotope-labeled proteins or peptides, selected to match proteins or peptides characteristic of target microorganisms, can also serve as internal standards for isotope ratio measurement. The addition of unmatched proteins or peptides is less reliable for either ESI or MALDI measurements because of unpredictable suppression in the variable mixture. 

As described above all samples were separated online using LCT ESI-TOF-MS then normalized for relative quantitation using a bovine insulin internal standard. Fractions were then collected for MAFDI-TOF-MS PMF, digested with modified porcine trypsin, and analyzed using the TofSpec2E. Following this analysis, three major classes of differentially expressed including proteins were revealed in these... 

Nonspecific protein binding to the solid phase complicates the method and is a selective pressure driving its evolution. The adaptive response has been the development of intrinsically comparative methods in which specific binding to an immobilized ligand is blocked in one out of two otherwise identical samples. When the respective protein components of the samples are compared, specifically bound proteins are present in one but severely depleted in the other. To allow relative quantitation, the two samples can be made isotopically distinct by a chemical or metabolic process and then mixed for an analytical step that avoids intersample variability [15]. 

Lattier D, Gordon D, Burks D, Toth G (2001) Vitellogenin gene transcription a relative quantitative exposure indicator of environmental estrogens. Environ Toxicol Chem 20 1979-1985... 

These relations are illustrated by Figure 6 various monodispersed polyethers with decreasing molecular weights, and hence increasing alcohol/ether ratios, behave like a polyether + 1-butanol mixture with the same alcohol content. The result obtained with monofunctional ethers and alcohols can be used to deal with the case of polyethers in a relatively quantitative manner, assuming an independence of the functional groups. 

A speciation of particular interest is to detect the presence of species in different oxidation states in the sample. The determination of the presence of two or even more oxidation states of a given species, and eventually their relative quantitation, is a highly important analytical target in a variety of applications. The determination of the Fe III)/Fe II) ratio in raw and ceramic materials is probably one of the most... 

In spite of its evident limitations, solid-state electrochemical methods can also be used for quantifying electroactive components in sparingly soluble solids. As discussed in this chapter, relative quantitative methods and absolute quantitations are available from voltammetric data. 

Relative quantitation can also be obtained from voltammetric data in the case of strongly overlapped peaks using generalized Tafel analysis of the rising portion of voltammetric curves. For quantitation of a mixture of two components, X and Y, one can combine the Tafel dependence for individual components ... 

These results illustrate the inherent capabilities of the voltammetry of microparticles for determining the absolute concentration of analytes in samples from works of art. Here, the most serious limitations are associated with (i) the need for well-defined electrochemical responses, and (ii) the need for relatively high amounts of sample. The second limitation, however, does not apply when relative quantitation procedures are used. As a result, a judicious use of such methodologies can provide valuable information for archaeometry, conservation, and restoration. 

Relative quantitative strengths of acids and bases are given either by their ionization constants, and K, or by their pK and pK values as defined by ... 

Numerous studies on the metabolism of 2,1t-dichlorophenoxy-acetic acid (2,1+-D) and related herbicides in animals have shown that these chemicals are absorbed and distributed rapidly in the body, and are excreted, undegraded, relatively quantitatively in the urine within a week after administration (M Pharmacokinetic studies with 2,1+,5-T in rats and dogs (5.) and in humans (6J supported these findings, and demonstrated that rates of clearance from plasma and elimination in urine depend on dosage level, animal species, and chemical structure of the phenoxy acid being studied ( + ). Corresponding chlorinated phenol metabolites were detected only in ruminants (M or in trace amounts in urine of rats fed very high doses of phenoxy herbicides (7.) ... 

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