Selectivity determination

Tirouflet et al. measured polarographic half-waves for a series of substituted thiazoles (143) in order to determine the ability of polarog-raphy for the selective determination of thiazoles mixtures. The values found for 2-amino-5-bromothiazole agree with the +M effect ot the NHj... 

Linear dependence of current of additional peak 1 on concentration of Zr(IV) can be used for elaboration sensitive and selective determination of zirconium with detection limit of 1.7x10 mol/1. 

We showed that these mesoporous silica materials, with variable pore sizes and susceptible surface areas for functionalization, can be utilized as good separation devices and immobilization for biomolecules, where the ones are sequestered and released depending on their size and charge, within the channels. Mesoporous silica with large-pore-size stmctures, are best suited for this purpose, since more molecules can be immobilized and the large porosity of the materials provide better access for the substrates to the immobilized molecules. The mechanism of bimolecular adsorption in the mesopore channels was suggested to be ionic interaction. On the first stage on the way of creation of chemical sensors on the basis of functionalized mesoporous silica materials for selective determination of herbicide in an environment was conducted research of sorption activity number of such materials in relation to 2,4-D. 

Since only relatively few substances are capable of emitting fluorescent radiation, they can be particularly selectively detected. This means that the selectivity of the chromatographic separation, which is always aimed at, is meaningfully extended by the selectivity of detection. Accompanying substances that absorb radiation but do not emit light do not interfere when the analysis is made by the selective determination of fluorescence ... 

The. tytt-selective process [d.r. (syn/anti) >90 10] is rationalized by a C(E,E) type of transition state 5. The diastereofacial selectivity, determined by converting /1-amino esters 3 to /1-lactams 4 and measuring their optical purity, is satisfying (70% ee)29. 

Zougagh, M., Valcarcel, M., and Rios, A., Automatic selective determination of caffeine in coffee and tea samples by using a supported liquid membrane-modified piezoelectric flow sensor with molecularly imprinted polymer. Trends Anal. Chem., 23, 399, 2004. 

Brinkman, U. A. Th., de Jong, G. J., and Gooijer, C., Use of luminescence techniques for sensitive and selective determinations in HPLC (high-performance liquid chromatography), Pure Appl. Chem., 59, 625, 1987. 

Somsen et al. [796] have reported the use of SERR spectroscopy for the in situ selective determination and semi-quantitative analysis of structurally similar dyes separated by TLC. The limits of identification of the TLC-SERRS method (ca. 5ng applied) were sufficient for acquisition of spectra of impurities present in the certified dye standards. SERRS may also be used for in situ identification of highly fluorescent molecules on HPTLC plates. 

Dieker et al.67 used a similar method but applied a dropping amalgam electrode (DAE) and followed amperometrically by means of pulse polarography the anodic dissolution wave of mercury in the presence of an excess of ligand by appropriate choices of pH and titrant they achieved selective determinations of metal ions at low concentrations. 

Diastereoselective hydroformylation can be achieved in special cases through passive substrate control in which conformational preferences are transferred in the corresponding selectivity-determining hydrometalation step [4-6]. A recent example is the highly diastereoselective hydroformylation of a kainic acid derivative (Scheme 17) [64], The selective formation of the major diastereomer has been explained via a reactive substrate conformation in which allylic 1,2-strain has been minimized. In this situation the czs-positioned methylene carbonylmethoxy group controls the catalyst attack to occur from the si face exclusively. 

Employing a molar excess of the alkyne over the active hydrogen of only 12 percent the selectivity (determined by proton NMR after work-up) may be expressed as the ratio of Si-C=CH- to Si-CH2-CH2-CO-, which is about 100 1 [16]. The same reaction, carried out with silicone polymers with 10 or more pendant silicon-bonded hydrogen atoms, proceeds similarly cleanly without noticeable crosslinking by acrylic hydrosilylation. A slight excess of triple bond over Si-H groups is mandatory to suppress this side reaction as well as hydrosilylation of the olefinic group of the 2-silylalkene isomer product (see Table 2.). 

B. Selected determinants of host response or susceptibility to microbial infection... 

The selective determination of Cu(II) was accomplished by making use of pyrocatechol violet indicator, dissolved in plasticized PVC membrane as a... 

M. Pontie, F. Bedioui, and J. Devynck, New composite modified carbon microfibers for sensitive and selective determination of physiologically relevant concentrations of nitric oxide in solution. Electroanalysis 11, 845-850 (1999). 

Figure 15.14 illustrates a typical voltammetric result for the determination of dopamine in the presence of ascorbic acid with a CNT-modified electrode. The selective voltammetric detection of uric acid [82] or norepinephrine [83] in the presence of ascorbic acid has been demonstrated with a (3-cyclodextrin-modified electrodes incorporating CNTs. Ye et al. [84] have studied the electrocatalytic oxidation of uric acid and ascorbic acid at a well-aligned CNT electrode, which can be used for the selective determination of uric acid in the presence of ascorbic acid. The simultaneous determination of dopamine and serotonin on a CNT-modified GC electrode has also been described [85],... 

M. Zhang, K. Gong, H. Zhang, and L. Mao, Layer-by-layer assembled carbon nanotubes for selective determination of dopamine in the presence of ascorbic acid. Biosens. Bioelectron. 20, 1270-1276 (2005). 

It follows from Equation 8.13 that aA/B can be expressed as the product of the diffusivity selectivity, DA/DB, and the solubility selectivity, SA/SB. Diffusion (or mobility) selectivity is governed primarily by the size difference between gas molecules and always favors smaller gas molecules. Solubility selectivity is controlled by the relative condensability of the gases in the polymer and their relative affinity for the polymer. Solubility selectivity typically favors larger, more condensable molecules. From Equation 8.13, it is seen that the product of gas mobility and solubility selectivity determines the overall membrane selectivity. It is clear that for a membrane to be C02 selective, it must have high diffusivity selectivity based on the affinity for C02 but it should be flexible enough to permeate larger molecules such... 

One way to get a representative product distribution for a specific period is to remove all FT products in the reactor system and replace them with a substance that will not influence selectivity determination. The FT reaction is then run for a specific period, after which a full analysis can be done that will represent only the products produced during that specific period. In Figure 13.8, data are presented for a run started with the catalyst suspended in a highly paraffinic wax (FT HI wax, C30-C90). After a certain time of synthesis, the FT run was stopped and the catalyst placed under inert conditions (argon). The reactor content was then displaced with degassed and dried polyalphaolefin oil (Durasyn). After restarting the FT synthesis, the total product spectrum was determined (HI run after displacement). It was found that the value of a2 was much lower than before the displacement of the HI wax. In fact, the a2 values were quite comparable to those measured when the FT synthesis was started up with Durasyn (compare with Durasyn runs 1, 2, and 3). This clearly illustrates the impact that the reactor medium used to start the FT reaction can have on the determination of the a-value. The results further show that there was no change in the value of a2 of the iron catalyst up to 500 h on-line. 

Buchberger et al. [104] carried out a selective determination of iodide in brine. The performance of a potentiometric method using an ion-selective electrode and of liquid chromatography coupled with ultraviolet detection at 230 nm were compared as methods for the determination of iodide in the presence of other iodide species. Satisfactory results were obtained from the potentiometric method provided the solution was first diluted tenfold with 5 M sodium nitrate, and external standards were used. Better reproducibility was, however, achieved with HPLC, provided precautions were taken to prevent reduction of iodine to iodide in the mobile phase, for which extraction of iodine with carbon tetrachloride prior to analysis was recommended. This was the pre-... 

With the exception of antimony (V), which requires the presence of iodide for its reduction, all species can be reduced in an acid medium at a pH of 1 -2. However, the reduction of some species, including antimony (III), arsenic (III), and all tin species, will also proceed at higher pH, where arsenic (V) and antimony (V) are not converted to their hydrides. This effect permits the selective determination of the various oxidation states of these elements [714, 716]. In the case of tin, reduction can be achieved at the pH of the Tris-HCl... 

Because of the importance of ATP in these mechanisms, the BL system was used as standard procedure for an extremely sensitive and selective determination of the cited molecule. The detection limit for the firefly assay method is about l(Tn mol/L ATP, allowing the detection of single cells in certain cases... 

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