Competition technique

The reactivity of pyridine relative to that of benzene has been measured using the competitive technique developed by Ingold and his schoool for corresponding studies of electrophilic aromatic substitution. The validity of the method applied to free-radical reactions has been discussed. Three sources of the phenyl radical have been used the results obtained are set out in Table II. 

The reliability of this work has also been questioned by Brown and Nelson339, who could not get any reaction at all with benzene under the conditions quoted by Szmant and Dudek382. If the benzene value is in error it could account for the low selectivities that were observed. By using the competition technique with a deficiency of paraformaldehyde (but an excess of hydrogen chloride) they obtained a toluene benzene reactivity ratio of 112 and partial rate factors of / Me = 117, /mMc = 4.37, /pM = 430. 

The first two advantages listed above allow an optical method like transmission or reflection IR spectroscopy to be used for studies which would be impossible for a widely used competitive technique, electron energy loss spectroscopy (EELS). EELS must... 

Traditional electrophoresis and capillary electrophoresis are competitive techniques as both can be used for the analysis of similar types of samples. On the other hand, whereas HPLC and GC are complementary techniques since they are generally applicable to different sample types, HPLC and CE are more competitive with each other since they are applicable to many of the same types of samples. Yet, they exhibit different selec-tivities and thus are very suitable for cross-validation studies. CE is well suited for analysis of both polar and nonpolar compounds, i.e. water-soluble and water-insoluble compounds. CE may separate compounds that have been traditionally difficult to handle by HPLC (e.g. polar substances, large molecules, limited size samples). 

SPME-GC and SPE-GC are complementary rather than competitive techniques. SPE requires at least 100 p,L, of which only a small fraction is actually used for subsequent GC or HPLC analysis. This problem is addressed by SPME. Although SPME-GC is an attractive technique, its precision (RSDs, 3-20%) is often not as good as in SPE-GC (RSDs, 1-10%). 

If we consider only a few of the general requirements for the ideal polymer/additive analysis techniques (e.g. no matrix interferences, quantitative), then it is obvious that the choice is much restricted. Elements of the ideal method might include LD and MS, with reference to CRMs. Laser desorption and REMPI-MS are moving closest to direct selective sampling tandem mass spectrometry is supreme in identification. Direct-probe MS may yield accurate masses and concentrations of the components contained in the polymeric material. Selective sample preparation, efficient separation, selective detection, mass spectrometry and chemometric deconvolution techniques are complementary rather than competitive techniques. For elemental analysis, LA-ICP-ToFMS scores high. 

Sunda and Hanson [247] have used ligand competition techniques for the analysis of free copper (II) in seawater. This work demonstrated that only 0.02 -2% of dissolved copper (II) is accounted for by inorganic species. (i.e., Cu2+, CuC03, Cu(OH)+, CuCl+, etc.) the remainder is associated with organic complexes. Clearly, the speciation of copper (II) in seawater is markedly different from that in fresh water. 

Although this reaction occurs too rapidly for direct observation, the relative rate constants for pairs of phosphines were determined by standard kinetic competition techniques. The rate constant ratios for P(CeH4R)3 relative to PPh3 correlate well with the Hammett constant 3a, giving p = -0.70. 

A simple extension of the competition technique is to the comparison of scavenger efficiencies. Thus pairs of spin traps have been allowed to compete for a variety of radicals, including t-butoxyl, phenyl, and primary alkyl. Much more revealing, however, is the type of experiment in which the bimolecular trapping of a radical is allowed to compete with some other reaction of that radical whose absolute rate constant is known. In this way, the rate constant for the trapping reaction itself is accessible. 

A partial resolution to some of the problems with the non-competitive technique is to carry out the reactions of the separated isotopomers at the same time, and under the same conditions, but in different containers (say in a common thermostat). In this fashion one can directly compare isotopic differences as the reactions progress. For example, if the concentration of product or substrate can be followed spectropho-tometrically, one might use a two-beam instrument with the two samples placed next to each other. The photometric signal, then, is proportional to the difference in the absorption, A, of light and heavy species, and therefore to the difference in their concentrations, (provided the experiment is carried out in a region where the Lambert-Beer law is valid, and the molar extension coefficients are equal for both isotopomers), see Fig. 7.1. 

Extinction coefficients are usually determined by competitive techniques, employing a well-documented transient as a reference. Many extinction coefficients can be traced to the ketyl radical from benzophenone, for which the data are available from pulse radiolysis studies. This value is in turn referred to the yields of the solvated electron, HO and H produced in the radiolysis of water. The value for Ph2C OH is 3220 M cm in water at 537 nm, at room temperature. ... 

Contrary to potentiometric methods that operate under null current conditions, other electrochemical methods impose an external energy source on the sample to induce chemical reactions that would not otherwise spontaneously occur. It is thus possible to measure all sorts of ions and organic compounds that can either be reduced or oxidised electrochemically. Polarography, the best known of voltammetric methods, is still a competitive technique for certain determinations, even though it is outclassed in its present form. It is sometimes an alternative to atomic absorption methods. A second group of methods, such as coulometry, is based on constant current. Electrochemical sensors and their use as chromatographic detectors open new areas of application for this arsenal of techniques. 

The early value of the relative rate of propylene reactions obtained by the molybdenum trioxide technique (column A) is very much out of line, and so is its subsequent confirmation (column B). This discrepancy was realized at the time and the value was obtained again by Bradley et al. (15) using a direct competitive technique. A much larger relative rate of about 1.6 was obtained (column C, Table VII), in reasonable agreement with the values found by the other techniques. 

The hydrated electron is characterized by its strong absorption at 720 nm (e = 1.9 x 104 dm3 mol-1 cm-1 (Hug 1981) the majority of the oscillator strength is derived from optical transitions from the equilibrated s state to the p-like excited state (cf. Kimura et al. 1994 Assel et al. 2000). The 720-nm absorption is used for the determination of its reaction rate constants by pulse radiolysis (for the dynamics of solvation see, e.g Silva et al. 1998 for its energetics see, e.g Zhan et al. 2003). IP only absorbs in the UV (Hug 1981), and rate constants have largely been determined by EPR (Neta et al. 1971 Neta and Schuler 1972 Mezyk and Bartels 1995) and competition techniques (for a compilation, see Buxton et al. 1988). In many aspects, H and eaq behave very similarly, which made their distinction and the identification of eaq" difficult (for early reviews, see Hart 1964 Eiben 1970 Hart and Anbar 1970), and final proof of the existence of the... 

Scarano, G., Bramanti, E. and Zirino, A. (1992) Determination of copper complexation in sea water by a ligand competition technique with voltammetric measurement of the labile metal fraction. Anal. Chim. Acta, 264, 153-162. 

The relative reactivities of pyridine, 3-picoline, and 3-ethylpyridine toward phenyllithium have been measured under various conditions by a competitive technique and found to be in the order 3-pico-line > pyridine > 3-ethylpyridine.252 By carrying out reactions using an equimolar mixture of pyridine and 3-picoline and a large excess of phenyllithium, it has been possible to obtain yields of the phenyl-pyridines of over 80%, provided short reaction times and low temperatures are used. It has also been shown that the low yields usually obtained in such reactions are due to the fact that the dihydropyridyl-lithium intermediates form by-products, probably by polymerization (the intermediate dihydropyridine is a ct s-butadiene-like system and, in the presence of a Ziegler-type catalyst, can be expected to polymerize readily). The a-complexes from 3-picoline and phenyllithium polymerize faster than that from pyridine and phenyllithium, but there is no selective removal of the isomeric dihydropicolyllithium intermediates to form by-products, both isomers undergoing side-reactions at virtually the same rate. 

CT venography of cerebral veins is a very reliable technique for the demonstration of intraluminal abnormalities of dural sinuses (Figs. 18.10a, 18.12a Ozsvath et al. 1997). In particular, multisection CT venography with subtraction of bone is a promising and competitive technique for the evaluation of veno-occlusive disorders of the cerebral veins and sinuses (Majoie et al. 2004). One disadvantage, however, is the radiation exposure. Conventional DSA is typically not needed unless endovascular treatment is necessary, since MRI, MRA and CT are usually sufficient to make a correct diagnosis. 

Laruelle M. 2000. Imaging synaptic neurotransmission with in vivo binding competition techniques A critical review. J Cereb Blood FlowMetab 20 423-451. 

Comparison of relative importance of IPC and competitive techniques from... 

Fig. 9 Schematic drawing illustrating the simulation method based on the competition technique...

In this example, the equilibrium between A, B and AB is affected by the addition of C. The popularity of the competition technique is due to the fact that it can be used to investigate interactions (in this case the binding of A + C = AC) without having to detect the participating species (free C and the complex AC). The method relies on using one interaction (here A + B = AB) as a reporter to monitor the other. 

The liquefaction of waste plastics is not an entirely new technique, but the challenge to operate commercial plants has been tried in Germany by pyrolysis and hydrogenation. However, the former, operated by BASF was stopped in 1996, and the latter by Veba Oel GmbH in 1999 [1]. It is well known that the reason is not a technical problem, but an economic one the cost is higher than those of competitive techniques, such as mechanical recycling or blast furnace application. 

Surface enhanced resonance Raman scattering (SERRS) is an analytical technique with several advantages over competitive techniques in terms of improved sensitivity and selectivity. We have made great progress in the development of SERRS as a quantitative analytical method, in particular for the detection of DNA. However, one of the main advantages over fluorescence and other optical detection techniques is the ability to multiplex. 

Sunda W. G. and Hanson A. K. (1987) Measurement of free cupric ion concentration in seawater by a ligand competition technique involving copper sorption onto Cl8 SEP-PAK cartridges. Limnol. Oceanogr. 32, 537-551. 

By using this competitive technique for the reaction of alkyl halides with magnesium, the plots of Eq. (5) exhibited a linear character as illustrated in Figure 2. The relative rates (/C1//C2) were independent of the concentration of alkyl halide or the type of magnesium used, the size, the purity, the surface area, and the quantity. 

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