Measures of Selectivity

Measurement of Analytes Biochemical reactions used in the measurement of selected analytes are commercially available as prepackaged kits of reagents. Measurement of the reactions given plus many other analytes can be made (10,11). 

Brown assumed that a good measurement of selectivity was the ratio of the para and meta partial rate factors in toluene. He defined the selectivity Sf of a reaction as... 

Although rum ammonia levels are not routinely measured, it is a useful indicator of Reye s syndrome and should be monitored in newborns at risk of developing hyperammonemia Ammonia is produced in many analytically useful enzyme reactions and the ammonium ISE has been used as the base sensor in several enzyme electrodes (see next section). In addition to valinomycin, other antibiotics such as the nonactin homalogs and gramicidins also behave as ionophores. The nonactin homolo were originally studied for their ability to selectively bind potassiiun ions It was then discovered that ammonium ions were preferred over potassium ions, and the selectivity coefficient Knh+ = 0.12 was reported. Since ammonia is present at fairly low levels in serum, this selectivity is not sufficient to to accurately measure NH4 in the presence of K. An extra measure of selectivity can be gained by using a gas permeable membrane to separate the ammonia gas from the sample matrix... 

The sensitivity achieved (LOD) is not normally presented. It is recognized that different laboratories determine dissimilar values for this parameter and even within a laboratory the repeatability of the LOD is low. Most often, the lowest validated concentration gives an impression about the lowest levels that can be analyzed generally with acceptable results. A measure of selectivity is the intensity of blank results. This intensity is discussed by the participants of inter-laboratory validation studies. However, results are not reported and limits are not defined by CEN TC 275. The results of method validations of the several multi-residue/multi-matrix methods are not reported in the same way, but newer methods with limited scope generate analogous tables with validation results (as an example, see Table 7). 

In a system of two parallel reactions (Fig. 10.4), selectivity is an important parameter to monitor. The measure of selectivity is defined here as the ratio of isobutene reacted to ethers / total amount of isobutene reacted . Glycerol is consumed only in the etherification reaction, so the selectivity is calculated with respect to isobutene. Depending on the extent of the reaction, the formation of an ether molecule consumes from one to three isobutene molecules, and oligomerization consumes from two to four isobutene molecules [8],... 

Singh HB, Salas LJ, Smith A, et al. 1979a. Atmospheric measurements of selected toxic organic chemicals. Report to U.S. Environmental Protection Agency, Environmental Sciences Research Laboratory, Atmospheric Chemistry and Physics Department, Research Triangle Park, NC, by SRI International, Menlo Park, CA, 3-4, 28, 33, 35. 

Harkov R, Kebbekus B, Bozzelli JW, et al. 1983. Measurement of selected volatile organic compounds at three locations in New Jersey during the summer season. J Air Pollut Control Assoc 33 1177-1183. 

Singh HB, Salas LJ, Shigeishi H. 1981. Atmospheric measurements of selected hazardous organic... 

Finally, in x-ray diffraction measurements the knowledge of the equation of state (EOS) of specific materials is used to calculate the pressure by the measurement of selected reflections. For this purpose, Au [266], NaCl [267], Re [268], Pt [269], and MgO [231] have been used in x-ray scattering measurements when no optical access was available. 

Letinski, D.J., Connelly, M.J., Jr., Petersen, D.R., and Parkerton, T.F. Slow-stir water solubility measurements of selected alcohols and esters, Chemosphere, 48(3) 257-265, 2002. 

However, even this simplified formula does not justify the use of the ratio of stability constants of the extracted complexes as the only measure of selectivity of extractive separations. Such a widely used approach is obviously based on an implicit assumption that the partition constants of neutral complexes ML of similar metal ions are similar, so that their ratio should be close to unity. This is, however, an oversimplification because we have shown that the ifoM values significantly differ even in a series of coordi-natively saturated complexes of similar metals [92,93]. Still stronger differences in the values have been observed in the series of lanthanide acetylacetonates, due to different inner-sphere hydration of the complexes (shown earlier), but in this case, self-adduct formation acts in the opposite direction [100,101] and partly compensates the effect of the differences in. Tdm on S T(see also Fig. 4.15). Such compensation should also be observed in extraction systems containing coordinatively unsaturated complexes and a neutral lipophilic coextractant (synergist). 

Van Warmerdam EM, Erape SK, Aravena R, Drimmie RJ, Elatt H, Cherry JA (1995) Stable chlorine and carbon isotope measurements of selected chlorinated organic solvents. Appl Geochem 10 547-552... 

Recognizing this, Richard and Jencks, proposed using azide ion as a clock for obtaining absolute reactivities of less stable cations. The basic assumption is that azide ion is reacting at the diffusion limit with the cation. Taking 5 x 10 M s as the second-order rate constant for this reaction, measurement of the selectivity fcaz Nu for the competition between azide ion and a second nucleophile then provides the absolute rate constant since feaz is known. The clock approach has now been applied to a number of cations, with measurements of selectivities by both competition kinetics and common ion inhibition. Other nucleophiles have been employed as the clock. The laser flash photolysis (LFP) experiments to be discussed later have verified the azide clock assumption. Cations with lifetimes in water less than about 100 ps do react with azide ion with a rate constant in the range 5-10x10 M- s-, " which means that rate constants obtained by a clock method can be viewed with reasonable confidence. 

The dynamic melt viscosity measurements of select star blocks and a similar triblock were carried out on a rheometric mechanical spectrometer, RMS. Circular molded samples of 2 cm diameter and -1.5 mm thickness were subjected to forced sinusoidal oscillations. Dynamic viscosities were recorded in the frequency range of 0.01-100 rad/s at 180 °C. Figure 10 shows the complex viscosities of two select star blocks and a similar linear triblock. The plots showed characteristic behavior of thermoplastic elastomers, i.e., absence of Newtonian behavior even in the low frequency region. The complex viscosity of the star block... 

TABLE 1. Dynamic and Static Contact Angle Measurements of Selected Experimental Agents... 

Singh, H.B., Salas, L.J., Smith, J.A., Shigeishi, H. (1980) Atmospheric Measurements of Selected Toxic Organic Chemicals. 

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