Selectivity Measurement

The selectivity of any chemical assay is its ability to provide a result unbiased by the presence of foreign species, and it is therefore crucial to the assay s practical application. Surprisingly, however, very few attempts have been made to describe selectivity of chemical assays in exact terms—except for ion-selective electrodes. To quantify the extent of interference, it is essential to be able to conduct all measurements at precisely and reproducibly maintained conditions so that the interfering species are treated, physically and chemically, in exactly the same manner as the substance to be determined (analyte). Such identical treatment can be achieved in FI A, and it thus makes sense to express the selectivity of a method for species A toward an interfering species by a numerical value. Since any interfering species will always appear as a pseudoanalyte, it was suggested [378] that the selectivity coefficient for any FI A method be defined by 

Zone penetration is an ideal tool for measuring selectivity coefficients, since the method allows readouts to be taken (1) at that vertical slice of the composite zone where the dispersion coefficients are equal (Da = Db at point M Fig. 2.26, bottom) and (2) at the peak maximum of the pure A component. The concentrations within such a composite zone and position of the point M at a time tM are readily established by an experiment where zone A is first injected alone and peak A is recorded by a detector of choice. (If chemical reactions are involved, like a reaction with a suitable reagent for colorimetric detection, a suitable manifold and colorimetric detector are used—cf. Section 4.5.2). Next, zone B is injected alone and peak B is recorded. Provided that the same solution of analyte is injected in both runs A and B, and that the detector responds linearly to the injected species, this experiment yields, (a) the isodispersion point M within the time concentration rnatrix, which is identified via time delay tM, and (b) the peak height Ha for the response of the pure species A at the time Im, because it equals the horizontal distance between point M and the baseline (Fig. 2.26). 

Furthermore, since the peak maximum for pure A has been found, and since the ratio between Ha and Ha is a constant, (because Da IDm = const), the selectivity constant kAB may be calculated readily from subsequent experimental runs where zone A (the analyte injected at concentration Ca) and zone B (the interferrent injected at concentration C%) are injected simultaneously and allowed to penetrate (Fig. 2.26, top). If species B is not sensed by the detector, the peak B will not show and, hence, Ha = Ha+b-1 the special case when species B will be sensed by detector exactly as species A, and when injected at the same concentration (Ca = Cb), a composite curve A + as shown in Fig. 2.26, bottom, will be obtained. As seen Ha will increase ( + ), so that Ha+b = 2Ha and k = 2. Generally, from the value of Ha+b and the known concentrations of A and B, kAB can be computed from Eq. (2.32). For negative interferences, Ha+b will decrease (-) below the value of Ha, and the kAB value will be less than 1. 

---

These equations apply when an entire population is available for measurement. The most common situation in practical problems is one in which the number of measurements is smaller than the entire population. A group of selected measurements smaller than the population is called a sample. Sample statistics are slightly different from population statistics but, for large samples, the equations of sample statistics approach those of population statistics. 

With respect to selecting measurements, emphasis should include measurements within the equipment such as tower internal temperatures and compositions, internal reac tor conditions, and intermediate exchanger temperatures in multipass exchangers. Trace component compositions provide particular insight into distillation-column performance. Those components that fall between the heavy and light keys and distribute in the products can usually be described by a variety of models and parameter estimates They provide little insight into the column performance. 

Table 1 Interactions in solid state NMR, parameters, their selective measurement, and...

If the assessment shows that the exposure approaches the limit values, so that the OEL is met but there is a probability of exceeding the limit values, subsequent measurements at appropriate intervals must be taken to ensure that the assessment situation continues to prevail. The frequency of these measurements will depend on the previous results, so that the nearer the concentration recorded comes to the limit value, the more frequently measurements must be taken under normal working conditions. However, if the values are borderline, the decision of whether exposures are below the limit values within the OEA is not clear, and a more comprehensive sampling exercise may be required using, for example, worst-case measurements. This becomes more important the fewer the measurements that have been taken, so in case of doubt the evaluation results should be verified through additional selective measurements. 

In order to reduce the effect, you can select measuring devices that are several orders of magnitude more accurate than your needs so that when the devices drift outside the tol-... 

Select measures that address the benefits expected from the integration project, such as reduced cycle times or number of hours spent on PSM and ESH management. 

Which measures you actually use will partly depend on the objectives of your integration program. You should be checking that the expected benefits of integration are being achieved. If improved efficiency was a key objective, it will be important to develop and collect measures of efficiency as early as possible. When selecting measures of efficiency, you should make sure you have baseline data from before the implementation of the integration project. If you don t have such baseline data, the full benefits of the project will never become apparent. 

The best method of ensuring these three factors are the same each time is to hard mount vibration transducers to the selected measurement points. This will guarantee accuracy and repeatability of acquired data. It will also increase the initial cost of the program. The average cost of installing a general-purpose accelerometer will be about 300 per measurement point or 3000 for a typical machine-train. 

The preceding paragraphs have been primarily devoted to a brief description of the methods of measuring detonation pressure and the presentation of selected measurement data. We have emphasized that both theory and measurements entail considerable uncertainty. Thus comparison between theory and observation is at best rather risky. Nevertheless, the P j vs loading... 

The equipment required for direct potentiometric measurements includes an ion-selective electrode (ISE), a reference electrode, and a potential-measuring device (a pH/millivolt meter that can read 0.2mV or better) (Figure 5-1). Conventional voltmeters cannot be used because only very small currents are allowed to be drawn. The ion-selective electrode is an indicator electrode capable of selectively measuring the activity of a particular ionic species. Such electrodes exhibit a fast response and a wide linear range, are not affected by color or turbidity, are not... 

In a more comprehensive follow-up work, the selectivity on OAOR-modified silver could be raised to 65%, still without the presence of promoters such as 1,2-dichloroethane [4]. This value is by far better than most values known in the literature for the same catalyst. The best value finally obtained was 69% and approaches the industrial limit of 80% that was obtained with promoters and a different, better catalyst, kglk 20y A similar catalyst type (Aluchrom catalyst) was also tested in the micro reactor, but so far yielding lower results, the best selectivity measured being 58%. 

Each column of a measurement table can be expanded into an indicator table. The rows of an indicator table refer to the same objects and in the same order as in the measurement table. The columns of the indicator table represent non-over-lapping categories of the selected measurement. Table 32.1 has been expanded into the indicator Table 32.2 for compounds and into the indicator Table 32.3 for disorders. In the indicator table for compounds, a value of one in a particular row is recorded if a person has used the corresponding compound. In the indicator table for disorders, one in a particular row indicates that the person has been treated for the corresponding disorder. All other elements of the row are set to zero. Note that the order of the columns in the indicator tables is not relevant. 

Applications Potentiometry finds widespread use for direct and selective measurement of analyte concentrations, mainly in routine analyses, and for endpoint determinations of titrations. Direct potentiometric measurements provide a rapid and convenient method for determining the activity of a variety of cations and anions. The most frequently determined ion in water is the hydrogen ion (pH measurement). Ion chromatography combined with potentiometric detection techniques using ISEs allows the selective quantification of selected analytes, even in complex matrices. The sensitivity of the electrodes allows sub-ppm concentrations to be measured. 

Catalogue the impacts and select measurement indicators (units)... 

Analysis of antioxidants (antioxidant status) for characterization of the anti-oxidative homeostasis in organisms by selective measurement of the ACP can be very meaningful for efficient supervision of antioxidant therapy as well. 

Lacking methods for increasing the acceptance of selected measurements and measures these methods are needed to convince politicians and support their decision making. The methods are also expected to be helpful in rising public awareness and understanding and hence to gain public support for the measures. 

Catalysts Catalysts used in the flow reactor study for activity and selectivity measurements are described in Table I. They were monolithic... 

Apparatus and Procedure Flow Reactor for Activity and Selectivity Measurements The experimental apparatus, testing procedures and... 

Aside from N2 adsorption, Kr or Ar adsorption can be used at low temperatures to determine low (<1 m2/g) surface areas [46], Chemically sensitive probes such as H2, Oz, or CO can also be employed to selectively measure surface areas of specific components of the catalyst (see below). Finally, mercury-based porosimeters, where the volume of the mercury incorporated into the pores is measured as a function of increasing (well above atmospheric) pressures, are sometimes used to determine the size of meso- and macropores [1]. By and large, the limitations of all of the above methods are that they only provide information on average pore volumes, and that they usually lack chemical sensitivity. 

By using ratio the number of edge (C7) atoms to the number of (111) face (Cg) atoms (Fig. 12) to define an effective particle size , the selectivity of n-butane hydrogenolysis as a function of particle size for the two surfaces could be plotted and compared to selectivities measured on supported Ir catalysts This comparison is shown in Fig. 13. Clearly, the results on single... 

---