Minimal required dilution

Assay in biologicaf matrix Calibrators and controls in biological matrix Selectivity and specificity Minimal required dilution (MRD)... 

FIGURE 3.9 Evaluation of minimal required dilution. Individual and pooled normal human serum samples are analyzed at multiple dilutions. The matrix dilution at which spike recovery falls within an acceptable range (80 120%) in at least 80% of the samples indicates the MRD. 

In the case of commercial kits, comparison of kit and reagent lots should be addressed early on and, due to cost, the scope and goal of the transfer and development should be clearly defined. For example, use of additional or modified controls, modified standard curves, and sample treatment options (dilution scheme, assessment of minimal required dilution depending on matrix concentration) should be included. 

Typical techniques for determining molecular weight are given in Table 3.3. The most popular techniques will be considered briefly. All classical molecular weight techniques require dilute solutions, generally 0.01 g/mL or 1% (1 g/100 mL) solutions. To further minimize solute interactions, extrapolation of the measurements to infinite dilution is normal practice. 

Once equilibrium has been reached, the height difference between the two liquid surfaces is all that remains to be measured. The primary factor to note here is that capillaries are used to minimize the dilution effects. This means that corrections for capillary rise must be taken into account unless the apparatus allows the difference between two carefully matched capillaries to be measured. We discuss capillary rise in Chapter 6, Sections 6.2 and 6.4. Finally, there is an extremely important practical reason, in addition to the theoretical requirement of isothermal conditions, for good thermostating in osmometry experiments. The apparatus consists of a large liquid volume attached to a capillary and therefore has the characteristics of a liquid thermometer The location of the meniscus is quite sensitive to temperature fluctuations. 

In the analysis of nucleosides, bases, and other low-molecular-weight compounds, the use of ultrahltration is the most reliable technique. In addition to minimizing the number of steps, the use of ultrafiltration does not require dilution of the original sample. 

As in the general case discussed in section 8.2, for any choice of a volume Va which is at least the size of correlation volume Vc, one can obtain all the KB integrals from the inversion of the KB theory. Hence, we can compute all the local compositions as well as the preferential solvation around any species in the system. To the best of our knowledge, such a complete computation has not been undertaken for any three-component system. However, there exists abundant information, both experimental and theoretical, on a three-component system where one solute say, s, is very dilute in the mixed solvents of A and B. Although one can define the local composition and PS around s, A and B, only one of these has been studied, the component s which is diluted in the mixed solvent. It is worthwhile noting that in the traditional approach to solvation thermodynamics, only very dilute solutions could be studied, i.e., a dilute solution of s in a mixed solvent of two components was a minimal requirement for studying PS. We shall see in the next section that PS can be studied in a two-component system as well. 

The amount of solvent required depends strongly on the extraction mode used. In the static mode, the sample is extracted with a minimum volume of solvent (usually < 15 ml), with no outflow. When the solvent volume used in the static mode does not ensure quantitative extraction of the target analytes, several extraction cycles or the dynamic mode must be used. In the dynamic mode, the extractant flows continuously through the extraction cell, and so the volume of solvent that comes into contact with the sample is a direct function of both the flow rate of the circulating extractant and the extraction time. Obviously, the dynamic mode uses larger volumes of solvent than the static mode, and so it is less well suited for trace analysis, although there are various ways of minimizing this dilution effect, as shown below. 

The CLND is the most critical component in the platform. With our current experience, the most crucial factor is the stability at the sprayer exit. This may well be the source of the large deviations (up to 80%) observed for some of the examined compounds. The unpredictable nature of such operational malfunction strongly suggests the need for replicate determinations as the minimal requirement for reliable quantitative estimates. Other factors (injection error of the autosampler, dilution error in the preparation of solutions or standards) may account for only a limited fraction of the total measured variance and could be easily corrected for by the introduction of an appropriate internal standard. 

The bottoms from the stripper (40—60 wt % acid) are sent to an acid reconcentration unit for upgrading to the proper acid strength and recycling to the reactor. Because of the associated high energy requirements, reconcentration of the diluted sulfuric acid is a cosdy operation. However, a propylene gas stripping process, which utilizes only a small amount of added water for hydrolysis, has been described (63). In this modification, the equiUbrium quantity of isopropyl alcohol is stripped so that acid is recycled without reconcentration. Kquilibrium is attained rapidly at 50°C and isopropyl alcohol is removed from the hydrolysis mixture. Similarly, the weak sulfuric acid process minimizes the reconcentration of the acid and its associated corrosion and pollution problems. 

When sihca volatilizes, vapors condense on cooler areas to form a white bloom that can be removed by heat or dilute hydrofluoric acid. Because dilute hydrofluoric acid also attacks the substrate, a mild, careful treatment is required. To minimize volatilization, the temperature should be as low as possible. 

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