Retention measurement

Equation (34) is generally quite correct and useful. However, if highly accurate retention measurements are important, then second order effects must be taken into account and equation (33) indicates that, for accurate data, equation (34) is grossly over simplified. From equation (33), a more accurate expression for solute retention would be... 

It is seen that the separation ratio must be greater than about 1.055 for a low efficiency column (2500 theoretical plates) before accurate retention measurements can be made on the composite curve. On the high efficiency columns (10,000 theoretical plates), the separation ratio need only be in excess of about 1.035 before accurate retention measurements can be made on the composite curve. It will be seen later in this chapter that to optimize a column for a difficult separation, accurate retention data must be obtained over a range of temperatures and solvent compositions. It follows that ... 

There is an interesting consequence to the above discussion on composite peak envelopes. If the actual retention times of a pair of solutes are accurately known, then the measured retention time of the composite peak will be related to the relative quantities of each solute present. Consequently, an assay of the two components could be obtained from accurate retention measurements only. This method of analysis was shown to be feasible and practical by Scott and Reese [1]. Consider two solutes that are eluted so close together that a single composite peak is produced. From the Plate Theory, using the Gaussian form of the elution curve, the concentration profile of such a peak can be described by the following equation ... 

It is seen that a very significant temperature increase occurs particularly at high pressures. It would appear that, due to the poor radial thermal transfer in the column, the thermostat had little effect on the temperature change. Katz et al. [15] concluded that increases in column temperature resulting from high inlet pressures could seriously affect the accuracy of retention measurements. The heat evolved on changing the flow rate from 4 to 18 ml/min., accompanied by a corresponding... 

The net retention volume and the specific retention volume, defined in Table 1.1, are important parameters for determining physicochemical constants from gas chromatographic data [9,10,32]. The free energy, enthalpy, and. entropy of nixing or solution, and the infinite dilution solute activity coefficients can be determined from retention measurements. Measurements are usually made at infinite dilution (Henry s law region) in which the value of the activity coefficient (also the gas-liquid partition coefficient) can be assumed to have a constant value. At infinite dilution the solute molecules are not sufficiently close to exert any mutual attractions, and the environment of each may be considered to consist entirely of solvent molecules. The activity... 

Membrane Retention Measurements High-phospholipid in Surfactant-free Solutions... 

After reaching its maximum productivity (after ca. 8 hours.) the [Gl]-Nii2 showed a fast deactivation when applied in continuous catalysis performed in a membrane reactor (Figure 4.12). The fast loss of activity cannot be due to a lack of retention of the catalyst. Due to the high retention measured, this process should be much slower. A model study revealed that this deactivation process probably takes place by the formation of insoluble Ni(III) species (see Section 4.5 for further details). 

Under certain conditions chromatographic retention measurements can be used to determine the magnitude of equilibrium constants for reversible associations between a sample component and a complexing agent present in the eluent. The use of migration rate data to obtain equilibrium constants for interactions between eluents of the solvent and the solute is not novel. It has been developed for paper electrophoresis (297, 292). 

It follows that retention measurements on silica based stationary phases for the purpose of obtaining thermodynamic data is fraught with difficulties. Data from solutes of different molecular size cannot be compared or related to other Interacting variables ideally, thermodynamic measurements should be made on columns that contain stationary phases that exhibit no exclusion properties. However, the only column system that might meet this requirement is the capillary column which, unfortunately introduces other complications wmcn will be discussed later. 

Since the retention is measured as distance in stead of time, the / y-value, which expresses the position of a substance on a developed plate, should be calculated differently than retention measures are calculated in HPLC. First we introduce the distances which are of importance. These are Zp the distance between the solvent source and the solvent front, Zg the distance between the solvent source and the place where the solutes start and the distance between the start and final places of the solutes. From these distances it follows that ... 

Planeta, J. and Roth, M., Partition coefficients of low-volatility solutes in the ionic liquid l-M-butyl-3-methylimidazolium hexafluorophosphate-supercritical CO2 system from chromatographic retention measurements,. Phys. Chem. B, 108, 11244-11249, 2004. 

An attempt to compare retention times on two different columns of the same type can be difficult, at best. Differences in packing density, liquid loading, activity of the support, age and previous use of the packing, and variations in the composition of the column wall can lead to large differences in retention measurements between the two columns. If one must use two separate columns of the same type, then relative retention data is preferred since this measurement is reasonably constant for columns of the same type, it is not as subject to temperature and flow changes, and it is easy to obtain. 

The other type of water retention measurement is water uptake ability. The basic question that this test tries to answer is, what is the amount of water a material can retain in the presence of an excess of solution One might consider this measurement as the sponge capacity of a food material. This property depends mainly on the properties of the insoluble phase and on the physical state of the food, i.e., the degree of particle disintegration. It is thus a question that can only be asked of an insoluble or partially insoluble material. 

Retention measurements coupled with measurements of microbial activity. 

Figure 3.24 Typical result of a retention measurement of an almost retention free sample.

Retention measured as fall-to-fall enrollment from first to second year. 

Unlike the retentivity measure, k, relatively minor changes in alpha can have very major effects on resolution as shown in Table III. The variances in alpha that can be tolerated from column-to-column to maintain a given separation (R >1.25) is dependent on the alpha value involved. Table IV sfiows these allowable variances for several values of alpha. This table... 

Particle density can alternatively be evaluated from a set of retention measurements made in carriers of different density (1J5). Such density modifications are typically accomplished by successive additions of sucrose to the carrier. The observed retention values yield particle size and density through Equation 2. A rearrangement of this equation leads to a graphical procedure for size/density evaluation. The rearranged form is... 

When the above As are substituted into Eq. 9.29 or 9.30, we again end up with a linkage between experimental retention measurements (as reflected in R) and various electrical (n), diffusive (D), and thermal (a) properties of the components of interest. It is clear that species are separated on the basis of differences in these properties. For a particular sample we can, in theory, choose the property with the largest differences (the highest selectivity) between species, then apply the corresponding FFF subtechnique to that sample. 

It was shown in the previous section that solute migrates at a velocity proportional to its fraction in the mobile phase. At equilibrium this fraction is R and the migration velocity is Rv. Since equilibrium is attained at a point very near the zone center, the center s velocity is Rv. Thus the zone center, which is the reference point for retention measurement, migrates at a rate governed by equilibrium, even though this point is surrounded by regions where solute is migrating in a nonequilibrium condition. 

Some of the test methods being used to measure the processing stability of polypropylene include melt flow drift measurements at elevated temperatures using an extrusion plastometer (melt indexer), melt viscosity retention measurements using a torque rheometer, retention of melt flow after repeated extrusions, and injection molded spiral test measured by the flow in inches at various temperatures and the retention of melt flow of the injected spirals. The nine commercial resins were evaluated by these methods. 

Another modification is to include probe trials, where no escape platform is present (Czech et al. 2000). The retention measure is the time spent in the maze quadrant associated with the escape platform. An animal which remembers the location of the platform will spend more time in that quadrant of the maze. An essential requirement is that the probe trial remains short, say 1 minute, to avoid the animal learning that the platform is no longer present. If new learning can be thus avoided, probe trials provide the most sensitive index of the animal s retention of the position of the platform, unconfounded by random elements (the animal finding the platform by chance). 

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