Retention factor relative

The above results only demonstrate the chromatographic behaviour of compounds with logP = 3. Their relative retention factors thus are influenced by the properties of solvents in the eluent. The relative values cannot be controlled by only one property of the solvents and chromatographic behaviour therefore also depends on the properties of the analytes. 

Figure 1.7—Separation factor (or selectivity factor) between two adjacent components, a alone does not determine if the separation is, in fact, possible. Here, the separation factor is in the order of 1.3. For two non-adjacent peaks, the relative retention factor can be obtained and it is designated by r. By definition, neither a nor r can be less than one.

To standardize reported retention times for a given column, manufacturers often report adjusted or relative retention factors rather than distribution constants. The adjusted retention time of a given compound is reported relative to the retention time of an unretained component. The relative distribution coefficient is reported relative to the distribution coefficient of a reference compound ... 

The ratio of individual distribution coefficients is often used as a measure of the possibility of separating two solutes and is called the separation factor, a, or relative retention factor. 

For non-adjacent peaks the relative retention factor r, is applied, which is calculated in a similar manner to a. 

The relative retention factor, a, for an adjacent component pair A and B will also depend on the column temperature. 

The degree of separation between successive peaks (of retained compounds) is known as the selectivity factor, a, or the relative retention. This is one of the most critical factors in chromatography the selectivity factor should be great enough so that each peak is sufficiently resolved (i. e. there are no overlaps at the baseline). However, if the relative retention factors are too great, then the GC will operate inefficiently—the runs will be unnecessarily long. The selectivity factor is calculated as the ratio of the capacity factors and these are usually concerned with adjacent peaks ... 

As in other variants of chromatography, the retention values used most in GSC are not absolute but relative (retention factor, k, relative retention, r, and Kovats retention index, /), characterized by a higher reproducibility than that of absolute retention values. 

Recovery factor Reduced column length Reduced plate height Reduced velocity Relative retention ratio Retardation factor d Retention time Retention volume Selectivity coefficient Separation factor... 

To accomplish any separation of two cations (or two anions), one of these ions must be taken up by the resin in distinct preference to the other. This preference is expressed by the separation factor (or relative retention), using K+ and Na+ as the example ... 

By comparing the ratio of the distribution coefficients for a pair of ions, a separation factor (or relative retention) is obtained for a specific experimental condition. 

The known models for describing retention factor in whole variable space ar e based on three-phase model and containing from three to six par ameters and variety combinations of two independent factors (micelle concentration, volume fraction of organic modifier). When the retention models are comparing or the accuracy of fitting establishing, the closeness of correlation coefficient to 1 and the sum of the squared residuals or the sum of absolute deviations and their relative values is taken into account. A number of problems ar e appear in this case ... 

The simplest mode of IGC is the infinite dilution mode , effected when the adsorbing species is present at very low concentration in a non-adsorbing carrier gas. Under such conditions, the adsorption may be assumed to be sub-monolayer, and if one assumes in addition that the surface is energetically homogeneous with respect to the adsorption (often an acceptable assumption for dispersion-force-only adsorbates), the isotherm will be linear (Henry s Law), i.e. the amount adsorbed will be linearly dependent on the partial saturation of the gas. The proportionality factor is the adsorption equilibrium constant, which is the ratio of the volume of gas adsorbed per unit area of solid to its relative saturation in the carrier. The quantity measured experimentally is the relative retention volume, Vn, for a gas sample injected into the column. It is the volume of carrier gas required to completely elute the sample, relative to the amount required to elute a non-adsorbing probe, i.e. 

Because a chemical step is imposed on top of the physical distribution process of partition, there is a great potential for selectivity, as noted by Schill et al, (49>50), Such factors as pH, type and composition of the organic phase, and ionic strength of the aqueous phase can be used to control relative retention. The concentration and type of counterion mainly control the absolute retention. 

Another relatively new lipophilicity scale proposed for use in ADME studies is based on MEKC [106]. A further variant is called BMC and uses mobile phases of Brij35 [polyoxyethylene(23)lauryl ether] [129]. Similarly, the retention factors of 16 P-blockers obtained with micellar chromatography with sodium dodecyl sulfate as micelle-forming agent correlates well with permeability coefficients in Caco-2 monolayers and apparent permeability coefficients in rat intestinal segments [130]. 

The variables that control the extent of a chromatographic separation are conveniently divided into kinetic and thermodynamic factors. The thermodynamic variables control relative retention and are embodied in the selectivity factor in the resolution equation. For any optimization strategy the selectivity factor should be maximized (see section 1.6). Since this depends on an understandino of the appropriate retention mechanism further discussion. .Jll be deferred to the appropriate sections of Chapters 2 and 4. 

By convention, the adjusted retention tine or the capacity factor ot the later of the two eluting peaks is made the numerator in equation (1.10) the s junration factor, consequently, always has values greater than or equal to 1.0. The separation factor is a measure of the selectivity of a chromatographic systn. The separation factor is sometimes called the selectivity factor, selectivity or relative retention. 

Establish control charts of instrumental performance. Day-to-day variations in pump flow rate, relative response factors, absolute response to a standard, column plate counts, and standard retention times or capacity factors are all useful monitors of the performance of a system. By requiring that operators maintain control charts, troubleshooting is made much easier. The maintenance of control charts should be limited to a few minutes per day. 

Chromatographic system (See Chromatography <621 >.) The liquid chromatograph is equipped with a 230 nm detector and a 4.6 mm x 30 cm column that contains packing L7. The flow rate is about 2 mL/min. Chromatograph the Resolution solution and the Standard preparation, and record the peak responses as directed under Procedure the resolution, R, between the dibutyl phthalate and miconazole peaks is not less than 5, the tailing factor for the miconazole peak is not more than 1.3, and the relative standard deviation for replicate injections of the Standard preparation is not more than 2%. The relative retention times are about 0.7 for dibutyl phthalate and 1 for miconazole. 

The distance that the spots or bands move up the plate is measured relative to the distance the solvent front has moved from its point of origin. Thus the retention factor (Rf) is ... 

Retention distance (or time) is normally used to aid the identification of a component of a mixture, provided that a known sample of the component has been subjected to separation under identical conditions. Because of the variations that can occur in the retention time due to technical factors, e.g. fluctuations in flow rate, condition of the column, the relative retention or selectivity factor (a) is sometimes used. This expresses the test retention time as a ratio of the retention time of another component or reference compound when both are injected as a mixture ... 

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