Analyte capacity/retention factor

In chromatography, the retention factor, or, equivalently, the capacity factor, is used to characterize the chromatographic equilibrium properties of an analyte. The retention factor of an analyte, k, is usually derived to be the product of the column phase ratio, Vr/Ve, and the distribution coefficient of the analyte between the stationary phase and eluent phase, Ca,r/ca,e, eqn [24] ... 

Yamamoto, A., Hayakawa, K., Matsunaga, A., Mizukami, E., and Miyazaki, M., Retention model of multiple eluent ion chromatography. A priori estimations of analyte capacity factor and peak intensity /. Chromatogr., 627,17,1992. 

While retention time is used for peak identification, it is dependent on the flow rate, the column dimension, and other parameters. A more fundamental term that measures the degree of retention of the analyte is the capacity factor or retention factor (k ), calculated by normalizing the net retention time (% > retention time minus the void time) by the void time. The capacity factor measures how many times the analyte is retained relative to an unretained component. ... 

The silanol induced peak tailing is also a function of the pH of the mobile phase. It is much less pronounced at acidic pH than at neutral pH. Therefore many of the older HPLC methods use acidified mobile phases. However, pH is an important and very valuable tool in methods development. The selectivity of a separation of ionizable compounds is best adjusted by a manipulation of the pH value. The retention factor of the non-ionized form of an analyte is often by a factor of 30 larger than the one of the ionized form, and it can be adjusted to any value in between by careful control of the mobile phase pH. This control must include a good buffering capacity of the buffer to avoid random fluctuations of retention times. 

Hong et al. applied capillary EKC with dodecyltrimethylammonium-bromide/sodium dodecylsulfate (12.7/21.1 mM) vesicles to the separation of alkylphenones (Fig. 8A) and obtained better resolution than with sodium dodecylsulfate micelles (59). The logarithms of the retention factors for 20 neutral compounds of similar structures showed an excellent linear correlation with log Poct (R2 = 0.98). Similarly, Razak et al. (60) showed that the log capacity factors for interaction between neutral and positively charged analytes and cetyltrimethylammoniumbromide/sodium octylsulfate vesicles correlated linearly with the log Poa values. 

Equation 10.12 is more informative than the equivalent Equation 3.21 for interpreting the influence of temperature under IPC conditions. Equation 10.12 indicates that the experimental capacity factor is actually the weighted average of the electrostatically modulated retention factors of the free analyte and that of the paired analyte. Notably, the global A7/° for IPC retention can be thought of as a weighted average of the retention of both free (electrostatically tuned) analyte and paired analyte. 

If the charge status of analyte and the HR is the opposite, an increase in retention is expected because of electrostatic attraction between solute and charged stationary phase. A parabola-Uke dependence of analyte capacity factors on... 

In the older literature, this constant was called the capacity factor and was symbolized by k . In 1993. however, the lUPAC Committee on Analytical Nomenclature recommended that this constant be termed the retention factor and be symbolized by k. 

Figure 2.31 Peak capacity as a function of analytical run time. The graph is valid for isocratic reversed-phase systems which are run at their van Deemter optimum. The maximum retention factor /ris 20, i.e. the maximum retention time is to 21, then the separation ends. The figure is only valid for small analytes with a diffusion coefficient ofapprox. 1-10 m s and not for macromolecules. Dotted lines represent the particle diameter, dashed lines the column length, and solid lines the pressure, respectively.

Perhaps the single most important term in any kind of chromatography is the retention factor (or capacity factor), k. Conditions must be adjusted so that there is a sufficient difference in the k values of the various analytes to give a good separation. It is also necessary to select conditions so that the range of k values is such that a separation may be completed within a reasonable time. A k range of 2 tolO has often been specified as desirable. 

Retention factor (k) or capacity factor, which is a measure of the relative speed of an analyte through a column and is calculated as ... 

The retention factor depends on the concentration of the competing ion in the mobile phase raised to the power of the ratio of charges of the analyte to the competing ion. The same dependence is observed for the capacity of the ion exchanger. If the charge of the competing ion is 1, Equation (117)... 

The retention factor (k ), which was previously known as the capacity factor, represents the capacity of a stationary phase to attract an analyte, k can be defined as the time during which the sample remains in the stationary phase relative to the time during which it resides in the mobile phase (t ), as shown in Figure 2.3. The retention factor is represented by the following equation ... 

The retention of solutes has also been considered in terms of their distribution coefficient (K), which is the ratio of the concentrations of the analyte in the mobile phase (C ob) in the stationary phase (Cstat) at equilibrium. In this section, we will also consider the retention factor or capacity factor. Like the distribution coefficient, k is dependent on the relative affinity of the solute in the respective phases, but it is expressed as the ratio of the amount of solute in stationary and mobile phases. Thus,... 

Capacity factor (retention factor) A measure of the time the analyte resides in the stationary phase relative to the time it resides in the mobile phase... 

The retention factor k, a common measure of chromatographic retention, depends on the equilibrium constant K describing the partition of the analyte between the mobile and stationary phases and the so-called phase ratio O, which is the ratio of the capacities of the two phases to accommodate an analyte (see Eq. 1). 

Analytical information taken from a chromatogram has almost exclusively involved either retention data (retention times, capacity factors, etc.) for peak identification or peak heights and peak areas for quantitative assessment. The width of the peak has been rarely used for analytical purposes, except occasionally to obtain approximate values for peak areas. Nevertheless, as seen from the Rate Theory, the peak width is inversely proportional to the solute diffusivity which, in turn, is a function of the solute molecular weight. It follows that for high molecular weight materials, particularly those that cannot be volatalized in the ionization source of a mass spectrometer, peak width measurement offers an approximate source of molecular weight data for very intractable solutes. 

The time taken for an analyte to elute from a chromatographic column with a particular mobile phase is termed its retention time, fan- Since this will vary with column length and mobile phase flow rate, it is more useful to use the capacity factor, k. This relates the retention time of an analyte to the time taken by an unretained compound, i.e. one which passes through the column without interacting with the stationary phase, to elute from the column under identical conditions (to). This is represented mathematically by the following equation ... 

A general approach to the problem of identification, should more definitive detectors not be available, is to change the chromatographic system , which in the case of HPLC is usually the mobile phase, and redetermine the retention parameter. The change obtained is often more characteristic of a single analyte than is the capacity factor with either of the mobile phases. 

Capacity factor The parameter used in HPLC to measure the retention of an analyte. 

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