The Capacity Ratio of a Solute

An eluted solute was originally identified from its corrected retention volume which was calculated from its corrected retention time. It follows that the accuracy of the measurement depended on the measurement and constancy of the mobile phase flow rate. To eliminate the errors involved in flow rate measurement, particularly for mobile phases that were compressible, the capacity ratio of a solute (k ) was introduced. The capacity ratio of a solute is defined as the ratio of its distribution coefficient to the phase ratio (a) of the column, where 

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The value of (k ), if not calculated by the data processing computer, is usually measured from the chart as. 

Distance between the dead volume peak and the peak maxima Distance between the injection point and the dead volume peak 

In qualitative work, it is clear that some caution must be shown in comparing (k ) values for the same solute from different columns and for different solutes on the same column. Both (Vm) and (Vs) will vary between different columns and may, 

Note that (Vm) is the volume of mobile phase in the column and not Vo the total dead volume of the column. 

Nevertheless it must be pointed out that, in calculating (k ), the value taken in practice is often the ratio of the corrected retention distance (the distance in centimeters on the chart, between the dead point and the peak maximum) to the dead volume distance (the distance in centimeters on the chart, between the injection point to the dead point on the chromatogram). This calculation assumes the extra column dead volume is not significant and, unfortunately, in almost all cases this is not true, (k ) values calculated in this way will be in error and should not be used for solute identification. Where computer data processing is used and no chart is available the distances defined above would be replaced by the corresponding times. 

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The capacity ratio of a solute, (k ), was introduced to develop a chromatographic measurement, simple to calculate, independent of flow-rate and one that could be used in solute identification. Although helpful, the capacity ratio is so dependent on the accurate measurement of extra column volume and on very limited solute exclusion by the support and stationary phase, that it is less than ideal for solute identification. An alternative measurement, the separation ratio (a) was suggested where, for two solutes (A) and (B),... 

The ratio, Vs/Vm, is often referred to as the phase volume ratio. Combining equations (2.16)-(2.18) allows the development of equations (2.20)-(2.22), which relate the capacity ratio of a solute to its retention time and the elution time, of an unretained compound ... 

The capacity ratio of a solute fk ) was defined as the ratio of the distribution coefficient K) of the solute to the phase ratio a) of the column. In turn, the phase ratio of the column was defined as the ratio of the volume of mobile phase in the column (Vm) to the volume of stationary phase in the column (V ) that is,... 

The capacity factor k is essentially a corrected retention time (Equation [3.18]) that takes into account variations in mobile phase flow rate and thus provides a more robust indicator of analyte retention for a given combination of stationary and mobile phases. As is a unique property of a given solute A for a given stationary-mobile phase combination, the adjusted (corrected) retention volume V,a (Equation [3.15] (or the corresponding retention time) can be used as a tag for analyte identification. Thus the precision and accuracy of measurement of Vja become important and depend on those of the measurement of flow rate U since in practice retention times rather than volumes are the measured quantities. In response to this limitation of the V,a parameter, the capacity ratio of a solute (k) ) was defined as the ratio of its distribution (partition) coefficient to the phase ratio (Vm/V s) of the column with respect to analyte A ... 

Equation [3.18] assumes that the extra-column volume Vg is negligible. However, there are two definitions of void volume, and thus also of the capacity ratio of a solute. The two void volumes are called the thermodynamic and the dynamic void volumes and they are not equal (Scott, www.chromatography-online.org) the two void volumes and capacity ratios are used for different purposes. Equations [3.16-3.18] incorporate the thermodynamic dead volume and all further discussion in this chapter assumes this definition. 

The capacity ratio of a solute is dependent upon the concentration of counter ion adsorbed on the stationary phase as well as on the concentration in the mobile phase when the stationary phase is saturated with adsorbed counter ion. As it is necessary to maintain a steady-state concentration of counter ion in the mobile phase, especially when the counter ion is UV-absorbing, the stationary phase is saturated with counter ion prior to application of the solute mixture. The concen-... 

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