Retention volume corrected

Solvent interaction model for normal-phase liquid chromatography. The solvent-interaction model of Scott and co-workers (Scott and Kucera, 1979) assumes that the analyte partitions between the bulk mobile phase and a layer of solvent absorbed onto the stationary phase. The quantitative description of the relationship between retention and the composition of the mobile phase in the solvent-interaction model requires the definition of the void volume corrected retention volume (V), which is related to the retention volume (F ) and the void volume (Fq) by... 

Fig. 1 Diagram depicting the retention volume, corrected retention volume, dead point, dead volume, and dead time of a chromatogram. Fq total volume passed through the column between the point of injection and the peak maximum of a completely unretained peak F total volume of mobile phase in the column F (a) retention volume of solute A F (a) corrected retention volume of solute A F extra column volume of mobile phase volume of mobile phase, per theoretical plate vy. volume of stationary phase per theoretical plate distribution coefficient of the solute between the two phases n number of theoretical plates in the column Q column flow rate measured at the exit.

The corrected retention volume (V r) is the volume of mobile phase passed through the column between the dead point and the peak maximum. It will also be the retention volume minus the dead volume. 

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... 

Probably the best compromise for silica based stationary phases is to use corrected retention volume data for solutes eluted at a (k ) of greater than 5 and only compare chromatographic data for solutes of approximately the same molecular size. 

Figure 2. Graph of the Reciprocal of the Corrected Retention Volume against the Moderator Concentration for a Series of Aliphatic Alcohols...

Now taking (Cm) as the concentration of the moderator and (cp) the total chromatographically available surface area, then the corrected retention volume (V r)... 

Thus, if the slope and intercept of the curve relating the reciprocal of the corrected retention volume to the concentration of the moderator are (p) and (tj)) respectively. 

It is seen, from equation (5), that a graph relating the reciprocal of the corrected retention volume to the concentration of the moderator can provide values for the adsorption/desorption coefficient and the surface area of the stationary phase. Scott and Simpson [1] used this technique to measure the surface area of a reversed phase and the curves relating the reciprocal of the corrected retention volume to moderator concentration are those shown in Figure 2. 

Curves relating the corrected retention volume to the concentration of moderator (methanol) in the mobile phase [3] are shown in Figure 4. In pure water, the hydrocarbon chains of the brush phase interact with each other and collapse onto the surface in much the same way as drops of an hydrocarbon will coalesce on the... 

Concentrations of moderator at or above that which causes the surface of a stationary phase to be completely covered can only govern the interactions that take place in the mobile phase. It follows that retention can be modified by using different mixtures of solvents as the mobile phase, or in GC by using mixed stationary phases. The theory behind solute retention by mixed stationary phases was first examined by Purnell and, at the time, his discoveries were met with considerable criticism and disbelief. Purnell et al. [5], Laub and Purnell [6] and Laub [7], examined the effect of mixed phases on solute retention and concluded that, for a wide range of binary mixtures, the corrected retention volume of a solute was linearly related to the volume fraction of either one of the two phases. This was quite an unexpected relationship, as at that time it was tentatively (although not rationally) assumed that the retention volume would be some form of the exponent of the stationary phase composition. It was also found that certain mixtures did not obey this rule and these will be discussed later. In terms of an expression for solute retention, the results of Purnell and his co-workers can be given as follows,... 

Figure 14. Graph of Corrected Retention Volume against Volume Fraction of Stationary Phase...

For chromatography purposes the product of the distribution coefficient and the volume of stationary phase, or stationary phase surface area, gives the corrected retention volume,/, e.,... 

Substituting for the corrected retention volumes in equation (9) for inverse phase system,... 

If the corrected retention volume in the pure strongly eluting solute is very small compared with the retention volume of the solute in the other pure solvent, i.e., V"a V"b, which is very often the case in practical LC, then equation (12)... 

Figure 18. Graphs Relating the Reciprocal of the Corrected Retention Volume to the Volume Fraction of the Polar Solvent in n-Heptane.

Figure 19 Graph of Corrected Retention Volume of the (S) 4-Benzyl-2-oxazolidinone against the Reciprocal of the Volume Fraction of Ethanol...

The column was operated in the normal phase mode using mixtures of n-hexane and ethanol as the mobile phase. Equation (13) is validated by the curves relating the corrected retention volume to the reciprocal of the volume fraction of ethanol in Figure 19. It is seen that an excellent linear relationship is obtained between the corrected retention volume and the reciprocal of the volume fraction of ethanol. 

When the relationship between the distribution coefficient of a solute and solvent composition, or the corrected retention volume and solvent composition, was evaluated for aqueous solvent mixtures, it was found that the simple relationship identified by Purnell and Laub and Katz et al. no longer applied. The suspected cause for the failure was the strong association between the solvent and water. As a consequence, the mixture was not binary in nature but, in fact, a ternary system. An aqueous solution of methanol, for example, contained methanol, water and methanol associated with water. It follows that the prediction of the net distribution coefficient or net retention volume for a ternary system would require the use of three distribution coefficients one representing the distribution of the solute between the stationary phase and water, one representing that between the stationary phase and methanol and one between the stationary phase and the methanol/water associate. Unfortunately, as the relative amount of association varies with the initial... 

Vg) is the corrected retention volume of the solute with respect to the pure solvent (B),... 

Scott and Beesley [2] measured the corrected retention volumes of the enantiomers of 4-benzyl-2-oxazolidinone employing hexane/ethanol mixtures as the mobile phase and correlated the corrected retention volume of each isomer to the reciprocal of the volume fraction of ethanol. The results they obtained at 25°C are shown in Figure 8. It is seen that the correlation is excellent and was equally so for four other temperatures that were examined. From the same experiments carried out at different absolute temperatures (T) and at different volume fractions of ethanol (c), the effect of temperature and mobile composition was identified using the equation for the free energy of distribution and the reciprocal relationship between the solvent composition and retention. 

Then, after (t ) seconds the mean effective corrected retention volumes of the two isomers will be given by... 

In a similar manner the elapsed time between the elution of the unretained solute and the peak maximum is called the corrected retention time (t r). The volume of mobile phase that passes through... 

The corrected retention volume and corrected retention time can be used for solute identification but more appropriate measurements for this purpose will be discussed later. 

The first chromatographic parameter that was employed for solute identification was the corrected retention volume (V r). However, the measurement of (V r) is not as straightforward as it might seem. Consider equation (11), which for convenience will be reiterated. 

The exact nature of the dead volume is complex and, in fact, will vary from solute to solute due to the exclusion properties of the stationary phase, particularly if the stationary phase or support is silica or silica based. Thus, to measure (Vo) accurately, a non-adsorbed solute of the same molecular size as the solute should be used and then the correct retention volume (V r) can be calculated and employed for identification purposes. 

For example for two solutes (A) and (B) their corrected retention volumes will be will be... 

However, it must be emphasized that retention data, whether they be corrected retention volumes, capacity ratios or separation ratios, do not provide unambiguous solute identification. Matching retention data between a solute and a standard obtained from two columns employing different phase systems would be more significant. Even... 

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