Retention volumes, measurement

If the mobile phase is a liquid, and can be considered incompressible, then the volume of the mobile phase eluted from the column, between the injection and the peak maximum, can be easily obtained from the product of the flow rate and the retention time. For more precise measurements, the volume of eluent can be directly measured volumetrically by means of a burette or other suitable volume measuring vessel that is placed at the end of the column. If the mobile phase is compressible, however, the volume of mobile phase that passes through the column, measured at the exit, will no longer represent the true retention volume, as the volume flow will increase continuously along the column as the pressure falls. This problem was solved by James and Martin [3], who derived a correction factor that allowed the actual retention volume to be calculated from the retention volume measured at the column outlet at atmospheric pressure, and a function of the inlet/outlet pressure ratio. This correction factor can be derived as follows. 

Equation (3) merely sums the two peaks to produce a single envelope. Providing retention times can be measured precisely, the data can be used to determine the composition of a mixture of two substances that, although having finite retention differences, are eluted as a single peak. This can be achieved, providing the standard deviation of the measured retention time is small compared with the difference in retention times of the two solutes. Now, there is a direct relationship between retention volume measured in plate volumes and the equivalent times, which is depicted in Figure 6. 

The compressibilities of solvents vary significantly from one solvent to another. The compressibility of cyclohexane is about 0.67% per thousand p.s.i. change in pressure [11] and, thus, for a column operated at 6,000 p.s.i. (mean pressure 3,000 p.s.i.), there will be an error in retention volume measurement of about 2%. In a similar manner, n-heptane has a compressibility of about 1.0% per 1,000 p.s.i. change in pressure [11] and, under similar circumstances, would give an error of about 3% in retention volume measurement. Fortunately, as already discussed in Part 1 of this book, there are other retention parameters that can be used for solute... 

There are two major factors that influehce retention volume measurement and they are temperature and solvent composition. In order to measure retention volume with adequate precision it is necessary to know the relationship between retention time and temperature so that the control limits of the column temperature can be specified. 

It is seen that in order to measure retention volumes with a precision of 0.1%, the temperature control must be +/- 0.04°C. This level of temperature control on a thermostat bath is not difficult to achieve but it is extremely difficult, if not impossible, to return to a specific temperature to within +/- 0.04°C after prior change. To achieve a precision of retention volume measurement of 1%, the temperature control must be +/- 0.4°C. This is far more practical as most column oven temperature can be set to a given temperature to within +/-0.25°C. Although the data was obtained for three specific solutes, the results can be taken as reasonably representative for all solutes and phase systems. In most practical analyses, the precision limits of retention volume measurement will be about 1% but this will not include the reproducibility of the flow rate given by the pump. As... 

The Effect of Solvent Composition on Retention Volume Measurement... 

Figure 7. Experimental high pressure gas chromatographic apparatus for retention volume measurements.

Unfortunately values cannot be directly calculated from retention volume measurements by Equation 3 since the interfacial surface area is changing as a function of pressure due to the carbon dioxide sorption. However, the relative magnitude of the equilibrium shift of the sorbate from the solid adsorbent into the gaseous phase can be estimated by calculating the capacity factor, k, according to Equation A as given below ... 

Two types of retention data, i.e., (i) retention volumes measured at an infinite dilution of the solute as a function of temperature, and (ii) retention data recorded at different solute s concentrations and a constant temperature, can be used to characterize surface heterogeneity of porous solids in terms of the distribution function F (K). This function, which is the result of inversion of the integral equation (18), can be easily converted to the adsorption energy distribution. 

Suppose that one has, from chemical evidence, a fair idea of what the structure of the major component represented by peak 2 in Figure 13.11 may be. The identity of the compound then becomes established as soon as one finds which member of its class has the literature value of retention volume equal to C, within experimental error. But this is not so simple as that, if one bears in mind that the usefulness of retention volume as read from Figure 13.11 is limited, in that it applies only to a particular column packing at a given temperature. Values of retention volumes are very sensitive to temperature—a temperature change of 1°C frequently causes a 5% change in these values. For this reason it is now usual to deal with retention volumes measured relative to some given pure substance. This substance is then added to a mixture in which unknowns are desired to be studied, retention volumes of the unknowns are measured relative to the retention of the added substance. This eliminates the need for accurate control of column temperature and flow-rate. 

Not only variations in the pressure at constant temperature influence column-to-column retention data the role of the column hold-up volume as well as the mass of stationary phase present in the column is also important. The net retention volume caleulated from the adjusted retention volume corrects for the column hold-up volume (see Table 1.2). The specific retention volume corrects for the different amount of stationary phase present in individual colunms by referencing the net retention volume to unit mass of stationary phase. Further correction to a standard temperature of 0°C is discouraged [16-19]. Such calculations to a standard temperature significantly distort the actual relationship between the retention volumes measured at different temperatures. Specific retention volumes exhibit less variability between laboratories than other absolute measures of retention. They are not sufficiently accurate for solute identification purposes, however, owing to the accumulation of multiple experimental errors in their determination. Relative retention measurements, such as the retention index scale (section 2.4.4) are generally used for this purpose. The specific retention volume is commonly used in the determination of physicochemical properties by gas chromatography (see section 1.4.2). 

Vg being the specific retention volume measured at time t. 

The parameters basic to this discussion are the resolution coefficients (a), defined as the ratio of the retention volumes, measured from the air or solvent peak in GC, and of the capacity factors in HPLC. From these data, readily obtained from the chromatograms (see e.g. Fig. 1), also the difference in free energy of solution or adsorption of the enantiomers can be calculated by the relationship AAG = -RTlna. 

Gas-liquid Chromatography (g.l.c.).—Letcher has reviewed the use of g.l.c. to obtain activity coefficients in non-poiymer systems. The method is claimed - to be an accurate means of obtaining thermodynamic quantities in binary solutions when the two components differ considerably in volatility. Clearly this applies to many polymer-solvent systems and then the pol3rmer is conveniently made to form the stationary (liquid) phase in standard equipment. The solvent of interest is introduced into the mobile (gas) phase and its specific retention volume measured, from which heats of mixing are calculated > in the limit of zero concentration of solvent (a limit of interest in connection with the removal of volatiles from polymeric materials - ). 

At a constant flow rate, all retention volume measurements can be replaced by retention times. 

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