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retention difference, if taken from the maxima of the envelope, will give a value of less than 80% of the true retention difference. In addition, as the peaks become closer this error  rapidly increases. Most data processing software do not take this type of error into account. Consequently, if such data was used for solute identification, or column design, the results may be in gross in error. Another serious source of error can arise when two peaks are unresolved, and the retention time of the maximum of the envelope is taken as the mean retention time of the two individual solutes. This measurement can only be accurate if the peaks are absolutely symmetrical and the two peaks are of equal height. The result of different proportions of each isomer on the retention time of the composite envelope is shown in Figure 11. It is quite obvious that the position of the peak maximum of the composite envelope is very different from the mean retention time of the individual peaks

Thus, Vo = Qto where Q is the flow rate in ml/min. The retention time (tr) is the time elapsed between the injection point and the peak maximum. Each solute has a characteristic retention time. The retention volume (Vr) is the volume of mobile phase passed through the column between the injection point and the peak maximum. Thus, Vr = Qtr where Q is the flow rate in ml/min. Each solute will also have a characteristic retention volume. The corrected retention time (t'r) is the time elapsed between the dead point and the peak maximum. 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. Thus, V'r = Vr - Vo = Q(tr - to) where Q is the flow rate in ml/min

in figure 4. Figure 4 Graph of against (g) Figure 4 shows that there is little advantage in employing inlet/outlet pressure ratios much above 5 as values in excess of this do not reduce elution time significantly. If the column is very long, and consequently has a high flow impedance, higher inlet pressures may be necessary to obtain the optimum flow rate but this may not significantly reduce the elution time. In figure 5, the log of the retention time is plotted against (g) for both compressible and incompressible mobile phases. It is seen that for a compressible mobile phase the retention time falls to a constant level when (g) is about 5 or 6. In contrast, for an incompressible mobile phase (i.e. in liquid chromatography), the retention time is continuously reduced as (g) is increased. The advantages of flow programming with a compressible mobile phases are much less than for incompressible mobile phases

Figure 6 Graph of Retention Time against Pressure Program Rate for a Series of Solutes.   Consider five solutes having actual retention volumes of 150, 300, 600, 900 and 1200 ml eluted under pressure programming conditions where, (g1) is 1.2 and at (p) seconds after the start, gp = g1 + pa, where (a) takes values that range from 0.0025/s (0.0375 psi/s) and 0.025/s (0.15 psi/s) Employing equation (7) the retention time of the solutes can be calculated for the series of different programming rates. The results are shown in figure 6. It is seen that. although the use of pressure programming does indeed reduce the retention time of all solutes, program rates much above 0.2255 psi/s (13.5 psi/min.) provides very little advantage as far as reduction of analysis time is concerned.   Injection Devices The basic injection devices that are used in chromatography, such as the

as, within the precision possible for practical retention measurements in GC, many substances would have very close retention characteristics and, thus, in practice, retention data would have very limited use for solute identification.   Another problem associated with solute identification from retention data is the assumed availability of suitable reference samples to provide reference retention data. For most unknown samples, reference compounds are not available and, thus, retention values for any unknown solute is of little use for identification purposes. Unfortunately, even today, a half a century later, neither the thermodynamic theory of retention nor the interaction theory of retention are sufficiently well developed to be able to calculate the retention of a specific solute on a specific phase system from basic physical chemical data. As a consequence, reference retention data can not be calculated as an alternative to using a reference sample. It follows

of temperature on solute retention  in LC (compared to that in GC), temperature is not nearly so critical in governing absolute retention time but is often essential in achieving adequate resolution, particularly between closely eluting solutes such as isomers. In contrast to the GC column, the thermal capacity of an LC column is much higher as the specific heats of liquids are much greater than those of a gas. As a consequence, a high heat capacity thermostatting fluid is necessary and if retention measurements need to be precise, air ovens would not ideal for thermostatting LC columns. On the other hand, liquid thermostatting media are rather messy to use and tend to make column changing difficult and lengthy. However, if accurate data is required, good temperature control may be essential. If precise retention measurements are not required, an air thermostatting oven might be a reasonable compromise