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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

volumes included in equation (39). The exclusion characteristics of the interstitial cavities were obtained by measuring the retention volumes of salts having different molar volumes. The salts were ionically excluded from the pores of the packing and, thus, only penetrated the interstitial cavities as they passed through the column. The results are shown as a curve relating retention volume against ion volume in Figure 6. Courtesy of the Analyst (ref.11)   Figure 6. Graph of Retention Volume of a Series of Ions against Their Ionic Volume   The retention volume decreases linearly as the ion volume increases. It should be pointed out that the retention i not related to the charge on the ion. The intercept of the curve on the retention volume axis gives a value for the total interstitial volume of the column, which differs only slightly from the retention volume of sodium nitrate. Thus, the retention volume of sodium nitrate would give a close approximation to

separation as both retentive processes are usually present to some extent, particularly in liquid chromatography (LC). Retention in gas chromatography (GC), using coated open tubes, is, perhaps, the exception, as, in this distribution system, exclusion processes (aside from chiral phases) are normally absent and, consequently, retention control is purely interactive. Interaction and exclusion processes, when they do occur together, act independently. Although, together they determine overall theretention of a solute, the exclusion properties of a stationary phase do not effect the magnitude of any of its interactive properties. The mechanisms of retention have been discussed briefly in Book 1, but will now be considered in greater detail.  Although both retention processes (interaction and exclusion) are usually active, because they contribute to retention independently, the two mechanisms will be considered separately. Chromatographic Interactions Solutes are retained in the

The relationship between the retention of ethanol on the brush phase with water/methanol mixtures, as shown in figure 35, can now be explained. In pure water the hydrocarbon chains of the brush phase are collapsed on the surface and thus, the effective surface area of the stationary phase is much reduced. Consequently, the retention volume of the solute, being proportional to the available surface area, is also reduced. As methanol is added to the solvent mixture, the solvent becomes more dispersive and the hydrocarbon chains can begin to interact with it and, as a consequence, begin  to unfold. The liberation of the chains from the surface results in an increase in the effective surface area of the stationary phase and the retention of the solute also starts to increase. This  process continues until there is

The actual 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

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