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

before accurate retention measurements can be made on the composite curve. It follows that,: Considerable care must be taken when accessing closely eluting peaks. If the resolution is inadequate, measurements must be taken on the individual solutes, chromatographed separately. Quantitative Analysis from Retention Measurements A consequence of the above discussion on composite peak envelopes is that if the retention times of a pair of solutes are accurately known, then the measured retention time of the composite peak will be related to the relative quantities of each solute present. It follows that an assay of the two components could be obtained from accurate retention measurements only. This method of analysis was shown to be feasible and practical by Scott and Reese (12). Consider two solutes that are eluted so close together that a single composite peak is produced. Employing the Gaussian form of the elution equation, the concentration profile of such a peak can

;             (41) Equation (41) is the Gaussian form of the elution curve equation and can be used as an alternative to the Poisson form in all applications of the Plate Theory. Retention Measurements on Close Eluting Peaks The retention data, are the most important measurements made in any chromatographic analysis. In addition to providing data for identification (using the capacity ratio or the separation ratio), retention times are also important in column design. It will be shown later that the column efficiency needed to ensure resolution of a pair of solutes can be calculated from the capacity ratio and the separation ratio of the two peaks. However, when the solutes are eluted close together, serious errors can arise in retention measurements. In addition, the error becomes particularly significant when the accurate calculation of the required retention and efficiency is particularly essential. The

The Retention Volume of a Solute The retention volume of a solute is that volume of mobile phase that passes through the column between the injection point and the peak maximum. Consequently, by differentiating equation (10), equating to zero and solving for (v), an expression for the retention volume (Vr) can be obtained. Restating equation (12),                          &

Retention Gap Sampling The first solution to the problem of sample splitting was the 'retention gap method' which is depicted in figure 10. Figure 10. The Retention Gap Method of Sampling In this procedure stationary phase is removed from the first few centimeters of column. The sample is injected into this section and, if the sample becomes split, on commencing development, each split portion will still vaporize in the normal way. However, as there is

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