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the mobile phase containing the standards and the sample injected onto the column. If the product contains the components in the specified proportion, no peaks will appear on the chromatogram, as the sample and mobile phase will have the identical composition. Any component that is in excess in the sample will give a positive peak. Conversely, any component that is present in the sample that is below specifications will give a negative peak. The peak area or peak height, for both positive and negative peaks, will give a quantitative estimation of the amount the component deviates from that specified. Temperature Changes During the Passage of a Solute Through a Theoretical Plate in Gas Chromatography Thermal changes resulting from solute interactions with the two phases are definitely second-order effects and, consequently, as such and, as Einstein predicted, their theoretical treatment is more complex. The theoretical treatment of temperature perturbations that result from

especially applicable to process monitoring. If a mobile phase that contains a solute at a given concentration is continually passed through a column until equilibrium is achieved, then the concentration of solute in the column eluent will be the same as that at the inlet. Let a sample of pure mobile phase be placed on the first plate of a column. This will cause a fall in the solute concentration in the first plate which, mathematically, will represent the injection of a sample having a negative concentration. A negative concentration profile will pass through a column in the same way as a positive concentration profile and at the end of the column will be recorded as a negative peak. The negative concentration profile will be described by exactly the same elution equation and its retention volume will be identical to that for a positive concentration profile. The generation of negative peaks by injecting a sample of pure mobile phase into an equilibrated mobile phase

The curves in Figure 19 show that as the injection volume is increased, so the retention volume of the peak also increases. The retention volume of the small negative peak produced by the smallest charge will be the same as that for a sample where Xi>X0 and the same as that for a solute chromatographed in the normal way with the column carrying pure mobile phase only. The significant dispersion that occurs with larger charges is clearly demonstrated.   Courtesy of th Journal of  Chromatography (ref. 21) Figure 19. Vacancy Elution Curves from Different Injection Volumes on a Column of 500 Theoretical Plates This theoretical

equilibrium, the eluent from the column will also contain the solute at a concentration (Xo). Let a sample containing the same solute at a concentration (Xi) be injected onto the column under where either (Xo<Xi) or (Xo>Xi),but (XoXi). Such an injection will produce a transient change in the concentration (Xo) of solute in the mobile phase. From the plate theory, this transient concentration change will be eluted through the column as a concentration difference and will be sensed as a negative or positive peak by the detector. The equation describing the resulting concentration profile of the eluted peak, from the plate theory will be,                                                           

A linear detector will provide a normal response and follow the Gaussian concentration profile of the eluted peak as shown in figure 1. If the normal signal is electronically integrated with respect to time then an integral output is obtained. Similarly if the normal output is differentiated then the differential of the Gaussian curve is produced. In a normal response the area of the peak is proportional to the total mass eluted whereas with the integral response the step height of the integral curve is proportional to the total mass eluted. The differential curve is often used to identify the retention time which is the point where the signal crosses from positive through zero to negative. The Dynamic Range of the Detector A detector has two response ranges, the dynamic range and the linear dynamic range and the two range are not synonymous. The dynamic range of a

mobile phase was a 0.14 M potassium phosphate buffer solution adjusted to pH 4.0. Figure 20 reveals that the positions of the peaks are accurately predicted by the theory; the peak heights differ because the relative responses of the detector to the different bases were not taken into account in calculating the theoretical curves. The sample with excess concentration of solutes over that in the mobile phase, shown as a chromatogram with positive peaks, is almost exactly the mirror image of the negative chromatogram produced from the injection of 115 ml of pure mobile phase