System Peaks in Linear Chromatography

When the sample size is small, and the equilibrium isotherms of the sample components can be considered linear in the range of concentrations achieved for these components, the chromatographic phenomenon remains linear regarding aU the 

System peaks arise when one or several components of the mobile phase are adsorbed on the stationary phase, and because their concentrations are such that their adsorption isotherms are not linear. Injection of the sample in the multicomponent mobile phase perturbs the equilibrium of the strong solvent(s) or other additives of the mobile phase, which is nonlinear. As a result of this perturbation, more peaks may be recorded than there are components in the injected sample. For example, the injection of a sample of the pure weak solvent may generate as many peaks as there are additives in the mobile phase, if these additives are all adsorbed by the stationary phase. These extra peaks have received a variety of names in the early literature. They have been called system peaks, pseudopeaks, ghost peaks, eigen peaks, vacancy peaks, induced peaks, etc. We call them system peaks. Note that not all system peaks are recorded this depends on the type of detector used. Also, system peaks may be positive or negative, depending on the experimental conditions. 

In this work we call all the n + p peaks, system peaks. To distinguish between them when necessary, we call the n system peaks that move with the analyte peaks the analyte system peaks. The other p system peaks that move at velocities depending on the additive concentrations and characteristics are called the additive system peaks, or more simply the system peaks. 

Although a general mathematical treatment of system peaks was published very early by Helfferich and Klein [8], most early experimental work ignored these theoretical explanations. The first experimental observation of system peaks was probably reported by Fornstedt and Porath [9], while Solms et al. [10] made the 

System Peaks with the Equilibrium-Dispersive Model 

---

We have discussed the theory of system peaks in linear chromatography [20]. The discussion is based on the use of the equilibrium-dispersive model. The mass balance equations are written for the n components of the sample and for the p additives ... 

From a theoretical point of view, the discussion of the profiles of the component and the additive bands at high concentrations is complicated because the perturbation due to the injection of the sample cannot be considered small and cannot be treated by assxuning the system to behave linearly aroxmd the steady equilibrium point, as is done in the study of system peaks in analytical chromatography. Band profiles are accessible only through numerical calculations. The experimental results are still difficult to account for because of the scarcity of studies and data on the phenomenon, and because of the strange and unexpected shape of the profiles obtained imder some sets of experimental conditions. 

As expected in linear chromatography, there is one peak for each sample component, and this peak elutes at the same time as a pulse of the pure component in the same system. For a Langmuir isotherm, the retention factor is... 

The upper curve shows the adsorption isotherm that normally occurs in liquid chromatography separations where the concentration of solute in the system is very low. The isotherm is linear and thus the distribution coefficient is constant at all concentrations of solute in either phase. It follows that as the peak velocity is inversely related to the distribution coefficient, all solute concentrations travel at the same velocity through the column and the peak is symmetrical. 

When a small sample is injected, the problem can be considered as a mere perturbation of the phase equilibrium, and simple solutions are easily derived. When large samples are injected, the elution profiles are more complex, sometimes surprisingly so. Thus, a separate discussion of these problems in linear and nonlinear chromatography is in order. Note that system peaks arise only when chromatography is carried out under conditions that, although they may be linear for the analytes, are not linear for the additive(s). 

Adsorption Chromatography. The principle of gas-sohd or Hquid-sohd chromatography may be easily understood from equation 35. In a linear multicomponent system (several sorbates at low concentration in an inert carrier) the wave velocity for each component depends on its adsorption equihbrium constant. Thus, if a pulse of the mixed sorbate is injected at the column inlet, the different species separate into bands which travel through the column at their characteristic velocities, and at the oudet of the column a sequence of peaks corresponding to the different species is detected. 

A highly versatile method for enantiomer analysis is based on the direct separation of enantiomeric mixtures on nonraceinic chiral stationary phases by gas chromatography (GC)6 123-12s. When a linearly responding achiral detection system is employed, comparison of the relative peak areas provides a precise measurement of the enantiomeric ratio from which the enantiomeric purity ee can be calculated. The enantiomeric ratio measured is independent of the enantiomeric purity of the chiral stationary phase. A low enantiomeric purity of the resolving agent, however, results in small separation factors a, while a racemic auxiliary will obviously not be able to distinguish enantiomers. 

---