Topics - Peak Shape
The peak shape in chromatography is the name given to the type of curve resulting from plotting the concentration of solute in the mobile phase (or the mass of solute per unit time) eluted from a chromatographic column, against time. Its shape can be described mathematically or empirically. If the curve can be described by the error function then it is often described as a Gaussian or Error Function curve. If the elution profile is broadly asymmetrical and can be described by a Poisson function it may be called a Poisson Curve. Unless extremely small samples are placed on the column, the elution curve can be asymmetrical for a number of reasons. If the adsorption isotherm is not linear the peak will be distorted. A non linear isotherm can cause different types of peak distortion: if the non-linearity results from column overload, then solute-solute interaction can occur in the stationary phase and the peak will have a sloping front and a sharp tail. For obvious reasons this is called an overload peak. If the support in a packed column has adsorptive properties, the peak will exhibit a long tail and, again for obvious reasons, is called a tailing peak. Most chromatographic peaks will exhibit slight asymmetry resulting from thermal effects. In the front of the peak there is a net amount of the solute dissolving into the stationary phase which results in the heat of solution being continuously released. At the rear of the peak there is a net amount of the solute desorbing from the stationary phase which results in the heat of solution being continuously adsorbed. Thus, the stationary phase at the front of the peak will be above the average column temperature and conversely the temperature of the stationary phase at the rear of the peak will be below the average column temperature. This effect can easily be observed by placing a thermocouple in the column packing and, in fact, was the basis of an early LC detector. This temperature difference between the front and back of the peak, results in the distribution coefficient of the solute with respect to the stationary phase being smaller at the front of the peak, and a little larger at the rear of the peak. As the speed of migration of the peak is inversely proportional to the distribution coefficient, the front to the peak will migrate a little faster through the column than the rear of the peak and, thus, the front of the peak will be compressed and the rear of the peak extended, producing a slightly asymmetrical peak.