Shapes of Peaks:

At this stage, it may be important to examine the peaks in a chromatogram and the bands on a column. They show a similarity to normal error or Gaussian curves when the values of a measurement are plotted as a function of their occurrence. An individual solute molecule undergoes thousands of transfers between the stationary and mobile phase. In a few cases, a residence time within a given phase might be transitory and in other case the period may be relatively long. The molecule is eluted during its residence in the mobile phase. Therefore, the migration is highly irregular down the column. Since there is variability in the residence time the average rate at which the individual molecules travel relative to mobile phase varies considerably. A few molecules travel faster since of their accidental inclusion in the mobile phase for a majority of time. At the contrary, others lag behind since they spend more time in the stationary phase. Since a consequence of these random processes, a systematic spread of velocities around the mean value is observed. The breadth of band increases because more time is allowed for spreading to take place. It can be concluded that the zone breadth is directly related to the residence time in the column and inversely related to the velocity of the mobile phase.