Terms Referring to Column Efficiency

Normal distribution Generally, the peak profiles in the chromatograms are assumed to be symmetrical, and are treated by the well-known normal or Gaussian distribution equation  

Peak a mmetry There are many factors that can produce peak asymmetry. In a number of cases, the peaks recorded with a gas chromatograph are not Gaussian. Several measures have been used to characterise peak asymmetry and to study the influence of experimental conditions on peak asymmetry. The most simple and widely-used measure is the asymmetric ratio. As. defined as follows  

Plate number The plate number, or the number of theoretical plates is widely used to described the efficiency of a column, it can be expressed as  

As discussed above, for Gaussian peaks, the standard deviation can be expressed in terms of the peak width (W = 4F), thus we have  

From equations (12), (13), and (14), it can be seen that the plate number is a measure of the relative peak broadening that has occurred while the sample component passed through the column in time tR. As retention time increases, the value of W increases, that means the peak broadens. If the adjusted retention time, tR, is used, the peak number is called the effective plate number and written as  

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