Theoretical plate numbers (N) and Determination of “N” in Chromatography

What are theoretical plate numbers (N)?

Number of theoretical plates (N): It is one index used to evaluate the performance and effectiveness of chromatographic columns. A number of theoretical plates (N) are an indirect measure of peak width (W) for a substance peak at a specific retention time (tR).

Theoretical plate numbers (N)

For Gaussian peaks, it is calculated by:

Number of theoretical plates N = 16 [Retention Time (tR) / Peak Width (Wb) ] 2


N= 5.545 [Retention Time (tR) / Peak Width (Wh) ] 2


tR =Retention time of the substance

W =Peak width at its base, obtained by extrapolating the relatively straight sides of the peak to the baseline.
Theoretical plate numbers (N):

  • The number of theoretical plates (N) is determined by the substance to be chromatographed as well as the working conditions,, such as
    • The flow rate and
    • Temperature (°C) of the mobile phase or carrier gas in GC,
    • The quality of the packing,
    • The uniformity of the packing within the column, and,
    • For capillary columns, the thickness of the stationary phase film and the internal diameter and length of the column.
  • The higher the number of plates in a chromatographic system, the more difficult it is to handle separation problems. The number of plates can theoretically be increased by utilising a longer column.

Related: Principle of HPLC (Liquid Chromatography)

The height equivalent to a theoretic (HETP)

The height equivalent to a theoretical plate (HETP) = L/N


  • L= Column length
  • N= Number of theoretical plates

The lower the HETP, the better the resolution (R) and the more efficient the substance separation. Efficiency is improved when N is maximized and HETP is minimized.

Related: Difference Between C18 and C8 HPLC Column

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