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Chemistry LibreTexts

N – Number of Theoretical Plates

One obvious way to increase the number of plates is to increase the length of the column.  Doubling the length doubles the number of theoretical plates.  One cautionary note about this is to consider the square root dependency on the number of plates in the equation.  Doubling the column length will double the analysis time, but will not double the resolution.  We would need a column four times longer to achieve a doubling of the resolution.  Increasing the number of plates will always give better resolution, but there are diminishing returns when the increased analysis time is considered.

Another thing to examine is factors that influence the band broadening equation.  That equation related broadening to h, and the smaller the value of h, the more plates in a column.  One thing we could do is optimize the flow rate of the system.  We mentioned how it was common to use a flow rate higher than the optimal one to shorten the analysis time.  Slowing down the flow rate may provide enough gains in efficiency to separate two compounds that are not fully resolved.

Another change to make is to use smaller particles.  In gas chromatography, this will lead to a thinner coating, improving mass transport broadening in the stationary phase.  In both gas chromatography and liquid chromatography, smaller particles will reduce the interstitial volume and reduce mobile phase mass transport broadening.  Smaller particles, provided they are packed well, also will reduce broadening contributions from eddy diffusion.