Topics - Required efficiency
Required efficiency is a very loose term which in chromatography usually means the number of theoretical plates a column must have, when used with a defined stationary phase, mobile phase, temperature and flow rate (assumed to be the optimum flow rate) to separate a given pair of substances eluted at a specific capacity ratios and having a specific separation ratio. It might be guessed that the theoretical treatment of this problem is quite complex and the equation that gives the number of plates required also complex. However, by using the equation, and having calculated the number of plates that are required, the next challenge is to obtain them. If the distribution system, flow rate and temperature, has been defined, then there are only a few simple procedures left to control the number of theoretical plates in the column. There is linear relationship between the number of theoretical plates and the column length and, thus, the column length can be adjusted to provide the required number of theoretical plates. The number of theoretical plates in the column is (approximately) inversely proportional to the particle diameter and so the particle diameter can be reduced to produce more theoretical plates (reducing the column diameter will not change the column efficiency but reduce the loading capacity). The extent to which the particle size is reduced will be limited by the available pump pressure. The same discussion also applies to the capillary column, however, the column diameter replaces the particle diameter.