[kimi85]

does anyone know what's the significance of the plate height in hplc?

thank you very much.

[Alpha-Omega]

HPLC-Plate Height:  synonym for number of theoretical plates....this is a theoretical caalculation...it is virtual....the more theoretical plates the better your resolution.

For example in IC I have used the IonPac AS14 and the AS14A columns for determining the 7 common anions.  One of them is sulfate.  The AS14A will give me double the theoretical plates for sulftae.  This is hight=ly desirable if I want to increase my resolution and am working at low levels of detection.

You can usually tell when your coulmn has degraded or requires cleaning-when the plate height decreases this is an indicator of a loss of column efficiency.  If you lose 15% it is time to chane out the guard column minimally....I would also replace the analytical.

Plate Height -syn. height equivalent to a theoretical plate, HETP; column length corresponding to a theoretical plate, normally found by dividing the column length by the number of theoretical plates.

Reduced Plate height (h) - Used to measure efficiencies of columns. An HPLC column with an h value 2 is considered to be well-packed, h = H/dp.

The plate number depends on column length: the longer the column, the larger the plate number. Therefore, the plate height term has been introduced to measure how efficiently column has been packed, h = L/N . The lower the plate height and the higher the plate number, the more efficient the chromatographic column.

From the literature at Dionex: 

Band broadening (column efficiency): 

After injection, a narrow chromatographic band is broaden during its movement through the column. The higher the column band broadening, the smaller the number of components that can be separated in a given time. In other words, the sharpness of the peak is an indication of how good, or efficient a column is.

The peak width is an indication of peak sharpness and, in general, an indication of the column efficiency . However, the peak width is dependent on a number of parameters (column length, flow rate, particle size). Flow rate is the only parameter which can be changed from run to run on the same column. Thus, it is better to consider a relative value to express column efficiency.

In absence of the specific interactions or sample overloading, the chromatographic peak can be represented by a Gaussian curve with the standard deviation (s). The ratio of standard deviation to the peak retention time (s/tR) is called the relative standard deviation, which is independent on the flow rate.

[enahs]

The plate height (as with plate counts/theoretical plates) has no physical relation to dimensions of the column in anyway, or any other physical attribution of the column. It is nomenclature carried over from distillation column.

Also, it is not a synonym for theoretical plates (they both help represent a arbitrary concept, but are not synonyms to each other, just parts of a whole). It is related to theoretical plates:
N = L/H
Where N is the number of theoretical plates, L is the actual length of the column, and H is the "plate height". Only L is an actual real number related to some physical dimension of the column.

A larger plate count gives better separation, just like in distillation. As you can see by the formula above, a longer column gives a larger plate count, as well as a smaller plate height gives a large plate count.


It is still used to describe efficiency in a column; and that is all it is, a simple method for descriping separation efficiency. But this method is inferior to the more modern rate theory, which can account for peak broadening in a mechanistic manner.