Instant Notes: Analytical Chemistry

N5.54

2

(10)

Some laboratories favor the use of eqn. (9), but some favour eqn. (10) on the

grounds that peak width at half height can be measured with greater accuracy

than the base width. To make valid comparisons, the same formula should

always be used.

An alternative measure of efficiency, which is independent of the length of a

chromatographic column, is the plate height, H(or HeightEquivalent of a

Theoretical Plate, HETP), and given by

H (11)

where Lis the column length, usually expressed in millimetres or centimetres.

NBNis a dimensionless numberand to ensure correct computation, the

units of tRand Wbor Wh/2mustbe the same. Most chromatography data systems

have software to perform the calculations. Plate numbers for solutes separated

by GCand HPLCare of the order of several thousands to several hundreds of

thousands.

Columns giving very high plate numbers are capable of separating multicom-

ponent mixtures, but it is their resolving power, as measured by the resolution,

RS, that is of prime importance. This is defined as the difference between the

retention times of two adjacent solute peaks, DtR, divided by their average base-

widths, (W 1 + W 2 )/2 (Fig. 5)

RS (12)

As in the case of the efficiency formulae, Gaussian peaks are assumed and all

measurements mustbe in the same units as RSis a dimensionless number.

Values of RSapproaching or exceeding 1.5, which is defined as baseline resolu-

tion, are deemed satisfactory for most purposes.

The following is an example of the calculation of efficiency and resolution

using the above formulae:

2 DtR





(W 1 W 2 )

L

N

tR




Wh/2

D2 – Principles of chromatography 127

Concentration axis

Injection

tR

Wh/2

Wb

Peak height

One-half

peak height

(4s)

Time axis

Fig. 4. Measurement of chromatographic efficiency from a Gaussian peak.