Basic Principles of Chromatography - Forensic Applications of Gas Chromatography - page 35

• w 1/2 = the width of the peak at half its height. In practical terms this
is done by halving the peak height and measuring the width of the
peak at this position.
• W 0.6065 = the width of the peak at the point of inflection (or curvature
as described in w b ) near the peak base. In practical terms this is done
by halving the peak height and measuring the width of the peak at
this position.
• k′ = capacity factor. (Sometimes it is deined by use of a small letter
k with a prime, i.e., k′ or simply k. It has no units.)
• N = column eiciency.
• L = column length (the dimension needs to be deined in appropriate
units, e.g., metres, centimetres or millimetres).
• HETP = height equivalent to a theoretical plate, expressed as column
efficiency (N), in units of millimetres.
• A s = asymmetry factor.
• R = a measure of the degree of separation of adjacent compound peaks.

The importance and use of some key terms will now be described.

3.2.1 Capacity Factor
In order to be able to compare the elution time of one compound between
one gas chromatograph and another (whether in the same laboratory or not),
the capacity factor for that compound must be calculated. Calculating the
capacity factor creates a unitless measure of the compound's retention time
irrespective of column length or flow rate. Capacity factor is therefore often
considered a more useful measure of retention time. It is calculated as follows:

k′ = (t r - t o )/t o (3.1)

(The terms have been defined in the previous section.)

3.2.2 Column Efficiency
The concept of plate theory was originally developed to evaluate the perfor-
mance of distillation columns (e.g., for the separation of crude oil into its
component fractions—that is, petrol, diesel etc.). The theory assumes that the
column is divided into a number of zones or plates; in reality, for a capillary
GC column this is clearly not the case. Nevertheless, the concept of the num-
ber of theoretical plates is a useful measure for GC because it gives a practical
numerical value that indirectly provides a measure of the peak narrowness.
In principle, therefore, the narrower the peak shape is the more peaks (or
compounds) can be separated. The number of theoretical plates is therefore