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process such as adsorption, partition, ion exchange, ion pairing and molecular exclu-

sion. These mechanisms involve the unique kinetic and thermodynamic processes that

characterise the interaction of each analyte with the stationary phase. The second

general process defines the other processes, such asdiffusion, which tend to oppose

the separation and which result in non-ideal behaviour of each analyte. These

processes are manifest as abroadeningandtailingof each analyte band. The analy-

tical challenge is to minimise these secondary processes.

11.2.2 Retention time

Achromatogramis a pictorial record of the detector response as a function ofelution

volumeorretention time. It consists of a series ofpeaksorbands, ideally symmet-

rical in shape, representing the elution of individual analytes, as shown in Fig. 11.1.

The retention timetRfor each analyte has two components. The first is the time it takes

the analyte molecules to pass through the free spaces between the particles of the

matrix coated with the stationary phase. This time is referred to as thedead time,tM.

The volume of the free space is referred to as the columnvoid volume,V 0. The value

oftMwill be the same for all analytes and can be measured by using an analyte that

does not interact with the stationary phase but simply spends all of the elution time in

the mobile phase travelling through the void volume. The second component is the

time the stationary phase retains the analyte, referred to as theadjusted retention

time,t^0 R. This time is characteristic of the analyte and is the difference between the

observed retention time and the dead time:

t^0 R¼tRtM ð 11 : 2 Þ

It is common practice to relate the retention timetRort^0 Rfor an analyte to a reference

internal standard (Section 11.2.5). In such cases therelative retention timeis often

calculated. It is simply the retention time for the analyte divided by that for the

standard.

0.607

s

tRB

tRA

whA wh 8

wA wB

hp

Recorder signal (^1) / 2 hp hp
Baseline
(a) (b) (c)
Injection
Time
Fig. 11.1(a) Chromatogram of two analytes showing complete resolution and the calculation of retention
times; (b) chromatogram of two analytes showing incomplete resolution (fused peaks); (c) chromatogram of an
analyte showing excessive tailing.
436 Chromatographic techniques