preface to fifteenth edition

11.28 SECTION 11
11.4.2.2 Partition Ratio. The partition ratio is the additional time a solute band takes to elute,
as compared with an unretained solute (for which k 0), divided by the elution time of an unre-
tained band:
Retention time may be expressed as
t t
k
t
V V
V
R M R M
M
L
tR
t M(1 k) (1 k)
u
11.4.2.3 Relative Retention. The relative retention of two solutes, where solute 1 elutes before
solute 2, is given variously by
M
k 2 V R,2 t R,2
k V t
1 R,1 R,1
The relative retention is dependent on (1) the nature of the stationary and mobile phases and (2) the
column operating temperature.
11.4.2.4 Column Efficiency. Under ideal conditions the profile of a solute band resembles that
given by a Gaussian distribution curve (Fig. 11.1). The efficiency of a chromatographic system is
expressed by the effective plate number N eff , defined from the chromatogram of a single band,
L t R
t
Neff 16 5.54
H W W
2 2
R
b 1/2
where L is the column length, H is the plate height, t R is the adjusted time for elution of the band
center, W b is the width at the base of the peak (Wb
4) as determined from the intersections of
tangents to the inflection points with the baseline, and W 1/2 is the width at half the peak height.
Column efficiency, when expressed as the number of theoretical plates N theor uses the uncorrected
retention time in the foregoing expression. The two column efficiencies are related by
N
k
2
eff Ntheor
k 1
11.4.2.5 Band Asymmetry. The peak asymmetry factor AF is often defined as the ratio of peak
half-widths at 10% of peak height, that is, the ratio b/a, as shown in Fig. 11.2. When the asymmetry
ratio lies outside the range 0.95–1.15 for a peak of k 2, the effective plate number should be
calculated from the expression
41.7(t/W ) R 0.1
N (a/b) 1.25
11.4.2.6 Resolution. The degree of separation or resolution, Rs, of two adjacent peaks is defined
as the distance between band peaks (or centers) divided by the average bandwidth using W b ,as
shown in Fig. 11.3.
tR,2 tR,1
Rs 0.5(W 2 W 1 )