Instant Notes: Analytical Chemistry

Example. A gas chromatographic method for the separation of a mixture of

cyclohexane, t-butanol and benzene on a 10 m capillary column gave the follow-

ing data:

Parameter Cyclohexane t-Butanol Benzene

tR 3 m 20 s 3 m 30 s 3 m 45 s

Wb 8s 9s 11s

Wh/2 4.6 s 5.1 s 6.2 s

Calculate (a) the plate numbers, using both plate number formulae, for each

solute, (b) the plate heights, and (c) the resolution between adjacent pairs of

solutes.

(a) plate numbers N (^16) 
2
N5.54
2
cyclohexane N (^16) 
2
10 000 N5.54
2
10 473
t-butanol N (^16) 
2
 8711 N5.54
2
 9393
benzene N (^16) 
2
 6694 N5.54
2
 7296
(b) plate heights H
cyclohexane H = 1.0 mm H = 0.95 mm
t-butanol H= 1.15 mm H= 1.06 mm
benzene H= 1.49 mm H= 1.37 mm
(c) resolution RS
cyclohexane/t-butanol RS = 20/17 = 1.2
2 DtR



(W 1 W 2 )
2 DtR



(W 1 W 2 )

10 000

N

225

6.2

225

11

210

5.1

210

9

200

4.6

200

8

tR




Wh/2

tR




Wb

128 Section D – Separation techniques

Concentration axis

tR, 1 tR, 2

DtR

W 1 W 2

Time axis

Fig. 5. Measurement of the resolution of two adjacent Gaussian peaks.