Page 60 - Subyek Encyclopedia - Encyclopedia of Separation Science

Sepsci*21*TSK*Venkatachala=BG
I / CHROMATOGRAPHY 55

General Elution Problem

Constant separation conditions, for example isother-
mal operation in GC and isocratic elution in LC,
are unsuitable for separating samples containing
components with a wide retention range. Employing
average separation conditions will result in a poor
separation of early-eluting peaks, poor detectability
of late-eluting peaks, and excessively long separation
times. In GC there is an approximately exponential
relationship between retention time and solute
boiling point under isothermal conditions. For mix-
tures with a boiling point range 'c. 1003Cit is
impossible to identify a compromise temperature that
will provide an acceptable separation. The solution in
this case is to use temperature programming, Sow
programming, or both. Temperature programming is
Figure 17 Variation of the resolution of two closely migrating the most common and usually involves a continuous
zones as a function of the R F value for the faster moving zone.
(Reproduced with permission from Poole CF and Poole SK (1991) linear increase in temperature with time, although
Chromatography Today, p. 669, copyright ^ Elsevier Science other programme proRles are possible, including seg-
B.V.) mented programmes incorporating isothermal peri-
ods. The reduction in separation time, increase in
approximation for the number of theoretical plates peak capacity, and nearly constant peak widths ob-
that a particular zone has migrated across. Relatively tained are illustrated by the separation in Figure 18.
small changes in selectivity have enormous impact on The general elution problem in LC is solved using
the ease of obtaining a given separation in TLC, since solvent-strength gradients. Here, the composition of
the total number of theoretical plates available for the mobile phase is changed as a function of time.
a separation is never very large. Separations in TLC Binary or ternary solvent mixtures are commonly
are fairly easy when R F2 !R F1 '0.1 and very dif- used as the mobile phase in which the relative com-
Rcult or impossible for R F2 !R F1 40.05 in the region position of the strong solvent (that solvent with the
of the optimum R F value for the separation. Max- capability of reducing retention the most) is increased
imum resolution is obtained at an R F value of about over time. In SFC it is usual to programme the den-
0.3 and does not change much in the R F range of 0.2 sity, mobile-phase composition or temperature as
to 0.5, as can be seen in Figure 17. Resolution is zero a single factor, but it is also possible for some combi-
for compounds that are retained at the origin or nation of parameters to be changed simultaneously.
migrate with the solvent front. The goal remains the same, as indicated by the

Figure 18 Temperature programmed separation of fragrance compounds by GC on a 30 m 0.25 mm i.d. fused silica open-tubular
1
column coated with DB-1, film thickness 0.25 m, helium carrier gas 25 cm s and temperature program 403C (1 min isothermal) then
1
40}2903Cat 53C min . (Reproduced with permission from J&W, copyright ^ J&W Scientific Inc.)