The separation of a series of n-alkanes isothermally at 150°C on a packed
column is shown in Figure 1(a), and illustrates the first two problems. The first
four alkanes, n-hexane (C6) to n-decane (C10), are incompletely resolved, C6
and C7 co-eluting, whilst later eluting peaks are smaller, broader and are
fronting (Topic D2). In the temperature programmed chromatogram (Fig. 1(b)),
a complete separation of all the n-alkanes up to C21 has been achieved and in
about a third of the time taken to separate only up to C15 isothermally. C6–C8
are fully resolved and the later peaks are sharper and more symmetrical. For
mixtures where the individual components are not, as in the case of the n-
alkanes, members of a homologous series, temperature programs are often more
complex. They may involve initial, intermediate and final isothermal periods
separated by temperature ramps of rates varying between 2°C and 30°C per
minute. The optimum conditions for a particular sample are generally estab-
lished by trial and error.150 Section D – Separation techniques
00 4 8 12 16
MinutesMinutes20 24 28 32 365 1015202530455055 9095C 8C 8C 9C 10 C 11 C 12 C 13 C 14C 15 C 16
C 17C (^18) C 19
C 6 C 7 C 20 C 21
C (^9) C
10 C 11 C 12 C 13
C 14
C 15
C 6 + C 7
Inject
(a)
(b)
Fig. 1. Isothermal and temperature programmed separation of a homologous series of n-
alkanes; column: 20 ft ¥1/16 in, 3% Apiezon-L on 100–120 mesh Varaport 30; flow rate:
10 cm^3 min-^1 He. (a) Isothermal; (b) Temperature programmed.