Page 61 - Subyek Encyclopedia - Encyclopedia of Separation Science

56 I / CHROMATOGRAPHY/ Derivatization

density- and composition-programmed separation of
oligomers in Figure 19.
Solvent-strength gradients in TLC are usually dis-
continuous and achieved through the use of uni-
dimensional multiple development. This is accom-
panied by zone refocusing resulting in a larger zone
capacity and easier-to-detect separated zones. All uni-
dimensional multiple development techniques em-
ploy successive repeated development of the layer in
the same direction with removal of the mobile phase
between developments. Each time the solvent front
traverses the sample zone it compresses the zone in
the direction of development because the mobile
phase contacts the bottom edge of the sample zone
Rrst where the sample molecules then start to move
forward before those molecules ahead of the solvent
front. Once the solvent front has reached beyond the
zone, the refocused zone migrates and is broadened
by diffusion in the usual way. When optimized, it
is possible to migrate a zone a considerable distance
without signiRcant zone broadening beyond that ob-
served for the Rrst development. If the solvent com-
position is varied for all, or some, of the development
steps during multiple development, then solvent
strength gradients of different shapes can be pro-
duced. With increasing solvent-strength gradients it is
usually necessary to scan the separation at a number
of intermediate development steps corresponding to
the development at which different components
of interest are separated, since in later developments
these zones may be merged again because of
the limited zone capacity in TLC. Alternatively,

Figure 19 Separation of Triton X-114 by SFC using pro-
grammed elution on a 10 cm 2 mm i.d. column packed with
3 m octadecylsiloxane-bondedsilica gel at 1703C with UV detec-
tion. (A) Carbon dioxide/methanol (2#0.125) mL min 1 at Figure 20 Separation of the 3,5-dinitrobenzoyl esters of
210 bar; (B) as for (A) with pressure programmed form 130 to poly(ethylene glycol) 400 by (A) a single conventional develop-
375 bar over 8 min; and (C) using a mobile-phase composition ment and (B) by incremental multiple development with a step-
1
gradient from 0.025 to 0.4 mL min methanol over 8 min at 210 wise gradient of methanol, acetonitrile and dichloromethane over
bar. (Reproduced with permission from Giorgetti A, Pericles N, 15 developments (Reproduced with permission from Poole CF,
Widmer HM, Anton K and Datwyler P (1989) Journal of Chromato- Poole SK and Belay MT (1993) Journal of Planar Chromatogra-
graphic Science 27: 318, copyright Preston Publications, Inc.) phy 6: 438, copyright Research Institute for Medicinal Plants.)