Page 283 - Multidimensional Chromatography

278 Multidimensional Chromatography

Mobile phase

Pump 1 UV
RP column R2
UV W
CB Organic R3
PS
TP1 FR
Aqueous W
PTFE coil PR
TP2
R1 3 2 1 Helium
Extraction solvent GC unit
pump 2 MS

Early vapour exit
Figure 11.11 Schematic diagram of the instrumental set-up used for on-line coupled
LC–GC–MS: 1, retention gap; 2, retaining pre-column; 3, analytical capillary; R1, LC col-
umn (restriction); FR, flow regulator; R2, needle-valve restrictor; R3, capillary (75 m i.d.);
PS, phase separator (sandwich type); CB, cooling bath; PR, pressure regulator; W, waste; TP1
*
and TP2, T-pieces; UV, UV detector. (Note that the UV detector can be positioned either
before or after the liquid–liquid extraction unit.) Reprinted from Journal of Chromatography,
626, J. Ogorka et al., ‘On-line coupled reversed-phase high-performance liquid chromatogra-
phy–gas chromatography–mass spectrometry. A powerful tool for the identification of
unknown impurities in pharmaceutical products’, pp. 87–96, copyright 1992, with permission
from Elsevier Science.

acetamide in the retention gap before the GC separation. After the reaction, the sol-
vent vapour exit was closed and the GC run was started. The yield of the on-line
derivatization was comparable with off-line derivatization. It should be noted that
the liquid–liquid extraction step in the applied system can offer extra selectivity. The
extraction yield was increased by the use of higher temperatures. The high total
selectivity is illustrated by Figure 11.12. The total analysis time was less than 60
min, which is much shorter than with more traditional analytical methods. The limits
of quantification were 61–92 ng/ml.

11.5 SOLID-PHASE EXTRACTION–GAS CHROMATOGRAPHY

In LC–LC and SPE–LC, the presence of water is commonly no problem at all.
Actually, the reverse is true because eluents in RPLC are typically water–methanol or
water–acetonitrile mixtures, and a high water content is mandatory during trace
enrichment in order to ensure strong retention. However, when such an SPE pre-
column or analytical column is coupled to a GC system, the introduction of water
should be avoided completely or, at best, be permitted under strictly controlled condi-
tions (see above). It will be clear that on-line trace enrichment (and clean-up) by SPE