Page 31 - Multidimensional Chromatography

Coupled HPLC with HRGC 21

The retention gap technique, due to the solvent effects explained above, allows the
analysis of compounds eluting immediately after the solvent peak. The limitation of
this technique is the need for long retention gaps and long analysis times, since the
solvent has to be completely evaporated prior to starting the elution of components of
interest. It is possible to find in the literature many applications which use the classi-
cal retention gap for the transfer from HPLC to HRGC. In particular, Figure 2.3 (a)
(4) shows the LC chromatogram of a bovine urine sample after formation of the
dipentaflorobenzyl ether of diethylstilbestrol (DES), which is a synthetic estrogen.

Figure 2.3 (a) LC chromatogram of a bovine urine sample of DES carried out on a 100 3
mm id glass column, packed with 5 m silica gel Spherisorb S-5-W, with cyclohexane/1%
THF as eluent at a flow rate of 260 l/min. (b) GC–ECD (electron-capture detector) chro-
matogram of the transferred fraction coming from a urine sample spiked with 5 ppb of DES
(two isomers). Reprinted from Journal of Chromatography, 357, K. Grob et al., ‘Coupled
HPLC–GC as a replacement for GC–MS in the determination of diethylstil bestrol in bovine
urine’, pp 416–422, copyright 1986, with permission from Elsevier Science.