Page 366 - Multidimensional Chromatography

Multidimensional Chromatography in Environmental Analysis 357

A general step is the coupling of a 10 mm long precolumn (2–3 mm i.d.), filled
with C 18 or polystyrene–divinylbenzene, to a 10–50 m RPLC column. A PLRP-s
precolumn achieved higher breakthrough volume for the more polar compounds and
is thus highly recommended. This set-up has been used to determine several groups
of pesticides (40, 55–62), phenols (41, 63–65), PAHs (66, 67), naphthalenesul-
fonates (68, 69), etc. The sample volume depends on the breakthrough volumes of
the compounds to be determined, with typical values being between 10 and 200 ml.
For example, Figure 13.13 shows the chromatogram obtained when 200 ml of tap
water was spiked at levels of 1 g l 1 of such pesticides. The limits of detection
1
achieved by using this method were between 0.05 and 0.5 g l , although more polar
compounds such as vamidothion or 4-nitrophenol could not be determined (61).
Various highly crosslinked polymers, with slightly different properties, such as
Envi-Chrom P, Lichrolut EN, Isolute ENV or HYSphere-1, have been applied in
environmental analysis, mainly for polar compounds. For phenol, for instance,
which is a polar compound, the recoveries (%) when 100 ml of sample was analysed
were 5, 16 and 6 for PLRP-s, Envi-Chrom P and Lichrolut EN, respectively (70).
Chemically modified polymers have been used to determine polar compounds in
water samples (37, 71). Chemical modification involves introducing a polar group
into polymeric resins. These give higher recoveries than their unmodified analogues
for polar analytes. This is due to an increase in surface polarity which enables the
aqueous sample to make better contact with the surface of the resin (35).

Figure 13.13 On-line trace enrichment–RPLC-diode-array detection (DAD) chromatogram
(at 230 nm) obtained from 200 ml of tap water spiked with various pesticides at levels of 1 g
1
l . Reprinted from Chromatographia, 43, C. Aguilar et al., ‘Determination of pesticides by
on-line trace enrichment–reversed-phase liquid chromatography–diode-array detection and
confirmation by particle-beam mass spectrometry’, pp. 592–598, 1996, with permission from
Vieweg Publishing.