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Figure 14.2: SFC of a PAH standard mix. Peak information: see Figure 4.1 (page 101).
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Figure 14.3: SFC of a pesticide mixture.
While the process of extraction by supercritical fluid has been known for some time, for example the removal of caffeine from coffee, supercritical fluid extraction has only recently been applied on an analytical scale, analysts rediscovering the technique as a powerful and selective sample preparation tool for off-line and on-line combination with other chromatographic techniques. The most important characteristics of supercritical fluid extraction (SFE) are the high recovery rates attainable with relatively short extraction times (typically 30 minutes) and the high degree of selectivity that can be introduced into the sample preparation step. The schematic in Figure 14.4 illustrates a modern SFE instrument with the four 'tools' used to introduce selectivity, namely the addition of polar or apolar modifiers, the control of supercritical fluid density and temperature during sample 'leaching', the selection of solid trap packing material and finally the different solvent polarities used to rinse the trap.
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SELECTIVITY 2
SELECTIVITY 3
MATERIAL
DENSITY TEMPERATURE
MODIFIER
SOLVENT 1 SOLVENT 2
SELECTIVITY 1
SELECTIVITY 4
Figure 14.4: Selectivity in supercritical fluid extraction.
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SFE lends itself to both off-line or on-line coupling with various separation techniques and there is still some controversy as to which technique requires the fewest compromises. With the introduction of robotic arms to transfer SFE collection vials to GC and LC autosamplers there is in fact very little difference between the two techniques. The comparison below may help readers to decide which technique is most applicable for their specific applications.
On-line Pros High sensitivity, less contamination (?),
full automation,...
Cons Memory effects, matrix effects, one-shot analysis (repeatability?), sample sizes too small to be representative,...
Off-line Pros Large sample sizes, different analyses possible
(repeatability), different separation methods may be applied, full automation via robotics, operational simplicity, further clean-up/derivatization steps possible,... Cons Lower sensitivity (larger injection volumes!),...
In environmental work, SFE is used mostly in the analysis of solid samples such as sediments, sludge, soil, solid waste etc., but in combination with solid phase extraction, may also be used in air analysis (SFE of Tenax, charcoal or poly-urethane foam filled cartridges) or water analysis (SFE of ODS cartridges or membrane discs etc.) where SFE's excellent selectivity applies equally well.
In sharp contrast to miniaturized liquid chromatography, which continues to evolve slowly, the number of publications based on 'electro-driven' separations is growing exponentially, supported by the rapid development of commercial instrumentation. The possible environmental applications of both capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography (MEKC) are documented in publications dealing with the separation of anions and cations, phenoxy acids, herbicides and pesticides, surfactants, disinfectants, polyaro-matic hydrocarbons, chlorinated phenols, cresols, nitrophenols, nitrotoluenes, nitronaphthalenes etc. Only lack of sensitivity hinders the wider acceptance of 'electro-driven' separations as routine procedures for analytical analyses. Developments of capillary electrophoresis in areas such as sample stacking effect, extended light path capillaries, laser fluorescence detection and the
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combination with moving boundary and isotachophoresis principles are steadily increasing sensitivity to the low ppb and even ppt levels for some applications. As an example, models which describe the dispersal of river water into the sea have been verified and calibrated using a rhodamine WT tracer, detected at, levels of less than 1 pg/1 using capillary electrophoresis with laser fluorescence detection.
The introduction of new, more sensitive analytical techniques for target compounds should not automatically lead to the adoption of more stringent regulatory norms, rather, the norms should be set on the basis of ecotoxicological demands. More useful than chasing the last degree of sensitivity in the analysis of target compounds is the application of new, sensitive methodologies for the analysis of non-target, unknown pollutants. All too often, the focus is on target compounds which are unlikely to be found, rather than on the metabolites and degradation products which may be equally toxic, for example glyphosate and AMPA.
When laboratory managers are asked to identify the greatest obstacle in the way of enhanced accuracy, reproducibility and productivity the answer is almost always a simple one, 'sample preparation'. The area where laboratories spend the most time and make the most errors, sample preparation is priority number one for the application of reliable and cost effective automation and as the gap between sample loads and the number of trained staff widens, labs are moving towards full automation of sample preparation, analysis and report generation. Instrumentation that goes beyond these needs to include sample preparation has recently been introduced.
Compared to the fully automated systems for the analysis of specific groups of pollutants such as the PAH or pesticide analyser discussed earlier, the next generation of analyser will, based on the development of systems for automatic measurement of organic micropollutants in surface water (SAMOS), offer a far broader range of applications. Standard on-line monitors for screening volatile organic compounds are based on the well known purge and trap method of a water aliquot pumped automatically into the extraction vessel and analysed by GC/FID or GC/MSD. Recently developed by the research teams of the institutes participating in the Rhine Basin Programme, SAMOS systems for on-line determination of polar pesticides in water include SAMOS LC, SAMOS GC and SAMOS LC/MS.
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In SAMOS the water sample is pumped through one or more precolumns filled with solid phase extraction material, on which the compounds of interest are adsorbed and enriched. Subsequently, the compounds are desorbed with a suitable solvent and introduced into an LC or GC equipped with an appropriate detector and data handling device which may in turn be connected to an alarm network.
A SAMOS system recently installed on the river Meuse was immediately successful in measuring 1.5 ppb of the pesticide diuron, resulting in a 1 month closure of the water intake used for drinking water production. The SAMOS (HP 1090 Win/SPE) system used comprised a solvent delivery/sample preparation system from Spark Holland combined with a Hewlett-Packard HP 1090 M HPLC with diode-array detector and HP Chemstation (Figure 14.5).
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Figure 14.5: System for the automatic measurement of organic micropollutants in surface water.
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Controlled by software designed to allow unattended on-line operation, the system traps polar pesticides on a precolumn filled with PLRP-S, a styrene-divinylbenzene copolymer from Hamilton. To prevent clogging and/or memory effects, the system automatically changes the precolumn between analyses.
For investigation, a Rhine water sample spiked with 27 polar pesticides
(Table 14.1) in concentrations ranging from lppb to 250ppt shown in Figure 14.6
indicates detection limits below the lppb alert and 3ppb alarm levels.
400-1
100 ML OF RHINE WHTER С1Э-3-1992, LOBITH) SPIKED WITH MIXTURE OF 27 PESTICIDES
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