Environmental Laboratory

European Water Solid Phase Extraction Methodology Advances

Dec 03 2015 Read 5468 Times

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Analysis is critical for environmental protection, and water is one of the most important media to monitor.  Water provides approximately 60% of the environmental samples by media and is especially important because it provides a large source of exposure for human and animal life.(1)  Water analysis for pollutants should be reliable and allow detection at low levels necessary for decision-making.  Extraction of the analytes from the water sample prior to instrumental analysis is generally required to separate any interferences and concentrate analytes.  Environmental analytical techniques generally used for semivolatile organic compounds are gas chromatography (GC) with a specific detector, such as an electrolytic conductivity detector (ECD) or more general mass spectrometer (MS) when a variety of compounds is to be determined.  Extraction can be accomplished with a liquid-liquid extraction with a solvent such as dichloromethane (DCM) or by using solid phase extraction to adsorb the analytes from the water and then elute them into a suitable solvent.

Solid phase extraction provides several advantages over liquid-liquid extraction including using less solvent and avoiding emulsion creation.  In addition, it can more easily be automated, further improving reproducibility between technicians and reducing exposure to solvent.  Solid phase sorbents are available in a variety of formats with cartridges being one of the more popular and very suitable for food and pharmaceutical analyses.  Disks are more suitable for larger-volume samples and for samples that may have particulates, such as many samples encountered in environmental analysis.
The European Union is moving to adopt newer technology based on solid phase extraction (SPE) that will allow larger samples to be easily extracted, making flexibility in achieving lower detection limits possible.(2)   Several methods that have been recently developed and their status are listed in Table 1.  These use SPE disks to allow the rapid extraction of semivolatile analytes for subsequent instrumental analysis.
These methods are designated for whole water which generally contains a small amount of particulate matter.  Solid phase extraction can also provide advantages for more heavily particulated samples, such as wastewater. Disk SPE can handle a larger percentage of solid material without clogging or taking an excessively long time to flow through the disk.  Pre-filtering the sample is impractical as it would remove some analytes bound to the particulates, resulting in a false, lower concentration.  Using SPE disks, the particulates are held on the disk and when the bottle is rinsed the solvent flows through the particulate as well, extracting analytes that may have been held by the particulate, in essence a one-step approach.   If the sample is very heavily particulated, a special disk holder (EZ Flow) may be used to allow easy placement of pre-filters and glass wool on top of the SPE disk to further facilitate flow.
Next we will discuss two compound classes that can be extracted from more particulated water samples with disk SPE.  Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants, naturally occurring in coal, crude oil, gasoline, and their byproducts (e.g., coal tar or creosote). In addition, PAHs are formed in the incomplete combustion processes of all organic materials, such as wood or fossil fuels. Consequently, the EU water framework directive (WFD) lists in its annex X the whole group of PAHs as priority hazardous substances and one of the new methods is devoted to these compounds in whole water.
Extraction with DCM is commonly used for PAH compounds, however increasing concern about the health effects of chlorinated solvents has made method developers look for less toxic solvents.  Werres, et. al., examined PAH compounds in whole water using acetone as the eluting solvent. (3) However; acetone creates a problem with residual water in the final extracts due to its miscibility with water.  The new CEN method specifies elution with DCM and solvent exchange to toluene before GC-MS analysis.  This work shows SPE using the high-particulate flow holder and elution with hexane as an alternative solvent.(4) Table 2 shows the prewet steps and elution on the SPE-DEX® 4790 automated disk extraction system.  The delivery of all steps in the table are completely handled by the 4790 system.
Table 3 shows good recovery for a variety of PAH compounds and good reproducibility with the system set up for higher particulate samples using a non-chlorinated solvent.  
Another example of SPE utility is in matching the elution solvent to the needs of the analytical step.  Considering the final analytical technique, pesticides are often measured with GC and an electrolytic conductivity detector (ECD) which is specific for organochlorine pesticides and provides a very sensitive measurement.  A non-chlorinated solvent is essential for the determinative step to preserve the detector.  Exchanging the solvent is a tedious step that can also cause analyte loss.  However, it is possible to use a solvent compatible with the detector for elution, eliminating the solvent exchange step entirely, saving time and preserving the analytes.  Table 4 shows the extraction method used for 1 L of a water sample. (5)
The recovery of pesticides using this method is shown in Table 5.
The primary column results are excellent.  The secondary column, used for confirmation, also shows recoveries within the method criteria.  Concentrating a 25 mL extract of hexane to a final volume of 5 mL will take approximately 10 minutes with the DryVap® Evaporation System.  Including the 10 minute sample run times on the GC/ECD, samples were extracted, concentrated, cleaned, and analyzed within two hours with excellent recoveries.

Conclusion
The European Union has taken the first steps to incorporate disk solid phase extraction as an option in their water sampling and analysis program.  These examples show that there are other options to also consider as they move forward to expand disk use and address more complex samples and issues.

References
1. Strategic Directions International, Environmental Market Report, 2001.
2. http://www.cencenelec.eu/Pages/default.aspx (accessed November 16, 2015).
3. Friedrich Werres , Peter Balsaa, Torsten C. Schmidt, Journal of Chromatography A, 1216 (2009) 2235–2240.
4. Jim Fenster, Kevin Dinnean,, David Gallagher, Michael Ebitson,  Determination of PAH Compounds from Aqueous Samples Using a Non-Halogenated Extraction Solvent and Atlantic C18 Disks, AN072 www.horizontechinc.com .
5. Michael Ebitson, Extracting Organochlorine Pesticides
from Water
With Atlantic® HLB-M Disks (EPA  Method 8081), AN067 www.horizontechinc.com .


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