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Cloud point extraction has been used to extract many different types of analytes from solution. It is based on the unusual behaviour of non-ionic surfactants in water, which become less soluble as the temperature is raised. Eventually, phase separation occurs and the surfactants "cloud out" to give surfactant-rich and aqueous phases with the target analytes partitioned between the phases. Subsequent analysis is typically carried out by spectroscopic, liquid chromatographic and capillary electrophoretic methods. GC or GC/MS is also possible but the surfactant-rich phase is problematical. If this phase is analysed without further treatment, the high levels of surfactant tend to become adsorbed onto the column walls, changing the retention characteristics and possibly leading to clogging. In addition, in GC/MS the surfactant peaks are plentiful and can interfere with the analyte spectra. The prerequisite treatment has included LC or liquid-liquid extraction to recover the analytes. Alternatively, volatile analytes can be extracted from the phase by headspace sampling procedures like SPME. Now, a new approach has been reported by Willie Hinze from Wake Forest University, Winston-Salem, NC, and Yoshitaka Takagai from Fukushima University. Instead of removing the surfactant, it was modified to become fully compatible with GC/MS. Their procedure was illustrated using Triton X-100 and Triton X-114, two poly(ethylene glycol) (tetramethylbutyl)phenyl ethers with average ethylene oxide chain lengths of 9.5 and 7.5, respectively. The surfactants were dissolved in acetonitrile and derivatised with the standard trimethylsilylation reagent BSTFA in the presence of pyridine. Reaction took just 5 minutes at 80°C. The cooled solution was injected directly into the GC/MS system. Good peak shapes were seen in the chromatograms, far better than with the underivatized surfactants and with greatly reduced peak tailing. Both surfactants displayed elution time windows during which no surfactant peaks were present. In the case of Triton X-114 this was over 5-30 minutes. So, any analytes eluting within that time frame would be in the "surfactant-free zone" with no interfering peaks, and would be suitable for cloud point extraction. The size of this window could be controlled by varying the GC temperature program. Mass spectrometric analysis confirmed that the peaks were from the surfactant homologues or BSTFA. Both the GC and MS data showed that the derivatisation reaction proceeded to completion. Apart from being required for the derivatisation, pyridine served the secondary purpose of reducing the viscosity of the reaction solution to aid injection via a microsyringe. The other major factor to consider was the presence of residual water in the surfactant-rich phase, which hydrolysed the derivatising reagents and prevented reaction. A sequence of up to three heating/centrifugation cycles separated the water and allowed it to be removed after each cycle with a microsyringe. The process was tested on a solution of six PAHs. They all appeared in the elution window for Triton X-114 and their retention times were the same as those from an underivatized solution in acetone and very similar to those from standard solutions. For a mixture of six herbicides, the retention behaviour was also good. Subsequent GC/MS quantification using a deuterium-labelled PAH internal standard showed a wide linear dynamic range over 0.5-4000 ppb and detection limits ranging from 6.6 ng/L for metolachlor to 480 ng/L for hexachlorocyclopentadiene. The enrichment factors were 17-33 and recoveries ranged from 50% for simazine to 100% for metolachlor and butachlor. A useful spinoff of surfactant derivatisation was illustrated in the extraction and analysis of the drugs ibuprofen and flurbiprofen. These are polar molecules which do not chromatograph well but the presence of a labile hydrogen atom means that they are also derivatised at the same time as the surfactants, ruling out the necessity of a separate derivatisation step. The researchers declared that this version of cloud point extraction is simpler and more convenient than other cloud point GC methods, requiring less sample manipulation and no additional separation steps. It is limited to some extent by the relatively high inlet temperatures required, which would be inappropriate for thermally labile compounds, but the use of different non-ionic surfactants might circumvent this. In general, it could lead to a wider use of cloud point extraction. Related links:
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. |
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