Chemistry Reference
In-Depth Information
extracted in this study was between pH 2.5 - pH 4.0. Sodium hydrogen carbonate (NaHCO
3
)
was added in the method to give a consistent pH during extraction independent of the
initial sample pH.
Aysal P. et al., 2007 [7] mixed the sample 1:1 (w:w) with anhydrous sodium sulfate (Na
2
SO
4
)
and used a 2:1 (v:w) ethyl acetate : sample ratio because it had been evaluated previously to
achieve high recoveries. It resulted in good extraction efficiency and is practical with regard
to achieving phase separation and avoidance of emulsions. [23-25].
The two conditions most relevant to extraction efficiency are the sample-to-solvent ratio and
the addition of salt, which in ethyl acetate-based multi-residue methods has always been
sodium sulfate. A study done by Mol, H.G.J et al. in 2003 [23] showed that the addition of
salt improves the extraction efficiency for polar pesticides.
3.3.3. Dispersive-SPE clean-up
The purpose of salt addition is to induce phase separation. The salting-out effect also
influences analyte partition, which is dependent upon the solvent used for extraction. The
concentration of salt can influence the percentage of water in the organic phase and can
adjust its "polarity". In the QuEChERS method, acetonitrile alone is often sufficient to
perform excellent extraction efficiency without the need to add non-polar co-solvents that
dilute the extract and make the extracts too non-polar. By using deuterated solvents in the
nuclear magnetic resonance studies, Anastassiades and colleagues [6] investigated the
effect of various salt additions on the recovery and other extraction parameters. They
studied the effect of polarity differences between the two immiscible layers. The use of
magnesium sulfate as a drying salt to reduce the water phase helped to improve
recoveries by promoting partitioning of the pesticides into the organic layer. To bind a
significant fraction of water, the amount of magnesium sulfate exceeded the saturation
concentration. The supplemental use of sodium chloride helps to control the polarity of
the extraction solvents and thus influences the degree of matrix clean up of the
QuEChERS method but too much of this salt will reduce the organic layer's ability to
partition polar pesticides.
Dispersive solid-phase extraction is similar in some respects to matrix solid-phase
dispersion developed by Barker [28-29] but in this instance, the sorbent is added to an
aliquot of the extract rather than to the original solid sample as in matrix solid-phase
extraction. In dispersive solid-phase extraction, a smaller amount of sorbent is used only
because an aliquot of the sample is subjected to the clean up. Compared with SPE,
dispersive solid-phase extraction takes less time and uses less labour and lower amounts of
solvent without the extra steps such as channeling, analyte or matrix breakthrough, or
preconditioning of SPE cartridges. Just as a drying agent is sometimes added to the top of
an SPE cartridge, magnesium sulfate is added simultaneously with the SPE sorbent to
remove much of the excess water and to improve the analyte partitioning to provide
better clean up.
