Chemistry Reference
In-Depth Information
of the large initial laboratory samples [20]. An extremely homogeneous sample also
maximizes surface area and ensures better extraction efficiencies.
30 g sub-sample was used by Aysal et al., 2007 [7] based on a study by Maestroni et al. 2000
[21-22] that showed results produced using the same chopper in the same laboratory gave
representative results within generally ≤ 8% relative error of the mean concentration of the
original sample.
3.3.2. Sample extraction
In contrast with acetone and acetonitrile-based methods, in which SPE is commonly
employed, it has been reported only occasionally or no clean-up for ethyl acetate-based
methods; however in this study dispersive-SPE clean-up was performed. Mol, H.G.J et al.,
2007 [23] showed that laborious steps in multi residue analysis can be replaced by more
efficient alternatives including the clean-up process. Solid-phase extraction previously used
in the clean-up procedure which involves less dilution and is less laborious can be replaced
by dispersive SPE, as described by Anastassiades et al., 20 [6].
SPE clean up used plastic cartridges containing various amounts of sorbent material and the
procedures involve conditioning, sample transfer, elution, and evaporative re-concentration
[23]. For this study, in the dispersive-SPE clean-up, 0.25 g primary secondary amine (PSA)
and 1.5 g of anhydrous magnesium sulfate (MgSO
4
) were added to a 10 mL aliquot of the
sample extract and the mixture is mixed using a vortex mixer to evenly distribute the SPE
material and facilitate the clean up process. The sorbent is then separated by centrifugation
and the supernatant is ready for analysis. The function of the sorbent is to retain matrix
components and not the analytes of interest. In some instances, other sorbents or mixed
sorbents can be used depending on the samples and analytes.
A difficulty that was encountered by using ethyl acetate is that some of the most polar
pesticides do not readily partition into ethyl acetate. It co-extracts with lipids and waxes,
giving lower recoveries for the acid-base pesticides, it is sufficiently polar to penetrate into
the cells of the matrix and it dissolves a great number of polar pesticides and their
metabolites. On the other hand, ethyl acetate is partially miscible with water and the
advantage is that it makes the addition of other non-polar solvents to separate water from
the extract unnecessary. To increase the recoveries of polar compounds, large amounts of
sodium sulfate (Na
2
SO
4
) are usually added in the procedures using ethyl acetate to bind the
water. Polar co-solvents, such as methanol and ethanol, have been used to increase the
polarity of the organic phase [23-26].
Different types of samples have different pH values that can affect the recoveries of pH-
susceptible pesticides and their stability in the extracts. Therefore the pH of the extracts for
some samples must be controlled [6, 20, 27]. Most pesticides are more stable at lower pH.
Problematic pesticides that are strongly protonated at low pH the extracts must be buffered
in the range of pH 2-7 [28]. The pH at which the extraction is performed can also influence
the co-extraction of matrix compounds and pesticide stability. The pH of the samples
