Sample Extraction and Clean Up
The QuEChERS sample preparation procedure consists
of the steps shown in Figure 1. There are three parts:
extraction, clean up, and solvent exchange. The solvent
exchange provides a final solvent that is more amenable to
splitless injection and concentrates the analytes to reach
lower detection limits. In addition, the solvent exchange
and final clean up removed caffeine and polyphenols from
the sample before injection. These compounds readily
dissolve in acetonitrile, as shown in Figure 2 (red trace).
However, they are not readily soluble in hexane:acetone
(9:1), as shown in the black trace in Figure 2. This helps
keep the analytical system clean.
Care must be taken to adequately and thoroughly
homogenize the sample. A large amount of water must be
added during the homogenization step when preparing the
tea for extraction. This must be taken into consideration
in the final calculations of spikes and standards. A total of
1200 mL of water was added to 200 g of green tea in
this experiment.
An observation was made during the extraction phase
of the sample preparation. If the MeCN extract was
poured into the MgSO
4
, poor spike recoveries were
observed. This was due to an exothermic reaction of any
water in the sample and the MgSO
4
. Although many vendors
offer the pre-measured powder reagents in a separate
capped centrifuge tube, it is recommended not to add the
sample to these tubes. Instead, reagents from these tubes
should instead be added directly to the sample containing
the acidified MeCN. Therefore, an empty 50 mL FEP
extraction tube was included in the list of consumables for
sample preparation. A thoroughly homogenized 15 g
sample of green tea and water were weighed into the FEP
extraction tube. Then 15 mL of 1% glacial acetic acid
MeCN extraction solvent was poured into the tube on top
of the sample. The surrogate and the pesticide solutions
were spiked into this MeCN layer for the method validation
(MVD) and method detection limit (MDL) samples.
Figure 1: Flow diagram of QuEChERS sample preparation steps
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