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C
HANG
ET AL
.:
J
OURNAL OF
AOAC I
NTERNATIONAL
V
OL
.
99, N
O
.
4, 2016
1051
Extraction
Weigh 5 g dry tea powder (accurate to 0.01 g) into a 80 mL
centrifuge tube, add 15 mL acetonitrile, and homogenize at
13500 rpm for 1 min. Centrifuge the mixture at 2879 ×
g
for
5 min, and transfer the supernatant into a pear-shaped Àask. Re-
extract the residue with 15 mL acetonitrile and centrifuge the
mixture. Combine the two extracts, and rotary evaporate in a
water bath at 40°C to about 1 mL for cleanup.
Cleanup
Place a pear-shaped Àask under the ¿ve-port Àask vacuum
manifold, and mount a Cleanert TPT cartridge onto the
manifold. Add about 2 cm anhydrous sodium sulfate onto the
Cleanert TPT cartridge packing material, prewash with 10 mL
acetonitrile–toluene (3 1, v/v) and discard the efÀuents to
activate the cartridge. Stop the Àow through the cartridge when
the liquid level in the cartridge barrel has just reached the top of
the sodium sulfate packing. Discard the waste solution collected
in the pear-shaped Àask and replace with a clean pear-shaped
Àask.
Transfer the concentrated sample extract (
see Extraction
section) into the SPE cartridge, rinse the sample solution bottle
with 2 mL acetonitrile–toluene (3 + 1, v/v), and repeat this step
thrice, transferring the rinsing liquids to the cartridge. Attach
a 50 mL storage device onto the cartridge, and then elute with
25 mL acetonitrile–toluene (3 + 1, v/v), collecting the efÀuent
into the pear-shaped Àask by gravity feed. Rotary evaporate the
efÀuent in a water bath at 40°C to about 0.5 mL. Add 40 ȝL
heptachlor epoxide ISTD to the sample. Evaporate to dryness
under a stream of nitrogen in a 35°C water bath. Dissolve the
dried residue in 1.5 mL hexane, ultrasonicate the sample to mix,
and ¿lter through a 0.2 ȝm membrane ¿lter. The sample is ready
for GC-MS/MS analysis.
Results and Discussion
Degradation of 271 Pesticides in Aged Oolong Tea
The residues of 271 pesticides in aged Oolong tea were
determined 25 times by GC-MS/MS over 120 days (every
5 days) to monitor their degradation behavior. To study the
degradation regularity of the 271 pesticides in aged Oolong tea,
scatter diagrams at
a
and
b
spray concentrations (
a
and
b
concns)
over 40 and 120 days were prepared, using determination days
as horizontal ordinates and concentrations of pesticide residues
as vertical ordinates. The degradation equations are summarized
in supplemental Table 1 (at
a
concn) and supplemental Table 2
(at
b
concn).
By comparing the degradation equations of pesticides at
a
and
b
concns over 40 and 120 days, it was found that their degradation
trends were different. The trends included the following
aspects: A—the residues of pesticide dropped exponentially
over both 40 and 120 days; B—the residues of pesticide
dropped exponentially over 40 days and logarithmically over
120 days; C—the residues of pesticide dropped logarithmically
over 40 days and logarithmically/polynomially over 120 days;
D—no trend over 40 days, showing only as scatter points
(R
2
<0.4), although a dropping trend was seen over 120 days;
E—some dropping trend over 40 days, whereas no trend over
120 days, showing only as scatter points (R
2
<0.4); and F—no
trend over either 40 or 120 days, showing only as scatter points
(R
2
<0.4). In addition, ¿ve pesticides were not detected at either
a
or
b
concn.
The degradation regularity of 271 pesticides was studied
according to the A–F trends. The ratios of pesticides associated
with degradation trends A–F in the 271 pesticides are shown in
Figure 1. It was observed that most of the pesticides followed
the A, B, or E degradation trends. At
a
concn, the ratios for A, B,
and E were 21.8, 35.1, and 24.4 , respectively, and their total
number was 220, accounting for 81.2 of the 271 pesticides. At
b
concn, the ratios for A, B, and E were 25.5, 33.9, and 14.8 ,
respectively, and their total number was 201, accounting for
74.2 of the 271 pesticides. These results demonstrate that A,
B, and E degradation trends could represent the main aspects
of the 271 pesticides. That is, most of the pesticides dropped
exponentially over 40 days, and they presented dropping trends
exponentially or logarithmically over 120 days. Although the
ratios of pesticides with other degradation trends were small,
they did represent a certain degree of degradation regularity.
Therefore, all trends (A–F) are discussed below.
Degradation trend A.—
Taking propachlor as an example, trend
A degradation rates over 40 and 120 days are shown in Figure 2a
and b, respectively. It is clearly observed that the concentration of
propachlor in aged Oolong tea exponentially decreased with the
increase of intervals. It is indicated that the degradation kinetics of
trend A is a ¿rst-order reaction by Equation 1:
e 1
0
-
C C
kt
)
(
=
where
C
is the concentration of each pesticide in aged
Oolong tea,
C
0
is the initial concentration of each pesticide
in aged Oolong tea,
k
is degradation rate constant, and
t
is the
determination time (day).
It can be concluded that without the inÀuence of other factors,
the degradation rate of pesticides in trend A has a direct ratio to
the initial concentration of pesticides in aged Oolong tea. Based
on the ¿rst-order reaction model, the half-life of pesticides in
trend A could be calculated by Equation 2:
ln2 2
1 2
t
k
)
(
=
where
t
1/2
is the half-life of the determined pesticide.
Half-lives were calculated according to the degradation
equations of pesticides in trend A listed in supplemental Table 1
Figure 1. Percentage of pesticides in accordance with degradation
trends A–F in 271 pesticides at
a
and
b
concentrations. Values
represent the number of pesticides in accordance with each trend
multiplied by 100 and divided by the total number (271) of pesticides
(% =
n
× 100/
N
).