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1052
C
HANG
ET AL
.:
J
OURNAL OF
AOAC I
NTERNATIONAL
V
OL
.
99, N
O
.
4, 2016
(at
a
concn) and supplemental Table 2 (at
b
concn). Degradation
rate constants and their half-lives are also shown in supplemental
Tables 1 and 2. By comparing the
k
values of
a
and
b
concns
over 40 and 120 days, it can be found that most of the pesticides
in trend A had higher
k
values over 40 days than over 120 days,
except 12 and 19 pesticides showed the opposite at
a
and
b
concns, respectively. This ¿nding indicated that the concentration
of most of the pesticides in type A dropped fast in ¿rst 40 days,
whereas they decreased slowly in the remaining days. However,
chlorfenapyr, bupirimate, fonofos, and furalaxyl had similar
degradation equations over both 40 and 120 days. For these four
pesticides, therefore, the degradation trend could be expressed by
the degradation equations over 40 days. At
a
concn, the half-life
of pesticides in degradation trend A ranged from 24.4–223.6 and
44.4–203.9 days according to the degradation equations over 40
and 120 days, respectively. Except for the previously mentioned
four pesticides, the half-life of the other pesticides varied over
40 and 120 days, with the biggest difference being156.9 days.
At
b
concn, the half-life of pesticides in degradation trend A
ranged from 40.8 to 315.1 and 46.8 to 330.1 days, according to
the degradation equations over 40 and 120 days, respectively.
On the whole, either at
a
or
b
concn, there were great differences
in half-lives over 40 and 120 days. Based on actual conditions,
the half-life calculated by degradation equations over 120 days
was considered to be reasonable. There were 28 pesticides in
accordance with degradation trend A at both
a
and
b
concns.
By comparing
k
values from their degradation equations over
120 days, 10 pesticides had higher
k
values at
b
concn than at
a
concn, whereas the remaining 18 pesticides showed the opposite.
Degradation trend B—
For degradation trend B, the
concentration of pesticide in aged Oolong tea dropped faster
over 40 days than over the remaining days. From the data in
supplemental Tables 1 and 2, it was clear that the exponential
equation was suitable for the ¿rst 40 days, whereas the
logarithmical equation was suitable for 120 days. Taking
chlorfenvinphos as an example, the degradation pro¿les of
trend B are shown in Figure 3a and b for 40 and 120 days,
respectively.
There were 95 pesticides at
a
concn and 92 pesticides at
b
concn in accordance with degradation trend B, accounting for
35.1 and 33.9 of the 271 pesticides, respectively. Among
them, 50 pesticides were in accordance with degradation trend
B at both
a
and
b
concns. The
k
values from the degradation
equations over 40 days were compared, showing that most of
the 36 pesticides had higher
k
values at
a
concn versus
b
concn.
Degradation trend C.—
Degradation trend C was similar
to degradation trend B; however, the difference was that the
concentration of pesticides decreased logarithmically over
40 days (Figure 4). There were 7 and 10 pesticides in accordance
with degradation trend C at
a
and
b
concns, respectively. It can
be seen from the raw data that the concentrations of pesticides
at day 5 or 10 day greatly differed from those on the ¿rst day.
This logarithmic decrease may be considered the explanation
for degradation trend C.
Degradation trend D.—
For degradation trend D, the
concentration of pesticides presented as scatter points with
R
2
values of <0.4 over 40 days, and most of the dropped trend
could be ¿tted by exponential and logarithmical curves and a
few ¿tted by polynomial curves over 120 days (Figure 5). At
a
concn, there were 23 pesticides in accordance with degradation
trend D; among them, 15 pesticides decreased exponentially
and 8 decreased logarithmically or polynomially over 120 days.
At
b
concn, there were 31 pesticides in accordance with
degradation trend D, and 23 pesticides decreased exponentially
over 120 days. By comparison, it was found that 10 pesticides
were in accordance with degradation trend D at both
a
and
b
concns.
Degradation trend E.—
Of the 271 pesticides, the ratios of
pesticides in accordance with degradation trend E were third-
ranked among the A–F aspects. The degradation pro¿les of
4,4ƍ-dibromobenzophenone over 40 and 120 days are shown
as an example of degradation trend E in Figure 6a and b,
respectively. For degradation trend E, the concentrations of
pesticides decreased exponentially or logarithmically over
40 days, whereas no suitable equations could be used to ¿t the
scatter points over 120 days. It can be seen from supplemental
Tables 1 and 2 that there were 66 pesticides in accordance
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chlorfenvinphos) over (a) 40 days and (b) 120 days. Determination
days were plotted as horizontal ordinates; residue concentrations of
pesticide were plotted as vertical ordinates.
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dichlorofop-methyl) over (a) 40 days and (b) 120 days. Determination
days were plotted as horizontal ordinates; residue concentrations of
pesticide were plotted as vertical ordinates.
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cycluron) over (a) 40 days and (b) 120 days. Determination days
were plotted as horizontal ordinates; residue concentrations of
pesticide were plotted as vertical ordinates.
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of propachlor are shown as an example) over (a) 40 days and
(b) 120 days. Determination days were plotted as horizontal
ordinates; residue concentrations of pesticide were plotted as
vertical ordinates.