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1054
C
HANG
ET AL
.:
J
OURNAL OF
AOAC I
NTERNATIONAL
V
OL
.
99, N
O
.
4, 2016
Practical Application of Degradation Regularity
Degradation regularity of 20 representative pesticides.—
The
single-laboratory validation results ofAOAC INTERNATIONAL
priority research project, “High-Throughput Analytical
Techniques for the Determination and Con¿rmation of Residues
of 653 Multiclass Pesticides and Chemical Pollutants in Tea by
GC/MS, GC/MS/MS and LC/MS/MS: Collaborative Study,
First Action
2014.09
,” showed that the method could be used for
determination of as many as 653 target pesticides.
21
To reduce the
workload and guarantee a smooth AOAC interlaboratory study,
an alternative “shrunken” protocol was proposed by AOAC.
Here, using the shrunken protocol, the pesticides commonly used
in growing tea, as well as those necessary to be determined for
the international tea trade, were selected from the 271 pesticides
determined by GC-MS/MS. It should be noted that these
pesticides, after being sprayed onto tea, are all of relatively good
stability and their polarities are widely representative. On the
basis of the degradation equations in supplemental Tables 1 and 2
and discussions regarding them, 20 representative pesticides
were optimized (
see
Table 1).
The degradation regularity of the 20 representative pesticides
in aged Oolong tea was studied at
c
and
d
concns (
d
>
c
>
b
>
a
)
over another 90 days byGC-MS/MS. Similarly, their degradation
equations were obtained by plotting the determination time
(every 5 days) on the
x
-axis and the concentration on the
y
-axis
(
see
Table 2). It can be seen from Table 2 that the degradation of
the 20 representative pesticides agrees well with the logarithmic
equation over 90 days, i.e., the pesticides degraded slowly in
aged Oolong tea over 3 months.
From supplemental Tables 1 and 2, it can been
seen that, except for pirimicarb, fenchlorphos, and
4,4ƍ-dichlorodiphenyldichloroethylene (DDE), the other 17
optimized pesticides dropped exponentially or logarithmically
over 120 days at
a
and
b
concns. At the higher
c
and
d
concns,
however, all 20 optimized pesticides dropped logarithmically
over 90 days. Therefore, it can be concluded that their
degradation regularity is in accordance with a logarithmic
equation with spray concentration increasing.
Prediction of pesticide residues in aged Oolong tea.—
The
discussion above (
see Degradation Regularity of 20
Representative Pesticides
section) indicates that the degradation
behavior of the pesticides in aged Oolong tea has certain
regularity. However, it should be noted that this process is
time consuming for multiple determinations. Therefore, it is
necessary to propose a method for predicting the residue of
pesticides in aged Oolong tea. Here, we develop and validate a
prediction method by taking the raw degradation data of
c
and
d
concns.
Based on the results of pesticides in aged Oolong tea
determined by GC-MS/MS over 90 days (from the raw data
of
d
concn), trend charts (eg, dimethenamid,
see
Figure 10)
were plotted, with determination time (day) on the
x
-axis and
the difference between each measured value and the ¿rst-
time-measured value (degradation value) of target pesticides
on the
y
-axis. The logarithmic equations were obtained by
¿tting the 90-day determination results. From these equations,
the degradation value of any of the 20 target pesticides at any
speci¿c day could be calculated and applied to the raw data
generated for that pesticide in a particular laboratory.
The logarithmic functions of the 20 pesticides at
d
concn (listed in Table 3) were applied to predict the residue
concentrations of pesticides in aged Oolong tea at
c
concn. The
predicted residue of each pesticide on a particular day could be
obtained by subtracting the degradation value of this day from
the concentration of the ¿rst day.
Accordingly, the residue concentrations of 20 pesticides
in aged Oolong tea at different 5-day degradation intervals
(days 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75,
80, 85, and 90) after spraying at
c
concn were predicted, and
they were compared with the measured results determined by
GC-MS/MS (
see
Tables 4 and 5). It can be seen from Tables 4
and 5 that the deviation ratios of triÀuralin, teÀuthrin, and
dimethenamid were higher as compared to other pesticides at
different intervals, with deviation ratio ranges of í23.1 to 21.5,
12.2–26.0, and 6.6–24.0 , respectively. They were followed by
pirimiphos-methyl, tolclofos-methyl, and fenchlorphos, with
deviation ratio ranges of í2.8 to 21.8, 5.8 to 23.1, and í2.0 to
24.4 , respectively. The remaining 14 pesticides had relatively
lower deviation ratios, except for part of the intervals. It can be
also seen that the lowest deviation ratios of the 20 pesticides
were different. The numbers of pesticides that had the lowest
deviation ratios at the 20-, 30-, and 45-day intervals were 4, 9,
and 3, respectively. In addition, the highest deviation ratio of
2,4ƍ-DDE, 4,4ƍ-DDE, and bromopropylate was found at days
85, 85 (which were close to the deviation ratios at day 80),
and 35, respectively, whereas for all the other pesticides, the
highest deviation ratio was found at day 80. This ¿nding could
be due to the results at day 80 being abnormal. To evaluate
Table 1. Retention time and monitored ion transitions for
the 20 pesticides by GC-MS/MS
No.
Pesticide
Retention
time, min
Quantifying
precursor/product
ion transition
Qualifying
precursor/product
ion transition
ISTD Heptachlor
epoxide
22.15
353/263
353/282
1
7ULÀXUDOLQ
15.41
306/264
306/206
2
7HÀXWKULQ
17.4
177/127
177/101
3
Pyrimethanil
17.42
200/199
183/102
4
Propyzamide 18.91
173/145
173/109
5
Pirimicarb
19.02
238/166
238/96
6
Dimethenamid 19.73
230/154
230/111
7
Fenchlorphos 19.83
287/272
287/242
8 Tolclofos-methyl
19.87
267/252
267/93
9 Pirimiphos-methyl
20.36
290/233
290/125
10
ƍ ''( 22.79
318/248
318/246
11 Bromophos-ethyl
23.16
359/303
359/331
12
ƍ ''(
23.9
318/248
318/246
13 Procymidone 24.7
283/96
283/255
14 Picoxystrobin 24.75
335/173
335/303
15
Quinoxyfen 27.18
237/208
237/182
16
Chlorfenapyr
27.37
408/59
408/363
17
Benalaxyl
27.66
148/105
148/79
18
Bifenthrin
28.63
181/166
181/165
19
'LÀXIHQLFDQ 28.73
266/218
266/246
20 Bromopropylate 29.46
341/185
341/183