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proving that they are relatively dispersed. No big systematic
deviations happened with the test results.
Feedback and Collaborators’ Comments
In the report of collaborative study results, a column for
collaborator’s comments on the method was included, in which
12 collaborators filled out what they found in the process. The
following are responses to each of the comments:
(a)
Regarding GC/MS analysis.
—Laboratory 19: N.D. =
“Below Limit of Quantitation” as described in supplied
Collaborative Study Method.
Study Director response: In the table of collaborative
sample results filled out by the laboratory, pesticides marked
with “BLQ” as test results refer to the samples free from these
pesticides. Such marking is correct, and these samples are
indeed free from pesticides.
Laboratory 25: For oolong tea, one point from the calibration
curve was taken out for propyzamide, pirimiphos-methyl,
pirimicarb, bromophos-ethyl, and bromopropylate to make the
R
2
value 0.995 or greater.
Study Director response: If, at the time of establishing the
five-point concentration matrix-matching calibration curve,
numerical values at one concentration point deviate obviously,
causing the R
2
value of calibration curve to fall below 0.995,
this concentration point can be eliminated and the four-point
matrix-matching calibration curve is adopted for quantification.
In the matrix calibration curves established by the laboratory,
propyzamide had eliminated the second concentration point,
primicarb the fourth point, prirmiphos-methyl the fourth point,
bromophos-ethyl the fifth point, and bromopropylate the fifth
point. Details are as in Supplemental Figure 4. The Study
Director considers such handling is scientific and reasonable.
Laboratory 28: No results for tolclofos-methyl due to falsely
determined multiple reaction monitoring (MRM) acquisition
window.
Study Director response: At the time of GC/MS determination
by this laboratory, deviations occurred in the setting of the
MRM acquisition window of tolclofos-methyl, so only part of
peaks were integrated, leading to the absence of any test results,
such as the diagram below. The Study Director considers that
when this problem was discovered by Laboratory 28, the MRM
acquisition window should have been reset according to the
retention time of tolclofos-methyl in order to be able to collect a
complete set of peaks and obtain accurate results. This probably
was easy to solve, but Laboratory 28 did not use any remedial
measures, which led to absence of any test results for tolclofos-
methyl. This loss should not have occurred, and such problems
will not occur if measures are adopted as described above (
see
Supplemental Figure 5).
(b)
Regarding GC/MS/MS analysis
.—Laboratory 18:
Bifenthrin has been found in all samples. In Samples 3 and 8
the area of bifenthrin is very low and similar to the blank used
for the calibration curve. For this reason, the quantification has
been made by standard addition method.
Study Director response: Bifenthrin is a very commonly used
pesticide in tea plantations in China. If using an instrument of
relatively high sensitivity for determination, it is understandable
for Laboratory 18 to say: bifenthrin has been found in all
samples. Laboratory 18 pointed out: “In Samples 3 and 8 the
area of bifenthrin is very low and similar to the blank used
for the calibration curve”. Because No. 3 and No. 8 are blank
samples and appeared as blind samples and are unknown to
any of the collaborators, Laboratory 18’s finding is correct.
Moreover, Laboratory 18 adopts a standard fortification method
Table 12. Raw data and corrected data of RSD
R
for aged oolong tea by GC/MS/MS
No.
Pesticides
Before correction, % After correction, %
Difference, %
Deviation rate, %
1
Trifluralin
34.7
29.0
5.7
16.4
2
Tefluthrin
31.5
24.3
7.2
22.8
3
Pyrimethanil
29.4
23.6
5.8
19.8
4
Propyzamide
22.1
19.1
3.0
13.6
5
Pirimicarb
25.8
20.2
5.6
21.7
6
Dimethenamid
28.1
22.3
5.8
20.7
7
Fenchlorphos
34.1
26.2
7.9
23.2
8
Tolclofos-methyl
26.8
21.7
5.1
19.1
9
Pirimiphos-methyl
32.7
25.8
6.9
21.2
10
2,4’-DDE
31.0
25.3
5.7
18.5
11
Bromophos-ethyl
30.2
23.4
6.8
22.5
12
4,4’-DDE
29.1
23.7
5.4
18.6
13
Procymidone
29.2
25.7
3.5
11.9
14
Picoxystrobin
22.8
19.3
3.5
15.5
15
Quinoxyfen
29.6
25.5
4.1
14.0
16
Chlorfenapyr
22.0
19.2
2.8
12.7
17
Benalaxyl
24.8
21.2
3.6
14.5
18
Bifenthrin
27.7
23.5
4.2
15.3
19
Diflufenican
21.7
18.5
3.2
14.9
20
Bromopropylate
21.9
18.8
3.1
14.3