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ournal of
AOAC I
nternational
V
ol.
98, N
o.
5, 2015
for quantification, and this thinking is very meticulous and
scientific, which is worth learning from.
Laboratory 26: We can’t develop the instrumental condition
for chlorfenapyr because
m
/
z
408 in chlorfenapyr is low
sensitivity.
Study Director response:
m
/
z
408 in chlorfenapyr is the
ion recommended in the collaborative method. No similar
problems have been found when determining this ion by other
laboratories. Our review of data provided by this laboratory has
found that the precursor ion monitored by Laboratory 26 was
m
/
z
480, so it is judged that the cause of this problem might
be that this laboratory entered wrongly 480 for 408 when the
method was established. Therefore, it can be said the cause was
this mistake.
Laboratory 28: No results for 2,4′-DDE due to falsely
determined MRM acquisition window.
Study Director response: When using GC/MS determination
of tolclofos-methyl by the afore-mentioned Laboratory 28, only
part of peaks were integrated for the target pesticide owing to
errors with setting of the MRM acquisition window, leading to
absence of test results. At the time of GC/MS/MS determination
by the laboratory, they made the mistake again when setting
the 2,4′-DDE MRM window. They established the collection
method according to the retention time of 4,4′-DDE, resulting in
no detection of 2,4′-DDE, and we also feel the result was caused
by this mistake.
(c)
Regarding LC/MS/MS analysis.—
Laboratory 17: For the
chromatography, our experience is that the maximum injection
volume on an Acquity column is 5 μL. The 10 μL injection to
follow your method could explain some bad chromatographic
peak shape; it was difficult to obtain R
2
>0.995. For several
pesticides one calibration point has to be removed (clearly an
outlier), but even with four points of calibration, the 0.995
criterion was not always satisfied.
The Study Director considers their experience is due to the
maximum injection volume being 5 μL for their chromatograph,
while 10 μL injection was adopted in the collaborative study
method, causing certain pesticide chromatograph peak shapes
to worsen. Laboratory 17 also states that they eliminated the
point of obvious deviation for calibration curves of some
compounds in the specific determinations so as to ensure R
2
to reach 0.995. However, although the four-point calibration
curve was adopted, the criterion of R
2
>0.995 could not be met.
The Study Director considers the injection quantity mainly
depends on the sensitivity of instrument, and the instrument
used by Laboratory 18 has very high sensitivity; the sample
was diluted five times before injection for determination. It is
also acceptable that the 5 μL injection adopted by Laboratory
17 can satisfy the technical indexes of the method. Regarding
the criteria of R
2
≥0.995 being hard to meet, there are 2113 R
2
values submitted by 30 collaborators, with 2059 in conformity
with the criterion of R
2
>0.995, accounting for 97%. As for this
requirement, it should be strengthened in future collaborative
studies, and in so doing, the accuracy of the quantification of
target pesticides will be greatly increased while measuring
errors will be reduced, minimizing outliers.
Laboratory 18: Before injection, the samples were diluted five
times with acetonitrile–water (3 + 2, v/v) because the system is
very sensitive and we have some pesticides that were saturated.
Study Director response: The Agilent 6490 adopted by this
laboratory is the latest model of the Agilent triple quadruple
mass spectrometer. This instrument adopts the i-funnel
technique, about 10 times higher than the sensitivity of the
6460 instrument. Therefore, it is understandable that dilution
is needed before injection. At the same time, it is also found
that Laboratory 22 used the old instrument from the 1990s, and
its sensitivity is relatively low. To ensure the calibration curve
reached above R
2
>0.995, it adopted the method of increasing
the injection volume to solve the problems encountered in
the experiment. Laboratory 18 used the dilution approach to
solve the problems encountered, while Laboratory 22 used
the method of increasing the injection volume to solve the
problems encountered. In summary, Laboratories 18 and 22
investigated the ruggedness of the method in two aspects.
Based on consideration of the method principle, they used
their skills to tackle the practical problems encountered in the
experiment, which is advisable and praiseworthy.
Laboratory 23: Retention times slightly change from the
green tea to the oolong tea matrix. A column has been added to
show the exact retention time for both of the matrixes.
Study Director response: When using LC/MS/MS for
respective analysis of green tea and oolong tea samples, the
laboratory discovered that there existed a system difference
of 0.03 s maximum for retention times of pesticides in the
two matrixes. Laboratory 23’s observation for the experiment
was so sharp that we should learn from their meticulous
scientific research attitude. Because green tea and oolong tea
are manufactured with different processing technologies, the
Table 13. Method efficiency for determination of 20 pesticides in incurred green tea by GC/MS, GC/MS/MS, and LC/MS/MS
GC/MS (16 laboratories)
No.
Pesticide
No. of labs
Avg. C., μg/kg
S
r
, μg/kg
RSD
r
, % S
R
, μg/kg
RSD
R
, %
HorRat
1
Pyrimethanil
13
613.3
20.2
3.3
74.5
12.2
0.7
2
Bifenthrin
15
77.8
3.1
4.0
17.9
23.0
1.0
GC/MS/MS (14 laboratories)
1
Pyrimethanil
12
575.4
32.2
5.6
80.9
14.1
0.8
2
Bifenthrin
14
78.6
4.4
5.6
17.5
22.2
0.9
LC/MS/MS (24 laboratories)
1
Acetochlor
22
14.1
1.4
9.8
3.3
23.6
0.8
2
Triadimefon
24
41.3
4.4
10.6
8.9
21.6
0.8
3
Trifloxystrobin
24
90.7
7.6
8.4
19.4
21.3
0.9