Pang et al.: J ournal of AOAC I nternational V ol. 98, N o. 5, 2015  1449

Table 12. Raw data and corrected data of RSD R

for aged oolong tea by GC/MS/MS


Pesticides Trifluralin Tefluthrin

Before correction, % After correction, %

Difference, %

Deviation rate, %

1 2 3 4 5 6 7 8 9

34.7 31.5 29.4 22.1 25.8 28.1 34.1 26.8 32.7 31.0 30.2 29.1 29.2 22.8 29.6 22.0 24.8 27.7 21.7 21.9

29.0 24.3 23.6 19.1 20.2 22.3 26.2 21.7 25.8 25.3 23.4 23.7 25.7 19.3 25.5 19.2 21.2 23.5 18.5 18.8

5.7 7.2 5.8 3.0 5.6 5.8 7.9 5.1 6.9 5.7 6.8 5.4 3.5 3.5 4.1 2.8 3.6 4.2 3.2 3.1

16.4 22.8 19.8 13.6 21.7 20.7 23.2 19.1 21.2 18.5 22.5 18.6 11.9 15.5 14.0 12.7 14.5 15.3 14.9 14.3

Pyrimethanil Propyzamide


Dimethenamid Fenchlorphos Tolclofos-methyl Pirimiphos-methyl

10 11 12 13 14 15 16 17 18 19 20




Procymidone Picoxystrobin Quinoxyfen Chlorfenapyr

Benalaxyl Bifenthrin



proving that they are relatively dispersed. No big systematic deviations happened with the test results.

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

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.

Made with FlippingBook flipbook maker