AOAC OMB Final Action Recommendation (December 2019)-2016.14

2016.14 (October 2019) – FOS-03 MANUSCRIPT

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Collaborator Comments 1 After the MLT study, laboratories were invited to provide any comments about the method. Of the 2 14 participants, five did not provide any observations and many of the comments received were 3 linked to deviations applied to the MLT protocol, which are described in detail later in the section 4 Method adaptations by MLT participants . A significant number of participants (8) mentioned that 5 the SPE step was the most difficult part of the method. Indeed, it was stated in the original method 6 that the SPE step should be performed under gravity. However, it turned out that this was not always 7 possible. According to the participants, the elution time was far too long, making the sample 8 preparation hardly feasible in one day. To further explore this, some experiments were conducted 9 at the Study Director’s laboratory in order to provide better guidelines for the SPE elution steps. 10 The results of these experiments confirm that positive pressure can safely be applied on the top of 11 the SPE column and this up to an elution flow of 2 mL/min, without compromising the results (a 12 Gilson GX-271 Liquid Handling robot was used to perform these tests). During these experiments 13 it was also noticed that passing air through the column should be avoided as this resulted in higher 14 blank measurements. Based on these data additional instructions on how to proceed with the SPE 15 steps have been added to the final method. 16 One participant noticed that there was no mention of the reaction coil in the instrument 17 configuration, as well as no indication of the nature of the working electrode to be used. The final 18 method has been amended to include these details. Whether area or height should be used for 19 calibration was also not mentioned in the original method, this has now been specified. 20 One lab reported occasionally obtaining negative values for the blank analysis. Given the limited 21 impact of the blank measurement, (see discussion on “Blank Measurements” later) a statement has 22 been added in the method to only consider the blank if the size of the signal is greater than 10 times 23 the signal to noise ratio in order to try to avoid this. One lab mentioned that they were unable to 24 achieve good chromatography with PA20 column, but they successfully used the PA1 instead. One 25 participant tested the sample preparation with and without using Carrez and observed that the only 26

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