Dear All
For a MRM method involve more than 300 pesticide, can one carry out a spiking /fortified sample daily by a representative number of pesticide(10 to 30) rather than spiking by 300 pesticides in order to minimize the consumbtion of the prepared mixture (300 pest) and used it only for preparing calibration. While, preparing a mixture of lower n. pesticide(representitive based on solupility, pKa, stability,..) to be used for spiks preparation.
for the validation, spiking will be carried out using all the pesticides.
Yes, you can use a selected pesticide or pesticides to add into blank samples and process the same way as the samples and recovery is evaluated to demonstrate that the set is under control. It proves that you follow the method accordingly e.g. using the same amount of solvent, extracting procedure, perform liquid transfer correctly. You can use the pesticide that is stable such as chlorpyrifos or compounds that may be sensitive to changing parameter (pH or injector temp) to test the worst case scenario. Another way is using internal standard like d10-chlorpyrifos into each sample to prove that every sample has good recovery. The reason is just in case you have unexplained poor QC recovery and you don't want to throw away the whole set, you can use the d10-chlorpyrifos recovery to demonstrate that it may be acceptable. If you use native chlorpyrifos in each sample, you will have high recovery for the sample containing incurred residue of chlorpyrifos.
Most multicomponent methods use a subset of compounds for matrix spiking (see USEPA method 8760 and 8270, for example), in combination with a series of surrogate standards like Dr. Chamkasem discusses. You would ideally like to use a series of pesticides that encompasses a wide range of pK values, planarity, volatility, thermal stability, etc so that you can show recovery for similar compounds as well as problematic compounds. Normally you would spike one sample per analytical batch with this mixture as well as a "blank" sample (called a fortified analytical blank) which would be a similar matrix to your samples (for example, dried mixed vegetable matter that you rehydrated prior to analysis).
Additionally, since you are using MS/MS you can easily incorporate deuterated analogs of compounds (like the d10-chlorpyrifos mentioned) as surrogates to demonstrate your ability to recover pesticides in each sample. AOAC method 2007.01 references deuterated terphenyl as a surrogate; I personally use a mix of 5 deuterated compounds that we carry through the entire extraction, cleanup/concentration and analysis process. We can then verify our analytical method by checking the surrogate recoveries for each of these compounds in every sample, whereas we would only check the recovery in one sample. If you have a wide mixture of samples that you are analyzing the surrogates add another layer of assurance that your analytical technique is valid.
I think Dr. Narong Chamkasem and Dr. Mark Krause have said enough and left nothing to add
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