What you are talking about is two things. 1. the use of surrogate standards (standards that you carry through the entire sample preparation procedure) as validation of the sample preparation and 2. internal standards, which are added just prior to injection and are used to calculate concentration of compounds identified. This is very common in multicomponent analysis and is typically the only viable means of performing multicomponent analysis. See, for example, most of the USEPA 8000 series methods.

Thanks mark, I know that internal inj std is usually added before injection to the instrument.However it may be added during steps of the analysis (Boeing added in the extraction solvent)and called internal std, in this case it will compensate for both instrument errors and errors during some procedure steps (errors in extraction).

I am asking about, using surrogates representative to pesticide groups. I will check the USEPA 8000 as you suggest, thank you.

I would not recommend to add internal standard at the beginning of the extraction to compensate for the error during the procedure or extraction. If you drop half of the sample on the floor or accidentally use a wrong solvent, your recovery will be good because the IS will have the same "poor" recovery as your target and it will cancel out and give you "perfect" recovery. Actually, you can use "any" solvent to extract your sample from methylene chloride to water, you will get good recovery as long as IS behave the same way as the native.

As for the surrogates, you may group into say a representative of organo chlorine, organophosphate, nitrogent-sulfur containing pesticides or carbamates as a suggestion.

Dear Dr Narong

Using internal injection standard prior to the injection add a step to the procedure . But, it's ok for using internal std in the extraction solvent for reasons that you have mentioned .

Now, you agree for using representative surrogates as internal std but you like to use an internal std just prior to injection ( internal inj std ), isn't it. What about using non native internal std like triphenyl phosphate and some surrogates all in the extraction solvent

Do you have an article that use surrogates as internal std for pesticide residue analysis.

Thank you

You rather have a method that is reliable and accurate rather than a quick and wrong. We add internal standard all the time just before injection (particularly true for GC to minimize matrix enhancement occurs at the injector port). We pipette 0.5 mL of the extract + 50 uL of IS (deuterated napthalene, chrysene, acetnaphthene to spreadout the retention time area). It takes less than 2 min. TPP was used by many labs as a surrogate standard (before extraction) but some labs don't like it because it may be present already in the lab as a contaminant. We use deuterated chlorpyrifos here. I am an oppoent of adding IS before extraction to correct for bad extraction procedure. It can hide bad extraction efficiency as I previously described. Again, it is just my opinion.

Dear Dr Narong

thanks for your continued useful discussions, I learned a lot

Agreed. You use the IS to correct for volume differences, injection technique, volatilization efficiency and other variables associated with the instrument, and you normalize your results to the IS. The surrogates assess the efficiency of your extraction/sample preparation method, and should be representative of the types of compounds you are analyzing.

I much prefer deuterated analogs of compounds for both surrogates and internal standards as they will not exist naturally. So, for example, in AOAC 2007.01 GC-MS (QuEChERS method) you would add deuterated terphenyl prior to extraction (this is a surrogate standard, not an internal standard). I personally don't use terphenyl since it is not representative of any class of compounds that I am analyzing. I use deuterated chlorpyrifos and deuterated bifenthrin as my surrogates. I then go through the QuEChERS extraction/solvent exchange and prior to making up to my final volume I add my internal standards. I use the USEPA 8270 internal standards because they are cheap and readily available. I use the internal standards as my quantification basis (comparing my compounds to the IS), and I assess the efficiency of my extraction/cleanup steps with my surrogates.

thanks a lot dr mark, I really appreciate your discussions and the same with dr narrong. thank you

In my experience, when using quechers aoac or slightly modified methods, best way when screening or quantitating for hundreds of pesticides from the same extract on both gc and lc systems is to use more internal standards. One can be added to the extraction solvent, or spiked as any recovery prior to extraction. This is important also because for GC analysis you usually need a solvent change from acetonitrile. For this purpose I would recommend using TDCPP ( Tris(1,3-dichloro-2-propyl)phosphate) or maybe a deuterated or isotope labeled compound with realatively stable characteristics (you had previously good recoveries from different matrices). This will correct any errors during extraction, diluton, solvent exchange and injection. Its a good idea to have an injection internal standard especially on GC, however when screening for a lot of pesticides recovery losses due to matrix effects can be a huge problem. I would recommend the SANTE guidelines for this, to use a representative mixture of pesticides that consist of different pesticides classes and it is important to include the compounds that usually have a bad or low recovery and some stabile ones as well to see a result of a worst/best case scenario and it is a good ide to use compounds that have differet retention times (short, mid and long RT)

in your method. Ongoing QC is vital for pesticide analysis and in every batch there should be at least one recovery at LOQ and X tims the LOQ with at least the representative mix. In the SANTE guideline there are suggestions how many compounds you should have in your representative mixture and that mainly depends on the number of compounds in your method. This way you could still have problems with recovery, since some tricky matrices or compound responses could individually differ from the representative compounds or internal standards. Usually its because of matrix effects so it is important to have a mixture with all your compounds that will be measured and do a total recovery for all analytes for all representative commodity groups at least once or twice a year in a rolling program. This way you can construct a database for representative matrices and individual compound recoveries, and can prepare blank matrices of representative commodity groups so you can do matrix matched calibrations if needed. My approuach usually is when a result is close to MRL (for example: 40% of the MRL) is to do a spiking with the compounds in question observe the recovery and if it is not satisfactory do a representative matrix matched calibration series and reevaluate or reanalyze the samples if needed. If the recovery is still good and the 50% of the MRL is still present than I do another extraction with another spiking at the level of the previously detected amount and if needed use the matrix matched calibration series. For this my reason is the 50% uncertainty rule of the SANTE guidleines. Another option of course is to do standard addition. Hope I've helped, if you have questions about pesticide residue measurements feel free to contact me.

dear Hamow

thanks for your discussion. You have said that " In the SANTE guideline there are suggestions how many compounds you should have in your representative mixture " I really didn't see that before, i will read it again

thank you