the diferrence in area between the sample and the standard

[forward]

i have a small problem, i am supposed to check the % of a pesticide in a product as labelled on the bottle of the pesticide, this is usually done by preparing 2 vials of a standard solution and sample solution to be tested both to 100 ppm, and then run them successively, if the % of the pesticide is correct then the areas should be similar or very near to each. sure their is a percent of error which is accepted by the HPLC and the product.

if the pesticide is viscous , the preparation of the sample is done by measuring weight and then do the needed calculations,

usually the preparations are done with ACN and the mobile phase is ACN:water 80:20.
i am using automated HPLC from agilent.

most times the results are perfect , but some times i get a perfect peaks of the standard and the sample , but their is much difference in the area and this makes the expected error more than the accepted.

i am sure from the preparation and the used method , and the product i am checking is a good quality one, so i know that it should pass and it is not that their is a cheat in the product.

the same used standard is used before for many times and the results were great , so i am sure also it is not the standard.

as i said the hplc is automated and the peaks are perfect , so their is nothing from overdose injection or contamination.
the thing that noticed was just the pressure was not constant all the time, it was decreasing with time from about 80 to 70 ,

i am really mixed , if anybody knows why it happens like that please help.

if you need more details i will be happy to paste it.

[WillyOne]

I feel that Pesticides analysis are not so easy.
If a liquid formulation can happens that ACN does not mix with the formulation solvent.
If a suspension concentrate ACN will not break the slurry mean you don't extract the active ingredient.
For some pesticides 80:20 ACN/water can be a good eluent, for others can be not so good.
The coelution of impurities or formulation coadyuvants can give to wrong results.
Can you be more precise?

[forward]

i will try to get sure the sample is mixed well, although i put the prepared volumetric flask of the sample in sonicator.

what details you like to know? mean precise for what?

[WillyOne]

I think it can be easier if we talk about a specific pesticide. And one or several formulations type. About sonication I would suggest you that after sonicating, a mix it in magnetic stirrer

[Victoria]

Hi everyone:
I saw your post and I found something similar in my analysis. The thing is that areas in standard are perfect and reproducible but when I started to analyze formulated pesticides I found that my areas are increasing slowly (I have an excellet reproducibility day by day) until everything goes out of normal variation. I have been talking with some colleagues and they told me about a good inyector cleaning between inyections. Now I am trying to solve this problem. Another thing is column and system cleaning, every day, but I am sure that I had been cleaning my system well because if not I would´t have achieved reproducibility.
Perphaps we have the same problem, do you analize a formulated and TG?
regards
Victoria

[WillyOne]

I don't understand what TG means, I'm sorry. Otherwise my job is analyzing pesticides formulations as Q.C. but I work for spanish administration.
So, I normally try use the CIPAC Methods but sometimes I need to develop my own separation.
Even Pesticides as forrmulations level is not really difficult, sometimes it's hard to obtain a good separation and peak shape.
To analyze pesticides my first step is reading physico-chemical properties in the Pesticides Handbook. There I can found its thermal stability to choose between GC and HPLC.
There I can found also the solubilities in the different solvents and if water soluble, if it is ionizable that means using buffers (as Glyphosate).
I normally work in the 500 mg/l conc but check linerity between 100 and 1000 mg/l.
Normally the choose is RP but sometimes I work in normal phase.
I calibrate each day I work (only one standar plus 0) but repeatability and accuracy are measured to validate the method.

I don't imagine a only-method to separate a wide range of pesticides.
This was the reason to ask for a specific molecule, in order to compare our ways to resolve it