Chromatography Forum

Hello.
Iam now on practice in pesticide laboratory.
My supervisor ask me to prepar (on start - virtually) pesticide stock solution
consist of about 200 pesticides (he gave me a list) and working solution - 10ug/ml concentration
for LC-MS/MS analyzes.

Thats what i plan to do:

1) Check the solubility of each pesticide.
2) Prepare solvents (probably i will need more than one
acetone/acetonitrile, make a mix of them acetone 1:3 acetonitrile
for slightly soluble pesticides) ?
3) 200 pesticides is a lot i dont want to weigh one by one
into separate flask. I plan to groupe them.
4) prepare proper number of measure flasks (25ml volume)
5) weigh number of pesticides into one flask to get 1mg/ml stock concentration of each compound
7) prepare working solution from stocks (500ul of each one
into 50ml flask and fill up with ACN)

Have some more question about correct way to weigh standard on analitical balance but
first want to know if my plan is good.

p.s.
Sorry for my child-like english.

what is your application for running the separation?

be aware that in chromatography you can make the results bad by doing a few things wrong
one major mistake for example is to forget that the sample injection volume is something that can matter

in the end the sample volume needs to mix within the mobile phase used to run the sample and compounds into the column
if the injection volume is very great and/or the difference between the 2 too great then the sample do not mix well enough into the mobile phase before reaching the column
when that occurs, then until the mixing occurs the sample solution becomes the mobile phase.
the result is seen as peak distortion. as bad as splitting the peaks to doublets

i am commenting on this because from what i understood you are going to do the sample in almost all acetonitrile.
and i am guessing you are going to use a RP C-18 column, which is usually ran from 100% water/buffer to 100% in a gradient.

Hello.
Thanks for reply.
We will use Quechers extraction method and C18 or C8 column probably.
I will keep in mind the need of proper sample dilution.
Is it true that MeOH is better for LC-MS/MS ESI+ than ACN ?
Better ionization ?

I've certainly met a few things that ionise in methanol but not in acetonitrile, but they've been the exception rather than the rule. Methanol is also cheaper, and there's never been a crisis in methanol availability as there was with acetonitrile a while back.

On the other hand, methanol gives much higher back-pressures, especially if you have to use it at its worst percentage (about 50% MeOH is much more viscous than either methanol or water alone), and acetonitrile seems to give me narrower peaks (no idea why?? anyone??).

and acetonitrile seems to give me narrower peaks (no idea why?? anyone??).

Lower viscosity = faster diffusion = faster equilibration on-column = lower C-term = more plates (most of the time!)

thanks!

3) 200 pesticides is a lot i dont want to weigh one by one
into separate flask.

Funny, we were just talking about that issue last week at AOAC. Methods with a large number of analytes mean that substantial effort goes into preparing the standards. If you are truly going to be doing QuECheRS [Journal of AOAC International, Volume 90, Number 2, 1 February 2007 , pp. 485-520(36)], I don't think you get to skip that step. The method says to prepare stock standards at around 2 mg/mL in MeCN with 0.1% HOAC, and store in the dark in the freezer. Some of the standards will be more stable than others. That is a reason to keep the stocks separate. Another is that if you are adding analytes not included in the original method, you may need to change the solvent for the stock solution.

I hope you have a comfortable chair in front of your balance

You've probably figured out by now that 200 x 500uL will not fit in 50-mL volumetric.

Like i write :
" 200 pesticides is a lot i dont want to weigh one by one
into separate flask. I plan to groupe them."
so wont be 500ul x 200

I check some literature EU and US also ask one of the auditors from my country about the pesticides standards
all i get is that iam not obligated to do separate standard solutions ... and the standard that are less stable than others
can be grouped.