loss in resolution with PTV-GC-MS

[willemdm]

Hello,

We are developping an analysis for pesticides on water with PTV-GC-MS (50µl injection)

After around 10 injections we experience a dramatic loss of resolution (broader peaks, critical analites are not seperated anymore...)

Problems only accure with samples (dissolved in dichloromethane) and not with standards (dissolved in dichloromethane) so PTV and other instrument parameters should be fine.

For the moment we only work with clean samples such as drinking water so mayor contamination is excluded.

The sample preparation is with SPE:

1) wash SPE cartridge
2) load sample
3) wash with water
4) dry under vacuum for 30 min
5) elute with dichloromethane

Any ideas on what is the cause for this loss in resolution.
Could any residual water may cause any trouble or is it something else.

Grtz

[Peter Apps]

Presuming that you are not ding SPE on your standards, then something coming off the SPE cartridge is one (of several) possibilities. Try washing an SPE cartridge, then eluting (i.e. leave out the sample), inject the eluate, then a standard, then the eluate, then the standard etc etc and track performance.

Peter

[willemdm]

hi,

can this also happen when you are cleaning the cartrigde before loading the sample?

currently i am cleaning the cartridge with DCM, MeOH and water before use.

so you minimilize the leaching problem.

grtz

[Peter Apps]

hi,

can this also happen when you are cleaning the cartrigde before loading the sample?who knows ?, that's why you have to do the experiment

currently i am cleaning the cartridge with DCM, MeOH and water before use maybe the methanol and the water are contaminating the cartridge, who knows ?, do the experiment.

so you minimilize the leaching problem.

grtz

[BMU_VMW]

You might try to inject in Full scan to have an idea of the contaminants comming of the SPE-cartridge. By changing the rinsing conditions you can see what will do best.

BMU

[willemdm]

thanks for the tips,

the full scan did not give any mayor peaks in the spectrum...
also the experiment with the different injections of eluate did not gave improvement of the resolution.

after the test i added an extra drying step with NaSO4 which eliminated the problem.


willem

[BMU_VMW]

since you are working on drinking water you might want to try and extract the water with DM in stead of using SPE.
works fine for us. simple liq-liq extraction, dry the extract with Na2SO4 and inject 50µl.

[willemdm]

how many ml of water and DCM do you use with your extraction method? how many times do you extract?

we are trying SPE because it consumes less chlorinated solvents

it is also a bit better for the analysis of wastewaters which will be the next step in the project.

[BMU_VMW]

willem,

please send me an email
bart.mulkens@dewatergroep.be

in dutch would be nice