OP Pesticides by KD (Low Recoveries)

[scrappy1984]

I have been having problems getting acceptable recoveries on some OP pesticides evaporated by KD equipment.

I have definetly ruled out my GCMS as the problem, and i am positive the problem is occuring in the evaporation step.

I am aware that OP pesticides are susceptible to degradation from light and heat and have taken this into account.

The Analytes affected are, dementon-S-methyl, Fenthion,
Pirimiphos ethyl, Fenamiphos, Prothiofos, Carbophenothion and Azinfos-methyl.

I have been achieving only 30 - 70% recoveries, at a level that is definetly achievable for these analytes on my equipment.

I have been evaporating about 80 to 100mLs of DCM containing my spikes on KD at 70 degrees, and takes about 1.5hr to 2hrs to evaporate. After this step i have been using Nitrogen Blowdown, at 70 degrees to achieve the final volume.

Nitrogen is used at a very low flowrate.

I have tried running without light, but with minimum effect.

I have used standard procedures to clean glassware and liners to remove any active sites, but also with minimum effect.

I am also spiking PAH's at the same time and am getting 90 - 100% recoveries using these settings on the KD, other OP pesticides like Methyl parathion, monocrotophos, Dichlorphos are also recovering nicely.

I am using fresh standards also

Can anybody offer some advice on where i may be loosing these analytes, or offer any advice on settings or different techiques i may be able to try.

If you are after any other information that may help you please post what you need

Cheers

[Peter Apps]

If DCM = dichloromethane you are evaporating way above its boiling point of 40C.

If you are working at atmospheric pressure this defies the laws of physics.

Peter

[scrappy1984]

the waterbath has to be at 70 degrees in order for the DCM to collect.

I am aware that DCM boils at 40 degrees

[gpronger]

First, for these types of problems, my recommendation is to work backwards. Spike into methylene chloride at roughly the volume you finish up at with the KD's and then blow them down following your procedure. This will tell you if the KD step or the blow-down step is the culprit. Once this is defined you'll be able to focus on where the loss is occurring.

Now, if I were to guess, the loss is at the blow down. In my experience, when using methylene chloride you do not need to add heat at this point. I don't have any experimental data, but my guess is that with the KD, when hot, the atmosphere above the sample is simply vaporized methylene chloride while with the blow-down, even though we're using nitrogen, it is sufficiently turbulent that there is significant exposure to atmospheric oxygen.

[scrappy1984]

I have tried isolating the problem, an i believe it is coming from the nitrogen blow down.

I ran a standard through the blow down and then ran it on the GC and there was evidence i have lost some of my analytes.

I have attached a synder column to the 10mL piece of glassware (KD) and used a water bath to vap down to my final volume and the results look much better.

Can anyone confirm that nitrogen blow down for op pesticides isnt the way to go, and is the technique using a synder column the best way to go.

I do believe you can by mini synder columns available.

Or is there another method i could use to get my final volume

Cheers

[gpronger]

My experience is by using the nitrogen jet at ambient we avoid a lot of analyte loss. For whatever reason, the addition of heat, though it speeds things along, has a significant adverse effect on recovery except for the least volatile and robust analytes.

[Peter Apps]

In the KD step you have a refluxing film of solvent that traps any analyte vapours and returns them to the pot.

At 70C with nitrogen flow you just get very rapid boiling, no reflux, and your analytes get blown off in a stream of nitrogen and solvent vapour.

Peter

[scrappy1984]

thanks everyone for your reply.

We have decided to use micro synder columns and steer away from nitrogen blowdown.

Cheers

[gpronger]

If nothing else the micro snyders are impressively cute. Auditors will be impressed.

Greg

[scrappy1984]

u love micro synder columns

[gpronger]

We all have our weaknesses