evaporation - reduction of loss of analyte

[Mr.Brown]

Does anyone have some hints how to reduce loss of analyte during evaporation steps.

What type of keeper solvent would you recommand (GC-MS analysis afterwards)

Is dodecane a good choise?

[Don_Hilton]

It depends on your analytes and column. I've seen decane and dodecane used. The tradeoff is the more resistant your keeper is to evaporating, the later it is likely to elute in the chromatogram. As long as the keeper elutes before the analytes of interst you have a good posibility. And, beware of the potential of the keeper tailing a little inlet/column activity with a lot of compound that can interact with those active sites offerers potential for a tail on the peak and, thus, elevated baseline for a while.

[Mr.Brown]

Tested with dodecane, i think i used to much because it took me a lot of time and several injections
to get rid of the dodecane again.

[Don_Hilton]

Dodecane is used as a keeper for a couple of reasons. One is the lack of volatiltiy, which keeps it from evaporating easily under a gentle stream of gas flow at room temperature and the other is that in a GC inlet, it is well behaved and does not have properties like ugly solvent tails or carryover.

Given that dodecane is not caught up on active sites and such and mixes well with things like MSTFA, I am curious.

How hot is the inlet? And how new is the inlet liner? And what injection volume are you using for your sample? As long as your inlet is hot enough to vaporize the dodecane and the liner is not fouled, the dodecande should transfer well to the GC column. If you are using too large an injection volume, it is possible to get flashback - which generates carryover.

If you give mroe details, we can perhaps give you a better answer.


Assuming these are OK, what solvent are you adding to the GC vial prior to injection. And what solvent(s) are you using to wash the syringe before and after injection. If you have dodecane, but are washign the syringe with methanol, the dodecane will not be washed from the syringe - dodecane will prefer the surfaces in the syringe to solution in methanol. And it will be there to mix into your next sample.

[Peter Apps]

If you are losing analyte during evaporation, those analytes must be quite volatile (unless you are using harsh conditions, or evaporating to dryness) - so I would expect that during chromatography they would not separate well from a heavier solvent such as dodecane. What specific analytes are you interested in, and what are your evaporation conditions ?

Peter

[Mr.Brown]

Trying to clean up honey to analyse Beerepellents with GC-MS.
(Benzaldehyde, Phenol, p-Dichlorobenzene, Phenylacetaldehyde, Nitrobenzene, Naphthalene, Thymol)

injector temp: 275 °C
MeOH used as solvent to reconstitute the concentrated sample
MeOH was also used to clean the syringe --> as Don_Hilton suggesting this was probably a bad idea on my part
Did not see a problem as the small keeper droplet of dodecane was easily solved in MeOH when reconstituted to 1 mL

The liner is/was not the newest (roughly 200 injections in total, mostly standards and some samples)

I usually inject 1 µL sample (some times 2 µL)

Mainly I do this to get rid of the ACN after QuEChERS extraction.

If anybody has other ideas how to clean up and analyse beerepellents in honey using GC-MS I am all ears.
Recovery is a bitch.

[Don Shelly]

After the Q extraction, load about 4g of clean sodium sulfate into a 1 g HIGHLY ENDCAPPED C18 cartridge that has been conditioned with acetonitrile. If you don't use a highly endcapped C18 you will loose analytes through hydrogen bonding.

Pass the extract through dropwise and rinse with more ACN for a complete elution.

Solvent exchange into toluene. You might see some highly polar coextractables precipitate out of solution. Filter with a syringe filter and inject.

This should give you a cleaner extract.

[Mr.Brown]

4g of clean sodium sulfate into a 1 g HIGHLY ENDCAPPED C18 cartridge that has been conditioned with acetonitrile.

could I also use the dispersive SPE cleanup very often used with QuEChERS with MgSO4, PSA and C18E?

Solvent exchange into toluene. You might see some highly polar coextractables precipitate out of solution. Filter with a syringe filter and inject.

I already recognized some precipitation when using toluene and honey extracts, is centrifuged for 5 min at 8000 g and collected the supernatant.
however syringe filter could be better.


Thanks for your help

[Don Shelly]

Dispersive SPE might work for you. My concern would be the potential loss of phenol to the PSA through hydrogen bonding. I'm not sure that acetonitrile would be strong enough to retain the phenol.

Be very careful in your selection of C18. No two manufacturers make them the same. It has been my experience that some work well while others retain the analytes. It is best to get samples from several manufacturers and compare.