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- Posts: 185
- Joined: Fri Jul 20, 2012 9:18 am
I’m trying to validate an automated liq-liq extraction of PAHs in water and I’m having some trouble with the extraction efficiency.
First of all: to avoid PAHs to stick to the glass wall of sample-bottles, vials etc we add 5% MeOH to the water.
Setup is as follows:
- 8ml water is transferred into an amber 10ml HS vial
- Sequence setup:
# Spike all QC and calibration standards with calibration standard only.
# extract QC-sample 1
* Add internal standard
* Add extraction solvent (dichloromethane)
* Vortex
* Centrifuge
* Extract solvent from the vial and transfer to a 250µl vial (I can’t inject with the long needle I need to extract the solvent from the bottom of the vial)
* Inject 75µl
# Extract a batch of unknowns (n15)
# Extract calibration curve
# Extract a batch of unknowns (n15)
# Extract QC_2
Mechanically this all works fine, but when checking the results I find that QC_2 only has 50% recovery for the heavy PAHs. Naphtalene, acenaphtylene etc are ok, but starting from Chrysene they are not good at all.
Time between spiking and extraction of QC_2 is approx. 25h.
My first thought is that they stick to the wall of the vial, but I’m extracting with dichloromethane directly in that same vial, so how can they stick??
Photodecomposition is another option, but they are in amber vials. What I couldn’t find is which PAHs are more susceptive to photodecomposition, the smaller ones of the heavy ones?
I can rule out a drift in the sensitivity of the MS as being the cause, since my internal standard response is constant throughout the sequence.
Any advice would be greatly appreciated
Bart