Turbovap - sample concentration

[MichaelVW]

Short version: what settings do you use on your turbovap and why?

Long version:

Our lab uses a Turbovap to concentrate extracts down for 8270. We got a new TurboVap II a couple months ago, and had been using it with the pressure set at around 5-6psi and the temperature set at 40 (for DCM). At these parameters it takes more than two hours for a sample to concentrate down from about 150-200 ml to 1 ml. (This compares to a half hour blowdown for our old Turbovap, which was so old and crusty that only God knows what settings we had it at.) But someone had tested it and determined that any increase in pressure would mean losing more analytes. It makes intuitive sense, and I also noticed in this forum's archives, people talk about minimising nitrogen flow when doing this.

But I decided to look into it a bit more... Biotage has a Application Note (#951) on their website, listing BNAsoil spike recoveries they were able to get using 21 psi and 40 degrees. 61% recovery for 2-chlorophenol, 94% for Pyrene; things generally trending upward toward the end of the run. Granted, the one thing they don't mention is the concentration/amount of spike they use, but that seems pretty good. I also thought it was interesting because the Turbovap II manual says never to go above 20 psi because it can be unsafe and damage the instrumetn, but whatever.

So I tried it: I spiked 150 ml of DCM with 100 ul of 1000 ug/ml BNA spiking solution and tried blowing it down a few different ways. I tried to be consistant with endpoints, as it does seem to be sensitive to that. And what do you know, 21 psi gave me better recoveries than 5 psi or 8 psi.

Does that make sense to anyone? One thought is that lengthening the time that the sample sits in the Turbovap gives you more time for the analytes to evaporate - maybe they aren't effected by gas flow as much as the DCM is. A nominal 40 degrees is near DCM's BP but still way below the analyte BPs. So maybe upping the flow increases DCM's evaporation rate proportional to the evaporation rates of phenol, pyrene, etc: so that even if those analytes are evaporating a little bit faster too, you're still going ot lose less of them during the concentration process.

[MichaelVW]

(Note that I direcetly spiked an evaporator tube of DCM, I didn't spike soil and then extract it like Biotage did in their application note).

Recoveries @ 5 PSI, average of six reps.
Phenol 61%
2-Chlorophenol 58%
1,4-dichlorobenzene 54%
n-nitrosodi-n-propylamine 57%
1,2,4-trichlorobenzene 56%
4-chloro-3-methylphenol 57%
Acenaphthene 54%
2,4-dinitrotoluene 64%
4-nitrophenol 70%
Pentachlorophenol 70%
Pyrene 82%

Recoveries @ 21 PSI, average of four reps.
Phenol 74%
2-Chlorophenol 68%
1,4-dichlorobenzene 66%
n-nitrosodi-n-propylamine 73%
1,2,4-trichlorobenzene 68%
4-chloro-3-methylphenol 76%
Acenaphthene 73%
2,4-dinitrotoluene 89%
4-nitrophenol 100%
Pentachlorophenol 99%
Pyrene 96%

[MichaelVW]

Just a note for future referance that Biotage says they typoed in their tecnical note... they meant to say 12 psi, not 21.

[twranger]

Michael,
If that is the same tech note we have, notice it states that the shorter time the better. The problem with letting it sit is the analytes are evaporating out with the solvent. The primary thing you're looking for is the 'vortex' action or swirling that washes the sides of the tube down as it evaporates. We start at about 8psi (at 180ml) so it doesn't splash, then bump that up as it goes down to about 15 psi. It's not completely automated, keep an eye on it. Also discard the spacers on the sensors so it will beep and stop evaporation at 1ml, not 0.75ml. Are you rinsing the tip as the tech note suggests? This makes a big difference in recovery.
Don't know how folks concentrated before TurboVap came out, they are great little workhorses that we have had little trouble with.