8330 Explosives Back Pressure

[ravenwork]

Doing explosives by 8330, using a 5um 250mm Merck Purospher C18 column with a water:methanol gradient, 90:10 to 20:80. The resolution is acceptable. However, the column degrades quickly, back pressure develops, and resolution for the critical pairs decreases. I am wondering if this could be due to the CaCl2 in the sample and standard solutions. Each one is 50:50 aqueous CaCl2 and acetonitrile, with a total concentration of CaCl2 at 2.5 g/L (dihydrate). I am wondering if I am getting some Ca precipitation on column. We inject 100 uL.

I could actually run the water samples without the CaCl2, but I am pretty sure I need it with the soils to help remove the excess solids.

Thoughts? Experience?

[JI2002]

Not sure if it's related to CaCL2, you can try using 1g/L CaCL2 solution instead of 2.5 g/L CaCL2 solution (it may take more time for particles to settle down). If the problem doesn't go away, it's probably something else.

[ravenwork]

Why should i even bother with the CaCl2 when doing SPE? There are particles to precipitate. All of that is removed in the SPE.

ravenwork

[JI2002]

If SOP requires using CaCl2 for water samples, maybe it's because CaCl2 is also added to standards in ICAL and QC samples. Are you using same curve for both water and soil samples? Just from sample preparation standpoint of view, I don't see why you can't get rid of CaCl2 for water samples.

[ravenwork]

Yes, there is CaCl2 in the ICAL standards, and we do share the ICAL between the soils and waters. This is easy enough to change.

[Mark Tracy]

Do you filter your samples after addition of MeOH and CaCl2? You might try letting the samples sit for a few minutes before filtration to let the partlcles aggregate. Also try a smaller pore filter; 0.5µ may not be good enough.

[ravenwork]

Yes, we do filter the soil extracts after addition of the CaCl2. For 8330 samples are extracted into acetonitrile, not MeOH.

However, and I mis-typed in an earlier post, there are not particulates in the water sample extracts. These are extracted by SPE disk. If the samples contain any particulates, they are removed by the disk. I suppose their could be some sloughing of particles from the disk or within the extraction manifold, but this seems negligible.

[Mark Tracy]

Calcium is easy enough to remove if that is the problem. 30% of methanol and 70% of 10mM Na2EDTA across the column for a half hour or so. If your column tolerates reverse flow, flush it backwards to waste. Follow that by unbuffered water/methanol.

I would not trust the SPE disk to do an adequate job of removing colloids.