[missaturn]

"before the extraction of the sample, THE SPE BED IS NOT ALLOWED TO DRY"

what does it mean by not allowed to dry? After the condition of the SPE cartridge, we need time to add the sample. The column somehow will dry. i dont understand the statement above. Thanks in advance for the explaination. 

[Arkcon]

Whenever you're doing column chemistry, it's important not to let the column dry, because air gets in, forms voids, and channels, and you lose resolution, and separation efficiency.

You've got to plug the bottom, or seal over the top, or do your workup first, so that a small amount of fluid is always remaining at the top of the column during all steps.

See if someone with a bit of experience can show you their method.  Once you've done it wrong, you'll know how to do it right.

[missaturn]

thanks.

One probelm is the process of concentrate the sample to a lower volume (200 microliter).

Is there any glassware to measure or control the volume during concentration of the extract. Thanks

[Arkcon]

thanks.

No problem.

One probelm is the process of concentrate the sample to a lower volume (200 microliter).

OK.  Wait, what?  You're starting with what volume, to get get to 200 uL?  And why?  Are you using the column for this process?

Is there any glassware to measure or control the volume during concentration of the extract. Thanks

Yes.

Ask the question again, as complete thoughts, from start to finish.

[JGK]

Most of the time for the concentration step, the analyte is eluted from the SPE cartridge with a volatile solvent  then concentrated by evaporation (normally using a centrifugal evaporator or nitrogen manifold) to dryness then resuspended in the required volume (200 µL). 

I have never seen a method where you attempt to concentrate to a volume direct from the cartridge.

[missaturn]

After all the SPE step, i get 15 ml of the extract. I use the nitrogen flow to concentrate the extracts. Its difficult to estimate such a small volume of extract.

[JGK]

After all the SPE step, i get 15 ml of the extract. I use the nitrogen flow to concentrate the extracts. Its difficult to estimate such a small volume of extract.

As I said previously, in most method I have seen and used, you evaporate to dryness - then resuspend in the correct volume. If you use a conical collection tube, the residue should be concentrated at the tip making resuspension easier.

[Arkcon]

If you use a conical collection tube, the residue should be concentrated at the tip making resuspension easier.

There are also conical tubes with graduations, so if you're down to 1 or 2 mL, you can reconstitute to your needed volume.  That's pretty much what I meant when I gave the O.P. a simple "yes"