SPME

[GregK]

All-

Does anyone have experience running residual solvents (specifically methylene chloride, chloroform, DMF) on an Agilent 6890 GC?

I'm trying to develop a method for in-house use, and don't want/ can"t afford to buy too much. I've read some literature on Supleco's site, and have their CD with info on SPME, but can't find much on the instrumentation side.

Do I need a special inlet? Is there a way to program ChemStation to have 0 flow at the beginning of the run to desorb the fiber in the inlet?

Any help would be greatly appreciated.

Thanks,
Greg K.

[Don_Hilton]

The only change you need to make to your GC is to replace the inlet liner. There is a special low volume liner. You can user a regular liner with no insert (no glass wool or such), but the low volume liner helps to avoid band broadening.

The GC is run just as you would with a liquid injection. The heat from the column causes compounds to desorb from the fiber.

Using the manual injection system there is one critical step - withdraw the fiber back into the holder before removing the holder from the inlet. If you try to pull an exposed fiber back though the septum - you lose the fiber. I've done it more than once.

Take care to equilibrate the fiber in the sample for a fixed time and at a fixed temperature.

[GregK]

So the analytes will desorb (is that the right word ?) more or less instantly? I thought it required some amount of time to come off the fiber? If it does happen instantly, then great, I can get started as soon as I can get the fibers and holders in.

Thanks again.

-Greg K.

[Schmitty]

I find that having the fiber in the inlet for a fixed amount of time will increase reproducibility. I will use the GC timer and go to 0.5, 0.75 or 1.0 minute.

[GregK]

Schmitty-

Doesn't that introduce the sample slowly onto the column? Maybe I'm not thinking clearly, but it would seem that one would want zero flow through the inlet while the analytes desorb from the filter. I guess a high enough inlet temp with a low initial column temp would concentrate everything on the column...

I've ran GC for a few years, but never had a lot of the theory explained to me, that's why I love this board.

-Greg K.

[Ron]

Zero flow through the GC inlet would not be a good thing. The first thing to worry about is air infiltration, which causes problems in column, liner, and fiber degradation. Second, with no flow the analytes would diffuse throughout the inlet after they were desorbed, probably causing carryover and poor peak shape, as well as loss of analyte.

The compounds usually desorb from the fiber quickly, so the injection can be thought of as a fast injection.

The reality is a little more complex, but compounds that are loosely held by the fiber will desorb rapidly, and compounds released more slowly are refocused at the head of the column.

[BZK]

So the analytes will desorb (is that the right word ?) more or less instantly? I thought it required some amount of time to come off the fiber? If it does happen instantly, then great, I can get started as soon as I can get the fibers and holders in.

Thanks again.

-Greg K.

Low boiling compounds are immediately desorbed ; Middle to high boiling requiring up to 20-30 sec. for full desorption, depending from temperature. Low boiling compounds are not "cold trapped" into the analytical column and then splitting is required to avoid peak broadening. You can avoid splitting and increase sensitivity by use of cryogenic device on the first part of GC colmn.
I have discovered that with Shimadzu 2010 GC ON-Column SPME is possible without modification, and my work is published into the application book of december, 2009, of LC_GC europe.

Robertino Barcarolo, Italy