Chromatography Forum

Is it possible to run a large volume injection (say 100uL) of semivolatiles extract into a GC column and have it split between an FID and mass spec?

Possible - yes.

Practicle is the qustion. Do you have in mind splitting at the inlet into two columns or after one column into two detectors? (Both configurations are possible.)

large volume injection can be done - but you either need a column that can handle the loading or the use of somethign like a PTV system to get rid of the solvent. And, a PTF system to get rid of solvent in large volume injections requires a solvent that boils significantly below the lowest boiling analyte of interest.

Practicality lies in the details. If you can give a bit more detail about what you are trying to accomplish and your contraints, someone may have some better details for you.

You can use stationary gap in the inlet end for large injection

Thanks for the replies. Yes, I was thinking of splitting after the column. I just found that SGE has a kit for this very purpose (MS/FID Splitter Kit). We'd like to ID gasoline, diesel, and motor oil range individual peaks with the MS. We use a megabore column to handle the large volume. Most of the solvent gets split off anyway. Would definitely have to make some method modifications. Any potential pitfalls I may come across?

Any potential pitfalls I may come across?

MS detector operates under a vacuum, FID doesn't. So if you're looking to match retention times: tougher.

That was my worry. Thanks.

I might be wrong, but i don't believe that there will be a big difference in the RTs (atleast not big enough for the purpose you are interested in). The column lengths (after you split from the main column) is going to be relatively short when compared to the main separation column and the linear flow velocity of carrier gas is of the order of ~30 cm/s. Also, you can always play around with the dimensions of the two pieces of columns used for connecting the main column to the two detectors.

As long as you have a more restrictive column going to the mass spectrometer you can match retention times to wtihin a few seconds. Do you realize that you will not be able to get a positive compound ID for many components based solely on mass spectrometry? Many isomers have very similar mass spectra, and you will need retention time as well as mass spectra to identify these compounds.

I have done this with a VG-Trio. You can split at the injection port using a simple two-hole ferrule. Using HP flow calculator you can even estimate the flow rates through both columns and also the split between them. One column went to the MS (0.25 m.m. ID one) and the other megabore (0.53 m.m. ID one) went to the FID.

However, why are you even bothering injecting so much sample? Sounds like you want to characterize a product and not a trace level so using 100 uL is excessive. Most instruments can detect nanogram levels and even an FID can get down to at least 5 to 10 nanograms on the column.

Forget about using the on-column PVT (large volume) unless you want to be changing columns frequently or cutting off a meter off the front end every few samples. RT's will change, updating your method frequently etc. PVT's are highly overrated and thats why I have never seen them commonly used. Too much trouble. Injecting 1-2 uL should be OK and worked fine for us.

Timothy

Thanks all for the tips. Not sure where we'll head from here yet.