Chromatography Forum

Hi. I have a Agilent 6890 GC with a front and back injector and FID detector. I get a very large difference in signal depending if I inject the sample in the front or the back using the exact same sample and column. I changed the liner, septa and FID jet and it made no difference. I checked the split ratio with a flow meter and it was the same on both injection ports and matched the setting. Retention times are the same. Any ideas? Thanks!

A little more information will be helpful.

1. How are you measuring the signal?

2. How much is the difference?

If you are taking the signals out from the back of the 6890 to a A/D converter, check that you are using the same range setting for each signal.

Gasman

Thanks. Signal is measured by FID into ChemStation. Area is about 700-fold higher in the front compared to the back (peaks are higher and broader as well, so it's not just a numerical thing). Signal comes out of GC through ethernet connection into computer, I don't see anywhere to set signal range. I should also add I replaced the gold seal on both as well and it made no difference. For some reason more sample appears to be getting on the column in the front (or less on the back), but I don't see any obvious reason for this after replacing the obvious parts.

So now you must be sure that you have the same liner in both injectors and the exact same injection parameters (split flow, purge time, injection volume, etc.)

Does this GC have one or two FID's ?

Gasman

Liners are the same. Method is the same as well.

Unit has 2 FIDs.

Based on running the sample on a totally different GC (albeit with a different column), it looks like front is getting too much on the column rather than the back getting less than normal.

Check that the column is inserted to the same height in both injectors. Also, with a back-pressure regulated split splitless inlet you get flow out of the split vent even when there is no actual splitting of the carrier flow and sample. A block between the inlet and back pressure regulator can cause this, and from posts on the forum Agilents seem particularly prone to this problem. Have you done any maintenance on either inlet's split vent lines and filter ?

How are you doing the injections - if with an autosampler check that there is nothing odd about needle insertion depth or alignment.

What is the sample / standard that you are injecting ?

Peter

Another obvious test: swap the columns so that the front inlet goes to the back injector and vice versa.

Peter

Thanks, Peter. I was suspecting it may be an issue with the split vent filter. The flow rate measurements threw me off, but the fact it is back pressure regulated makes me think more this may be the issue. I'll check it out.

Good idea about swapping the inlets/outlet. The column height is the same in both inlets. Injection is done with autosampler; depth looks to be the same in both cases.