Chromatography Forum

What is the best way to prevent column bleed such as this and what is the best approach to cleaning it up once it does appear?

I presume the images below are due to column bleed? This is a relatively new 5% phenyl column (Rxi-5MS 30m x 0.25mm x0.25um) with a 1uL splitless injection (purge at 30 seconds) of hexane. The column was conditioned at 340oC for 2 hours prior to use (as per manufacturer instructions). The GC oven ramps at 80 to 320oC over 15 minutes, the injector is set at at 300oC, and transfer line to the MS at 325oC.

TIC


Peak at 7.4 mins

Do you see these peaks in instrument balnk (run without injection)? If you don't, then it's not column bleeding. Eleveted baseline, not individual peaks, is the indication of column bleeding.

All peaks have the same mass profile, and this site suggests its either septum or column bleed:

http://www.chem.agilent.com/Scripts/Gen ... &prodcol=Y

73, 147, 207, 221, 281, 295, 355, 429
Dimethylpolysiloxane - Septum bleed or methyl silicone column coating

Will try running without an injection to see what happens...

I'd say inlet/septum as cause, maybe with hexane solvent. Agree that bleed from column wouldn't give discreet retention times.

If it's septum bleed, would sticking some tinfoil between the septum and the injection port help reduce this? Is it just because I am using a splitless injection? What causes this, is it just the hexane coming into contact with the septum during injection or could there be bits of septum stuck in the injector port?

The peaks are too sharp to be column bleed. It is something being washed out of the injector, and a bit of septum in the injection liner is a prime suspect. It could also be that the septum in use is not a high temperature variety as the 300°C is not a mild temperature for many types of silicone rubber.

Another place to check for silicone contamination is the sample vial. If you are injecting hexane from a septum sealed vial that has been pierced previously you may be seeing silicones in this hexane vial solution that have been rinsed from the needle during previous injections. In other words, it might NOT be from the injector port septum or injection liner (bit of septum) at all.

best wishes,

Rod

OK, so I changed to a different column and then cleaned the injector port out. Most of the large peaks have gone but I still get quite a messy TIC baseline. If I run with no injection I get the below. Does this mean its stuck in the column somewhere? Top one is SIR of two masses (147 and 213) of interest, bottom is TIC 50-500

I guess I'll do a complete overhaul, clean source, different column, etc etc

Anything that appears as a sharp peak must come from upstream of the column. Accepting that the inlet liner is clean, check that there are no bits of septum in the bottom of the inlet body (how you do that depends on what hardware you have). Then make sure that you have a flow of 3 - 4 ml/min out through the septum purge.

If both of these are OK it is bad news becuase it probably means that the contaminants are coming in with the carrier gas. Do you have a gas scrubber on your carrier gas supply ?, when was it last changed ? Any plastic pipe in the supply line - replace it with metal. Problem is that any muck in the gas supply will have contaminated the GC plumbing and it can take weeks to bleed out.

Peter

If your septum purge has a suitable flow, have you changed your septum?

Or the brand of septum?

If you cannot find a piece of septum where it should not be, and you have changed the septum then I have to agree with Peter the contamination is coming from upstream.

Replace the septum but do not pierce it.

Do you still get the silicone peaks?

Replace the column with a new column.

Do you still get the silicone peaks?

If possible bypass the gas pneumatics of the GC and connect the inlet directly to a clean gas source.

Do you still get the silicone peaks?

Silicone grease is used often for seals in the pneumatics of the GC. That could be what you are seeing.

Good luck in finding and eliminating the silicone source.

Rod

Not sure what kind of insreument you have, but check the trap in the split vent line if there is one. If you run a lot of nasty samples, maybe you also need to clean the split vent line.