Chromatography Forum

Need to eliminate front end/low molecular weight contamination or “background” on a GC-FID using a splitless injection. It doesn’t appear to be disappearing with successive injections of sample or solvent (i.e. over 50 injections). The system is an Agilent 7890 with dual FID. Contamination is significantly greater on the front column which has been freshly installed and conditioned. We had a similar problem on an older 7890 which was only rectified by changing the whole inlet. The instrument in question is new though.

This contamination may be from using defective or poor quality septa.

best wishes,

Rod

we use high quality septa so it cant be that.

anybody else got some advice i would appreciate it

I would try running the instrument with no actual injection - set up for a manual injection and push start. Do this several times to see if the problem continues or if the junk will start to go away. This will ensure the problem is in the instrument and not introduced in the solvent.

Assuming the problem is in the instrument, there is a possibility that the small molecule contamination is coming from the plumbing and gas supply to the back of the instrument. What type of supply system do you have and do you have an inline filter at the back of the instrument. (I like to have the last filter in the system within half a meter of the back of the instrument.) And, how old are the filters? And what grade of gas are you using?

I've seen improperly assembled gas supply systems give problems. You only need someone to "helpfully" use pipe dope, a drop of oil to "clean" the end of a tube, or soldlering flux to make a gas supply system unusable.

Sometimes a new GC can have oil from its manufacture in its pneumatics, usually from controllers. This can contaminate the gas lines.

Silicone grease, I hate that stuff.

Rod

After doing what Don suggested and you find the problem is in the instrument, check to see if your carrier gas regulator has a stainless steel diaphram. If not, that could be causing the problem.

Rinsing and / or replacing the split line may help, though if this is seen on both inlets that may be unlikely.

thats great....i will try those and let you know what happens...thanks again