Terrible, Lingering, Un-removable GHOST PEAKS!!!

[g-star]

PLEASE HELP!!!

I am an experienced chromatographer and have been using a new 7890 for the past 8 months. We run several high temperature analysis (semi-volatiles, triglycerides, etc) using the split inlet and FID.

Recently I have been invaded by ghost peaks that I CANNOT rid the system of. I have tried all the traditional fixes but nothing has worked so far. Details:

-The problem is not related to the ALS/syringe, as the ghost peaks appear simply when cycling the GC up to 320C with no injection.

-The ghost peaks appear on several different columns, and after trimming both ends of the column and/or reversing it.

-The FID has been cleaned and the jet swapped for a new one, the ghosts reappear.

-The liner, septum, o-ring, gold seal have all been changed to new pieces....and the ghosts return.

-The inlet shell weldment has been cleaned using a gun brush and solvents (CH2Cl2, acetone, MeOH). No improvement.

-The split vent trap has been changed. The copper tubing leading from the inlet shell weldment to the split vent trap has been changed. The ghosts are still hanging around.

-High purity solvents have been used, and backflash calculations show that I am well within the volume limits of my liner.

I am charged to figure this out as my small company is averse to costly service visits. I am at a total loss and turn the GC community for any offer of wise advice/experience. I am at the point where I am thinking about changing out the entire injector body shell, split vent assembly, and/or inlet assembly...but this is also costly and in the realm of service engineer expertise.

PLEASE HELP!!!!!!

Any advice would be much appreciated. Thanks!

[giacomo56]

Change your gas tank or traps....

[Peter Apps]

Do as giacomo says, but if you have been running dirty gas into the GC plumbing it will take a while before you see any improvement.

Peter

[chromatographer1]

Your situation appears to have the source of your ghosts in the gas supply lines.

The ghosts can be from deposits from flashback of samples (or even a single sample) from the injector or from contamination of the gas cylinders.

Changing the traps and cleaning of the lines to the GC and within the GC upto the inlet is recommended. As Peter says it can take a LONG time for these ghosts to disappear if you do not service the lines.

You can do it or pay someone to do it, but get it done.

If you use cheap septa you can cause these ghosts and deposits. Use the best, ignore the rest.

Good luck,

Rod

[Charles A. Burger]

When you said split vent trap, did you actually clean and sonicate all of the lines with solvent including those involving the spetum purge?

[Jumpshooter]

This is truly an Enigma.
Yet, we must resolve it.
When you say "ghost peaks" we are assuming that: 1) the peaks appear in your solvent blank injection and 2) the peaks appear in an injection in which you simply task the GC to inject an als vial that is empty (i.e., air blank). Are these assumptions valid? Yes or No.
If "no" then the peaks are related to your sample prep in that they are coming from some reagent or moiety involved in the chemical work up actions. If "yes" then there is bona fide hardware contamination--somewhere in the GC. For the sake of discussion, we will assume "Yes" meaning that the source of contam. is in the GC hardware.
As the other writers have suggested, the air supply is contaminated. However, if you have the same supplier and never had this problem before then it is unlikely to be due to bad gases. What could've gotten into the gases--small metallic filings of the screwed in copper tubing? Are the peaks early eluters or late eluters? Pls post a chrom pic of the results.

[R13]

You did not mentioned septa purge flow. It is set in the method and did You check (measured) that the real flow escaping septum purge outlet is in accordance with the setting?

[aldehyde]

What is your inlet pressure, temperature, split flow (or splitless purge flow -- and time), total flow. What solvent is your sample dissolved in, and what is the injection volume?

I know you said you're sure you're not exceeding injection volume but I am just curious--the more info the better.

I have some customers who do pestacides, tobacco residues, and other gross high boiling oils that have this problem. Its good that you have replaced the split vent trap AND line, thats what I was going to recommend. The sample can get up into those unheated lines and then it very very slowly comes out over time. Was the split vent line dirty? Its possible that the same thing has occurred but into the even smaller diameter lines for the septum purge and carrier gas.

Does this 7890 have one channel, or two? If you have a 2nd inlet is it having the same problem? Slim chance, but if you have another inlet and it is OK try switching the top insert (the part that contains the septa and screws on to the top of the inlet. It includes those 2 lines, which you can trace back to where it is screwed into the EPC). I am guessing that you have something in these lines and if you do it will be quite hard to get out. I have replaced them before but never had to clean, I suppose you could sonicate it or try to force solvent through somehow but I think this would be pretty difficult. If we can somehow prove that this is where the contamination is you could give cleaning a try before you break down and buy a new top insert weldment.