Chromatography Forum

Hello,

I am running organochlorinated pesticides on 6890 HP GC-ECD with dual column configuration db-5 and db-17. I notice there is a ghost peak around the same time as endrin on db-5 and the same peak is on db-17 but does not interfere with any analyte. I just cannot get rid of the peak. I have tried scrubbing the inlet with DCM and methanol, scrub the gas inlet line, changed liners/gold seals/ septum/ guard columns and no change is seen. I perform maintenence every other day and some days the peak will be very small < 1.55ppb and other days it will be around 3.125, but it never completely goes away. Also throughout the sequence, the peak stays the same size and it is in samples/solvents and standards.

I have tried injecting solvent/ no syringe and the peak is still there so I know that it is not the solvent or syringe?

Does anyone have any suggestions?

Thanks in advance

Ghost peaks are a hardy perennial on this forum, and the troubleshooting has been laid out several times - search the archives, try what is in there and if that does not work then get back to us.

Peter

Try to think about the problem.

Material is condensing on your cold column and forms a plug which carries through the column as it is heated.

Where is this material coming from?

The carrier gas brings it to the column. It is in the gas supply? Is it in a line bringing the gas to the GC? It is in the pneumatics of the GC? It is in the septum in the injector? Is it from material that may have been injected and backflashed in your injector?

Eliminate one by one the possible sources and clean wherever your source of the contamination resides.

And SUDDENLY, no ghost peak. It is that simple. You have to do the elimination process yourself.

It it were easy anybody could do it.

good luck,

Rod

Hi Dalarie,

welcome to the club .
I have the same problem, just with variations. (And I also tried almost all you have described. Plus tried to find the source of cross contamination. That included testing working surfaces, blanc solvents, cleanup gels alone, rinsing glassware, name it... With no success. Except full preparation of black sample without any inner standarts — peaks were there... ) The big difference with me is I never had those peaks within standarts/blanc solvent. Only in the heating runs and samples. But not every single sample, but mainly fat or vegetables origin. Almost never (or very small compered to other cases) in feed or cereals origin samples. While preparation and cleaning technics are quite simmilar. For vegetables and feed/cereals at least.
What seems strange is my problem seem to dissapere within a year or so. It went down gradually (and RT of those peaks used to extend gradually to a time of b endosulfan) and dissapeared at all, untill I trim the columns. Ha. And then it returned back!?. So I am totally confused.
My columns are HP5 and DB5, though.

Hi Dalarie,

What seems strange is my problem seem to dissapere within a year or so. It went down gradually (and RT of those peaks used to extend gradually to a time of b endosulfan) and dissapeared at all, untill I trim the columns. Ha. And then it returned back!?. So I am totally confused.
My columns are HP5 and DB5, though.

This sounds as if you were eating the phase off the first bit of the column, and once it was all gone the problem went away, until you removed the section of column that had no phase on it

Peter

Well, I had same thoughts. But my gues was more about the tubing degradation products. With time it could cover with any type of residues and stay inert, but when I trimed the column it was opened in the liner again. I mean end not covered with stationry phase. And ready for some kind of interaction. Maybe.
My coleage in the other lab (Chlorinated pesticides in water and water animals.) sufers the same ghost peak as well. And by the way, we all use Agilent???

Well, I had same thoughts. But my gues was more about the tubing degradation products. With time it could cover with any type of residues and stay inert, but when I trimed the column it was opened in the liner again. I mean end not covered with stationry phase. And ready for some kind of interaction. Maybe.
My coleage in the other lab (Chlorinated pesticides in water and water animals.) sufers the same ghost peak as well. And by the way, we all use Agilent???

By tubing degradation products, do you mean the polyimide coating on the outside of the column ?

Peter

By tubing degradation products, do you mean the polyimide coating on the outside of the column ?

Peter

I gues. What else, if it has anything to do with a column at all?

I was thinking of the stationary phase on the inside.

Peter

dalarie,

I would start pursuing phthalates. They respond on an ECD and they are ubiquitous. Look everywhere in your prep for contact with plastics, pipettes, auto-pipettes, caps, etc.

Ryklys,

Not to rain on your parade, but an HP-5 and a DB-5 are not really a confirmation column pair since they are both very, very similar stationary phases. Probably better to move to the DB-17 as noted by dalarie.

Best regards,

AICMM

Ryklys,

Not to rain on your parade, but an HP-5 and a DB-5 are not really a confirmation column pair since they are both very, very similar stationary phases. Probably better to move to the DB-17 as noted by dalarie.

Best regards,

AICMM

You are welcome,

but this not gonna happen. And the reason is not me. I inheritated the method and since it... "works", nothing gonna change. By the way, we were equiped (columns selected as well) by our Agilent representatives soon 10 years ago. They have developed the method. (AS far as I was told.) Then, I work for state run lab, we count every penny. Overuse liners and septas, name it... Column change would mean a method implementation and testing. Loads of expences my management will never go for.

How sad is this situation. So much for the value of the 'truth'. It makes one wonder why the analysis is done. It is done to protect the people or to protect the politician: "not my fault" ?

My condolences to you in your situation.

best wishes,

Rod

Thanks for all he advice,

Last week I tried using dichloromethane clean the inlet and other parts while doing instrument maintenence. It seems to be working for now. The peak is still there but very small, way below my lowest standard.
I also thought it was phalates from the methanol bottle so maybe it is.

You are using MeOH from a plastic bottle?

yes, we use all solvents from a plastic squeeze bottle but it only seems to affect my machine, there are older 5890 gc's so maybe they are not as sensitive