Chromatography Forum

Long story, but I'll make it as short as possible:

I'm having troubble with one of my GC methods. The GC sequence looks something like this:

2 wash
4 standards
2 wash
10 unknowns
2 standards
2 wash
10 unknowns
repeating 3 last rows depending on amount of samples to be analyzed.

We have a minimum requirement for the area of the standard (which is 2,8).
The 4 first standards are usually around 4 in area.
The problem is then the first of the 2 next standards: 1,7!
Second of the 2 will typically be 3,3.
The same will occur on the next set of standards in the sequence.

It seems the analyte gets absorbed in the GC somewhere. I change syringe, liners, septum and even columns pretty often to keep the GC in shape cause of this. Usually either conditioning the inlet on high temperature, changing liner or changing (or cutting) the column helps when the area starts going downhill. But this time nothing works!

I would be thankful for any advice!

It sounds like some kind of residue is building up in the liner and/or head of the column. And, if areas beginto drop between the set of four standards and the next two, it would be a residue of something in your standards. Are the standards made in matrix? What is the solvent? And what kinds of analytes are you looking at?

Well, I left one detail out about the sequence. It actually looks like this:

2 wash
4 standards
2 wash
4 standards in matrix (must be at least 50 % of standard area)
2 wash
10 unknowns
2 standards
2 wash
10 unknowns
repeating 3 last rows depending on amount of samples to be analyzed.

The solvent is ethyl acetate and the analyte I'm looking at is 3-chloro-1,2-propanediol.

I can add that I've tried running
2 standards
10 ethyl acetate
2 standards
and I get the same thing.

We've had this method for 10-15 years now. It's always been troublesome, but lately I can't get a column to last longer than 1 month. Before it could last 6-12 months at least. Wondering if the whole inlet "house" is worn out or too "active".

A through cleaning of the inlet might help some. But I would look at the sample preparation. A change in matrix over time - or a change in something in the sample preparation could be leaving more active residue in the inlet.