Peaks in run with solvent

[DanaeJ]

Hi everyone,
I'm having some problems with an HP5890 (II). I'm using an HP-Innowax column (30m x 0.32mm x 0.50um) for analysis of FAMEs. When I do a no injection run everything is clear. When I inject plain methanol or petroleum ether (which is what we usualy use for our samples) I get peaks at specific times that correspond to known FAMEs that are often found in most samples we run. I thought it must be contamination of some sort so I got new solvents, cut the column, used a brand new syringe, changed the septum, but I still get the same 3 peaks (at around 25-30k counts). When I run hexane, however, the run is clear. Anyone have an idea what could be causing this??
Also, I haven't changed the liner because at the moment we don't have a spare, but a technician looked at it and said that it seems fine. In any case it was changed quite recently.

[Consumer Products Guy]

When I inject plain methanol or petroleum ether (which is what we usualy use for our samples) I get peaks at specific times that correspond to known FAMEs that are often found in most samples we run.

When I run hexane, however, the run is clear. Anyone have an idea what could be causing this??

I think you may have isolated the issue to your solvents. But it could be possible that you have some fatty acid salt (soap) built up in your injector or column head and that the injected methanol is forming FAME in-situ.

[chromatographer1]

CPG is describing the 'buildup' of dried FAMEs in the injector, usually deposited due to too large an injection for the volume of the injection liner.

These deposits can be backflashed into the pneumatics of the GC and may require the replacement of the entire pneumatic system, or at least the copper lines bring gases to your injectors. These 'deposits' can even be in the lines going to the traps of your injection splitter.

Clean, clean, clean, if you want flat baselines for solvent blanks.

best wishes,

Rod

[DanaeJ]

I think you may have isolated the issue to your solvents. But it could be possible that you have some fatty acid salt (soap) built up in your injector or column head and that the injected methanol is forming FAME in-situ.

At this point I can't imagine the problem is the methanol or the petroleum ether I'm using since I've used new bottles of both, borrowed from other labs, used new a new syringe, new everything. I'm just baffled by the fact that hexane is not giving any peaks. If the problem is in the injector, wouldn't hexane give peaks too? The column was cut so it's probably not to blame.

Thanks for answering!

[chromatographer1]

It depends upon the flash volume of the solvent and the surface upon which the deposits are laid.

Rod

[DanaeJ]

CPG is describing the 'buildup' of dried FAMEs in the injector, usually deposited due to too large an injection for the volume of the injection liner.

These deposits can be backflashed into the pneumatics of the GC and may require the replacement of the entire pneumatic system, or at least the copper lines bring gases to your injectors. These 'deposits' can even be in the lines going to the traps of your injection splitter.

Clean, clean, clean, if you want flat baselines for solvent blanks.

best wishes,

Rod

Thanks Rod,

If I understand you correctly this would be caused by too large an injection combined with infrequent liner changing?
Is there some way I can test the pneumatics? or is it just a case of eliminating other possibilities?

It depends upon the flash volume of the solvent and the surface upon which the deposits are laid.

Rod

I assume you are refering to the hexane here, but I'm not sure I'm getting what you're saying. I'm sorry but I haven't worked with GC too much and I don't really have someone I can ask too many questions, so I've been trying to figure it out by myself. I really appreciate your help.

[chromatographer1]

If your samples are clean you may go a LONG time without replacing the liner. But ONE injection that is too large (it forms a vapor cloud larger in volume than the internal volume of your liner) can deposit material that can later be reintroduced into the the column.

Looking at a macro example: a pressure cooker hold a quart of water. If it failed and exploded, the water vapor would fill the kitchen and might deposit 'dew' throughout the cool countertop of the kitchen.

Likewise, if your 1 microliter explodes into a cloud of 1 milliliter of vapor, and the liner only holds 0.8 mL, then where does the 0.2mL go? It goes backward into places from where the carrier gases came, or into the tube outside the injector which leads to the splitter trap. The vapor is vented into the column and into the trap but if the vaporized FAMEs meet a cool spot, they drop out of vapor phase and coat the tubing.

Then through diffusion when this coating contacts the next hot vapor cloud they can be 'steam cleaned off' partly into the vapor and through diffusion vent onto the column.

And then, what do you see? little bitty FAME peaks when you only injected solvent. Some solvent vapors are better at removing the deposits than others, hexane is probably weaker than methanol or ether.

Not all solvent vaporize at the same conversion from the liquid phase. You can search the internet and find sources of this information. Water expands at a huge change in volume, for example.

Disconnect the lines and clean them by flowing methanol and acetone through them. Then blow dry and reconnect.

Good luck,

Rod

[Bigbear]

Before washing disconnect the line going to the split solenoid. They do not like liquid. If it has been mis treated for a long time it may need replacing.

[DanaeJ]

Thanks everyone, I'll try washing the lines and see if it helps!