Where do these peaks come from?

Question

We are running a method to analyze assay and some impurities in a pharmaceutical material. We are running under the following conditions:

Agilent HP-1 30m x 0.53mm x 3.0 um column

200 C inlet

1:1 split

5 mg/mL (high concentration to detect impurities)

2.5 mL/min flow with temp gradient with 250C bakeoff for 20min

2 uL inj

Diluent: ACN

Liquid inject

Liner: 4mm deactivated straight through with wool (Restek cat # 20782)

Septum: Bleed/Temp Opt. non-stick 11mm (Agilent PN 5182-3413)

We have peaks that look like "impurities" peaks but do not increase and decrease with same magnitude as the sample concentration and are inconsistent between injections of the same sample. They are not in the ACN blank. They increase with inlet temperature increase to 250C.

I also see them (with lesser magnitude) with an injection of just hydrogen peroxide in ACN only.

These peaks are seen at 7.8 min and 8.2 min in the attached chroms.

Any clues where these are coming from? Interaction with injection and something in the inlet? Is there a more inert liner or septum I should be using?

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garyb007 · Answer

With that low a split what is total flow? It is recommended at least 20 ml/ min total flow I think. Try cleaning off the gold seal and cleaning the inlet with cotton swabs wet with methylene chloride. We use a tapered liner at the bottom.

gabcaron · Answer

So I see you say your method is a split method 1:1 (so 50% of sample goes through). Is there a reason for you to not use splitless and do injections of 1uL instead? Also, your liner looks to me like a liner I would use in a splitless method and not splitless unless you have the cross gold plate in your inlet instead of regular gold plate. Also, what is the pressure of your inlet with that flow? So you say it is not part of your ACN injections but it is part of your sample, and you say you are looking for impurities in your sample. So is the issue that it does not seem to be linear with the concentration of your samples? If so, if it is not part of your ACN Blank injections, are you sure it is impurities from the sample and not something like sample degradation from high temperature of the GC?

gabcaron · Answer

If you want to try Agilent's deactivated liners, this is a part number for it 5183-4693. These are for splitless injections. Also we have deactivated split/splitless inlet that you might want to look into if that could interest you if these are reactions with something in the inlet, if it is strictly liner then just a change of liner should fix it, but I cannot say what is creating that issue. I am not aware. If the peak increase with temperature, I would suspect there is a degradation or a reaction of some kind in the inlet that is happening for sure.

Where do these peaks come from?

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