Chromatography Forum

I am usual to inject an organic solution of 23 PAH (2ug/mL in dichloromethane:hexane misture solvent) on GC-MS by liquid autosampler injection on a 5-phenyl column 0.25mm 0.25um 30m. I use this method since many years. The column in new, the liner is new. Last night I injected one standard solution, some samples and at the end of the batch I injected 4 times the same initial standard from the same vial.
I saw something strange: peak Area of the first 18 PAH is the same in alle the standard injection (all of 5 chromatogram), while the last 6 PAH have a double Area in the last 4 injection respect to the first injection.
I can not understand why.

Can you help me?
Thank you

I am usual to inject an organic solution of 23 PAH (2ug/mL in dichloromethane:hexane misture solvent) on GC-MS by liquid autosampler injection on a 5-phenyl column 0.25mm 0.25um 30m. I use this method since many years. The column in new, the liner is new. Last night I injected one standard solution, some samples and at the end of the batch I injected 4 times the same initial standard from the same vial.
I saw something strange: peak Area of the first 18 PAH is the same in alle the standard injection (all of 5 chromatogram), while the last 6 PAH have a double Area in the last 4 injection respect to the first injection.
I can not understand why.

Can you help me?
Thank you

Were the samples dirty?

Sometimes dirty samples can coat the liner and prevent any of the PAHs from adsorbing on the glass and increase the transfer efficiency to the column. Other times (more often then the first instance) the samples will deposit something in the liner that will adsorb the PAHs and cause the areas to be lower.

I have also seen that if something is co-eluting with the peaks that is not really seen on the chromatogram, it can enhance the responses for things like Benzo(a)pyrene and the other heavier PAH compounds.

Have you tried to recalibrate after those samples and run the sequence again and see if they all stabilize at the higher response?

Was it exactly double ? Was is exactly the same for all of the peaks that were affected ?

Are your peak areas from the total ion chromatogram, or from specific ions for each compound ?

Peter

I am usual to inject an organic solution of 23 PAH (2ug/mL in dichloromethane:hexane misture solvent) on GC-MS by liquid autosampler injection on a 5-phenyl column 0.25mm 0.25um 30m. I use this method since many years. The column in new, the liner is new. Last night I injected one standard solution, some samples and at the end of the batch I injected 4 times the same initial standard from the same vial.
I saw something strange: peak Area of the first 18 PAH is the same in alle the standard injection (all of 5 chromatogram), while the last 6 PAH have a double Area in the last 4 injection respect to the first injection.
I can not understand why.

Can you help me?
Thank you

Were the samples dirty?

Sometimes dirty samples can coat the liner and prevent any of the PAHs from adsorbing on the glass and increase the transfer efficiency to the column. Other times (more often then the first instance) the samples will deposit something in the liner that will adsorb the PAHs and cause the areas to be lower.

I have also seen that if something is co-eluting with the peaks that is not really seen on the chromatogram, it can enhance the responses for things like Benzo(a)pyrene and the other heavier PAH compounds.

Have you tried to recalibrate after those samples and run the sequence again and see if they all stabilize at the higher response?

Thank you.
Anyway there aren't samples but standard solutions. So clean standard solution from CRM standards. This is the strange thing.

Was it exactly double ? Was is exactly the same for all of the peaks that were affected ?

Are your peak areas from the total ion chromatogram, or from specific ions for each compound ?

Peter

Thank you for the reply.
They are more or less the double. The acquisition is only SIM with 2 ions for each compouns (one target and one reference m/z).

Sorry for the delayed reply - I did not realise that you had responded.

That is is not exactly double for all the peaks rules out detector electronics.

If you acquring only 2 ions fro each peak there could be major interfering co-elutions that are otherwise invisible, and anything that generates one of your SIM ions would do it. If you haven't solved the problem yet, try running in full scan to see if the background is dirty.