Chromatography Forum

Hi,

I am a beginner in gas chromatography and I still have difficulties with the interpretation of peaks in the chromatogram.

My question on this would be, how can I interpret this measurement from my standard.



What can the smaller peak be and how should I integrate?

And what I also still don't understand is, why does the base line rise so much, after the peak of the solvent?

You have given nowhere near enough information to allow specific answers, but in VERY general terms:

1. the mystery peak could be anything:
- a degradation product from your standard ?
- a contaminant in your standard or your diluent ?

2. the rising baseline is most probably associated with the increase in temperature (assuming you are temperature programming, not isothermal):
- some detectors are more sensitive to temperature effects than others
- some stationary phases are more volatile than others and "bleed" as the temperature increases.

I'll be less "politically correct" than Tom:

Your chromatogram/peaks looks worse than a chromatogram from the 1960s !!!!

I wasn't trying to be "politically correct". The chromatogram is obviously a hand tracing, but both that and the questions are pretty much what I started from 55 years ago!

I should probably move the thread over to the "student projects" section.

tom jupille wrote:
You have given nowhere near enough information to allow specific answers, but in VERY general terms:

1. the mystery peak could be anything:
- a degradation product from your standard ?
- a contaminant in your standard or your diluent ?

2. the rising baseline is most probably associated with the increase in temperature (assuming you are temperature programming, not isothermal):
- some detectors are more sensitive to temperature effects than others
- some stationary phases are more volatile than others and "bleed" as the temperature increases.

Hi and sorry that I am so late in getting back to you. Thank you very much for your explanation. The impurity argument could be, the higher the concentration from my standard gets, the more insignificant the small peak becomes, until it disappears completely at 8 mg/ml from a standard solution.

Yeah, the column is already very old, column bleeding cannot be excluded, but my professor also said that the increase is related to the temperature.

tom jupille wrote:
I wasn't trying to be "politically correct". The chromatogram is obviously a hand tracing, but both that and the questions are pretty much what I started from 55 years ago!

I should probably move the thread over to the "student projects" section.

Yes, thats true

Yeah, the column is already very old, column bleeding cannot be excluded, but my professor also said that the increase is related to the temperature.

Those are not incompatible explanations. Any degradation or bleed would probably increase with temperature. That would account for the rising "lumpy" baseline.
Another possibility for the "contaminant" peak that I hadn't thought of before is a low-lever contaminant in your carrier gas that accumulates on the head of the column and then is eluted as a single peak when the temperature increases. That is fairly easy to test: do a blank run (no injection). Assuming the peak appears, do a series of blanks with increasing equilibration times between runs. If the contaminant is coming in via the carrier gas, you would expect the size of the peak to increase. If it doesn't appear at all, or if it doesn't increase, that points the finger at something related to the injection process (septum? syringe? vials?)

tom jupille wrote:

Yeah, the column is already very old, column bleeding cannot be excluded, but my professor also said that the increase is related to the temperature.

Those are not incompatible explanations. Any degradation or bleed would probably increase with temperature. That would account for the rising "lumpy" baseline.
Another possibility for the "contaminant" peak that I hadn't thought of before is a low-lever contaminant in your carrier gas that accumulates on the head of the column and then is eluted as a single peak when the temperature increases. That is fairly easy to test: do a blank run (no injection). Assuming the peak appears, do a series of blanks with increasing equilibration times between runs. If the contaminant is coming in via the carrier gas, you would expect the size of the peak to increase. If it doesn't appear at all, or if it doesn't increase, that points the finger at something related to the injection process (septum? syringe? vials?)

Thank you!

This is a good theory, but I found the cause, it was probably the solvent. The error was systematic and was present in all measurements.

The first measurements for practice were OK, now for further analysis a fresh solvent is used.