How to calculate yield and conversion from peaks in GC?

[rtrt228]

Hello,

I am a student and I need to use this GC for some basic experiments. Although i have understood how it works, and the principle behind the technique, I am not able to get good answer that how can I get yield and conversion from the peak area and retention time in GC?

I have access to a SRI 8610C GC with TCD and FID detectors. After reading some posts here in the forum I learned that I need to first get the standard peaks, from which I surmised that I need to inject pure sample of each and every component that I am expecting to see in the mixture of my test sample, right?

And once I save them, and then inject my test sample and then what?

Could anyone please guide me through the steps to be followed? In case, there a post and I missed it, kindly direct me there. I will really appreciate your help.

Thank you.

[Peter Apps]

Hello,

I am a student and I need to use this GC for some basic experiments. Although i have understood how it works, and the principle behind the technique, I am not able to get good answer that how can I get yield and conversion from the peak area and retention time in GC?

I have access to a SRI 8610C GC with TCD and FID detectors. After reading some posts here in the forum I learned that I need to first get the standard peaks, from which I surmised that I need to inject pure sample of each and every component that I am expecting to see in the mixture of my test sample, right?

And once I save them, and then inject my test sample and then what?

Could anyone please guide me through the steps to be followed? In case, there a post and I missed it, kindly direct me there. I will really appreciate your help.

Thank you.

Welcome to the forum.

I don't want to sound churlish, what with it being your first post, but these really are the sorts of questions that, as a student, your supervisor / lecturer / practical demonstrator should be able to answer in a lot less time that it will take people on the forum to type it all out. They get paid to teach you - we are all doing it for nothing.

If you are genuinely stuck, and have asked the questions and got no answers than come back for another try.

Peter

[Don_Hilton]

Yield and conversion are the easy part of your question. Once you know the concentration of each compund in the mixtures, it is an easy caclulation. The hard part is that you need to:
- know how to oprerate the GC system in a way that you get reliable answers
- ensuring the instrument is properly set up
- performing routine maintanance
- properly introducing samples to the instrument
- performing QC checks
- understand how to make up standard solutions to generate a calibration curve
- be able to develop a separation of the various components in the mixture
- select and internal or external calibration method (and know why you selected one or the other)
- propery sample the reaction mixture
- use the instrument software to properly integrate chromatograpic peaks
- develop a calibration curve using the instrument software or a spreadsheet and then obtain analytical results for your samples.
- for good research work, you may need statistical analysis of your data to ensure that variability in sampling and measurement do not lead you to unsuportable conclusions. This may be somethign that you do or someone else does.

Some of these items take specific knowledge of the particular type of instrument you are using. Others are things that you would be learning in analytical chemistry classes. Together it is a lot of material.

Where do you stand with these topics? Let us know which things you know how to do - and for the rest, we can help you with details. In other things, you need to have someone look to see what you are doing to be sure you are doing it correctly, such as weighing a sample, installing a chromatographic column, or integrating a chromatographic peak.

[rtrt228]

Hi Don,

First of all, thank you very much for the much needed detailed reply. I am now grateful to get your help.

Out of the things you have listed I dont know (or have less knowledge) about the following things:

-be able to develop a separation of the various components in the mixture
-select and internal or external calibration method (and know why you selected one or the other)
> I have what they are theoretically. Internal cali. method you add known substance at a constant concentration to all standards and samples. And that is always present at a const amt. While in external cali.one can get the concentration of the unknown by = [(area of unknown)/(area of known)]*(concentration of unknown)
> correct me if I am wrong.

- So, overall, getting the concentration is also more of a guessing game for me since I am uncertain about the correct method to be used.

- I think I have good grasp of the instrument in terms of its operating mechanisms, its parts and their functionality, importance, materials and safety.

But as I have mentioned earlier, the problem is getting the results and doing correct analysis.

Thank you again for helping me out.

[Don_Hilton]

A good part of developping methods comes from familiarity with the instrument and how it responds to changes in condition. Thus, you need hands on time with the instrument before you do method development.

You know how to do the math for calculating results for an interal standard. Picking one involves knowlege of the analytes of interest and the chromatographic system you are using. (I think that selection of internal standards has been discussed in the forum before. And, I made some comments.)

You say that you have access to the instrument. I presume that it is an instrument that another person or group owns. Can you get them to help you develop the separtion technique?

Developing analytical methods (correctly) is not trivial. Doing just that function has been my paycheck for a good part of my working life.

I would suggest that if you know how to get a chromatogram out of the instrument, step one is to get a good chromatogram for the starting mixture and a reaction mixture. You will need to be sure the mixture is in solution - which may requrie adding a suitable solvent. You will need to select inlet and oven starting temperatures and oven ramp rate. These depend on the solvent and what you expect to find in the mixture. The injection will be split or splitless, depending on the conentration of the samples - split vs. splitless has been discussed on this forum as well.

While you may be able to use any of several chromatographic columns, you need to select something appropriate and start with it. This also depends on the kinds of compounds you are separating. The chromatographc peaks must be adequately separated and you need to be able to identify the various components of interest to you.

If you can take that paragraph as guidance and come back with a couple of chromatograms, we're off to a start. If this needs further explanation, you need someone there who can give you instruction. Just an overview of the first goal is two paragraphs. And taking it to the next level of detail heads in the direction of becoming a book chapter.

[rtrt228]

Hi Don,

Thank you for your advise. After your last reply I did some work on my GC. I was able to get good chromatographs.

I have attached the results for two runs: one is before the reaction and one is after the reaction. Do you think I can calculate, just based on the area counts, the conversion of C2H6?



Thank you,
Rajan

[Don_Hilton]

The signal on the FID shows an decrease in area going from before to after the reaction, but the TCD shows a greater resonse for after the reaction. So, there is something else going on here that is confusing things. I can think of one or two possibilities, but without knowing how the experiment is done and what chromatographic conditions are being used, I may be thinking of things that are entirely irrelevant.

On the other gasses, without knowing response factors for the other gases, you can not say much about conversion from C2H6.

To say anything about how much of some other gas was generated from the ethane, you need to have some information about response factors, which requries running some kind of calibration with all of the gasses involved.