Chromatography Forum

Good day all.

I need some help, I don't know if I'm missing something. I set up a standard curve on the GC-FID using the FAME-37 standard. When I do analysis of an internal standard I get roughly 60% of the expected concentration. This is the case for many concentrations, not just one.

Does anyone know what I can do to resolve this?

1. What are you using as internal standard?

2. Is the 60% of the expected concentration on a sample? If so, is the sample triglyceride, fatty acid methyl ester, fatty acid, or fatty acid salt?

3. What is the sample preparation? How is the internal standard added?

I am not getting CO2 peak after methanizer by FID ( baseline is shifted and not coming back to originsl position)but i am getting CO and CH4 peaks.
Please advice

Thanks for the response. To answer your questions:

1. The internal standard I'm using is Undecanoic acid. The only fatty acid that I have never detected in our samples.

2. The 60% of the expected concentration is both present when the internal standard is analyzed by itself and with samples. The fatty acids are extracted from microalgae.

3. The fatty acids are extracted with a dionex ASE350 and then derivatized with 5% H2SO4-Methanol. I have added the internal standard to the algae before extraction to ensure complete extraction and to the extracted fatty acids before derivatization and the 60% result occurs under both these procedures.

... I need some help, I don't know if I'm missing something. I set up a standard curve on the GC-FID using the FAME-37 standard ....

Did you choose right calibration type ?

Yes, I believe that I have selected the correct calibration type.

You're using the Supelco 37 FAME mix that's provided as a solution in dichloromethane? So I understand - you inject that standard mixture, which is 200 ug/mL C11:0 FAME, then prepare the same concentration of C11:0 FAME in the same solvent, and the response is 60%?

Yes, that is correct.

How are you preparing your dilutions of C11? In detail please, the more you don't tell us the more we can't help you.

Peter

How are you preparing your dilutions of C11? In detail please, the more you don't tell us the more we can't help you.

Peter

Yep - that derivatization process is pretty crucial for FAMEs, temps have to be right, extraction has to be done carefully...

How are you preparing your dilutions of C11? In detail please, the more you don't tell us the more we can't help you.

Peter

Yep - that derivatization process is pretty crucial for FAMEs, temps have to be right, extraction has to be done carefully...

But if I'm understanding the issue correctly, this has nothing to do with derivatization. Read my response, which explains the basics. Agree with Peter - please provide exact specifics for:

1) What is being done with the Supelco FAME 37 mixture (injected neat? If diluted, with what solvent?). As that mixture is a solution in methylene chloride, which is EXTREMELY volatile, how carefully has it been handled?

2) How is the solution of C11:0 FAME being prepared.