Chromatography Forum

I am performing analysis of lubricants formulated from animal and vegetable oils using a gc/FID with a packed column. My current column phase is 3% Dexsil 300. My problem is that, compared to most of the oils we have, the column is relatively insensitive to the highly refined peanut oil that is a key component of one of our manufacturing processes. Can anyone suggest a different phase or setup that would be more sensitive to refined peanut oil?

What do you mean by insensitive? Also can you give a bit mroe information about the GC conditions?

You are using a polycarboranesiloxane packing which allows extremely high temperatures and in your case the elution of triglycerides.

You might change your analysis to the determination of fatty acid content but that will be a drastic change. You don't want to do that I would guess.

PDMS phases don't tolerate the temperatures you are using, not for long anyway.

I do not know any phase better suited to your purpose as you are doing the analysis presently.

Size exclusion LC might be a solution but then you will lose the history of your analysis comparisons.

Good luck,

Rod

We use metal (not fused silica) capillaries for triglycerides, designed for large molecules and high temperatures. See Quadrex, Phenomenex

But CPG

I remind you that I said "PDMS phases don't tolerate the temperatures you are using, not for long anyway."

There may be issues of sample decomposition with the matrix and the temperatures which I can suppose but which he has not disclosed.

I suspect we are talking about temperatures in excess of 400°C. PDMS chemistry does not tolerate these temperatures for any length of time without being destroyed.

best wishes,

ROd

More Information:
The maximum temperature my column sees is 395 C.
Thre are no peaks evident after 300 C.
I am not nearly as knowledgeable about GC as I would like to be. The comments I have seen already are helpful.
I have the hardware to switch to a direct injection capillary column. If there are some good options there, I would like to know what they might be as well.

Most PDMS phases are limited to 360° and if temperatures above that are encountered they degrade.
Si-O-Si-Me bonds have a temperature limit.

Capillary columns are sample size limited before overload conditions are reached. I would suggest 5-15 meter lengths and about 0.15 µ film thickness for minimum coverage of the fused silica surface.

Packed columns are CHEAPER, last longer with impure samples, and have much larger sample loading capacity.

This may translate to lower detection levels or it may not. It may be the sharper peaks seen with capillary columns more than compensate for the smaller sample capacity, or not.

I believe there are vendors who have the carborane phase columns. I have no recommendations as far as if they are suitable for your application.

good luck with your work,

Rod

What inlet temperature are you running? You will not see much elute above your inlet temperature.

My injection port is set at 300 C., FID detector at 200 C. The manufacturer says that the Dexsil phase I am using has a max temperature of 400 C which I do not believe I have exceeded. I did condition the column at 395 C prior to use.

I would increase the FID to 300°C if not 350°C

I have heard of folks using 425 as a max limit but follow your vendor's recommendation.

If you have any oxygen in your carrier you will (or already have) damage your phase and make it reactive to the triglycerides, which can cause their peaks to disappear.

Good luck,

Rod