Peaks tailing-problem on fatty acid and ester analyses

[July_M]

Hi everyone,

I’m working on varian 3900 GC with an injector system 1177 and FID detector. I’m working with hydrogen for carrier gas and N2 for the make up.

I have a tailing problem a fatty acid and ester analyses with polar and apolar columns on the GC system. I’m using this method for a long time now and this problem occurs overnight.

I have worked on several parameters:

I changed the column and put in place new one. I have also checked the ferrules and leaks.The detector and gas flow have been cleaned and checked.
I have also cleaned the injector with solvent, change the septum and insert.
The problem is still occurred.

Another point, I’m working with other analytical methods (notably, to quantify alcohol ) and I don’t have any repercussion of peaks tailing on that method.

Have you any advice for me?
I’m stuck with this problem for a long time now. I don’t have any more idea to test.

Thanks.

[Peter Apps]

Welcome to the forum.

Please post all your operating conditions; inlet temperature, column temperature programme, detector temperature, gas flow rates, split or splitless, if splitless what is the splitless time. What are the column dimensions and stationary phase ? What design of inlet liner are you using ?

When you say the "problem occurs overnight" do you mean that it works alright during the day, but runs at night have a problem, or that it works OK one day, but not the next ?

Does the tailing affect both esters and free acid peaks ?

What are you analysing - which FAs are your targets. What esters are you making, and how ?

The more you tell us the better we can help.

Peter

[Consumer Products Guy]

Please respond to Peter's questions, he is quite knowledgeable. But we need more information.

Myself: I've analyzed thousands of fatty acid methyl (and other esters) by GC, mostly in the 8 to 20 carbon range like used in soaps, detergents, and foods. I've used DEGS columns, PEG columns, but primarily I've used cyanopropylsilicone columns, both packed and capillary columns, with both helium and hydrogen carrier gases, and with both helium and nitrogen make-up gases.

1. Do you have a purchased fatty acid methyl ester standard mix, so you can ascertain that you actually are making the esters?

2. Exactly what are your preparation steps to prepare such FAME?

[July_M]

Sorry for the delay.
Thnaks you for your previous answers.

The problem occurred without any modification in the method or in the system. And since this time, I didn’t manage to obtain results on the quantification of fatty acid and esters. The tailing affects both esters and fatty acid. To the preparation, it depends of the raw material I have to analyze. Generally, I’m doing a saponification with methanol and KOH, after the methylation with a solution Boron trifluoride in methanol and I’m doing the extraction with cyclohexane. I can also see the tailing when I’m just diluted in cyclohexane for some raw material.

I don’t have any standard for fatty acid methyl ester. I have to quantify FAME to C12-C21.

I’m working with capilar polyethyleneglycol column as Stabilawax (30m, 0.25µm, 0.25mm) and with a capilar 100% Dimethylpolysiloxane column as EC-1(30m, 0.32mm, 0.25µm)
Injector/detector : 230°c
Columns temperature : 90°C to 230°C
Split 1/50e
Gas flow: 28ml/min N2, 30ml/min H2 and 300 ml/min air.
Inlet liner of 4mm with deactivated wool (78.5mm, 6.3mm).

Yesterday, I try to change the methylation method for quantify esters. But the peaks are still tailing.

[Peter Apps]

There may be fragments of septum or O-ring in the bottom of the inlet body, Changing the liner does not get rid of these. Remove the septum and top part of the inlet, and the inlet liner, and look down into the inlet cavity using a small flashlight. You might see particles in there. If you do the easiest way to remove them is to suck them up with a narrow tube on a vacuum pump.

For trouble-shooting you really need a test solution containing methyl esters or hydrocarbons.

When you install columns do you cut about 2 cm of the column end off after you have threaded it through the ferrule ?

Peter

[Consumer Products Guy]

Suppliers such as Supelco or NuChek Prep have fatty acid methyl ester mixes, I would get some of those to start. Or grab a bar of soap (plain old soap, not one containing lauroyl isethionate or other synthetic anionic surfactant and make methyl esters from that, dissolve in the BF3-methanol, heat on steam bath 5 minutes, go from there. You'd get C8 to C18 saturated esters, with significant oleic FAME too and some smaller unsaturates, tiny amounts of C15 and C17.

But I think that your issue is a GC one. The PEG columns I didn't like so much for these, and PEG columns can degrade from oxygen in the carrier gas. The plain-jane silicone type column you mentioned will will have the unsaturated FAME for each carbon number elute before the saturated FAME, not typically what is done in the industry, why I used cyanopropylsilicone columns like Rtx-2330, SP-2330.

But I still think column or inlet cleanliness/installation is the key factor for you.