N2 as a carrier gas in Fatty Acid and essential oil analysis

[Chris Jones]

Any comments or recommendations?

We have used Helium in the past as a carrier gas with no make-up gas.

I'm not convinced that the make-up gas does anything anyway, but I will gladly be proven incorrect.

Regardless, our helium cyllinder is empty and a new cyllinder of ultra high purity He is about A$1300. Surely ultra high purity nitrogen will be cheaper and do just as good a job??

I'm doing essential oil analysis, and a colleague is doing lipid analysis on oilseed crops like Pongamia and canola. I understand they have a different column, but since we have N2 plumbing in place, could we not use N2 as a carrier?

Any comments or advice here?

Cheers,
CHRIS

[Peter Apps]

Hi Chris

Assuming that your are not using a mass spectrometer;

You can use nitrogen as carrier gas. Because the flow rate needs to be lower the separations will take about twice as long.

Hydrogen is cheap and faster than helium.

There is a lot of material in the forum archives, try a search.

Peter

[Chris Jones]

Cheers.

I did a bit of a search and it seems He is still used as H2 tends to reduce unsaturated oils, resulting in new peaks. N2 could be a good alternative despite the long run times, which are already close to 1 hour anyway.

So I guess my next question would be; does the $300 cylinder of HP helium do the same job as the $1300 cylinder of UHP helium?

[Consumer Products Guy]

So I guess my next question would be; does the $300 cylinder of HP helium do the same job as the $1300 cylinder of UHP helium?

I'd get the less-expensive grade of helium and a decent trap for your application.

[larkl]

Re - make-up gas. YOu can get by without it if your column flow is high enough. Without it and with low (true capillary flows), you'll see poorer sensitivity with no makeup.

[HW Mueller]

There seems to be a myth here which keeps reappearing. The unsaturated oils which will be saturated by H2 should be extremely rare. One generally needs to use catalysts to do this.

[Bricevan]

HW Mueller is correct. You will not have any problems using hydrogen as the carrier gas. I have many years experience doing fatty acid analysis on canola oil using hydrogen. In addition, official AOCS methods are written with hydrogen as the carrier.

[Bruce Hamilton]

The issue of hydrogen with unsaturated fatty acids originated in the days of stainless steel packed and capillary columns. The solution was glass columns, then fused silica capillary columns.

It apparently was a big issue for PUFA-containing fish oils. Note that it was not just hydrogenation, but also polymerisation and degration due to activity buildup. Some people mistakenly thought the increase in saturated FAs was due to conversion, but usually it was due to polymerisation and degradation of the unsaturated acid esters.

If you allow your injector/column to become active, you will also see a disproportionate reduction in unsaturated acids - even as "inert" esters.

I've been using hydrogen for FAMEs since 1984 on fused silica columns with no evidence of hydogenation, polymerisation or degradation.. It's still a very good idea to use a reference standard to confirm the system is working OK.

It doesn't have to be expensive, I used commercial menhaden and cod-liver oils for fish PUFAs, and borage and evening primrose for C18 oils. The OCS, and some suppliers, also offer standard mixes of FAMEs, but they can be expensive compared to just making your own from cheap triglycerides that have similar FAME profiles to your samples..

Please keep having fun,

Bruce Hamilton

[Don_Hilton]

How about hooking up nitrogen as the makeup gas and using helium or hydrogen) as the carrier. If I recall correctly (and it's been a while) you should get better response from an FID with nitrogen makeup gas.

The reason for the expensive helium is the background stuff in the cylinder. Helium is a byproduct of natural gas production - and the less background from hydrocarbons and other stuff that you are going to have in the helium, the more work you have to do. And, for UHP grades the supplier analyzes the filled cylinder - which is a good idea because I've seen one researcher use a handy, empty gas cylinder to mix up a mixture for an experiment - and then it went back to the vendor to be refilled.

Do you need the really expensive grace for GC-FID? Possibly not. I would look at going with a lower grade. I've never used UHP helium until I was working with mass specs.

[Bruce Hamilton]

How about hooking up nitrogen as the makeup gas and using helium or hydrogen) as the carrier. If I recall correctly (and it's been a while) you should get better response from an FID with nitrogen makeup gas.

{on helium carrier }
Do you need the really expensive grace for GC-FID? Possibly not. I would look at going with a lower grade. I've never used UHP helium until I was working with mass specs.

I should have commented on these..

My recollection is that instrument manufacturers recommend using Nitrogen as makeup for flame detectors with He and H2 carrier gases and capillary columns. Unless there is a special detector design that requires carrier gas as makeup, I would use nitrogen.

I used H2 carrier wherever possible - because it was the cheapest and would provide the fastest analysis.

I used technical grade carrier gases, along with suitable gas purification traps. You need to review the cost/benefit analysis, as prices of grades may be different.

If you only use small quantities of gases, cylinders of purest grades might be a preferable option to purification trap stream that may have to be individually regenerated annually with impure grades.

The purification modules I used were from Alltech - and the largest capacity available, and were:-
Carrier gases - Moisture ( Drierite/Molecular sieve ), Oxytrap, Indicating Oxytrap, Activated Carbon.
Detector gases - Mpoisture ( Drierite/Molecular Sieve ) Activated Carbon. The air was factory air that went through large scale driers and activated carbon before hitting another set of Moisture/Activated Carbon traps.

I also fitted a shutoff toggle valve on the instrument - so gas lines were always pressurised, and I didn't mess with the temperamental instrument controls for detector gases.

Please keep having fun,

Bruce Hamilton

[Chris Jones]

Thanks guys and or gals,

Great advice.

Cheers,
CHRIS