Chromatography Forum

We have some hydrocarbons to analyze C12 to C20, mostly C17, C15 that our chemist wants us to analyze using GC. We currently use GC for biodiesel analysis and we use direct injection. What kind of method should be used for analyzing hydrocarbons? Do you need a splitter? What kinds of standards are used, if internal standard methods are utilized? What kind of column/length/diameter should be used? What kind of stationary phase?

thanks all!

You are using a 0.32mm ID or 0.53mm ID 10 to 14 meter 5% Phenyl capillary column for biodiesel analysis.

You can use it for the hydrocarbon analysis. You will only have to dilute your HC sample 10:1 to 50:1 in pentane, and inject 0.5µL or so.

You can slow your oven ramp down but still start at 50°C but with a longer initial hold if you wish.

Your oven rate can be 2°C or 4°C/ min and you will have an excellent analysis and an inexpensive solution.

If you need more plates (and I doubt that you will) use a split injection and a Supelco Cat# 24282 100 meter 0.25mm ID 0.5µm Petrocol n-Octyl methyl silicone (or a Supelco Cat# 24133-U 50 meter 0.2mm ID 0.5µm) if you wish to duplicate a Squalene capillary selectivity as closely as possible. You could also use a 100% methyl silicone, or even a Squalene SCOT column from Supelco Cat# 25535.

Only two weeks until retirement !

best wishes,

Rodney George
Senior Research and Development Scientist
Gas Separations Research
Supelco
595 North Harrison Road
Bellefonte, PA 16823

814-359-5737 voice
814-359-5459 fax
rodney.george@sial.com

Thanks for the response! Could I dilute it in heptane instead of pentane? That's the solvent we have available right now.

Do I need a splitter? Doesn't seem so. Do I need a silyating reagent? As volatile as the hydrocarbons are I hope I do not need that either

Thanks!

Em

You can try heptane.

I assume that your sample is hydrocarbons: dodecane through eicosane

as you described earlier.

No you do not need a splitter if you dilute your sample adequately and reduce your injection to as small a volume as you are capable.

No reagent is needed as the hydrocarbons require no derivitization.

This will be easy as long as you keep your injector cool initially and allow it to track with the oven, just as you do with your biodiesel samples.

best wishes,

Rodney George
Senior Research and Development Scientist
Gas Separations Research
Supelco
595 North Harrison Road
Bellefonte, PA 16823

very good, i will try heptane (yes the hydrocarbons: dodecane-eicosane for sample I'm analyzing)

thank you rod, congradulations on your retirement.

Rodney I'm going to milk you for everything you got before you retire

What is the best temperature program to use for this? I tried the 15 deg. C/min up to 180 deg. C (to drive off the heptane) then 2 deg C per minute up to like 280 deg. C, held it there for five minutes, then 2 deg C/min to 325 held for five minutes.

The entire sample had eluted by the time I got to 160 deg. C. I got a couple of very broad peaks (aside from the solvent of course) and a couple of small peaks. The resolution was not very good as they were very close to each other, and it looked like I had some split peaks also.

The sample, to clarify, is actually mostly saturated and unsaturated C15 alkanes/alkenes.

What is the best temperature profile to analyze this? Also, for direct injection should I utilize the same linear velocity of the carrier gas for this analysis?

Thanks

Milk, huh?

Did you grow up in the city? not on the farm? Ya got to find a heifer for milk, really, ya do, no bull !

My thoughts?

You may be injecting too much sample for this column (remember it was designed to look for trace impurities in a balance of components that don't require good separation). It is a very thin film and you may only be using 0.32mm ID. So I would reduce even more the sample size and/or increase the dilution. I would hold the initial temperature longer too if you can to help refocus the higher boiling point components.

If that doesn't work (it was worth a shot, wasn't it?) then plan B:

then use a longer column with a thicker film if possible.

Or plan C:

split the sample with the original column and adjust your temperature program.....

or do both B and C.

Ain't research FUN ! I know I will miss it.

best wishes,

Rod with 1 week to go

Well, I tried. My guess is I"m going to have to get a dedicated column for this method. Ideally, I'd have both a splitter/dual column. Unfortunately, we are low budget

Can you tell I'm a city resident?

Thanks anyway for your help!

Bulletin 868A from Supelco discusses hydrocarbon analyses with chromatograms.

good luck,

Rod

perfect! thanks