Chromatography Forum

I hope somebody can help me, i am new to GC techniques and i have been trying to quantify hexanal and other aldehydes in meat. We have PE HS 40XL connected to PE Autosystem XL. My first problem is even the standards (20 ul each) mixed with 10ml water do not produce peaks. My parameters for my GC is as follows: Equilibration is about 15 minutes, Constant Mode, 30 minutes thermostatting time, 5ml sample injection, 80 C oven, 90 C needle, and 110 C transfer. My GC FID is 200C and 200C column (30m, 250 mm id), oven program is 40C for 5 min, 15C/min to 200C hold for 18min. With all these parameters, there are no peaks coming out. Any suggestion please?

You are heating aldehydes at 80°C for a total of 45 min., right? I would not suggest these conditions.

You are injecting for 5 min? 5 min? isn't that a bit broad of an injection plug? Why aren't you injecting for 5 sec?

Are you placing the column up and into the transfer line into the HS 40? or do you have an actual transfer line (made out of metal) ?

Discuss in a bit more detail the type of vial, seal, and the amount of sample (water, meat) that you place into the vials. The details are where your problems lie.

best wishes,

Rod

hi Rod, thank you very much for your quick reply. My sample injection is by volume, not by time that's why its 5ml. Do you think it's too much? I tried 1ml but i got no peaks. Yeah you are right, my conditions I guess are not appropriate.

I heated the standards (aldehydes mixed in 10 ml water) to 90C, i havent analyzed meat yet because I wanted to check how my standards respond to the conditions I set. I have a fused silica tubing (2.5mx0.32mm) inside the transfer tube that is directly connected from the headspace sampler to the injector in the GC. I have a 22ml PE vial capped with septum and CRIMP cap, star spring, and septum. The cap is fitted tightly on the vial. I tried split and splitless but i am not sure about the split flow.

Please advice what parameters should I set.

Thanks again,
Hazel

First I would not heat the aldehydes at the temperature or the time you are presently using.

The FSOT tubing inside your transfer line is a good idea. But how is it connected to your injection port onthe GC? What headpressure are you setting on the HS 40 to get the linear flow rate? You should be checking your split ratio carefully, you could be splitting a million to one, so no surprise you are seeing no peaks !

I don't know why you are heating 10mL of water, why is that necessary?

At most you should be heating 0.5mL, perhaps even 0.25mL would be better.

I would put your aldehydes into dodecane or some other solvent which will not react with your aldehydes. Put from 1µg to 5µg of each into the empty vial when you make your stds.

But first set your linear flow rate and the split ratio. Do that by putting pentane or hexane into an empty vial (1µL) and injecting that onto your GC. Repeat injections from that same vial until you get a suitable split ratio and linear flow rate.

I have used the HS 40 and it is a fine instrument. I did direct timed injections, no split, and was very successful. Remember you have to have the pressure inside your vial at the same headpressure of your column or you will not get any sample injected. If you have a leak, NO PEAKS.

Good luck with your trouble shooting.

Rod

Thank you Rod, I finally got peaks. Your advice was really of great help. I come across a technique called "quadrupole mass spectrometer with ammonia negative chemical ionization and selected ion monitoring", do you have any idea about this technique? Where can I find help in setting this up? Thank you once again!