baseline jump - help

[afischer]

Hello,

Has anyone seen anything like this before...a strange jump in the baseline, consistently at 4.4 minutes. Please look at these chromatograms

Chromatogram of sample
http://img423.imageshack.us/img423/577/example4pa.png

Chromatogram of solvent injection
http://img409.imageshack.us/img409/6286/example20dg.png

I am running an Omegawax 320 capillary column with a temperature ramp from 65 to 225C. Hexane is the solvent and helium is the carrier. I have cleaned and sonicated the injector and detector, as well as replaced all replaceable injector and detector components. I am about to try another column and see if that changes anything.

Any input would be greatly appreciated. I have been stumped for weeks. Thanks

Andy

[pi3832]

I am running an Omegawax 320 capillary column with a temperature ramp from 65 to 225C. Hexane is the solvent and helium is the carrier.

What kind of detector are you using? FID?

What's the sample you're running?

If you make repeated solvent injections does the size of the jump go down?

[afischer]

Yes, I am using a FID detector and I am running a FAME sample in hexane. The jump has not gone down any yet.

[Radish]

Andy,

What happens when you run your program without injecting anything? Have you tried another source of hexane?

[afischer]

I haven't tracked down another source of haxane yet. I have injected a clean methanol and the jump disappears. Also, the Supelco column test mix runs smoothly without any jumps. I also ran a FAME sample that I processed with a different batch of hexane. The jump is still there but is smaller.

Possibly impure hexane or something on the column/injector that does not react well with the hexane?

[Radish]

Hexane is not a "reactive" solvent and there should be no interaction between it and your Omegawax column. Is your hexane a mixture of isomers, or could it have become contaminated with another solvent?

[Peter Apps]

Hi Andy

This could be a ghost peak left over from earlier injections, but it is not likely becuase it does not get smaller with repeated solvent injections.

I have seen very similar baseline humps when working with PTV inlets. Could you post details of your inlet conditions; temperature, split ratio, splitless time, type of liner, syringe operation etc.

Peter

[afischer]

Thanks for the reponse. Here are the conditions I am running under.

Gas Flows
He(Carrier): 2.0ml/min
H2(combustion): 32ml/min
Air: 385ml/min
N2 (make-up): 26ml/min

Split injection: 1-1.5ul
Split ratio: 1:25
Splitvent/inlet purge: 47 ml/min
Septum purge: 2.5 ml/min

Injector temperature: 250C
FID temperature: 260C

Column temperature program
65C for 0.5min
40C/min to 195C, hold 15min
2C/min to 225C, hold 15 min

Spit Injection sleeve cup design, packed w/deactivated GL wool

I inject the sample manually with a Hamilton 10ul syringe (I have tried different needles).

[chromatographer1]

afischer

Something is missing in this story.

How are you preparing your sample?

Are you taking FAME stds and dissolving them in your sample of hexane?

Are you making an extraction of FAMEs using hexane as the extraction solvent?

Are you using this hexane when you derivitize FA to make MEs of them?

Tell us the steps this hexane solution undergoes before you inject it.

I think then the Forum will be able to give you a good answer.

best wishes,

Rod

[afischer]

Hi Rod,

Thanks for the response. I am preparing my own FAME samples using a modified method of Folch et al (1957). Sample are suspended marine sediments, filtered onto a GF/F filter. The filter with the sample is ground up in a centrifuge tube with MeCl and methanol and lipids are extracted with a MeCl and Methanol wash, 3 times. The bottom organic layer is transfered to a new centrifuge tube. I then extract the FAME with Hexane from this organic layer, washing the organic layer 3 times with hexane.

Hope this helps.

[Lucap]

And where is the methylation step?

[afischer]

It is part of the last step...evaporate to near dryness ..add 1.0ml of BF3/MeOH...heated bath for one hour...etc.

[Peter Apps]

Hi Andy

OK, there is nothing that looks funny in your inlet conditions, so here goes with a couple of mildly improbable possibilities.

You have a very rapid temperature ramp at the begining of the programme. This might be giving some column bleed (or something similar) that then flattens out as the temperature rises more slowly. The solution would be to start the column at a higher temperature - because you are doing split injections the analytes get onto the column quickly (or not at all) and so you do not need a low start temperature to focus analytes coming slowly out of the inlet as they might with a splitless injection.

Some GCs have a gas saver function that reduces the split ratio a couple of minutes after the injection. Add another couple of minutes for the hump material to get down the column and you could have a baseline rise at about 4 min. If you have a gas saver, try turning it off. The real solution is not to play with the gas saver, because the hump points to a very slow departure of material from the inlet, which would call for some sample clean up. Given that the hump does not appear with methanol or test mix, heavy muck in the sample is the most likely explanation.

Peter

[afischer]

Peter,

Thanks for the thorough reply.