Discussions about GC and other "gas phase" separation techniques.

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Dear Chromoatos,

I am a PhD candidate and my project involves trying to develop an electronic sensor to detect and quantify volatile fatty acids (acetic, propanoic, butryic). My experiment involves bubbling nitrogen through dilute aqueous acid samples and measuring the electrical response of the gases.

I am trying to utilize GC methods to calibrate, quantify and verify my results. The equipment to do so is available to me within my university but there are no experienced users or training available to me.

The GC is a varian 450 with autosampler and the column i have been using is a ZB-FFAP (Nitroterephthalic Acid Modified Polyethylene Glycol) 60mx25mmx25um /w 15m column guard. I also have a ZB-waxplus column available to me, but haven't tried it yet.

For now I have been trying to measure a liquid standard that we made consisting of ranging from ~1mM of each acid up to ~0.8 M, diluted in DI (mili-q) water, Although I think it may be necessary to do headspace type measurements in the future (and compare it to a direct injection from my bubbling experiment?).

I have been using a method developed by a previous user a few years ago, that was based on EPA wastewater testing methods seemed to agree with published literature, the parameters are;

Injector: 200C, Detector(FID): 350, Oven 100 for 2 min, 8C/min to 180 and hold 180 for 10 minutes. The injection is pressure pulsed at 10 psi to homogenise the sample. carrier flow is N2 at 20-30 (~1.6mL/min) cm/s, 1:50 split, 1uL injection, i think approximately 30 cm/s H2 and 300 cm/s air for the FID.


The results so far have not been successful, in the image are two samples; one at 3mM at 0.8M, the responses are barely above noise levels. Obviously something is wrong but due to my limited experience I am at a loss of where to start. (I suspect the baseline drop at the start of the test is from the pressure pulse)

I am not sure if this is;

a solvent issue, in the previous method as i have also seen online methanol, not water was used at the solvent

a column instalation issue, i have read low peak formation like this could be as a result of the column being inserted too far away from the FID

a column maintenance issue, the column has previously been used for waste water samples, the same guard was used and has been stored in open air for ~2 years.

Any guidance, advice or insight you could provide would be invaluable to me, i am truely desperate here.

Edit:if this is, as I am beginning to suspect a solvent issue (I plan to test a different solvent on Monday) what options do I have to eventually test my experimental headspace sample ? Would a GC with mass spec be the way to go?
I work a long time with the equipment and as I checked your chromatogram the same is appearing only noise.
Do you know the configuration of the injector? Which liner is installed?
I would make the first changes.
1 - Change Liner.
2 - Cut the column and adjust the correct lengths on the injector and detector.
3 - Make a solvent injection and make the calculation of noise and Drifit.

If you need help I can help you.
Sorry, my English is not very good.

Ítalo Sangaletti
Service and suporte Brazil.
Engineer Specialist in chromatography, ICP-MS, AA, TOF, Maldi-TOF, SQ, TQ and Iontrap.
2 posts Page 1 of 1

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