methane in seawater using headspace equilibration method

[Steve P]

Hello, I'm new to this forum so I'd first like to introduce myself. I work in an oceanography lab in Nova Scotia, Canada.

I'm setting up a method for measuring methane in seawater samples. I have an SRI GC/FID. Column is 2 m of 1/8" SS packed with HayeSep A, carrier is UHP N2 at 20 mL/min. Oven temp is 50 C. I have made some preliminary injections of air and methane standards using a 1 mL sample loop and have nice symmetrical CH4 peaks, retention time is 0.97 min.

This setup will eventually linked to an equilibrator for sea surface measurements and continuously cycling. I'm expecting that water vapour and other C compounds in seawater will eventually cause a problem with ghost peaks and baseline deterioration in a single column config. so I'm considering installing a pre column with backflush to vent to stop those compounds entering the main column. I'm looking for advice on whether this is the best way go and what would be a good adsorbent for the pre-column. I have been very impressed with a two column system in a Peak Labs H2 analyser under continuous operation (injections of equilibrated headspace). I think that they used Unibeads in the pre column and water vapour didn't seem to be a problem.

Thanks
Steve

[chromatographer1]

Your backflush column can be of several packings, but the easiest choice is the A polymer you have for the analytical column. The primary issue is polymer bead containment within the column. Supelco's integrated 5 or 10 micron screens are the best choice for long term use and ease of cleaning later when deposits grow at the head of the column.

Make sure you have enough flow to clean the bf column (2-4x forward flow). A 9-18 inch column should meet your needs. Also, Unibeads 3S, or a CWax on Chrom P are alternate choices.

Rod

[Steve P]

Hi Rod,

Thanks for the quick reply, that is most helpful. I wonder if Porasil B would be a straight swap for Unibeads 3S, we already have some Porasil in our lab.

Steve

[chromatographer1]

That is very similar. Yes, go ahead and use it.

Rod

[AICMM]

StevP,

I don't know if it is necessary because you are using a backflush column, but have you thought about using a drying agent in front of the sampling valve. I have seen several different versions of this which work well for this (at least for the applications I have worked with.) White Drierite for one seems to work very well.

Best regards.

[Steve P]

AICMM,

I had indeed thought about incorporating a small drying tube packed with magnesium perchlorate in front of the sample loop and I have used such a setup some years ago when making manual injections of headspace gas. The aim here is to have the equilibrator/GC/FID running 24/7 with minimal operator time so I thought I might be able to eliminate the drying tube by running a two column system.

Cheers

Steve