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Stable Isotope Geochemistry

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Subject:
From:
"Anthony M. Faiia" <[log in to unmask]>
Reply To:
Stable Isotope Geochemistry <[log in to unmask]>
Date:
Fri, 8 Jul 2005 12:20:47 EDT
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Bill,

We have been running lake waters for DIC with concentrations around 15ppm C. It is based upon the Gas Bench DIC procedure for ocean waters and gives reproducible results (std dev=0.15 permil). Our procedure differs in that we inject more water than for ocean samples. We use 5mL and also remove some of the helium from the exetainer before injection of water. The removal of He gives a better response for the mass spec and makes it much easier to inject 5mL. You might be able to get away with a lower He pressure when flushing instead or evacuate the containers instead as Noah suggested. The detailed procedure follows.

1. Prepare gas bench vials for sampling.
        add 150uL of phosphoric acid (H3PO4) to vial
        cap vial
        flush vial with helium using Gas Bench for 5 minutes at 100mL/min flow

2. Sample water.
        rinse 15mL syringe with sample
        fill syringe with 6mL of water
        keep syringe vertical and remove 5mL of helium from vial
        inject 5mL of water sample into vial, leaving 1mL of water as air lock in syringe

3. Analyze DIC.
        put samples in Gas Bench tray at 25ˇăC
        wait at least 1 hour
        run mass spec sample sequence for measurement of DIC



--- You wrote:
Hi all,

Can anyone direct me to a reference (or give some advice) on using the gas
bench to analyze DIC in ground waters? I have a student who would like to
make some measurements this summer, but we have not tried this on the gas
bench and would prefer not to reinvent the wheel.

Thanks,
Bill
--- end of quote ---

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