We put a piece of plastic wrap over the top of the tray, and tape it
down so it is taught. The sampling needle has no trouble piercing it.
We'll shake or spin down the tubes before setting up the analysis--and
it's good to go. There can be problems after running carbonates or DIC
samples. IF a little phosphoric acid gets into the line, it will suck
in some of the water vapor which will lead to a clog--so I try to rinse
the line before switching to water.
The Isogeochem wiki has a how to for making sampling needles, and others
have simpler ways to make needles too (epoxy--but can't seem to find
publication off hand)
Just a reminder, condensation, as long as it does not clog the line,
doesn't cause big problems for 18O from CO2-water equilibration in the
closed system. However, the Pt only does the H-H2O exchange with the
water it sees, and it doesn't see the water condensed on the cap, so
that leads to problems. As Marilyn said, TC/EA better for D/H
(H-Device/chromium reduction works well too)
p.s. Press-fit connections work in the transfer line and remember not to
tighten caps too tightly--more likely to clog needle...
On 10/31/2016 1:16 PM, Raquel Mingote wrote:
> Hi All,
> We are using the equilibration method H2/CO2 - water with GasBech (CF-IRMS) for d18O and d2H in water. USGS LIMS is used for drift correction, normalization and data management.
> We observed that the sample condenses in the septum, which can increase the carry of water to the GasBench. The temperature of GasBench is 28 oC and the room temperature 24 oC. We are working with the minimum of 18 hours equilibration.The background m/z=18 is increasing to high values, until 15000 mV after the run. Usually we are carrying the baking of the GC Column (120 oC) ,as suggested by paper of Rangarajan and Ghosh (2011), after the batch of samples. We are monitoring the background of m/z=18 before and after the run for d18O.
> We appreciate all comments about this issue.
> Rangarajan R. and Ghosh, P.; Role of water contamination within the GC column of a GasBench II peripheral on the reproducibility of 18O/16O ratios in water samples. Isotopes in Environmental and Health Studies, v. 47, n. 4, 498-511, 2011.
> Raquel Mingote