The directly measured D/H values can be good, ca. +/- 0.5 per mil, but then the correction to the water D/H values means that error will be around +/- 2 per mil. If the on-off pulses of the reference gas are giving good results, then its the equilibration that's causing problems. I've taken to soaking our Pt sticks in 0.1 N nitric for ca. 30 min, then lots of rinsing/soaking with nanopure water. before letting them dry. Test with your setup--I'm not too sure how well the glue holding the Pt beads onto the stick holds up.
[log in to unmask]">Hi Dominic -
We used to make these measurements on the GB using the Pt rods, but have moved over to the TC/EA because the precision is better. When things were running well, we could achieve ~2 ‰, but it was not uncommon to see some really unacceptable variability (up to 5 ‰ or more). We found that baking the column prior to a run really improved precision. Funky chemistry (salty, high organics or sulfide) has been attributed to variability and the gasbench manual suggests adding some Cu wire and/or activated C. We did this because we run some funky waters, but were not convinced it made a measurable difference - it made us feel better though.
I don't remember what water backgrounds did, but I don't think the increases in m18 that you observe should be a problem for the d2H measurement, but maybe someone else knows differently.
It is probably normal that your peak intensity drops as much as you observe during a run due to dilution by the He flow. But 3 ‰ variability within a run is too much (I think we were pretty happy when it was 0.5). But we were running at much higher sample amplitude - ours dropped from ~9 V to 7 V over ten peaks.
best of luck sorting this out.
Dennis L. Newell
Associate Professor of Geology
Department of Geology
Utah State University
4505 Old Main Hill
Logan, UT 84322
From: Stable Isotope Geochemistry <[log in to unmask]> on behalf of Dominic Ángel Becerra Serrato <[log in to unmask]>
Sent: Saturday, September 7, 2019 2:25 PM
To: [log in to unmask] <[log in to unmask]>
Subject: [ISOGEOCHEM] Problems with high deviations in H2 results.
Hello everyone, we´re running deuterium measurement tests with GasBench and so far we have not been able to obtain the desired results.
The deviations are very high, even among the ten peaks of the same sample (between 0.8 - 3 ‰) and also when we compare the results of different vials containing the same sample (we´re introducing 5 vials per sample) the deviations reach values between 1 - 5 ‰.
We´ve noticed that after finishing a test, the level of mass 18 increases considerably. For example, we started running the test with 1400 mV in mass 18 and at the end of the test the intensity increased to 3700 mV. Could this be affecting the deltas? If this is the case, how can we fix it?
We also observe that the intensity of the 10 peaks decreases significantly, around 50% between the first and the last one (4500 mV to 2300 mV). Is it normal for it to fall so abruptly?
We would greatly appreciate your support. Regards.