Why the D47 is always higher (~0.1 permill) than expected for the first sample measured everyday ?

28 Sep 2019

Hi! everyone

 

We have this problem for very long time. We routinely measure six sample (include one or two standards) every day. We will bake out our off-line after we finished the measurements. After the long baking time, the off-line should be cleaner than before. However, the Delta 47 value of the first carbonate standard (like Carrara marble) processed on the off-line every day is always higher than expected. We recollected the gas from the 253Plus and cleaned it again. Interestingly, the D47 value of the sample becames normal and this  situation is reproducible. Note that there was no significant change on d13C and d18O value after twice treatments on the off-line. If any one of you have this situation before, please let me know the possible causes(s).

Best regards

 

Yangrui Guo

State Key Labortary of Isotope Geochemistry
Guangzhou Institute of Geochemistry
Chinese Academy of Sciences

Cedric's picture
Cedric
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Joined: Feb 6 2011
Have you checked the 49

Have you checked the 49 parameter? Is it high for the samples wiht high mass 47?

We have not had this issue, but I wonder if your problem has to do with either (1) a subtle leak in your vacuum line or (2) the fact that you may not effectively pump out the contaminants liberated from your trap during the nights bake out. The fact that it is systematically the first sample after a few hours of baking on the trap overnight makes me tink that (2) is not unlikely.

One thing you can do to check this is to eliminated the acid bath, and run a clean gas through the trap as your first sample. Does it come with high contamination parameters?

Cedric

Yangrui Guo's picture
Yangrui Guo
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Joined: Jun 26 2015
It seems like we need to

It seems like we need to install an oil mist trap on our vacuum line.  Otherwise, we need to close all valves or only isolate the Porapak trap during the long time bake out. Only in this way can we solve the problem.

Yangrui

Yangrui Guo