Am 16.09.2013 um 05:55 schrieb Christopher Maupin:

> As far as air peaks they are typically extremely small or absent, and well separated. Here's a typical carbonate chromatogram for a ~100 microgram sample:

What is the pre-peak at 189 sec?
Is this peak present in all samples, i.e. also in the samples that run fine?
Hard to say from the screenshot, but the ratio does not point to air (air should have an increased 46/44 ratio).
If it is CO2 - why is it there? There should be no peak. As this peak does not precede the other Co2 peaks I suspect the problem in the sample transfer or the needle, not in the sample vial or the MS.
Sometimes I saw something like that on a partly clogged needle with the first sample peak lower than the others.

You might try to increase the transfer time in the method to see if this peak disappears. If timing is bad than it can happen that the sample loop is not completely flushed by the sample gas.
Is the isotope value of the first sample peak (at 212 sec) is significantly different than the other?

If the pre-peak is not your problem and your erratic 18O values are related to the vial material (as you suspect; there was a discussion on isogeochem on this issue; check the archives) then you can do the following:
- Use a completely new batch of Labco vials and see of you still have the problem.
- Lower the temp to 25 C and react for 24h


Dr. Robert van Geldern
GeoZentrum Nordbayern
Applied Geosciences
University of Erlangen-Nuremberg
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