Hi AMMRLers,
We ran across a problem in our lab that you may want to be aware of. We were doing some trace analysis and found that one of the curses/blessings of a cryoprobe is its excellent sensitivity. Of course, this usually falls in the blessing category, but it sometimes turns up signals that catch you by surprise. On and off for years we had been detecting a very small signal at 8.459 ppm. Usually it was so small we just ignored it. With the higher sensitivity of our Bruker AV600 equipped with a cryoprobe, the signal became very easily visible. We thought that the signal was coming from our D2O solvent, but we were able to prove conclusively that it was coming from the Pasteur pipettes used to transfer the solvent to the vial in which the solution was made and then transfer the solution to the NMR tube. With a little detective work (spectral ID on a single peak is difficult), we were able to show that the peak was actually due to formate. At the same time we also determined that there was a second peak at 1.929 ppm that was also due to the pipette and was identified as acetate. The level of the formate was estimated at 5 ppm by comparison to the TMSP signal in the D2O which was present at 0.02% wt/v.
Not all pipettes are born equal. We found this impurity in two sources of pipettes but it was absent in those provided by two other sources. So, if you are using a Pasteur pipette in the process of looking at low levels of materials, we suggest that you wash the pipette before using it or at least run a blank to see if it is a problem.
Regards,
Joe
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G. Joseph Ray, PhD
Senior Research Scientist/Technology Resources/PCS
Baxter Healthcare Corporation
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Received on Wed Jan 18 2012 - 12:21:00 MST