We have see a similar effect here using a FM H/Device, and I know another researcher using a Micromass continuous flow system who has a similar effect.

I assume the results you show below are then normalized to the correct value of the standards.  However, that means that the difference between them should be the same.  Your data shows the difference between the standards has changed from -281 to -289, an 8 delta difference when I presume the precision of your system is better than 1 delta unit.


I assume that most analysts are using a two point calibration for D analysis as recommended in:

Brand, W. A, T. B. Coplen.  2001.  An interlaboratory study to test instrument performance of hydrogen dual-inlet isotope-ratio mass spectrometers.  Fresenius J. Anal. Chem. 370:358-362.

Doing a two point calibration should take care of any change in each standard if it is consistent over the course of the analysis run.  However, what seems to happen is that during the course of an analysis using chromium,  if one calibrates with one standard, the other standard of a different isotope ratio does not always drift or change at the same rate.  I calibrate with a +3.5 standard and then, if analyzing samples from 0 to -80 which is our usual range, us a -95 standard for the two point calibration.  I was originally using two standards so close together because memory effects were a problem from the syringe in the auto-sampler, a problem we have now overcome.

However, just calibrating with the 3.5 the -95 standard would change during the course of a 23 hour 100 injection analysis, as shown below.

   run number  1    2  3  4   5   6   7     8
beginning of run-96.3-95-96-96.2-96.4-94.9-96.2-95
end of run-93.6-95-94-93.7-93.5-95.1-93.8-95

One can see here that -95 standard has a tendency to be more negative at the beginning of a run than at the end, but not consistently. I eventually created a a spreadsheet that fits a curve to both the 3.5 and -95 standards, and then for every injection does a two point fit between the 3.5 and  -95 curve.  Quality controls since I started this procedure have been excellent, better than plus or minus 0.4 delta units long term external precision.

I was able to modify my spreadsheet to work with 30 hour analysis runs that a colleague was doing with a Micromass continuous flow system and was seeing the same effect.  Therefore the effect would not seem to be from the instrument, but some effect of the chromium.

I would be interested in any other researchers who have seen a similar effect. 

Paul Brooks.

At 10:43 AM 12/5/03 -0800, you wrote:
I use IsoPrime to run dD of water by Cr reduction method. The raw values of the working standards sometimes fluctuate day by day. The values of  -360 and -79 for the first day can be -354 and -65 the next day. There is no big change of the machine condition except a slight shift of peak center. However, the calculated dD values of the repeated samples are perfectly match from day to day. It seems not a real big problem. I will feel better if more people telling me they have the similar experiences.
Dachun Zhang

Paul D. Brooks,
Center for Stable Isotope Biogeochemistry,
Dept. Integrative Biology MC3140,
3060 Valley Life Sciences Building,
UC Berkeley, Ca. 94720-3140.

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