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
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
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
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.
| run number|| 1
7 || 8|
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
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.
Paul D. Brooks,
Center for Stable Isotope Biogeochemistry,
Dept. Integrative Biology MC3140,
3060 Valley Life Sciences Building,
UC Berkeley, Ca. 94720-3140.