I suspect variable memory to be the cause of the observed raw dD
fluctuations. We have been struggling with memory in water analysis and
it seems to vary quite a bit. There is changing memory in the syringe we
use to inject into the TC/EA sytem. We see the same with the H-device.
There is also memory associated with the chromium, the quartz and,
probably most importantly, with the surface that the water can interact
with before reduction.
In our sequence runs we therefore always include a large isotope jump
(>100 per mill for hydrogen) and follow the fate of the measurements for
about 6 analyses. The memory correction (usually around 2-4 % of a
mixture of the preceding measurements) is then modelled (e.g 2% of the
previous analysis + 1% of the one before that) to correct the jump in an
optimal way. The correction is then applied to all samples.
The memory correction is applied prior to a drift correction that can be
attributed to several drift sources including depletion of reference gas
in the bellows (H-Device).
The last step is the scaling to a given difference as mentioned by Paul
Paul Brooks wrote:
> 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.
> -360 -79 difference -281
> -354 -65 difference -289
> 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
> difference -2.7 0 -2 -2.5 -2.9 0.2 -2.4 0
> 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
> 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.
> [log in to unmask]
> phone (510)643-1748,
> FAX (510)643-1749
Willi A. Brand, Stable Isotope Laboratory [log in to unmask]
Max-Planck-Institute for Biogeochemistry
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