One question concening the drift correction of hydrogen values.
Since our MS is not equipped with an automatic reference refill system
I observe a drift during a sequence of 50 samples measured for dD
using a Finnigan H/Device coupled to a delta S. The drift is the
result of a fractionation of the reference gas in the standard
bellow. To correct the data for this drift we carry out repeated
measurements of the same standard sample during the sequence.
Theoretically, the drift should follow a Rayleigh fractionation and
one should observe a (more or less) continuous drop of the measured
standard sample values. The sequence values can be corrected for the
drift by fitting a function to the data points of this "drift
standard". This is exactly what I observe in most of the sequences.
However, on some days the drift seems not to follow the theoretical
model. Instrument parameters (filling of the bellow, temperature,
etc.) are essentially the same as on the other days but in some
sequences I observe a convex shaped function instead of a concave
shaped Rayleigh function or a step-like function with a fast drop of
the values at beginning a stable phase for several hours in the
middle and a second drop at the end of the sequence. Memory effects
can not account for the observed trends.
I expect I am not the only one observing such a phenomena from time
to time. The question is how you handle this kind of data? The first
option is to discard the whole sequence. The second would be to fit a
(polynomial) correction function to the measured values assuming that
my data points represent the original drift of the gas (even if the
measured drift differs from theoretical assumptions). The third
option would be to fit a strict Rayleigh function (or alternatively a
linear regression) to the data points which results in a very bad
fitting of the function to the measured values.
Robert van Geldern
Leibniz Institute for Applied Geosciences (GGA)
S3: Geochronology and Isotopehydrology
Stilleweg 2, 30655 Hannover, Germany
phone: +49-(0)511-643-2313, fax: +49-(0)511-643-3665
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