Dear List Members,

If anyone could give some advice on the following, it'd be appreciated...I
am sure they are common questions after a new installation.

We have a new Delta-V and gas bench that we are using mostly for carbonate
digestion and water, and I am curious about drifting values for C and O
within sample runs.  We usually run 64 samples in a batch which includes 14
stds.  

What I observe is the C and O values in each run systematically lighten by
0.1 to 0.2 per mille w/i the run...say from 2.05 to 1.85 max for NBS-19
13-C...with a similar trend for oxygen.  By the next std, the first pulse is
back at the heavier value...so there is no trend over all the stds, just
within runs.

We have been able to correct this trend by using a ref peak at the beginning
and end of each run...is this cheating?  Any ideas on how to fix the drift
or why it occurs?  We are small internal and external errors over all the
stds, but just want to know why the drift is there.

Also, hope I phrase this correctly, but since our ref gas is anchored to
NBS-19, re are running a diff std to correct for samples that vary a lot
from NBS-19 values.  I have been using both LSVEC and NBS-18, but having a
lot of problems getting consistent 18-O values with precision...is this a
heterogeneity problem with these stds?  

Any advice is appreciated,

Cheers, Dave Mrofka

Earth Science Dept.
UC Riverside