Hi Anne,

As long as your samples and standards are subject to the same principal
treatment (i.e. seeing the same dilution) the wt% calculation should
work.  You would still want to build a standard curve with a range of
masses to get the most accurate equation.  

I compared elemental% with a TCD undiluted and IRMS, diluted.  With the
dilution off, the IRMS brought more precise estimates.  But with the
dilution on, IRMS estimates were typically less precise than TCD
estimates. Ideally you would calculate elemental% without dilution.
You'd have to see if the error with dilution is acceptable.  For me it
was up to +/- 1% IRMS vs. +/- 0.1% TCD.

The slightly greater error in elemental % when using the diluter could
be due additional sources of variability (physical actuation of helium
capillary and bypass valve, slight changes in gas flow, etc.).  I don't
think you could correct for these effects because they don't seem
consistent.

Regards, 

Ben Harlow
Washington State University

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
Behalf Of Fname Lname
Sent: Thursday, November 08, 2007 1:54 PM
To: [log in to unmask]
Subject: [ISOGEOCHEM] wt% calculation

Dumb question # 347.

Why is the wt% calculation incorrect when using the dilutor and how do I
correct for this in Isodat (or spreadsheet)?

Thank you, Anne
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Anne M. Cotter
U.S. Environmental Protection Agency
Mid-Continent Ecology Division
6201 Congdon Blvd
Duluth, MN 55804
218-529-5183
fax 218-529-5003
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