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IsoSource: stable isotope mixing model for partitioning an excess number of sources
IsoSource version 1.3 is a Microsoft Visual Basic™ software package which calculates ranges of source proportional contributions to a mixture based on stable isotope analyses when the number of sources is too large to permit a unique solution (> number of isotope systems + 1). [It may also be used for other non-isotopic tracers such as concentrations of trace chemicals.] Examples include partitioning of pollutant sources in a waste stream, food sources in an animal’s diet, water sources for plant uptake, carbon sources in soil organic matter, and many others. This software provides the distribution of source proportions which are consistent with isotopic mass balance. See Phillips and Gregg (2003), Source partitioning using stable isotopes: coping with too many sources, Oecologia 136:261-269, for further details about the calculations. A link to a PDF file for this paper is provided below. PDF files can be read and printed using Adobe Acrobat Reader™ , which is available for downloading free of charge at www.adobe.com.
The user supplies the isotopic signatures (e.g., atom %, isotopic ratios, or d values) for the mixture and each of the sources. The user also supplies the source increment (e.g., entering a value of 1% specifies examination of all possible combinations of sources contributions from 0-100% in increments of 1%), and the mass balance tolerance (e.g., entering a value of 0.1l specifies that all source combinations that result in predicted mixture signatures within 0.1l of the observed signature are considered as feasible solutions). IsoSource provides output files which list each feasible solution, descriptive statistics about the distribution of these solutions (number of solutions, mean, standard deviation, minimum, maximum, 1%ile, median, and 99%ile for each source), and histograms of these distributions. Output files can be viewed on-screen, printed, or read into other applications such as Microsoft Excel™ for further viewing or manipulation.
As mentioned in the Phillips & Gregg (2003) paper, additional non-isotopic constraints can be incorporated by extracting appropriate subsets of solutions from the IsoSource output. Additional information on this post-processing is given in the "Additional constraints.pdf" link shown at the bottom of the page." The Phillips et al. (2005) paper attached below also describes alternative methods for combining sources. One of these methods also involves post-processing IsoSource output.
NOTE - Each of the individual solutions represents a
combination of source proportions which satisfies isotopic mass balance in the
mixing model. Descriptive statistics are provided simply as a way to
characterize this entire distribution of feasible solutions. To avoid
misrepresenting the results, users of this procedure should report the
distribution of feasible solutions rather than focusing on a single value such
as the mean.
This software
is provided free of charge with the understanding that it will not be used for
any commercial purposes. It is reasonably reliable, but has not been
exhaustively tested and must be applied at the user’s own risk. Mention of
trade names or commercial products does not constitute endorsement or
recommendation for use.
Several minor changes have been made since the original version of this software
(1.1).
Version 1.2 - Corrected bugs found with the implementation of IsoSource for 8 or 9
sources.
Version 1.3 – Corrected
“Run time error 6, overflow” error when there was only one solution (n=1) due
to division by zero (n-1) in the formula for standard deviation.
Version 1.3.1 - Corrected
problem of IsoSource crashing when you minimized the help window.
Please
use only the latest version below.
The software
is designed to run in a Microsoft Windows™ environment. It was
developed and tested in Windows 98 and 2000, but also runs in other versions of Windows™ such as XP. To install the software, click on the IsoSource link below and follow
the installation instructions. Instructions on use of the software can be found
on its Help menu.
Phillips & Gregg 2003.pdf [paper describing procedure]
Phillips et al 2005.pdf [paper describing alternative methods for combining sources]
IsoSource Version 1.3.1 (IsoSource software in a ~8.1MB zip file - Unzip the file and run Setup.exe]
For further information, questions, or suggestions for improvement of the software, contact Don Phillips at Phillips.Donald@epa.gov.
Accesses since April 20, 2006..