IsoSource Help
Overview
IsoSource is a program written in Microsoft Visual Basic®
for Windows® operating systems by Robert Gibson (Computer Sciences
Corp.) to perform the stable isotope mixing model calculations described in:
Phillips
DL, Gregg JW (2003). Source partitioning
using stable isotopes: coping with too many sources. Oecologia 136: 261-269.
This procedure is designed to
use isotopic signatures (ratios) to determine the range of feasible source
contributions to a mixture when there are too many sources to allow a unique
solution (>n+1 sources
when n isotope signatures are
used). All possible combinations of each
source contribution (0-100%) are examined in small increments (e.g., 1%). Combinations that sum to the observed mixture
isotopic signatures within a small tolerance (e.g., +0.1 l) are considered to be feasible solutions, from which
the frequency and range of potential source contributions can be determined.
See the Phillips & Gregg (2003) paper for details.
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.
IsoSource allows the use of up to 5 isotope signatures (e.g., d13C, 208Pb/207Pb,
etc.) and up to 10 sources. The user
specifies:
·
title
·
source increment (in %)
·
mass balance tolerance
(same units as isotopic signatures)
·
labels for isotope
signatures
·
labels for sources
·
isotope signature
values for the mixture and each source
Note: If there is any
isotopic fractionation between sources and the mixture (such as “trophic fractionation” or “tissue-diet discrimination” in
dietary studies) the source signatures should be adjusted for this before they
are input.
These input data may be
printed [Print] or saved [File/Save] in a file that can be
reopened [File/Open] for future use
(.DAT file name suffix). The [Calc]
function starts the program, creating all possible combinations of the sources
which sum to 100%, using the specified increment. Combinations which give predicted mixture
isotopic signatures within the specified tolerance of the observed
mixture signatures are identified as feasible solutions, and are stored in a
raw data output file (.OUT file name
suffix). In addition, summary histogram
and descriptive statistics (n, mean, minimum, maximum, 1%ile, 50%ile, 99%ile,
and standard deviation) are stored in a summary output file (.TOT file name suffix). Both files may be viewed with the [View] function. Using the [Graph] function, the histograms for each source can be graphed for
display on screen and for printing.
The number of combinations to
examine increases with more sources, according to the formula:
where i = increment (%) and s =
number of sources. Up to 6 or 7 sources
can be run at 1% increments within a few minutes on a modern PC. Higher numbers of sources will take longer
unless the increment size is increased (see discussion in Phillips &
Gregg).
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. For questions or
problems, contact Don Phillips (phillips.donald@epa.gov).