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).