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Model Report

IsoConc

Last Revision Date: 01/21/2014 View as PDF
General Information Back to Top
Model Abbreviated Name:

IsoConc
Model Extended Name:

IsoConc
Model Overview/Abstract:
Stable isotope analyses are often used to quantify the contribution of multiple sources to a mixture, such as proportions of pollutant sources to a waste stream, proportions of food sources in an animal’s diet, etc. Linear mixing models can be used to partition three sources when there are two isotopic tracers used (e.g. δ13C and δ15N. However, the standard model assumes that the proportional contribution of a source to the mixture is the same for both elements (e.g., C and N). This may be a reasonable assumption if the concentrations are similar among all sources. However, one source is often particularly rich or poor in one element (e.g., N), which logically leads to a proportionate increase or decrease in the contribution of that source to the mixture for that element relative to the other element (e.g., C). IsoConc is a concentration-weighted mixing model which assumes that for each element a source’s contribution is proportional to the contributed mass times the elemental concentration in that source.
Keywords: Stable isotope analysis, concentration, estimating source proportions
Model Technical Contact Information:
Agency Contact:
Donald Phillips
EPA/ORD/NHEERL/WED
phillips.donald@epa.gov
541-754-4485
Model Homepage: www.epa.gov/wed/pages/models/stableIsotopes/isoconc/isoconc1_01.htm
Substantive Changes from Prior Version: N/A
Plans for further model development: None

User Information Back to Top
Technical Requirements
Computer Hardware
PC
Compatible Operating Systems
Windows 98 or later
Other Software Required to Run the Model
Excel 2000 or later
Download Information
www.epa.gov/wed/pages/models/stableIsotopes/isoconc/isoconc1_01.htm
Using the Model
Basic Model Inputs
Isotopic signatures (e.g., atom % or δ) for each of two isotopic tracers (e.g., δ13C and δ15N) for each source and the mixture, and concentrations of each element (e.g., C and N) in the sources. (For dietary studies, appropriate isotopic tissue-diet discrimination corrections should be made first.)
Basic Model Outputs
Point estimates for the proportions of each source’s contribution to the mixture considering both isotopic composition and concentrations.
User Support
Other User Documents
Phillips DL and Koch PL (2002) Incorporating concentration dependence in stable isotope mixing models. Oecologia 130: 114-125.

Koch PL and Phillips DL (2002) Incorporating concentration dependence in stable isotope mixing models: a reply to Robbins, Hilderbrand and Farley (2002). Oecologia 133: 14-18.

Availability of User Support
e-mail Don Phillips at phillips.donald@epa.gov
User Qualifications
Basic familiarity with stable isotope analysis

Model Science Back to Top
Problem Identification
Stable isotope analysis is often used to estimate the proportional contributions of sources to a mixture based on isotopic composition as measured by two isotopic tracers (e.g., δ13C and δ15N), but ignoring the effects that differential concentrations among the sources may have on the mixing proportions for the two elements. This model incorporates concentration effects and for each source gives separate mixing proportions for each element and total mass.
Summary of Model Structure and Methods
The model is an Excel spreadsheet. The user supplies information about the isotopic signatures and elemental concentrations for each source and mixture in highlighted cells on the spreadsheet. The source proportion estimates are calculated and shown on the spreadsheet, along with a ternary diagram of the mixing space and source proportions.
Model Evaluation
The model and its constituent equations are described in the peer-reviewed journal publications:

Phillips DL and Koch PL (2002) Incorporating concentration dependence in stable isotope mixing models. Oecologia 130: 114-125.

Koch PL and Phillips DL (2002) Incorporating concentration dependence in stable isotope mixing models: a reply to Robbins, Hilderbrand and Farley (2002). Oecologia 133: 14-18.

The code was verified by comparison of results to those from an independent program written in SAS for the same purpose. Sensitivity analyses were performed for the effect of concentration differences among the sources on the source proportion estimates for each element and total mass, as described in Phillips and Koch (2002).

Key Limitations to Model Scope
The model as currently configured is restricted to computing the proportional contributions for three sources using two isotopic signatures.
Case Studies
The Phillips & Koch (2002) paper shown above gives two examples of application (dietary studies for mink and grizzly bears). A wide variety of additional examples can be found in 179 other papers that have cited this paper (per Google Scholar, 11/11/2009).


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