Working with isotope measurements

Data from the LI-7825 CO2 Isotopes/NH3 Trace Gas Analyzer include measurements of CO2 isotopologues. SoilFluxPro Software can evaluate this data using two methods: Slope Regression and Keeling Regression. To use these features, the data must include the major and minor gases.

The Isotope tab allows the assignment of a Major Gas and Minor Gas for the regression.

  • Major Gas: The major species to use for the regression calculation. Use total CO2 measurements from the dataset and 12C_CO2 from the LI-7825.
  • Minor Gas: The minor gas species to use for the regression calculation. Use Delta13C and 13C_CO2 from the LI-7825.
  • Standard: Isotopic standard value. Only relevant to the Slope method. Default value is 0.0112372.

To add a new regression:

  1. Select a Major Gas and Minor Gas from the available variables, then click Create.

  2. Two new regressions are created - Keeling and Slope.

    For each regression, the deadband and stop time can be optimized.

Regression options

Each Regression object presents options - expand the object to view them.

Which observations? If Selected is chosen, SoilFluxPro will display a plot and table of data, along with options to adjust the parameters for the measurement selected in the data display. If All is chosen, SoilFluxPro will show options that can be adjusted for all measurements.

Apply button: Click to Apply changes.

Revert button: Click to discard and revert to the prior state.

Metadata: The Deadband, Stoptime, and Standard are configurable.

Plot: Major gas over Minot gas (or 1/Minor Gas) is displayed when a single observation is selected. The graph shows the deadband (vertical green line). You can adjust the deadband by clicking and dragging the line. It also shows the stoptime (vertical red line). You can adjust the stoptime by clicking and dragging the line. The regression fit is shown in green.

Table: Shows parameters when a single observation is selected.

Keeling regression

Minor gas is expressed as 1/minor gas on the x-axix of the regression plot. The source isotopic value is calculated from the y-intercept of a linear fit of Major Gas over 1/minor Gas.

Table 2‑1. Keeling regression variables.
Interface
variable		 Units Data file
variable		 Description
Source Major Gas Same as gas SOURCE_Major_Gas Source isotopic value
r2   R2 R2 or linear fit
Slope   SLOPE Slope of linear fit

Slope regression

The source isotopic value is calculated from the sloe of a linear fit of the Major Gas over Minor Gas standardized to the isotopic Standard Value:

source isotopic value = (slope/standard − 1) × 1000

A new slope regression is automatically set to 0.0112372.

Table 2‑2. Slope regression variables.
Interface
variable		 Units Data file
variable		 Description
Source δMajor Gas ‰ SOURCE_Major_Gas Source isotopic value
r2   R2 R2 or linear fit
Slope   SLOPE Slope of linear fit
Intercept Same as major gas Intercept Y-intercept of linear fit