Data Acquisition

For all continuous-flow applications the reference/monitoring gas isotopic value is set to 0.000 ‰ in Isodat. At least two reference materials tied to relevant international isotopic scales and with contrasting isotopic values are analyzed repeatedly alongside samples during each acquisition.

Each set of samples is analyzed with matrix-matched (where possible) blind standards that either have manufacturer-certified isotopic values, or that have been ring-tested with other labs to produce consensus isotopic values.

Data Normalization

As of August 2018, we use LIMS (Laboratory Information Management Systems) for Light Stable Isotopes, version 9.205 and LIMS for Lasers 2015 to manage workflows and correct and normalize all raw isotopic data. The software is developed and provided free of charge by the USGS. For more information, visit https://isotopes.usgs.gov/research/topics/lims.html.

Raw isotopic data are normalized to international scales in LIMS using linear least squares regression. In the linear equation y = mx + b, y is the final delta value, m is the expansion coefficient, x is the penultimate delta, and b is the additive correction factor.

QA/QC

Long-term data for some laboratory reference materials (updated 8 December 2018):

Reference Materials Under Evaluation

We're always working to expand our library of isotopic references, with the dual goals of: (1) having matrix-matched blind standards for most types of samples; and (2) widening the isotopic range of our bracketing normalization standards. Below is a list of materials currently under evaluation: