Inorganic Geochemistry

IsoFind's inorganic geochemistry module covers major and trace elements from field samples, their redox speciation, diagnostic source ratios, and isotopic fractionation. It is distinguished from the molecular module by the nature of the pollutants involved (elements rather than molecules), the data source (concentrations in mg/kg from standard elemental analyses), and the engine that processes them (Nexus bridge instead of CSIA bridge). This page provides an overview of the module and its three detailed sub-pages.

Two Complementary Storage Systems

Inorganic data within an IsoFind project is stored in two distinct locations based on its chemical nature. This distribution reflects an operational distinction: pure elements on one side, and structured chemical species (ions, complexes, oxyanions) on the other.

This bipartition follows a practical logic. ICP-MS or ICP-OES produces total elemental concentrations (e.g., total Cr in mg/kg) which are directed to the Geochemistry view. Conversely, Ion Chromatography produces concentrations of distinct species (e.g., nitrate in mg/L, chlorate in µg/L) which are directed to the Molecules view, even if these species are inorganic. IsoFind's inorganic module covers both.

Elements Supported by the Nexus Bridge

The IsoFind geochemical engine (Nexus bridge) features tabulated isotopic fractionation factors for eleven metallic and metalloid elements commonly targeted in environmental forensics. The engine's _EPSILON_TABLE contains epsilon values for each element under abiotic, biological, mean, and adsorption conditions.

Elements Without Variable Redox States

An important distinction, hard-coded into the engine via the _NORED_ELEMENTS constant, separates elements with active redox speciation from those that remain in a single chemical form in aqueous solution. This distinction determines which engine functions can be activated for a given element.

For redox-active elements, the engine solves the distribution between oxidized and reduced forms based on local physicochemical conditions (pH, Eh, dissolved oxygen, organic matter). This speciation is then used to calculate the isotopic fractionation of the residual (Rayleigh equation for the reduced fraction) and the differential adsorption of forms onto solid phases.

The Three Sub-pages of the Module

The inorganic module is divided into three dedicated pages, each addressing a distinct aspect. This breakdown reflects how data is actually utilized by users: entry and interpretation of raw concentrations on one side, source diagnostics calculated from ratios on the other, and redox speciation for elements with multiple oxidation states in between.

The 12 Inorganic Species in the Molecular Catalog

Beyond raw elemental concentrations, IsoFind integrates twelve named inorganic species into the molecular catalog (table ref_molecules with pollutant_type = 'inorganic'). These species are managed as full molecules because they carry their own regulatory thresholds, dedicated analytical methods, and sometimes specific isotopic interpretations.

Common Inorganic Isotopic Elements

Beyond the carbon fractionation that dominates organic molecule studies, inorganic studies utilize a wider array of isotopic systems. IsoFind supports δ measurements for the main elements, using their conventional reference scales.

These isotopic measurements are entered in the Isotopes view of the sample sheet, independent of concentration storage. The concentration/isotope coupling occurs during calculation, when the Nexus engine utilizes both to compute diagnostics.

Inorganic Data Entry and Import

Elemental concentrations are recorded in the Geochemistry view with a specific structure. Each measurement includes the element, its value, native unit (mg/kg, mg/L, µg/g, pct...), normalized value in mg/kg, uncertainty, and analytical method.

CSV imports accept both common orientations: one element per row (long format, typical of laboratories) or one element per column (wide format, typical of Excel spreadsheets). The IsoFind parser automatically recognizes the structure and switches to normalized storage.

Integration with the Simulation Engine

The inorganic module feeds the 3D simulation engine via the simulator's "element" mode, as opposed to the "molecule" mode used for the molecular module. The engine consumes inorganic data in three ways:

Explicit Nexus processes (nexus_process_hint such as redox, adsorption, biological, dissolution, precipitation, evaporation) allow forcing a process category to query the fractionation database. These overrides are particularly useful for testing alternative hypotheses on the same site.

Why a Separate Module from Molecular?

The separation between inorganic and molecular geochemistry in IsoFind is not just a filing convention. It reflects five operational differences that require distinct treatments:

• Elemental data comes from total analyses (ICP after digestion) which do not distinguish chemical species; molecular data comes from specific analyses (GC-MS, LC-MS) that identify each compound.
• Native units differ: mg/kg for solids and mg/L for liquids on the inorganic side, while molecules are mostly reported in ng/L or µg/L.
• Analytical frameworks differ: ISO, EPA, and NF standards for inorganic methods vs. ISO 21675 and EPA 537 for PFAS, for example.
• Transformation mechanisms differ: redox speciation and adsorption for elements vs. kinetic degradation and metabolization for molecules.
• Isotopic signatures do not overlap: δ¹³C, δ²H, δ³⁷Cl for molecules vs. δ⁵³Cr, δ⁵⁶Fe, δ⁸²Se, δ¹²³Sb for inorganics.

Further Reading

• Majors and Traces: Entry, units, normalization, and typically monitored elements.
• Diagnostic Ratios: Elemental ratios for source attribution.
• Redox Speciation: Cr(VI)/Cr(III), Fe(II)/Fe(III), and other redox couples.
• Molecular Module: The organic counterpart to the inorganic module.