Source: https://guthlabs.ai/docs/instruments/isotope-average-atomic-mass-calculator/mechanism/

instrument mechanism / verified-clean

# Isotope / Average Atomic Mass Calculator mechanism

The inspectable method record for Isotope / Average Atomic Mass Calculator, kept separate from implementation and qualification.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Model scope

The weighted-average computation is correct for supplied isotope masses and valid fractional abundances. The verdict is NEEDS_REVIEW because the specification simultaneously defines the formula using fractional abundances and the inputs using percentage abundances without explicitly defining the conversion or validation rules. Also, the label 'average atomic mass' should clarify whether it means the supplied-composition weighted average or a standard terrestrial atomic weight from a cited data source.

- The formula is standard when abundanceᵢ is a fractional abundance between 0 and 1, but the stated inputs are percentage abundances; each percentage must be converted as abundanceᵢ=percentageᵢ/100 before multiplication.

- The specification does not state the required constraint that abundances must be nonnegative and sum to 100% (or 1.0 after conversion).

## Units and conventions

- All isotope masses must use the same unit, conventionally unified atomic mass units (u).

- The output has the same mass unit as the isotope masses; if masses are in u, the result is an average atomic mass in u.

- If masses are expressed numerically in g/mol instead, the numerical weighted average is a molar mass in g/mol, not an atomic mass in u.

- Abundances supplied as percentages must be divided by 100; fractional abundances must not be divided by 100 again.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "isotope_masses_u": [
      34.96885,
      36.9659
    ],
    "abundances_percent": [
      75.78,
      24.22
    ]
  },
  "expected_outputs": {
    "average_atomic_mass_u": 35.45253551
  },
  "working": "Convert percentages: 75.78%=0.7578 and 24.22%=0.2422. Compute 34.96885×0.7578=26.49939453. Compute 36.96590×0.2422=8.95314098. Add: 26.49939453+8.95314098=35.45253551 u."
}
````

## Failure boundary

- Reject negative abundances.

- Validate that abundances sum to 100% when percentage inputs are used, or 1.0 when fractional inputs are used; otherwise the result is not a valid weighted average.

- If an implementation normalizes abundances by their sum, an all-zero abundance list causes division by zero.

- Require isotope masses and abundances arrays to have equal nonzero length.

- Do not silently mix percentage and fractional abundance representations.

- Built-in isotope data must identify its NIST/IUPAC source, reference composition, and revision because natural isotope abundances can be reference-dependent.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/isotope-average-atomic-mass-calculator/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/isotope-average-atomic-mass-calculator/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/isotope-average-atomic-mass-calculator/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche5/tranche5-results.json`