Assay Methods and Detection Limits

An assay method defines how a constituent is measured in a prepared sample and how the result is expressed. It includes the test portion, any decomposition or extraction, the measurement technique, calibration and calculation. A detection limit describes one aspect of analytical capability; it does not establish that a method measures the intended constituent completely or is suitable across all concentrations and sample types.^[1]

Method selection should therefore begin with the geological or engineering question, expected mineral hosts, material matrix and reporting basis. The lowest advertised detection limit is not necessarily the best basis for selecting an assay.

Define What the Result Is Intended to Measure

The measurand is the quantity intended to be measured, including necessary qualifications. An elemental concentration on a dry-mass basis is different from an as-received concentration. Total content and the fraction released by a specified extraction are also different quantities, even if both reports use the same element symbol and unit.^[2]

The distinction affects comparison and interpretation. If a project requires total concentration, a well-calibrated measurement of an incomplete extract may not answer that question. If the intended property is explicitly method-defined extractability, incomplete dissolution is not necessarily a method failure. The procedure defines part of the property being reported.

A request for analysis should consequently identify the constituent, intended fraction, concentration range, material types and required use. Geological descriptions can help a laboratory recognize conditions outside the demonstrated method scope. The project should retain the complete method identity, rather than reducing it to a broad technique label such as inductively coupled plasma (ICP) analysis.^[3]

Preparation, Decomposition and Measurement

Solution-based methods first bring relevant constituents into a form the instrument can measure. The decomposition and instrumental finish are separate stages. An instrument can measure a solution consistently while resistant minerals remain in the residue; analytical precision alone does not establish complete extraction.

ALS's technical note distinguishes partial aqua regia digestion from near-total four-acid digestion and identifies minerals that can remain incompletely attacked by the latter. Fusion or another specifically appropriate procedure may be necessary for particular elements and mineral hosts. These are method-selection considerations, not a claim that one decomposition is universally superior.^[4]

Precious-metal work provides another example of linked preparation and assay. In screen fire assay, the screened fractions are treated separately and the final sample result is reconstructed using their masses. The approach addresses the heterogeneous material submitted to that procedure, not an arbitrary larger geological volume or material previously discarded.^[5]

Changing the method can change the measured fraction as well as the numerical performance. Where new results are to be combined with historical assays, a comparison should investigate the material, preparation, decomposition, concentration range and reporting basis. Agreement on a limited set of easy samples does not establish equivalence for every domain.

Validation and Working Range

Method validation provides evidence that performance meets the requirements of the intended use. Relevant characteristics can include selectivity, working range, precision, bias and measurement uncertainty. Verification concerns an adopting laboratory's ability to achieve the required performance of an already validated procedure under its own conditions. A changed method or use outside its validated scope may require additional validation rather than verification alone.^[1]

Matrix effects and interferences can alter response. Calibration establishes a relationship between response and known quantities under stated conditions; it does not by itself validate every preparation step or prove that unknown samples behave like the calibration materials. Appropriate reference materials and other checks are needed to examine that wider claim.

The working range has upper as well as lower limits. A result exceeding the validated range may require dilution, another test portion or a different analytical finish. That subsequent result needs its own method and qualification records. A displayed number beyond the validated range should not be treated as valid merely because the instrument software calculated it.^[1]

Detection, Quantification and Reporting Limits

A critical value or decision threshold is applied to an observed signal or result to decide whether detection is established. The limit of detection, or LOD, instead describes capability: the true concentration that can be detected under specified false-positive and false-negative error probabilities. It is not simply the threshold applied to an individual result. The limit of quantification, or LOQ, concerns the lowest concentration meeting specified quantitative performance. Conventions and estimation conditions must be stated; these limits are not fixed properties of an element.^[1]

An instrument detection limit can omit sample preparation. A method detection limit should reflect the whole specified measurement procedure and the calculation used for routine samples, not merely instrument response. That laboratory procedure still does not automatically quantify field-sampling uncertainty. The concentration limit for the original solid depends on test-portion mass, final volume and dilution; clean-solution performance cannot simply be assigned to a difficult geological sample.^[1][2]

“Reporting limit” is not automatically a synonym for either LOD or LOQ. Laboratories and programmes use the term differently, sometimes incorporating practical reporting conventions. Users should establish which limit appears on the certificate and whether sample-specific adjustments have changed it.^[1]

These distinctions separate detection from defensible quantification. A response may provide evidence of presence while remaining too uncertain for an unqualified quantitative result. Conversely, a reported non-detection does not prove an exact zero concentration. The result describes the procedure's finding under its stated capability and reporting convention. A reported inequality is not a guaranteed bound on the unknown true concentration.^[1]

Using Qualified and Out-of-Range Results

The database should preserve the reported value, inequality or qualifier, applicable limit, unit and method. “Not analysed”, “not detected”, “below quantification” and “above the working range” are different outcomes. Collapsing them into a common zero destroys information needed for later interpretation.^[6][1]

Any numerical treatment of censored data belongs in a documented interpretation step, not an undocumented alteration of the laboratory record. Substituting a fraction of the limit is an assumption, not a measurement. Its effect depends on how many results are censored, the distribution of values and the engineering decision. Retaining the original qualifications allows alternative treatments to be checked.

For an over-range sample, an appropriate follow-up determination may replace the original qualified value in a selected-result field. Both results and the selection rule should remain traceable. Repeated analysis should not become a search for the most convenient grade.^[6]

Demonstrating Fitness for Engineering Use

Reference materials help assess agreement with assigned values, but their suitability depends on the certified property, matrix and instructions. The Joint Research Centre distinguishes total element values from extraction-specific values and explains why the intended measurement must match the reference value's definition.^[7]

Blank and duplicate results provide additional evidence about contamination and variability. Checks should cover relevant concentrations and materials, including deleterious constituents where these affect the decision. CIM's guidance recommends timely review and clear identification of the calculation and sample type used to describe precision.^[6]

Suppose, hypothetically, that two laboratories report different concentrations for a resistant-mineral-bearing sample. Before ranking laboratory quality, compare the decompositions and intended fractions. If the methods measure different fractions, a reproducible difference may be expected. If the measurands and procedures are equivalent, the investigation can then address calibration, matrix response, subsampling and other possible causes.

The final conclusion should state what was measured and how well it was supported. A lower detection limit, an extra decimal place or agreement between repeat solutions cannot independently demonstrate representative field sampling, complete decomposition or suitability for mine planning.