Aqueous Dimethylamine Assay: Reading Results Near a Specification Limit

China Amines Co., Limited

A laboratory reports an aqueous dimethylamine assay just above the agreed minimum. Another reviewer asks whether measurement uncertainty has been considered.

Both are looking at the same result, but the acceptance decision needs an additional piece of information: the decision rule.

That rule is best agreed before testing begins. Discovering it during a delivery dispute gives a small decimal place a surprisingly large administrative workload.

1. Separate the Result from the Acceptance Decision

A measured value reports the laboratory’s estimate of the quantity being assessed. A conformity statement applies a defined rule to decide whether the result meets a specified requirement.

JCGM 106 discusses measurement, comparison with requirements and the subsequent decision as related steps in conformity assessment.[1] bipm.org

For aqueous DMA, the parties should therefore identify both the required assay specification and the basis for making an acceptance statement.

2. Understand the Reported Uncertainty

Expanded measurement uncertainty is commonly expressed alongside a result as an interval. Its interpretation depends on the laboratory’s evaluation and stated coverage information.

ILAC G8 discusses expanded uncertainty and the role of decision rules when laboratories report conformity with specifications.[2] ilac.org

When reviewing a report, clarify whether uncertainty is expressed in the same units as the assay or relative to the result. Percentage points and relative percent are different ways of expressing a quantity and should be labelled clearly.

3. A Hypothetical DMA Example

The following numbers are illustrative only. They are not China Amines specifications or actual batch results.

The corresponding numerical interval is 39.4–39.8 wt%. Its coverage interpretation still requires the laboratory’s accompanying information.

Now consider two possible decision rules:

• Simple acceptance: the reported value is compared directly with the specification limit. Here, 39.6 is above 39.5.
• An illustrative guarded rule: acceptance requires the reported value minus the expanded uncertainty to meet the minimum. Here, 39.4 is below 39.5, so that acceptance criterion is not met.

ILAC discusses simple acceptance and guard-band approaches.[2] The example shows why the selected rule matters; it does not prescribe either rule for a DMA purchase. ilac.org

4. Agree on the Rule Before the Result Arrives

A testing agreement can identify the specification, method, reporting basis, relevant uncertainty information and decision rule.

ILAC’s guidance emphasizes defining and documenting the applicable rule, including agreement with the customer where required.[2] ilac.org

For a commercial DMA qualification, also clarify rounding conventions and the process for reviewing disputed results. These details help prevent different departments from applying different acceptance assumptions.

5. Investigate Disagreements Consistently

If two laboratories report different DMA assays, review the sample identities, methods, preparation records, reporting units and available uncertainty information.

Apply the agreed investigation procedure. Any repeat testing or independent assessment should have a defined purpose and a documented basis.

This gives purchasing and laboratory teams a clearer route from a reported number to an acceptance decision.

References

1. JCGM 106 — The Role of Measurement Uncertainty in Conformity Assessment
2. ILAC G8 — Guidelines on Decision Rules and Statements of Conformity

About China Amines

For aqueous dimethylamine enquiries, contact China Amines with your concentration requirement, intended use, quantity and destination. Include any specific analytical or acceptance requirements so that the proposed specification and supporting documentation can be reviewed before qualification.