Certified Reference Materials for Combustion Analysis
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At LabSupplies.com, the most expensive mistake we see in combustion laboratories is using the same reference material to calibrate the instrument and to verify it. If the calibrant and the control come from the same certified value, the check cannot detect a calibration error — it can only confirm that the arithmetic worked. Calibrate on one material, verify on a different one.
This guide covers what "certified" actually means, what traceability buys you, how closely the matrix needs to match, and how to structure calibrants and controls so your QC has real diagnostic power.
What "Certified" Means
A certified reference material is not simply a material with a number on the label. Under ISO 17034:2016 — which replaced ISO Guide 34 — producers must demonstrate competence in homogeneity assessment, stability assessment, value assignment and uncertainty estimation. The certificate that ships with the material is the deliverable, not the bottle.
Read the certificate for three things before you use it: the certified value and its stated uncertainty, the minimum sample mass for which homogeneity was established, and the expiry or recertification date. Weighing below the stated minimum mass invalidates the certified value, because homogeneity was never demonstrated at that scale.
Traceability: What It Actually Buys
NIST defines a Standard Reference Material as a certified reference material that delivers at least one certified property value with metrological traceability to the SI or another internationally recognised higher-order reference system, produced consistently with ISO 17034:2016.
Practically, traceability is what lets two laboratories compare results and what an assessor will ask you to demonstrate. If your work is accredited, feeds a certificate of analysis, or supports a material specification, the traceability chain is not optional. If you are doing internal process control and nobody outside sees the number, a working standard may be sufficient — but know which situation you are in, because the cost difference is real and so is the audit risk of guessing wrong.
Matrix Matching
Combustion behaviour depends on the matrix, not only on the analyte concentration. A carbon value certified in a low-alloy steel does not transfer cleanly to a cast iron, a nickel alloy or an ore, because the melt behaviour, the flux requirement and the gas release kinetics all differ.
Match on three axes, in this order of importance:
- Matrix type — steel, cast iron, nickel alloy, ore, soil, coal, cement. This matters most.
- Concentration range — bracket your working range rather than sitting at one end of it. A calibration anchored only at high carbon will not serve low-carbon work.
- Physical form — chip, pin, powder or ring. Form affects both the weighing step and the combustion, and it should resemble how your real samples present.
Calibrant vs Control: Use Different Materials
This is the point worth taking away. Build your QC around two distinct materials:
- The calibrant sets the instrument response. It should sit near the centre of your working range and match your dominant matrix.
- The control is analysed as an unknown to verify that calibration. It must be a different material from the calibrant — ideally a different lot and a different producer — or the check has no independent information in it.
Run the control at the start of a run, periodically through long runs, and after any change of crucible lot, accelerator lot or reagent tube. If the control drifts while the calibrant still reads correctly, you are usually looking at a consumable problem rather than a calibration problem.
Storage and Shelf Life
Metal CRMs oxidise. Keep chips and pins in their original sealed container, in a desiccator if your environment is humid, and take only what you need for the run. Do not return unused material to the container. Powders and organic CRMs are more sensitive again — follow the certificate, which will state storage conditions, and treat the recertification date as real rather than advisory.
Record the lot number with every result. When a QC excursion turns up months later, the lot number is often the fastest route to the cause.
Selecting the Right Reference Material
| If your work is… | You need | Because |
|---|---|---|
| Accredited testing / certificates of analysis | CRM with documented SI traceability | Assessors require the traceability chain |
| Method validation | CRM, matrix-matched, bracketing the range | Bias is only visible against a certified value |
| Daily instrument QC | Control material distinct from the calibrant | A shared value cannot detect calibration error |
| Internal process control only | Working standard may suffice | No external claim is being made |
We stock certified reference materials for carbon, sulfur, oxygen, nitrogen and hydrogen determination across steel, iron, ore, soil and organic matrices, cross-referenced to the OEM part numbers your method names. Browse certified reference materials, or start from your instrument: LECO-compatible, ELTRA-compatible and Horiba-compatible consumables.
Related reading: crucible selection for carbon and sulfur analyzers, tin vs silver capsules for CHNS, choosing a lab balance, and chemical storage best practice.
Frequently Asked Questions
What is the difference between a reference material and a certified reference material?
A certified reference material carries a certificate with an assigned property value and a stated uncertainty, produced under ISO 17034:2016, which requires the producer to demonstrate competence in homogeneity, stability, value assignment and uncertainty estimation. An uncertified reference material has no such documented basis and cannot support a traceability claim.
Can I use the same CRM to calibrate and to verify?
No. If the calibrant and the control share a certified value, the verification cannot detect a calibration error — it only confirms the arithmetic. Calibrate on one material and verify on a different one, ideally a different lot from a different producer.
What is metrological traceability and do I need it?
It is an unbroken chain of comparisons linking your result to a recognised higher-order reference such as the SI. NIST defines an SRM as a CRM with at least one certified value traceable in this way. You need it if your work is accredited, feeds a certificate of analysis, or supports a material specification. Purely internal process control may not require it.
How closely does the CRM matrix need to match my samples?
Closely, because combustion behaviour depends on the matrix and not only on concentration. Match matrix type first — steel, cast iron, nickel alloy, ore, soil, coal — then bracket your concentration range, then match physical form. A carbon value certified in low-alloy steel does not transfer cleanly to cast iron.
Why does the certificate specify a minimum sample mass?
Because homogeneity was only demonstrated at or above that mass. Weighing below it invalidates the certified value and its uncertainty, since the material is not guaranteed to be representative at a smaller scale.
How should I store metal reference materials?
In the original sealed container, in a desiccator if your environment is humid, taking only what the run requires and never returning unused material to the container. Metal CRMs oxidise, and surface oxidation changes the very values you are relying on. Record the lot number with every result.
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Edited by Ray Dillman-Neu, Co-Founder, LabSupplies.com. LabSupplies.com supplies aftermarket consumables, certified reference materials and replacement parts for elemental analysis laboratories across the United States.
Specifications in this guide were verified against the published standards cited below. Always confirm consumable compatibility against your instrument manual and your validated method before substituting any part.
Sources and Technical References
- Standard Reference Material definitions, National Institute of Standards and Technology.
- Metrological Traceability: Frequently Asked Questions and NIST Policy, National Institute of Standards and Technology.
- ISO 17034:2016 — General requirements for the competence of reference material producers, International Organization for Standardization.
- ASTM E1019 — Standard Test Methods for Determination of Carbon, Sulfur, Nitrogen, and Oxygen in Steel, Iron, Nickel, and Cobalt Alloys, ASTM International.