How do reference materials improve the reliability of combustion analysis?

Reference materials improve the reliability of combustion analysis by providing a known, independently verified benchmark against which instrument performance can be measured. When a certified reference material with a documented elemental composition is analysed alongside unknown samples, any deviation from the expected result reveals whether the instrument is performing correctly or whether a systematic error is present. The sections below unpack the most common questions labs raise about using reference materials effectively in combustion workflows.

What types of reference materials are used in combustion analysis?

Combustion analysis relies on two broad categories of reference materials: certified reference materials (CRMs) and in-house or working standards. CRMs are produced by accredited organisations and carry a certificate stating the accepted elemental values along with their associated uncertainty. Working standards are typically well-characterised materials prepared or sourced internally, used for routine monitoring between CRM runs.

Within these categories, the specific matrix matters considerably. Common reference materials used in combustion elemental analysis include:

Organic compounds with certified C, H, N, S, and O content, often used to verify detector response and combustion completeness
Soil and sediment CRMs, relevant for environmental labs measuring total carbon, nitrogen, or sulphur
Coal and coke reference materials, widely used in energy and petrochemical quality control
Biological tissue CRMs, applied in food, agricultural, and pharmaceutical laboratories
Pharmaceutical excipient standards, used to verify elemental purity in drug substance and product testing

Selecting a reference material whose matrix closely resembles the unknown sample is essential, because combustion behaviour, gas release profiles, and potential interferences differ significantly between organic, inorganic, and mixed matrices.

How do reference materials verify instrument calibration accuracy?

Reference materials verify calibration accuracy by serving as an independent check on the entire analytical chain. After an instrument is calibrated using standards, a certified reference material is analysed under identical conditions. If the measured value falls within the certified value’s accepted uncertainty range, the calibration is confirmed as fit for purpose. If it falls outside, the calibration requires investigation before results can be trusted.

This process separates two distinct functions that are sometimes confused. Calibration establishes the mathematical relationship between instrument signal and elemental concentration. Reference material verification then tests whether that relationship holds true for a real, complex sample, not just for the pure standards used to build the calibration curve. Because CRMs carry independently assigned values, they provide an objective, traceable check that is not influenced by the instrument’s own calibration data.

In regulated environments such as pharmaceutical or environmental testing, this traceability to recognised reference values is not optional. It forms a documented record demonstrating that results are comparable across instruments, laboratories, and time periods, a requirement under GMP and GLP frameworks.

What happens to combustion analysis results without reference materials?

Without reference materials, systematic errors in combustion analysis can go undetected for extended periods. Instrument drift, incomplete combustion, detector degradation, or reagent depletion may all shift results away from true values, but if there is no independent benchmark being measured, the laboratory has no mechanism to identify the problem. Results may appear internally consistent while being systematically wrong.

The practical consequences are significant. In quality control settings, unreliable elemental data can lead to incorrect acceptance or rejection of batches. In environmental monitoring, undetected errors may result in misreported compliance data. In research and development, biased results can misdirect experimental conclusions.

Reference materials act as an early warning system. A trend of gradually increasing or decreasing recovery in a CRM, even within specification, can signal that maintenance or recalibration is needed before the instrument drifts out of control. Without this data, corrective action is reactive rather than preventive.

How often should reference materials be run during combustion analysis?

The frequency at which reference materials should be run depends on the analytical context, but a widely adopted practice is to include at least one certified reference material at the beginning of each analytical sequence and one at the end. For longer runs or high-stakes analyses, additional reference material checks are placed at regular intervals throughout the sequence.

Several factors influence the appropriate frequency:

Regulatory requirements, some frameworks specify minimum CRM frequency; always consult the applicable standard or pharmacopeia for exact requirements
Sample volume, higher throughput sequences benefit from mid-sequence checks to catch drift early
Matrix variability, when a run includes samples of very different compositions, additional reference material checks help confirm that matrix effects are not distorting results
Instrument age and stability, older instruments or those with known variability may require more frequent verification
Consequence of error, in GMP pharmaceutical environments, more frequent CRM runs are standard practice to support robust audit trails

The goal is not simply to pass a check at the start and end of a run, but to build a quality control dataset over time that allows statistical monitoring of instrument performance trends.

What’s the difference between a calibration standard and a reference material in combustion analysis?

A calibration standard is used to construct the instrument’s response curve, it defines the relationship between signal and elemental amount. A reference material, particularly a certified reference material, is used to verify that the calibration is performing correctly on real samples. The two serve complementary but distinct roles, and using one does not replace the need for the other.

Calibration standards in combustion analysis are often pure organic compounds with precisely known stoichiometry. They are selected for their stability, purity, and predictable combustion behaviour. They tell the instrument what a known amount of carbon, hydrogen, nitrogen, or sulphur looks like in detector terms.

Certified reference materials, by contrast, are typically complex matrices, soils, biological tissues, fuels, or pharmaceutical materials, with elemental values assigned through interlaboratory studies or primary reference methods. They are not used to build the calibration; they are used to challenge it. A result that is accurate for a pure calibration standard but inaccurate for a CRM reveals a matrix-dependent error that calibration alone cannot correct.

How should reference materials be selected for different combustion matrices?

Reference materials should be selected based on matrix similarity, elemental range, and the specific analytes being measured. The closer the CRM’s physical and chemical properties are to the unknown sample, the more meaningful the verification result becomes. A reference material that combusts very differently from the sample provides limited assurance that the method is working correctly for that sample type.

Key selection criteria include:

Matrix type, organic, inorganic, or mixed; solid, liquid, or gaseous
Elemental concentration range, the CRM’s certified values should fall within the expected range of the samples being analysed
Analyte coverage, the CRM should be certified for the specific elements under measurement, not just broadly characterised
Certification body, prefer materials certified by recognised national or international metrology institutes or accredited producers
Homogeneity, particularly important for solid reference materials, where poor homogeneity introduces sub-sampling uncertainty that masks true instrument performance

When no commercially available CRM closely matches a specific sample matrix, laboratories sometimes develop in-house reference materials through characterisation by multiple independent methods. While these do not carry the same traceability as a commercially certified material, they provide a practical quality control tool when used alongside periodic checks with a traceable CRM.

How SciSPX supports reference material use in combustion analysis

For laboratories in the Benelux region, establishing a reliable reference material strategy for combustion elemental analysis requires both the right instrumentation and the application knowledge to implement it correctly. SciSPX supports labs at both levels:

Instrument qualification and validation, IQ/OQ installations and calibrations performed by certified specialists, aligned with GMP, GLP, and 21 CFR Part 11 requirements
Application-level guidance, Product Specialists who work directly with lab technicians and researchers to design appropriate reference material protocols for specific sample matrices
Ongoing service and maintenance, preventive and corrective maintenance programmes that keep instruments performing within validated parameters, so reference material checks remain meaningful over time
Training, hands-on training that helps laboratory teams understand how to interpret CRM data and respond appropriately when results fall outside expected ranges

If your laboratory is reviewing its quality control approach for combustion analysis, contact SciSPX to discuss how our team can support your specific analytical requirements.

Frequently Asked Questions

Can I use the same reference material for multiple elements, or do I need separate CRMs for each analyte?

Many certified reference materials are certified for multiple elements simultaneously, such as a soil CRM certified for C, N, and S, making them efficient for multi-element combustion workflows. However, you should always verify that the CRM’s certified values cover every specific analyte you are measuring, not just some of them. If a material is only informatively characterised (rather than certified) for a particular element, it cannot serve as a traceable verification standard for that analyte. Where gaps exist, a second CRM certified for the missing element should be incorporated into the run sequence.

What should I do if my reference material result falls outside the accepted uncertainty range?

A single out-of-range CRM result should trigger an immediate investigation before any sample results from that run are reported. Start by checking the most common causes: reagent condition, combustion tube integrity, carrier gas purity, and whether the CRM was handled and weighed correctly. If a root cause is identified and corrected, re-run the CRM to confirm recovery before proceeding. If the cause cannot be determined, the entire analytical sequence should be flagged and the samples re-analysed after the instrument has been serviced and re-verified.

How should reference materials be stored to ensure they remain reliable over time?

Storage conditions vary depending on the matrix, but most solid reference materials should be kept in a cool, dry environment away from direct light, with containers tightly sealed between uses to prevent moisture uptake or contamination. Always follow the storage instructions provided on the certificate of analysis, as some biological or organic CRMs have specific temperature requirements. It is also good practice to record the date a container is first opened and monitor cumulative usage against the certified minimum sample mass, since taking sub-samples below the recommended minimum can introduce homogeneity-related errors.

Is it acceptable to use an expired certified reference material for routine quality control checks?

Using a CRM beyond its certified expiry date is generally not acceptable in regulated environments such as GMP or GLP laboratories, because the certified values can no longer be guaranteed and the traceability chain is broken. In less regulated research settings, an expired CRM may still provide useful trending data for instrument monitoring, but results should not be used to formally validate or report analytical accuracy. The safest approach is to maintain a stock management system that flags CRMs approaching expiry and ensures replacements are ordered and qualified in advance to avoid gaps in your quality control programme.

How do I build a statistically meaningful control chart from my reference material data?

Start by running your chosen CRM consistently across a sufficient number of analytical sequences, typically a minimum of 20 independent measurements, to establish a reliable baseline mean and standard deviation under your specific laboratory conditions. Plot these values on a control chart with warning limits set at ±2 standard deviations and action limits at ±3 standard deviations from the established mean. Over time, patterns such as gradual drift in one direction, increasing variability, or repeated results near a warning limit are often more informative than individual out-of-range events, as they signal developing instrument issues before a hard failure occurs.

Can in-house reference materials replace certified reference materials entirely in a combustion analysis workflow?

In-house reference materials are a valuable complement to CRMs, particularly for routine between-run monitoring and for matrices where no commercial CRM exists, but they cannot fully replace certified reference materials in a robust quality system. Because in-house materials are characterised internally rather than through independent interlaboratory studies, they do not provide external traceability and may carry undetected systematic biases. Best practice is to anchor your quality control programme to at least one traceable CRM and use in-house materials for higher-frequency monitoring checks between those anchor points.

What are the most common mistakes laboratories make when implementing a reference material strategy for combustion analysis?

The most frequent mistakes include selecting a CRM whose matrix is too different from the actual samples, running reference materials only at the start of a sequence and overlooking mid-run drift, and failing to maintain a longitudinal control chart that would reveal gradual performance trends. Another common oversight is treating a passing CRM result as confirmation that all samples in the run are correct, without considering whether matrix-specific interferences could still affect individual sample types. A well-designed reference material strategy addresses each of these points by combining appropriate CRM selection, sufficient run frequency, and systematic trending of results over time.

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