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ISO 16968:2015 and CSA ISO 16968‑15 — Solid Biofuels: Determination of Minor Elements
Technical requirements, analytical methods, and compliance notes for the determination of trace elements in solid biofuels
Scope and Purpose
ISO 16968:2015 — adopted in Canada as CSA ISO 16968‑15 — specifies methods for the determination of 13 minor (trace) elements in solid biofuels. Solid biofuels such as wood pellets, chips, briquettes, and agricultural residues are increasingly used for heat and power generation. The presence of minor elements (often referred to as heavy metals or trace elements) influences combustion behaviour, ash utilisation, and environmental emissions. This standard provides harmonised analytical procedures to ensure consistent, comparable results across laboratories and jurisdictions.
Tip: CSA ISO 16968‑15 is an identical adoption of the ISO 16968:2015. Laboratories operating in Canada should reference the CSA number, while the technical content remains unchanged.
The standard covers the determination of the following elements: arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), mercury (Hg), manganese (Mn), molybdenum (Mo), nickel (Ni), lead (Pb), antimony (Sb), vanadium (V), and zinc (Zn). It is applicable to all solid biofuels, including chemically untreated and untreated wood, bark, and herbaceous biomass.
Technical Requirements
Elements Covered and Performance Characteristics
ISO 16968:2015 defines specific performance requirements for each element, including recommended wavelengths for optical emission spectrometry, detection limits, and linear ranges. The table below summarises the elements, typical quantification limits, and recommended techniques.
| Element | Symbol | Recommended Techniques | Typical Quantification Limit (mg/kg, dry basis) |
|---|---|---|---|
| Arsenic | As | ICP‑OES, HG‑AAS | 0.05–0.2 |
| Cadmium | Cd | ICP‑OES, GF‑AAS | 0.01–0.05 |
| Cobalt | Co | ICP‑OES, GF‑AAS | 0.02–0.1 |
| Chromium | Cr | ICP‑OES, FAAS | 0.1–0.5 |
| Copper | Cu | ICP‑OES, FAAS | 0.1–0.5 |
| Mercury | Hg | CV‑AAS, ICP‑OES | 0.005–0.02 |
| Manganese | Mn | ICP‑OES, FAAS | 0.5–2.0 |
| Molybdenum | Mo | ICP‑OES, GF‑AAS | 0.02–0.1 |
| Nickel | Ni | ICP‑OES, FAAS | 0.1–0.5 |
| Lead | Pb | ICP‑OES, GF‑AAS | 0.05–0.2 |
| Antimony | Sb | ICP‑OES, HG‑AAS | 0.05–0.2 |
| Vanadium | V | ICP‑OES, GF‑AAS | 0.02–0.1 |
| Zinc | Zn | ICP‑OES, FAAS | 0.5–2.0 |
Important: Quantification limits depend on the sensitivity of the instrument and the quality of sample preparation. The standard requires laboratories to verify these limits using spiked samples and certified reference materials.
Analytical Methods
The standard describes two main approaches:
- Oxidative acid digestion in a closed vessel (microwave‑assisted) using a mixture of nitric acid (HNO₃) and hydrogen peroxide (H₂O₂). Digestion conditions (temperature, pressure, time) are specified to ensure complete mineralisation of the biofuel matrix.
- Instrumental determination by inductively coupled plasma optical emission spectrometry (ICP‑OES) or atomic absorption spectrometry (AAS) with either flame (FAAS), graphite furnace (GF‑AAS), or hydride generation (HG‑AAS) for specific elements.
For mercury, a separate cold vapour technique (CV‑AAS) is mandatory because of its volatility. The standard also includes instructions for handling and preserving samples to prevent contamination or loss of volatile elements.
Implementation Highlights
Sample Preparation
Proper sample preparation is critical for achieving reliable results. ISO 16968:2015 requires that the laboratory sample be dried at 105 °C to constant mass, then milled to a particle size of ≤ 1 mm. Digestion must be carried out in a closed system (microwave or high‑pressure autoclave) to avoid loss of volatile elements (As, Hg, Sb).
Warning: Use of open‑vessel digestion is not permitted for the determination of Hg, As, or Sb. These elements require closed‑vessel microwave digestion to maintain recovery within the required range (90–110 %).
Quality Control
The standard mandates the following quality control measures:
- Blank determination for each batch.
- Recovery check using a suitable certified reference material (CRM) of solid biofuel or a spiked sample.
- Duplicate analysis for at least 10 % of samples; the relative standard deviation must not exceed 10 % for levels above the quantification limit.
- Linearity verification of the calibration curve (correlation coefficient ≥ 0.995).
Good Practice: Laboratories are encouraged to participate in interlaboratory comparisons (e.g., through regional proficiency testing programmes) to validate their performance and maintain accreditation to ISO/IEC 17025.
Interferences and Correction
Spectral interferences in ICP‑OES (e.g., iron and aluminium on arsenic lines) must be corrected using inter‑element correction factors or by selecting alternative wavelengths. For GF‑AAS, matrix modifiers must be used to control background absorption. The standard specifies acceptance criteria for interference checks.
Compliance Notes for Canadian Adoption
CSA ISO 16968‑15 is recognised by the Standards Council of Canada and is referenced by several provincial biofuel quality programs. Compliance with this standard is often required for:
- Export certification of solid biofuels under the Canadian Biofuel Quality Assurance Program.
- Meeting emission limits under the Canadian Environmental Protection Act (CEPA).
- Ash disposal or land application of biofuel combustion residues.
Because the standard does not set maximum permissible concentrations, individual contracts or regulations may specify limit values. The laboratory’s report must clearly state the method used and the measurement uncertainty. Any modification to the digestion or detection procedure (e.g., use of ICP‑MS instead of ICP‑OES) requires full validation and documented equivalence.
Non‑compliance risk: Failure to follow the closed‑vessel digestion requirements for Hg may result in inaccurate low results, leading to non‑compliance with regulatory limits and potential legal liability.
Conclusion
ISO 16968:2015 / CSA ISO 16968‑15 provides a robust framework for the determination of 13 minor elements in solid biofuels. By specifying both digestion and instrumental analysis conditions, it ensures comparability of results across laboratories and supports the safe and sustainable use of biofuels. Adherence to the quality control requirements and attention to matrix‑specific interferences are essential for reliable data. Laboratories should keep their equipment calibrated and staff trained to maintain accreditation.
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Q: What is the difference between ISO 16968 and CSA ISO 16968‑15?
A: There is no technical difference. CSA ISO 16968‑15 is an identical adoption of ISO 16968:2015 by the Canadian Standards Association. Laboratories and manufacturers in Canada should reference the CSA number for regulatory compliance, but the test methods are the same.
A: There is no technical difference. CSA ISO 16968‑15 is an identical adoption of ISO 16968:2015 by the Canadian Standards Association. Laboratories and manufacturers in Canada should reference the CSA number for regulatory compliance, but the test methods are the same.
Q: Can ICP‑MS be used instead of ICP‑OES for the determination of minor elements?
A: The standard specifically lists ICP‑OES and AAS techniques. If ICP‑MS is used, the laboratory must demonstrate that the method meets the equivalent performance criteria (quantification limits, precision, bias) described in the standard. Full validation and documentation of the alternative method are necessary.
A: The standard specifically lists ICP‑OES and AAS techniques. If ICP‑MS is used, the laboratory must demonstrate that the method meets the equivalent performance criteria (quantification limits, precision, bias) described in the standard. Full validation and documentation of the alternative method are necessary.
Q: Does the standard include maximum permissible limits for any elements?
A: No. ISO 16968:2015 is purely an analytical test method standard. It does not define maximum allowed concentrations. Such limits are set by product specifications, regulatory bodies, or purchasing agreements. The standard only ensures that the measurements are accurate and reproducible.
A: No. ISO 16968:2015 is purely an analytical test method standard. It does not define maximum allowed concentrations. Such limits are set by product specifications, regulatory bodies, or purchasing agreements. The standard only ensures that the measurements are accurate and reproducible.
Q: What sample pre‑treatment is required for mercury analysis?
A: For mercury, the standard requires closed‑vessel microwave digestion with a special oxidising mixture to retain volatile Hg. The digestate is then analysed by cold‑vapour atomic absorption spectrometry (CV‑AAS) or inductively coupled plasma optical emission spectrometry (ICP‑OES) with a cold‑vapour generation accessory. Drying step must be performed carefully to avoid loss.
A: For mercury, the standard requires closed‑vessel microwave digestion with a special oxidising mixture to retain volatile Hg. The digestate is then analysed by cold‑vapour atomic absorption spectrometry (CV‑AAS) or inductively coupled plasma optical emission spectrometry (ICP‑OES) with a cold‑vapour generation accessory. Drying step must be performed carefully to avoid loss.