Test certificates for liquids and gases serve the same fundamental purpose as MTCs for metals: they document that a batch of material meets the specification it was supplied to. The document types, required fields, and traceability requirements differ significantly from metal certificates.
Quick Answer
Quick Answer
Test certificates for industrial liquids and gases are either a Certificate of Analysis (CoA), which includes actual analytical test results for each lot, or a Certificate of Conformance (CoC), which is a declaration of meeting specification without test data. A CoA is the standard for chemicals, gases, and reagents where composition directly affects product quality or safety.
Certificate of Analysis (CoA) vs. Certificate of Conformance (CoC)
| Attribute | CoA | CoC |
|---|---|---|
| Contains test data | Yes, actual measured values for each lot | No, declaration only |
| Traceability | To batch/lot number | To product type or catalogue entry |
| When required | Most industrial gases, pharmaceutical chemicals, calibration gases, reference materials | Commodity chemicals, general-purpose solvents, lower-risk applications |
| ISO/GMP requirement | ISO 17025, GMP, Pharmacopoeia require CoA | GMP may accept CoC only for non-critical materials |
For any liquid or gas used in a quality-critical process (calibration, NDT, pharmaceutical manufacturing, food processing, or analytical measurement), a CoA is mandatory.
Certificate of Analysis: Key Fields
A well-structured CoA for a liquid or gas must include:
- Product name and trade name (if applicable)
- Chemical name and formula (for pure substances)
- CAS number. Unique chemical identifier; critical for regulatory compliance.
- Grade / specification. E.g., "Nitrogen, Purity Grade 5.0 (≥99.999%)", "HPLC Grade", "EP Grade".
- Batch / lot number. The primary traceability identifier.
- Expiry date / retest date. Particularly critical for gases and unstable compounds.
- Quantity and packaging. Volume (L or m³), cylinder pressure (bar), container type.
- Analytical results. For each tested parameter:
- Parameter name
- Test method reference (e.g., ISO 6142, USP <621>)
- Specification limit (min, max, or range)
- Measured value
- Pass/Fail
- QC release statement. "Released for use" signed by QC Manager.
- Testing laboratory identification. If analysed by an external lab, include lab name and accreditation number.
Industrial Gas Certificates
Bulk and Cylinder Gas (Nitrogen, Argon, Oxygen, CO₂, Helium)
Industrial gas suppliers issue a CoA for each cylinder or batch. The primary parameters reported:
| Gas | Key parameters |
|---|---|
| Nitrogen (N₂) | Purity (% N₂), O₂ content (ppm), moisture (H₂O ppm), CO content (ppm) |
| Argon (Ar) | Purity, O₂ content, N₂ content, moisture |
| Oxygen (O₂) | Purity, moisture, hydrocarbons (for medical/food grade) |
| Carbon Dioxide (CO₂) | Purity, moisture, SO₂, acetaldehyde, other organic impurities |
| Hydrogen (H₂) | Purity, O₂, moisture, CO |
Purity grades are standardised:
- Grade 2.0 = 99% purity (minimum)
- Grade 4.0 = 99.99% purity
- Grade 5.0 = 99.999% purity
- Grade 6.0 = 99.9999% purity
The grade number corresponds to the number of nines in the purity specification.
Shielding Gas for Welding
EN ISO 14175 defines shielding gas classifications (e.g., M21 for Ar/CO₂ mix). The CoA must state:
- Exact composition percentages of each component gas
- Moisture content (H₂O ppm)
- Oxygen content (O₂ ppm)
- Sulphur compound content (SO₂ ppm)
For ASME code welding, the shielding gas CoA is retained as part of the welder's material traceability record.
Calibration Gas Certificates
Calibration gases are certified reference materials used to calibrate gas analysers, atmospheric monitors, and process instruments.
Requirements
- ISO 17025 accreditation of the preparation laboratory is mandatory
- Certified reference value. The stated composition with uncertainty (e.g., 100 ppm ± 2% CO in N₂).
- Traceability chain. Stated reference to a national metrology standard (NIST, PTB, NPL).
- Stability data. Demonstrated stability throughout the shelf life.
- Certificate expiry date
- Cylinder pressure at fill and at delivery
A calibration gas certificate that lacks a stated uncertainty or traceability chain is not a valid reference material certificate under ISO 17025 requirements.
Chemical / Liquid CoA: Pharmaceutical and Process Chemistry
For chemicals used in pharmaceutical manufacturing, GMP regulations (ICH Q7, 21 CFR Part 211) require:
- CoA for every lot of starting material and reagent
- Results performed on the specific lot supplied (not reference to a general product spec)
- Comparison to compendial methods (USP, BP, EP, JP)
- Date of analysis and date of manufacture
- Retest date or expiry date
- Signature of QC Release Officer
For industrial process chemicals (cleaning agents, pH adjustment chemicals, scale inhibitors), the required rigour depends on the application: food contact or pharmaceutical contact applications require full CoA, while general utilities (cooling water treatment) may accept a CoC.
Traceability: Linking the CoA to the Physical Batch
The batch/lot number on the CoA must match the marking on the container or cylinder. This check is frequently missed:
- For liquids: lot number printed on the label and stated on the delivery note
- For gas cylinders: lot number and cylinder serial number on the tag, valve collar label, or valve
- For bulk gas deliveries: tanker delivery note with batch reference
Loss of traceability (e.g., label defaced, certificate filed without matching lot number) constitutes a non-conformance that may require testing the material or rejecting the lot.
What is the difference between an expiry date and a retest date on a CoA?
An expiry date means the material should not be used after that date without re-testing or disposal. A retest date means the material should be re-analysed to confirm it still meets specification; it may remain in service if it passes the retest. Pharmaceutical and calibration gas CoAs typically specify expiry dates. Some industrial chemicals specify retest dates for long shelf-life products.
Does a welding shielding gas always require a CoA?
ASME IX and most structural codes do not explicitly require a CoA for shielding gas, but they require the shielding gas to conform to the qualified procedure. In practice, quality plans for code construction typically include retention of the shielding gas CoA as a material traceability record linked to the weld journal.
What traceability is required between a gas cylinder and its CoA?
The cylinder serial number or batch/lot number on the CoA must match what is physically marked on the cylinder valve, tag, or collar. The delivery receipt should reference both the cylinder serial number and the CoA lot number. Without this link, the CoA cannot be traced to the specific cylinder used.
Are there international standards for calibration gas CoAs?
Yes. ISO 6141 defines general requirements for the preparation and certification of gas mixtures used as reference materials. ISO 17025 is the overarching accreditation standard for the preparation laboratory. For cylinder gases, ISO 6142-1 covers gravimetric preparation and ISO 6143 covers comparison methods. NIST and NPL are the primary national metrology institutions for traceability in the USA and UK respectively.
Can a CoA from a previous lot be reused for a new delivery?
No. A CoA is specific to the lot/batch it was issued for. Reusing an earlier CoA for a new delivery is a GMP violation and breaks material traceability. Each delivery must be accompanied by a CoA issued for that specific lot. If the supplier provides the same CoA for consecutive deliveries, demand lot-specific documentation.
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