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Category: Gas Analytics | Process Analytical Technology | Emissions Monitoring | PAT
Author: Dusko Kadijevic, Publisher Analytic Journal
Last updated: May 2026
Reading time: approx. 10 minutes
Target audience: Process engineers, instrumentation engineers, PAT specialists, plant safety managers, environmental compliance officers
Is the target component clearly defined, does the concentration range fit, is in-situ measurement feasible, are temperature, dust, moisture and corrosion manageable, and are regulatory requirements met?
| Question | ✅ Green | ⚠️ Yellow | ❌ Red |
|---|---|---|---|
| 1. Target component known? | Single component clearly defined | Multiple components, but prioritisable | Target unclear or variable |
| 2. Concentration range suitable? | Range matches the principle well | Borderline range or high matrix influence | Measurement range unsuitable |
| 3. In-situ feasible? | Direct measurement in process possible | Possible with limitations only | Extractive only |
| 4. Process conditions manageable? | Temperature, dust, moisture, corrosion uncritical | Individual interference factors present | Heavily loaded or aggressive matrix |
| 5. Regulatory/safety requirements met? | ATEX/UKCA, SIL, CEMS, QAL1 or EPA PS clearly achievable | Additional testing required | Regulatory compliance not demonstrable |
Process gas analytics refers to the continuous or discontinuous measurement of the chemical composition of gas streams in industrial processes — with the objectives of assuring product quality, improving process efficiency, maintaining compliance with emission regulations, and detecting safety hazards in time.
Unlike laboratory analysis, process gas analytics operates under real industrial conditions: fluctuating temperatures, pressures, moisture contents, particulate loads, and aggressive interfering components. The robustness of a measurement principle under these conditions is at least as important as its laboratory accuracy.
| Principle | Target Components | Measurement Location | Key Characteristics |
|---|---|---|---|
| NDIR | CO, CO₂, CH₄, SO₂, NO, N₂O, H₂O | Extractive (online) | Established, cost-effective, broad application |
| TDLS | O₂, H₂O, CO, NH₃, HF, HCl, CH₄ | In-situ (inline) | Highly selective, low maintenance, no sample gas |
| Paramagnetic | O₂ | Extractive (online) | Specific to O₂, robust, long track record |
| Zirconia (ZrO₂) | O₂ | In-situ or extractive | Very fast, ideal for combustion control |
| Electrochemical | O₂, CO, H₂S, Cl₂, NH₃ | Extractive | Low cost, limited service life |
| FID | Total hydrocarbons (THC), VOC | Extractive | Very sensitive, requires H₂ fuel gas |
| Process GC | Multicomponent, individual species | Extractive | Highest specificity, discontinuous |
| FTIR | Multicomponent simultaneous | Extractive or in-situ | Broad spectrum, complex calibration |
| Mass Spectrometry | Multicomponent simultaneous | Extractive | Highest sensitivity, high investment |
| Thermal Conductivity (TCD/WLD) | H₂, He, CO₂ as major components | Extractive | Simple, non-specific |
| Chemiluminescence (CLD) | NO, NO₂, NOₓ | Extractive | Specific for nitrogen oxides, CEMS standard |
| UV Photometry | SO₂, NO, NH₃, H₂S | Extractive or in-situ | Selective in UV range |
Principle: Infrared light passes through the sample gas. Each gas molecule absorbs infrared radiation at characteristic wavelengths. The attenuation of the light beam is proportional to the concentration of the target component.
Strengths: Robust, cost-effective, decades of proven performance, suitable for a wide range of industrial gases, good long-term stability, largely maintenance-free.
Limitations: Cross-sensitivities due to spectral overlap — CO₂ interferes with CO measurement, H₂O interferes with many components. Sample conditioning (cooling, filtration, drying) is typically required.
Typical applications:
Principle: A tunable diode laser emits light precisely targeted at a characteristic absorption line of the target molecule. Since each gas has unique absorption lines, selectivity is exceptionally high. Measurement occurs in-situ — the laser beam crosses the process gas stream directly in the pipe or duct, without sample extraction.
Strengths: Highest selectivity among optical methods, no sample gas, no sample conditioning, fast response time, integrated self-diagnostics, low maintenance, suitable for the most demanding process environments.
Limitations: Heavily particulate-laden streams can attenuate the laser signal. Higher capital cost. Not suitable for liquid media. Pressure dependence of absorption line width must be accounted for in system design.
Typical applications:
Principle: Oxygen is paramagnetic — it is attracted by magnetic fields. Most other gases are diamagnetic. This physical property is used to measure O₂ selectively without spectroscopic methods.
Strengths: Very specific to O₂, proven over decades, good long-term stability, wide measurement range (ppm to %).
Limitations: Online principle — requires sample conditioning (filtration, cooling, pressure regulation). Cross-sensitivity to NO and NO₂, which are also paramagnetic. Not suitable for in-situ measurement.
Typical applications:
Principle: A heated zirconia ceramic cell (operating at approximately 650–850°C) generates an electrochemical voltage proportional to the difference in oxygen partial pressure between the sample gas and a reference air stream.
Strengths: Extremely fast response (seconds), direct in-situ measurement possible, robust at high temperatures, preferred principle for combustion control applications.
Limitations: Requires reference air or reference gas. Cross-sensitivity to combustible gases (H₂, CO, CH₄) that consume O₂ at the sensor surface, causing low readings. Special design required for hazardous area use.
Typical applications:
Principle: The sample gas is burned in a hydrogen flame. The resulting ions generate an electrical current proportional to the carbon content of organic compounds — regardless of molecular structure.
Strengths: Very high sensitivity (ppb range), measures virtually all organic compounds as a sum parameter (THC), largely unaffected by water and most inorganic gases.
Limitations: Requires hydrogen as fuel gas — safety requirements and gas supply logistics. No component identification (sum parameter only). CO and CO₂ are not detected. Careful sample conditioning required.
Typical applications:
Principle: The gas mixture is passed through a separation column. Different components travel at different speeds and are detected sequentially. Common detectors include TCD, FID, and mass spectrometric detectors.
Strengths: Highest specificity — individual components in complex gas mixtures can be identified and quantified. Multicomponent measurement in a single analytical cycle. Optimised for continuous industrial operation.
Limitations: Discontinuous principle — cycle times ranging from minutes to one hour. High capital and maintenance cost. Requires trained personnel for method development and upkeep.
Typical applications:
Principle: An interferometer generates a broadband infrared spectrum that interacts with the sample gas. Mathematical transformation and spectral library comparison enable component identification and quantification.
Strengths: Simultaneous multicomponent measurement without hardware changes, broad component spectrum, high flexibility when measurement requirements change.
Limitations: Complex calibration and maintenance. H₂ and monoatomic gases (He, Ar, N₂, O₂) are infrared-inactive. Cross-sensitivities between spectrally similar components require extensive matrix calibration.
Typical applications:
Principle: Gas molecules are ionised, separated by their mass-to-charge ratio (m/z), and detected. Each compound produces a characteristic fragmentation pattern.
Strengths: Highest sensitivity (sub-ppb range), simultaneous multicomponent measurement, identification of unknown substances, very short measurement cycles possible.
Limitations: High capital and operating cost, high-vacuum system required, demanding sample conditioning, specialist personnel needed.
Typical applications:
Legend: ✅ Well suited | ⚠️ Conditionally suited | ❌ Not suited
| Measurement Task | NDIR | TDLS | Para-mag. | ZrO₂ | FID | GC | FTIR | MS |
|---|---|---|---|---|---|---|---|---|
| O₂ monitoring — inerting | ❌ | ✅ | ✅ | ⚠️ | ❌ | ❌ | ❌ | ❌ |
| Combustion control (O₂) | ❌ | ✅ | ⚠️ | ✅ | ❌ | ❌ | ❌ | ❌ |
| CO/CO₂ — flue gas | ✅ | ✅ | ❌ | ❌ | ❌ | ⚠️ | ✅ | ⚠️ |
| Total hydrocarbons (THC/VOC) | ❌ | ❌ | ❌ | ❌ | ✅ | ⚠️ | ⚠️ | ⚠️ |
| NOₓ emissions monitoring | ✅ | ⚠️ | ❌ | ❌ | ❌ | ❌ | ✅ | ⚠️ |
| SO₂ emissions monitoring | ✅ | ❌ | ❌ | ❌ | ❌ | ❌ | ✅ | ⚠️ |
| NH₃ process / SCR | ⚠️ | ✅ | ❌ | ❌ | ❌ | ⚠️ | ✅ | ⚠️ |
| H₂O / dewpoint — natural gas | ⚠️ | ✅ | ❌ | ❌ | ❌ | ⚠️ | ✅ | ❌ |
| Natural gas / heating value | ❌ | ❌ | ❌ | ❌ | ❌ | ✅ | ⚠️ | ⚠️ |
| Noble gas purity control | ❌ | ❌ | ❌ | ❌ | ❌ | ✅ | ❌ | ✅ |
| Multicomponent simultaneous | ❌ | ❌ | ❌ | ❌ | ❌ | ✅ | ✅ | ✅ |
| H₂ measurement | ❌ | ❌ | ❌ | ❌ | ❌ | ✅ | ❌ | ✅ |
| HF/HCl trace measurement | ❌ | ✅ | ❌ | ❌ | ❌ | ⚠️ | ✅ | ⚠️ |
| SIL-relevant safety monitoring | ⚠️ | ✅ | ✅ | ⚠️ | ❌ | ❌ | ❌ | ❌ |
This matrix provides initial orientation. Final principle selection always requires an application-specific assessment — particularly regarding interfering components, process conditions, and regulatory requirements.
Single-component measurement at percent level: NDIR, Paramagnetic, ZrO₂ — cost-effective and robust. Trace measurement at ppb level: TDLS, MS, FID — higher investment, higher specificity. Multicomponent measurement: GC, FTIR, MS — depending on cycle time requirements and budget.
Inline (in-situ): The sensor measures directly in the process stream — no sample gas, no sample line, no conditioning. Fastest response, fewest failure sources. Suitable: TDLS, ZrO₂, certain UV systems.
Online (extractive): A sample conditioning system extracts, cools, filters, and conditions the gas before analysis. Enables more sensitive analysers. Suitable: NDIR, Paramagnetic, FID, GC, MS, FTIR.
| Criterion | Inline | Online (extractive) |
|---|---|---|
| Response time | Very fast (seconds) | Slower (minutes) |
| Sample conditioning | Not required | Required |
| Failure sources | Few | More |
| Maintenance burden | Lower | Higher |
| Compatible principles | TDLS, ZrO₂, UV | NDIR, GC, FID, MS, Paramagnetic |
| Component | Function | Typical Failure Mode |
|---|---|---|
| Filter cartridge | Particulate removal | Blockage, bypass leakage |
| Sample cooler / Peltier cooler | Moisture removal | Condensate accumulation |
| Diaphragm pump | Gas transport | Membrane wear |
| Heated sample line | Condensation prevention | Heating failure |
| Flow meter / controller | Constant sample gas flow | Drift, blockage |
Chemical Industry: Process gas composition monitoring (synthesis gas, reactor gas), combustion gas analysis (heating value, Wobbe index), inerting control (O₂), continuous emissions monitoring (CEMS). Central challenge: aggressive, corrosive, or toxic process gases require careful material selection and specialised sample conditioning. Cross-sensitivities in complex gas matrices must be excluded through validation studies.
Oil, Gas and Petrochemicals: Heating value measurement and custody transfer (process GC, calorimeter), gas quality monitoring (GC), H₂S monitoring (electrochemical, UV), BTX measurement (GC, FID), CEMS at flares and combustion units. In the US, 40 CFR Part 60 Subpart J applies to petroleum refineries; in the UK, permits under the Environmental Permitting Regulations govern refinery emissions.
Pharmaceutical Industry: Purity control of process gases and compressed air (O₂, CO₂, H₂O), fermentation gas analysis (O₂, CO₂, ethanol), inert gas monitoring, CIP/SIP process gas monitoring. FDA 21 CFR Part 11 data integrity requirements apply to automated monitoring systems in the US; UK equivalent guidance applies under MHRA.
Power Generation and Utilities: Combustion optimisation (O₂, CO via ZrO₂ and NDIR), CEMS for SO₂, NOₓ, CO (mandatory under 40 CFR Part 75 in the US; IED permit conditions in the UK), SCR ammonia slip measurement (NH₃ via TDLS or FTIR), combustion gas heating value measurement.
Food and Beverage: CO₂ purity monitoring (NDIR, GC), protective atmosphere monitoring in packaging lines (O₂, CO₂), fermentation gas analysis in breweries, ethanol vapour measurement.
| Standard / Regulation | Region | Scope |
|---|---|---|
| 40 CFR Part 60 / Appendix B | USA | EPA CEMS performance specifications (PS-1 to PS-19) for stationary sources |
| 40 CFR Part 75 | USA | Acid Rain Program — SO₂/NOₓ continuous monitoring for electric utilities |
| EPA Methods 6C, 7E, 10, 25A | USA | Reference methods for SO₂, NOₓ, CO, THC |
| BS EN 14181 | UK / EU | Quality assurance for automated measuring equipment — QAL1, QAL2, QAL3, AST |
| BS EN 15267 | UK / EU | Certification of automated measuring systems for emissions monitoring |
| EA Guidance M2 | UK | Environment Agency — stack emissions monitoring to atmosphere |
| Industrial Emissions Directive (IED) | EU / UK (retained) | Framework for industrial emissions permitting |
| TA Luft / 17. BImSchV | Germany | German emission limit values — reference for EU comparison |
| ISO 10723 | International | Performance evaluation framework for gas analysers |
| ASTM D1945 / D1946 | USA | Natural gas composition analysis by GC — custody transfer standard |
| IEC 60079 / ATEX | EU | Equipment for explosive atmospheres |
| UKCA marking | UK | UK conformity assessment post-Brexit — replaces CE/ATEX for UK market |
| NAMUR NE 107 | International | Self-monitoring and diagnostics of field devices |
What is the difference between inline and online measurement in gas analytics?
Inline (in-situ) means the sensor measures directly in the process gas stream — without sample extraction. Online (extractive) means a sample is withdrawn, conditioned, and fed to the analyser. Inline is faster and has fewer failure sources. Online enables more sensitive analysers and is often the only practical solution for aggressive or condensing media.
Why is TDLS increasingly the first choice for safety-critical monitoring?
TDLS measures inline, without sample conditioning and without consumables. Its high selectivity and integrated self-diagnostic functions make undetected failures less frequent than with other principles — key criteria for SIL-relevant safety applications.
When is a process GC the right choice?
When individual components in complex gas mixtures must be specifically and quantitatively determined — and when a cycle time of minutes is acceptable. Typical applications: heating value measurement, synthesis gas analysis, purity control in petrochemicals, and natural gas custody transfer per ASTM D1945/D1946 (US).
What is CEMS and which regulations apply in the US and UK?
CEMS stands for Continuous Emission Monitoring System. In the United States, 40 CFR Parts 60 and 75 govern CEMS requirements with EPA performance specifications and mandatory RATA audits. In the UK, BS EN 14181 (QAL1/QAL2/AST) and BS EN 15267 apply, administered through site-specific environmental permits issued by the Environment Agency.
Why is sample conditioning so critical?
The measurement result of an extractive analyser is only as reliable as its sample conditioning. Filters, coolers, pumps, and lines can alter the sample gas through condensation, chemical reaction, or pressure loss. In SIL applications, the entire sample conditioning system must be included in the safety assessment.
Which principles are suitable for hazardous areas (ATEX/UKCA)?
All principles in principle — provided they are installed in appropriately certified enclosures per IEC 60079. In the UK, UKCA marking applies post-Brexit for the domestic market. In-situ principles such as TDLS and ZrO₂ have the advantage that analyser electronics can be located outside the hazardous zone.
What is QAL1 under EN 14181?
QAL1 (Quality Assurance Level 1) is the certification that a measuring device is suitable for continuous emission monitoring under European standards. It is granted by accredited third-party bodies (e.g. TÜV, MCERTS-approved bodies in the UK) and is a prerequisite for regulatory-compliant operation of CEMS installations under IED and UK EPR permits.
What is a RATA and when is it required?
RATA (Relative Accuracy Test Audit) is a US EPA requirement under 40 CFR Part 60 Appendix F. It compares CEMS measurements against certified reference method measurements taken simultaneously. RATA must be conducted at installation and annually thereafter to confirm that the CEMS produces results within acceptable accuracy limits.
Speciality methods for ultra-pure and trace gas analysis
For applications in the ppb-to-ppt range — particularly in semiconductor manufacturing, specialty gas production, and pharmaceutical environments — measurement principles exist beyond the scope of this overview: CRDS (Cavity Ring-Down Spectroscopy), QCL (Quantum Cascade Laser), PAS (Photoacoustic Spectroscopy), IMS (Ion Mobility Spectrometry).
These methods are covered in a dedicated Analytic Journal article: "Speciality Methods in Gas Analytics — Trace Measurement and Ultra-High Purity Applications" (forthcoming)
Fixed and portable gas detection systems
Gas warning systems follow their own selection logic: response time, robustness, ATEX/UKCA certification, alarm thresholds per OSHA/HSE requirements, and the distinction between personnel protection and process monitoring.
This topic is covered in a dedicated Analytic Journal article: "Fixed and Portable Gas Detection Systems — Personnel Safety, ATEX/UKCA and Measurement Principles" (forthcoming)
Gas analytics in the process industry is not a question of a single measurement principle — it is a balance between target component, process conditions, measurement location strategy, regulatory requirements, and economics. The application matrix in this article provides initial orientation; the final decision always requires an application-specific assessment.
Three developments are shaping process gas analytics in the coming years: In-situ measurement continues to grow — TDLS and related laser technologies are displacing extractive systems in more applications because they are more robust, lower-maintenance, and SIL-friendly. Regulatory pressure is increasing — tighter emission limits under the Clean Air Act, IED, and emerging carbon monitoring requirements raise the bar for CEMS systems. And data quality is becoming a differentiating factor — analysers with integrated self-diagnostics (NAMUR NE 107), digital communication, and auditable calibration histories are no longer optional.
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This article has been written to the best of our knowledge based on publicly available technical information, standards, and manufacturer data. Specifications may change — for binding information, consult current manufacturer datasheets and relevant standards bodies. Analytic Journal produces editorial content independently of manufacturers and advertising partners.
Author: Dusko Kadijevic — 20 years of experience as a PAT practitioner in the process industry
Dusko Kadijevic | Publisher Analytic Journal | analyticjournal.com
Written May 2026. All content formulated in our own words, created with AI assistance.
First published on 8 June 2026 on analyticjournal.com