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Category: Gas Analytics | Trace Analytics | Process Analytical Technology | High-Purity Gas | PAT
Author: Dusko Kadijevic, Publisher Analytic Journal
Last updated: May 2026
Reading time: approx. 10 minutes
Target audience: Process engineers, PAT specialists, analytical chemists in semiconductor, pharmaceutical, chemical and high-purity gas industries
Speciality methods in gas analytics refer to measurement principles deployed for trace concentrations in the ppb-to-ppt range, for particularly aggressive process gases, for high-purity gas quality control, or for safety applications with the highest demands on selectivity and sensitivity. They complement established standard methods — NDIR, TDLS, paramagnetic, GC — in applications where those reach their physical or practical limits.
The four most important speciality methods: CRDS (Cavity Ring-Down Spectroscopy), QCL (Quantum Cascade Laser Spectroscopy), PAS (Photoacoustic Spectroscopy) and IMS (Ion Mobility Spectrometry).
Standard methods are sufficient for the majority of industrial measurement tasks. Speciality methods become necessary when:
CRDS is an optical absorption technique that exploits a fundamental physical characteristic: instead of measuring the attenuation of a light beam passing through the sample gas, CRDS measures the decay time (ring-down time) of a light pulse in an optical resonator formed by two highly reflective mirrors.
A short laser pulse is coupled into the resonator. The light bounces between the mirrors — thousands of times — losing a small fraction at each pass through mirror transmission and gas absorption. The time for the signal to decay to a defined fraction of its initial value (the ring-down time) is directly related to the gas concentration.
The decisive advantage: since the ring-down time is an absolute temporal measurement, CRDS requires no reference gas for calibration. The method is self-calibrating — a significant practical advantage in applications where calibration gases are difficult to obtain or expensive.
Detection limits in the ppt range (10⁻¹²), absolute measurement principle without calibration gas, very high long-term stability, low cross-sensitivity through narrowband laser light sources.
The highly reflective mirrors are sensitive — contamination or deposits shorten the ring-down time and simulate higher concentrations than are actually present. This is the critical weakness for process applications with particulate-laden or condensing gases. For this reason, CRDS in the process industry is almost exclusively used extractively with careful sample conditioning — not in-situ.
Practical note from the field — CRDS for H₂O in HCl gas: In one facility, H₂O in HCl gas was measured in the concentration range of 200–600 ppb using CRDS. The sensitivity of CRDS was ideal for this task. Since HCl is extremely aggressive and would immediately destroy the sensitive resonator mirrors, an online measurement with careful sample conditioning was designed — electropolished lines to minimise wall adsorption effects. Despite these measures, a pronounced memory effect appeared: the measured H₂O concentration responded sluggishly to genuine concentration changes — the pipe walls adsorbed and desorbed H₂O slowly, producing a day-night cycle in the measured values. The solution was heating the sample line to a constant temperature — this stabilised the adsorption equilibria and eliminated the memory effect. The lesson: in trace measurements at the ppb level, the sample line is as much part of the measurement system as the analyser itself.
QCL is a laser spectroscopy technique operating in the mid-infrared (MIR) — typically in the wavelength range of 3 to 15 µm. This is the range of the so-called fundamental vibrations of gas molecules — the physically strongest and most characteristic absorption transitions of most compounds.
For comparison: TDLS operates in the near-infrared (NIR) and captures overtones — considerably weaker transitions. QCL accesses the fundamental transitions directly. This yields substantially stronger signal intensity and thus greater potential sensitivity compared to NIR-TDLS.
Why QCL is not yet as widespread as TDLS:
QCL lasers may require cooling depending on wavelength — not all wavelengths operate stably at room temperature. Manufacturing costs for QCL lasers are higher than for NIR diode lasers. Detectors for the MIR range are more complex. Hybrid systems combining QCL and TDLS are gaining importance — particularly for multicomponent measurements at highest sensitivity.
Very high signal intensity through fundamental transitions, broad spectral coverage, excellent for molecules with weak NIR signals (e.g. N₂O, SO₂, many organic compounds), increasingly operable at room temperature through ongoing development.
Higher system costs than NIR-TDLS, partial cooling requirements, more limited field data base than TDLS for SIL applications, etalon effects can be more pronounced in the MIR.
PAS exploits a different physical effect than classical optical absorption measurement. Modulated light is absorbed by the sample gas. The absorbed photons are converted to heat — and this periodic heating generates pressure waves detected by a microphone as an acoustic signal. The measurement signal is proportional to gas concentration.
The particular advantage of PAS: the signal only originates where light is absorbed — non-absorbing background gases generate no signal. PAS is therefore highly selective and well suited for compact, robust systems.
Compact system design possible, no complex optical reference beam path required, very sensitive in the ppm-to-ppb range, low maintenance, well suited for portable or automated systems.
Ambient noise can interfere with measurement — PAS measures acoustically and is therefore sensitive to vibrations and acoustic disturbances. Not suited for high-pressure applications without design adaptation. Cross-sensitivities from other absorbing gases in the spectral range of the laser used.
Practical note from the field — PAS in automated air separation analytics: In an air separation plant, an automatic analysis of the tank fill is performed when vehicles are loaded with gases such as liquid nitrogen or oxygen. The analysis certificate is provided to the driver automatically — without any personnel involvement. PAS enables precisely this kind of automated, low-maintenance inline analytics: compact, reliable, without continuous staffing. The entire process chain — filling, analysis, certificate generation — runs fully automatically.
IMS ionises gas molecules and separates the generated ions according to their mobility in an electric field. Different molecules have characteristic mobility values — the "fingerprint" signal for identification. IMS operates at atmospheric pressure and delivers results within seconds to milliseconds.
IMS is extremely sensitive — detection limits in the ppt range are achievable. The technology is best known from security applications (explosive detection at airports) but is increasingly important for industrial trace gas analytics and quality control in pharmaceutical manufacturing.
Very high sensitivity (ppt range), very fast response times (seconds to milliseconds), compact and portable systems available, no carrier gas required, well suited for field applications without laboratory infrastructure.
Cross-sensitivities are the greatest challenge — IMS separates by mobility, and different compounds can have similar mobility values. In complex gas matrices, identification can become uncertain. High ambient humidity influences ionisation and can reduce selectivity. For quantitative measurements in complex matrices, IMS is more demanding than other methods.
Practical note from the field — IMS handheld device for DMS leak detection: At one facility, an IMS handheld device was used for rapid detection of DMS (dimethyl sulphate, a methylating agent) to quickly locate leaks in the plant. Sensitivity was impressive — even minor leaks were rapidly localised. The experience also demonstrated the cross-sensitivity challenge: IMS responded similarly to several compounds. Unambiguous identification required experience and contextual knowledge. For leak searching in a known facility with a defined target substance, the device was very useful — for identifying unknown substances in complex matrices it would be too uncertain.
The four speciality methods are so different in their fundamental logic that a direct comparison in a single matrix would be misleading. The right question is not "which is better" — but which was designed for the relevant context.
| Method | Typical Context | Core Strength | Typical Limitation |
|---|---|---|---|
| CRDS | High-purity gas quality control, atmospheric research, aggressive gases in ppb/ppt range | Absolute measurement without calibration gas, highest sensitivity | Mirrors sensitive — extractive only, with careful sample conditioning |
| QCL | CEMS at highest sensitivity, marine exhaust monitoring, fugitive emissions, multicomponent | Strong fundamental transitions in MIR — more sensitive than NIR-TDLS | Higher cost, partial cooling required, limited field data base |
| PAS | Automated high-purity gas certification, SF₆ leak detection, compact continuous operation systems | Compact, low maintenance, well suited for autonomous continuous operation | Sensitive to vibrations and ambient acoustic noise |
| IMS | Rapid field measurement, leak detection, security applications, CBRN protection | Portable, very fast, ppt sensitivity without laboratory infrastructure | Cross-sensitivities complicate identification in complex matrices |
Practical decision logic:
| Characteristic | CRDS | QCL | PAS | IMS |
|---|---|---|---|---|
| Detection limit | ppt | ppb–ppt | ppb–ppm | ppt |
| Selectivity | Very high | Very high | High | Medium (cross-sensitivities) |
| Calibration gas required | No (absolute) | Yes | Yes | Yes |
| Compactness | Medium | Medium | High | Very high (portable) |
| In-situ capability | No (extractive) | Conditional | Conditional | No (extractive/portable) |
| Robustness in process environments | Low (mirrors sensitive) | Medium | Medium | Medium |
| Cost | High | Very high | Medium | Medium |
| Typical application domain | High-purity gas, atmosphere | CEMS, trace gas, process | Compact systems, automation | Security, leak detection, field |
| If... | Then speciality method | Standard method sufficient |
|---|---|---|
| Concentration < 1 ppm | CRDS, QCL, IMS | — |
| Absolute method without calibration gas | CRDS | — |
| Aggressive gas destroys contact sensors | CRDS (extractive with conditioning) | TDLS (in-situ) |
| Mobile field use without laboratory | IMS, PAS | — |
| Automated high-purity gas certification | PAS | — |
| Concentration > 1 ppm, standard matrix | — | NDIR, TDLS, paramagnetic, GC |
For CEMS applications involving QCL or other speciality methods, acceptance depends on the applicable EPA Performance Specification under 40 CFR Part 60. Method-specific approval is required — QCL-based systems for SO₂ or NOₓ monitoring must demonstrate compliance with PS-2 or PS-7 as applicable.
For ship emissions monitoring, the US Coast Guard enforces MARPOL Annex VI requirements in US waters and Emission Control Areas (ECAs) along the US coastline. Shore-based and portable monitoring of vessel emissions increasingly uses QCL-based systems.
For high-purity gas applications in semiconductor manufacturing, SEMI standards govern purity specifications — CRDS is a recognised measurement technique for trace moisture and trace impurity monitoring in specialty gases.
Following Brexit, UK ship emission monitoring operates under the Marine Pollution Regulations implementing MARPOL Annex VI domestically, enforced by the Maritime and Coastguard Agency (MCA). QCL-based monitoring systems used for CEMS require MCERTS certification where applicable.
For high-purity gas applications, UK Pharmacopoeia and MHRA guidance apply in pharmaceutical contexts. CRDS and QCL are accepted analytical techniques for trace impurity determination where suitable validation has been performed.
| Standard / Regulation | Relevance |
|---|---|
| ISO 14687 | Hydrogen quality for fuel cell applications — CRDS and QCL as recognised analytical techniques |
| ASTM D7607 | Moisture measurement in gases — relevant for CRDS applications |
| EN 15267 | Certification of automated measuring systems — applicable when CRDS/QCL/PAS used for CEMS |
| IMO MARPOL Annex VI | Ship emission limits — driver for QCL-based SO₂/NOₓ monitoring |
| 40 CFR Part 60, PS series | US EPA performance specifications — applicable to QCL CEMS use |
| SEMI standards | Semiconductor industry gas purity specifications — relevant for CRDS trace monitoring |
| NAMUR NE 107 | Self-monitoring and diagnostics of field devices |
What is the difference between CRDS and TDLS?
Both are optical absorption techniques using laser light sources. TDLS measures the direct attenuation of laser light passing through the gas — and requires a reference gas for calibration. CRDS measures the decay time of a light pulse in a resonator — an absolute measurement principle requiring no calibration gas. CRDS achieves considerably lower detection limits (ppt vs ppb) but is less robust for harsh process environments due to its sensitive mirrors.
Why is QCL more sensitive than TDLS?
TDLS operates in the near-infrared (NIR) and captures overtones of molecular vibrations — physically weaker transitions. QCL operates in the mid-infrared (MIR) and captures the fundamental vibrations — the physically strongest absorption transitions of most molecules. This yields substantially stronger absorption signals and therefore potentially lower detection limits.
What is the memory effect and why is it critical in trace measurements?
The memory effect describes the adsorption and delayed desorption of gas molecules at pipe walls and sample conditioning surfaces. In trace measurements at the ppb level, even electropolished stainless steel lines can store enough H₂O or other polar compounds to cause sluggish measurement response and apparent day-night cycles in readings. The solution is typically heating the sample line to a defined constant temperature to stabilise adsorption equilibria.
When is IMS useful despite cross-sensitivities?
IMS is well suited when the objective is to rapidly and sensitively detect a known substance in a known environment — without requiring complete identification or quantification. For leak detection in a known facility with a defined target substance, for explosive detection at airports, or for contamination screening in cleanrooms, IMS is excellent. For identifying unknown substances in complex matrices it is less suited.
Can PAS be used for continuous process monitoring?
Yes — PAS is well suited for continuous, automated process monitoring when vibrations and ambient noise are controlled. In compact, well-shielded systems, PAS delivers very stable long-term measurements. The automated high-purity gas certification in air separation filling operations is a good example of successful continuous unattended operation.
Which speciality method is suited for H₂O in aggressive gases?
CRDS is the preferred method for H₂O trace measurements in aggressive gases such as HCl or Cl₂ — because the sensor has no contact with the gas and because sensitivity in the ppt-to-ppb range is ideal for this task. The critical challenge is sample conditioning: heated lines, electropolished surfaces and careful conditioning are essential to control the memory effect.
What drives QCL adoption in marine emissions monitoring?
IMO MARPOL Annex VI sets sulphur emission limits for ships — 0.1% in Emission Control Areas (ECAs) since 2020, and 0.5% globally since 2020. Port state control authorities and flag states require demonstrable compliance. QCL-based continuous SO₂ monitoring in ship exhaust provides the sensitivity and response speed needed for real-time compliance verification — conventional NDIR systems often lack sufficient sensitivity at the required detection levels.
CRDS for atmospheric measurements and climate research
CRDS is widely used in atmospheric science and climate monitoring (CO₂, CH₄, N₂O, δ¹³C isotope ratios). These applications differ substantially from Process Analytical Technology (PAT) and are treated in a separate context.
Further speciality methods
SIFT-MS (Selected Ion Flow Tube Mass Spectrometry), FAIMS (Field Asymmetric Ion Mobility Spectrometry) and other specialised techniques for breath gas analytics, food quality control and research applications are not covered in this overview.
Speciality methods in gas analytics solve measurement tasks that standard methods cannot address — primarily in the trace concentration range, with aggressive media, and in automated applications without laboratory infrastructure.
CRDS is the method of choice when absolute accuracy without calibration gas at the highest sensitivity is required. QCL opens the mid-infrared range with its strong fundamental transitions — increasingly relevant for marine emissions monitoring and high-sensitivity CEMS. PAS enables compact, automated systems for unattended continuous operation. IMS delivers the fastest field responses with simple operation — with the limitation that cross-sensitivities complicate interpretation in complex matrices.
The practical challenges lie less in the measurement principle itself than in sample conditioning and system integration — as the memory effect in H₂O trace measurements and the mirror sensitivity in CRDS demonstrate.
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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