Principles of tunable diode laser absorption spectroscopy (TDLAS)

Tunable diode laser absorption spectroscopy (TDLAS) is a laser-based technique for detecting and quantifying gas concentrations with exceptional precision. It is widely used in industries such as natural gas, petrochemicals, refining, and environmental monitoring, where accurate, real-time gas analysis is critical for safety, compliance, and process optimization.

• In-situ TDLAS - measures gas concentrations directly across the full diameter of a smokestack or duct, providing real-time data without diverting the process flow
• Extractive TDLAS - diverts the process gas through a bypass line to an analyzer, allowing the system to be isolated for calibration, verification, and maintenance

In this article, we take a closer look at extractive TDLAS analysis for quality and process control.

TDLAS works by tuning a diode laser to a specific wavelength that corresponds to an absorption line of the target gas. As the laser passes through the gas sample, molecules absorb light at that wavelength. The amount of absorption reveals the gas concentration—often down to parts-per-billion (ppb).

TDLAS is rooted in Beer-Lambert’s Law, which describes how light is absorbed by a gas:

A = – ln (I/I₀) = X ● P ● S ● ϕ ● L

Where:

• A = absorbance
• I₀ = incident light intensity
• I = transmitted light intensity
• X = mole fraction of the gas
• P = pressure
• S = line strength
• ϕ = line shape
• L = path length

This relationship allows TDLAS systems to calculate gas concentrations with high accuracy, even in complex or variable environments.

TDLAS uses tunable diode lasers—compact, rugged devices that emit light at extremely narrow linewidths. These lasers can be finely tuned across specific absorption lines of target gases. By scanning across wavelengths, TDLAS systems generate a spectral fingerprint that enables precise gas identification and quantification. This tunability is key to avoiding cross-interference and achieving selectivity, especially in multi-component gas streams.

While both TDLAS and non-dispersive infrared (NDIR) techniques are used for gas detection, they differ significantly in precision and performance. TDLAS uses a narrow-linewidth laser tuned to specific absorption lines of the target gas, enabling highly selective and sensitive measurements—even at parts-per-billion (ppb) levels. In contrast, NDIR employs a broadband infrared source and optical filters to isolate absorption bands, which can result in lower resolution and greater susceptibility to cross-interference from other gases. TDLAS also offers faster response times and long-term stability without the need for frequent recalibration, making it ideal for demanding industrial applications where accuracy and reliability are critical.

• Laser source: Tunable diode laser emitting in NIR or mid-IR
• Optical cell: 2-pass cell (simple design for short path measurements) or Herriott cell (multi-pass design for enhanced sensitivity up to 28 meters)
• Detector: Measures transmitted light intensity
• Modulation system: Adds sine wave modulation for improved signal-to-noise ratio
• Signal processor: Uses algorithms to extract gas concentration from spectral data
• Enclosure: Heated and insulated to prevent condensation and stabilize measurements

To improve sensitivity, TDLAS often employs wavelength modulation spectroscopy (WMS) with second harmonic (2f) detection. This technique:

• Modulates the laser at high frequency (e.g., 7.5 kHz)
• Uses a lock-in amplifier to detect the 2f signal
• Filters out noise, and enhances trace gas detection

This approach enables detection of gases at extremely low concentrations, even in complex backgrounds. It also compensates for laser drift, mirror fouling, and intensity fluctuations.

In environments with high background interference, TDLAS systems use differential spectroscopy:

• A scrubber removes the target gas from the sample, creating a “dry” spectrum
• The system compares this to the “wet” spectrum (with the gas present)
• Subtracting the two isolates the target gas signal

This method is especially useful for measuring H₂O (water/moisture), H₂S (hydrogen sulfide), NH₃ (ammonia), and CO₂ (carbon dioxide) in hydrocarbon-rich streams, where overlapping absorption bands would otherwise obscure the signal. 

Differential measurement:

• Absorptivity of analyte is low
• Analyte signals are very weak vs. background
• Spectral background changes significantly due to composition and other factors

Non-differential measurement:

• Analyte absorptivity is high
• Analyte signals are strong compared to background (good signal-to-noise)
• Spectral background changes are inconsequential

To achieve high sensitivity, TDLAS systems often use Herriott cells, which fold the laser beam multiple times through the sample gas. This creates a long optical path (up to tens of meters) in a compact volume, enhancing the signal without increasing system size. Unlike cavity-enhanced spectroscopy, Herriott cells are less sensitive to mirror fouling and maintain consistent path length, making them ideal for industrial environments.

TDLAS offers several advantages:

• High selectivity: Targets narrow absorption lines
• Low detection limits: Down to ppb levels
• Fast response: Real-time measurements (sub-second)
• Minimal maintenance: No moving parts or consumables
• Robust performance: Stable over years without recalibration
• No wet-up/dry-down delays: Unlike surface-based sensors

TDLAS is a cornerstone technology for modern gas analysis, offering unmatched sensitivity, selectivity, and stability. Whether you are optimizing a refinery process, ensuring pipeline compliance, or monitoring emissions, TDLAS provides the data you need—accurately and in real time.

The combination of advanced spectroscopy, rugged design, and minimal maintenance makes TDLAS ideal for demanding industrial environments. With proven performance across trace and percentage-level applications, TDLAS is the go-to technology for reliable gas measurement.

• Challenge: Hydrocarbons and other gases can obscure target signals
• Solution: Select lines carefully using high-resolution transmission molecular absorption (HITRAN) database; differential, and multi-peak spectroscopy

• Challenge: Pressure and temperature can affect line shape and intensity
• Solution: Use real-time compensation algorithms and temperature-controlled enclosures

• Challenge: Mirror fouling can reduce signal intensity
• Solution: Normalize 2f signals and use automated diagnostics to detect optical power loss

• Challenge: It can be difficult to maintain accuracy over time
• Solution: Use permeation devices, cylinder standards, and factory calibration with National Institute of Standards and Technology-traceable (NIST-traceable) gases

Natural gas: Perform on-line, real-time measurements of impurities in natural gas streams

• H₂O (moisture) in CH₄ (methane): TDLAS can detect water vapor down to <5 ppb, even with strong methane interference
• H₂S (hydrogen sulfide) monitoring: Ensures compliance with pipeline tariffs and environmental regulations down with detection limits below 1 ppm
• CO₂ (carbon dioxide) and CH₄ (methane) detection: Supports emissions monitoring and process optimization

Biogas / Biomethane

Natural gas processing: Monitor contaminants throughout the gas treatment process with selective & specific measurements

LNG: Perform critical measurements supporting LNG production and on-time shipments

Refinery: Monitor contaminants in refinery gas streams (for example, in refinery fuel gas and hydrogen recycle loops)  

Syngas: Perform highly selective and accurate laser-based measurement for carbon dioxide in syngas

Petrochemical:

• High purity ethylene and propylene streams: Measures trace moisture and HCl (hydrochloric acid) to protect catalysts
• C₂H₄ (ethylene) production: Detects C₂H₂ (acetylene), NH₃ (ammonia), and CO₂ (carbon dioxide) for product quality control
• Caustic wash towers: Monitors acid gases like CO₂ (carbon dioxide) and H₂S (hydrogen sulfide) at inlet/outlet

Environmental:

• Greenhouse gases: Real-time detection of CO₂ (carbon dioxide), CH₄ (methane), and N₂O (nitrous oxide)
• O₂ (oxygen) in hydrocarbon streams: Prevents combustion risks during storage and transport

• H₂O (moisture) in N₂ (nitrogen): Repeatability of ±3 ppb
• H₂S (hydrogen sulfide) in sour gas: Range up to 50%, repeatability ±1%
• CO₂ (carbon dioxide) in synthesis gas: Range up to 40%, repeatability ±0.02%
• NH₃ (ammonia) in C₂H₄ (ethylene): Repeatability better than ±50 ppb, with potential <20 ppb
• CO (carbon monoxide) in H₂ (hydrogen)*: Detection limit <10 ppb
• CH₄ (methane) in H₂ (hydrogen): Repeatability of ±4 ppb

These capabilities vary by product but nevertheless demonstrate TDLAS’s significant precision across a wide range of concentrations and gas types.

^{*Requires quantum cascade laser}

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