Open-Path Spectroscopy Overview

Open-path spectroscopy has been gaining popularity since it was first deployed in the 1980s, but these analyzers come with a unique set of challenges. We touched briefly on the key factors and considerations of this technology in our Fenceline Monitoring Overview article, but dive deeper into what is necessary for successful fenceline monitoring for industrial facilities in this discussion.

What is Spectroscopy?

Spectroscopy is an area of scientific study that began in the 17th century with Isaac Newton’s early experiments. In broad terms, it is the measurement of spectra that is produced when light interacts with matter. Modern-day instruments employ this idea when they examine the unique “fingerprint” wavelengths of light that are absorbed by molecules in the air. Absorption by compounds in the sample path are then used to determine (1) what compounds of interest are present and (2) their respective concentrations according to Beer's Law.

Spectroscopic instruments have historically used enclosed sample cells and extractive sampling, meaning they remove a sample from a source for analysis. In the 1980s, open-path analyzers extended these measurements to long pathways open to the environment, allowing for real-time sampling of air quality around industrial facilities.

Open-Path Spectroscopic Instruments

A variety of spectroscopic instruments are used for open-path monitoring. They all emit light from a source and send it to a receiver at the end of a pathway, known as a bistatic configuration, or to a mirror that reflects it back to a receiver collocated with the source, known as a monostatic configuration. Compounds present in the path absorb light at specific wavelengths depending on their composition, which results in changes between the light that is emitted and the light that is detected by the receiver. Absorbance measured at various wavelengths results in a spectrum with unique contributions, or “fingerprints,” from each compound present in the beam. In this configuration, the sample pathway is the entire column of air between the instrument and the receiver or mirror, enabling facilities to cover their perimeters completely if they wish.

Open-path analyzers differ primarily in the light source that is used, which determines the wavelengths for analyses and therefore the compounds that can be measured. The following are three common examples of open-path analyzers.

• Ultraviolet Differential Optical Absorption Spectroscopy (UV-DOAS) – UV-DOAS analyzers contain an ultraviolet (UV) light source, either a xenon or deuterium lamp. Compounds measured with this instrument include benzene, toluene, ethylbenzene, and xylenes (BTEX), sulfur dioxide, and other air toxics. This technique allows for simultaneous measurement of multiple compounds in real time at a time resolution as fast as once every 30 seconds.
• Fourier Transform Infrared (FTIR) – FTIR analyzers contain an infrared (IR) laser. Similar to UV-DOAS analyzers, this can be used to measure multiple compounds simultaneously in real time at a time resolution of approximately once every two to three minutes. Compounds measured with this instrument include alkanes, aldehydes, ammonia, hydrogen cyanide (HCN), methane, and other volatile organic compounds (VOCs).
• Tunable Diode Laser Absorption Spectroscopy (TDLAS) – TDLAS uses a diode laser that emits a narrow wavelength of light tuned to a specified range of interest. This technique is typically used to measure a single compound, including hydrogen sulfide (H₂S), hydrogen cyanide (HCN), and other industrial compounds of interest.

Minimum Detection Limits (MDLs)

One of the biggest challenges for open-path spectroscopy is the ability to measure compounds of interest at sufficient detection limits, including relevant health thresholds. To address this, the U.S. Environmental Protection Agency (EPA) released a standardized procedure for processing spectra obtained with an FTIR analyzer titled Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air, or TO-16. The draft was released in 1992 and was followed by a final edition in January 1999 that is still in use today.

TO-16 is one of several methods for determining MDLs, but its historic application in laboratory environments does not represent actual performance in the field. Real-world MDLs are dependent on operating conditions and interference from other similarly absorbing compounds. An example of this is the known interference from water and carbon dioxide (CO₂) in the infrared region, which may impact compounds analyzed with FTIR and TDLAS. MDLs are a primary consideration for the type of open-path analyzer that is selected for different compounds of interest.

Final Thoughts

Open-path spectroscopy continues to evolve with additional use, particularly in industrial settings. While this technology is a powerful tool for industrial monitoring, it also requires careful consideration of key factors.

For a more in-depth look at this topic, you can read a peer-reviewed article written by Sonoma Technology scientists titled Real-World Application of Open-Path UV-DOAS, TDL, and FT-IR Spectroscopy for Air Quality Monitoring at Industrial Facilities, published in Spectroscopy.