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FTIR Gas Analysis

20 April 2018

FTIR (Fourier Transform InfraRed) spectroscopy is the most popular analytical technology for industrial applications requiring the continuous measurement of multiple parameters simultaneously. This article, by Nenne Nordström of Gasmet Technologies, looks at the advantages and uses of FTIR technology in these different applications. 

Typically, FTIR analysers are employed for process control and emissions monitoring, however, due to the robustness and flexibility of this technique, it can also be applied in a wide variety of applications.

FTIR gas analysers identify and measure gaseous compounds by their absorbance of infrared radiation. This is possible because every molecular structure has a unique combination of atoms, and therefore produces a unique spectrum when exposed to infrared light.

Instrumental analysis of the spectrum (2 to 12 micrometer wavelength) enables the qualitative identification and quantitative analysis of the gaseous compounds in the sample gas. Importantly, FTIR analysers are able to simultaneously measure multiple analytes in complex gas matrices, detecting virtually all gas-phase species (both organic and inorganic, except diatomic elements N2, O2 etc. and noble gases He, Ne, etc.).

FTIR gas analysers can collect a complete infrared spectrum (a measurement of the infrared light absorbed by molecules inside the sample gas cell) 10 times per second. Multiple spectra are co-added together according to a selected measurement time (improving accuracy by raising the signal-to-noise ratio). The actual concentrations of gases are calculated from the resulting sample spectrum using a patented modified Classical Least Squares analysis algorithm.

Typical applications

The ability of FTIR to monitor multiple gases simultaneously, even in hot, wet aggressive gas streams means that it is ideal for emissions monitoring in the power sector, energy from waste, incineration plants and in industries such as cement and aluminium. These processes have to demonstrate compliance with emissions limit values specified in regulatory permits using instrumentation with appropriate certification.

A number of typical configurations exist for monitoring emissions including parameters such as CO, NO, H2O, SO2, HCl, NH3, NO2, N2O, CO2, HF, CH4 and CHOH. However, with the ability to monitor thousands of compounds, the opportunities for industrial process monitoring with FTIR are endless.

The emissions from all types of engines are monitored for compliance purposes and also to improve engine efficiency. Again multigas capability is required and FTIR is therefore commonly applied. With increasing concern over the impact of vehicle emissions on ambient air quality, FTIR is a popular monitoring technique with engine developers; providing an opportunity to refine engine performance whilst measuring the effects on the individual components of emitted gases.

In addition to emissions monitoring, Gasmet FTIR analysers are also employed in other environmental applications, particularly where multigas monitoring is required or where it is necessary to be able to identify unknown gases. Portable and ambient versions of the same FTIR technology are therefore available and are employed in applications such as greenhouse gas monitoring in soils, hazardous gas analysis in contaminated land and chemical leaks and spills, and also for forensic investigations following fires etc.

FTIR analysers are employed in occupational safety applications such as anaesthetic gas monitoring and analysis, shipping container investigation, fumigation monitoring and the testing of compressed breathing air for impurities.

FTIR gas analyser versions

FTIR analysers are available in a variety of different formats to meet the requirements of different applications, including:
1.  Fixed Continuous Emissions Monitoring System
2.  Portable heated FTIR for measuring extractive gases
3.  Portable ambient gas FTIR analyser
4.  Fixed multipoint ambient FTIR gas analyser
5.  Stack/duct mounted FTIR gas analyser for measuring gases in-situ

FTIR – common questions

1. What is a Reference Spectrum?

A reference spectrum is a spectrum of known concentration of one IR absorbing gas diluted in nitrogen. Reference spectra are used to analyse measured sample spectra. The absorption peaks in a sample gas are compared with those of reference spectra to determine simultaneously the concentrations of multiple gas components of the sample. For instance, if the absorption due to Methane in the sample is 1.2 times that of a 10 ppm Methane reference spectrum, the concentration of Methane is 12 ppm.
The reference spectrum is recorded using a long measurement time to eliminate noise from the spectrum and the instrument is carefully purged to eliminate traces of moisture and carbon dioxide (the two main atmospheric IR absorbing gases) from the spectrum. In order to model moisture and carbon dioxide in the sample, reference spectra of these gases are used.

2. Which gases can be measured by FTIR Gas Analysers?

The gases measured by FTIR include:
•  Inorganic gases: Water, CO2, CO, NO, NO2, N2O, NH3, SO2, HCl, HF, …
•  Volatile organic compounds: hydrocarbons, alcohols, aldehydes, ketones, freons, …
The main exceptions are:
•  noble gases (He, Ar, …)
•  metals (Hg)
•  molecules with just two atoms of the same element (N2, O2, H2, Cl2)
•  molecules with very small dipole moment change (H2S)
•  low volatility organics (high boiling point or room temperature solid)
•  particulate matter or aerosols (not a gas)

3. What are the detection limits for commonly measured gases?

FTIR gas analysers can measure from low ppm levels up to vol-% with sub-ppm detection limits for almost all gases. Exact detection limits depend on the type of sample cell and detector used.

4. How many gases can be measured and analysed at the same time?

Typically up to 40 or 50 gases can be analysed from a sample spectrum. The exact number of gases depends on the availability of non-saturated absorption peaks in the spectrum (areas of spectrum with absorbance stronger than 0.8 absorbance units should be avoided) and in samples with strong absorbance across the spectrum the number of gases may be lower.

5. What is the Reference Library Collection?

The Gasmet reference library collection contains IR spectra and concentration information for over 250 gas species, enabling library searches and the use of non-instrument specific references. To complement this collection of spectra, the NIST/EPA vapour phase IR library is also available in a form compatible with Gasmet IR spectra and this allows the identification of an additional 5000+ gases.

Summary

FTIR analysers offer the ability to monitor multiple gases simultaneously, reliably and accurately in a wide variety of applications. The best emissions monitoring analysers comply with international performance standards and can even measure the components of hot, wet aggressive gas mixtures.

Comprehensive certification provides regulated processes with the assurance that monitoring data from FTIR analysers will meet or exceed all regulatory requirements across an expanded range of compounds.

One of the most important features of FTIR analysers is their flexibility. If users need to monitor new gases; no extra hardware is necessary because the addition of new compounds to the list of measured species is a relatively simple procedure. This means that the monitoring equipment is future-proof. In addition, software enables users to retrospectively analyse recoded spectra, which means that it is possible to measure compounds in previous data, that were not originally of interest. Many users have found this to be an extremely useful tool in the identification of process related problems.

The portable versions of the analysers offer a range of unique advantages. For example, many stack monitoring locations can be difficult to access so small lightweight equipment is preferred.

In conclusion, FTIR is a powerful but flexible monitoring technology; able to provide comprehensive monitoring data in an enormous variety of monitoring applications.

About the author box

Nenne Nordström graduated from Aalto University School of Science and Technology (School of Chemical Technology) and has an MSc in Technology from the University of Technology in Finland. She now works as a Sales Channels and Marketing Manager at Gasmet Technologies, a leading global manufacturer of industrial gas analysers. In this position Nenne has become an expert in FTIR technology and portable gas analysers, regularly giving presentations on gas analysis to international audiences.

Gasmet has published a White Paper providing more detailed and comprehensive information on FTIR Gas Analysis and this is available as a free download via the News page of www.gasmet.com.


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