FTIR for Toxic Gas Analysis
(ISO 19702; EN 455452; IMO)
Toxicity analysis of fire effluents is an important aspect for developing modern materials used in aircrafts, trains and buildings to ensure public safety. Analytical techniques and performance criteria have been specified in various fire safety standards and regulatory codes. FTIR is the technique that has been chosen by the ISO, EN & IMO standardisation committees as the most suitable method for measuring toxic fire effluents. The commonly targeted toxic species are CO, CO2, HCN, SO2, NOx, HCl, HBr and HF.
Fourier Transform InfraRed (FTIR) Spectroscopy
FTIR spectroscopy is a fullspectrum analytical technique that allows all IR absorbing species to be detected and measured by a single instrument.
The infrared light from the source is modulated by the interferometer. This device allows for the light to be split into two different paths and recombined, producing an interference wave known as an interferogram. The light is split via Classic Michelson Interferometer an optical device known as a beam splitter.
The use of a monochromatic or single wavelength light source, typically a laser, is used to provide a reference signal in the interferometer. Measurement of the interference pattern of the single wavelength allows the speed of interferometer’s mirror movement & alignment to be controlled precisely.
The light is passed through a sample compartment which is often referred to as the sample cell or gas cell. Sample cells can be of various designs in order to achieve the most suitable pathlength i.e. total length that the IR light passes through the absorbing medium. For long pathlengths (of the order of meters) this usually involves the use of mirror arrangements to bounce the light through the sample medium. As the sample cell contains the extracted sample medium, care has to be taken that the sample cell is constructed of suitable materials and operates at the required temperature and pressure.
An infrared detector, e.g. deuterated triglycine sulfate (DTGS) detector, and associated electronics are required to make single point measurements of the infrared signal as the interferometer scans.
An FTIR analyser does not directly produce a spectrum for analysis; an interferogram is produced. This is timedomain measurement of IR signal and contains the modulated wave of the entire broad band source. To extract the IR spectrum a mathematical manipulation called a Fourier Transform must be applied to the interferogram. The mathematics of this are all handled in software in realtime.
The resulting singlebeam or intensity spectrum is then compared against a zero or background spectrum to produce an absorbance spectra. This absorbance spectrum is what we need to run a spectral analysis, applying Beer’s Law.
Beer’s Law describes the linear relationship between IR absorbance and concentration when variables such as temperature, pressure and path length are kept the same. With absorbance spectra collected and saved, chemometric techniques can be applied to extract concentration information. FTIR spectroscopy is considered the most suitable analytical technique for measuring toxic gas species in fire effluents because:
- a variety of gases across wide concentration ranges can be determined by a single method;
- monitoring of species development throughout the fire is possible with time resolved measurements;
- toxicants can be identified or reanalysed retrospectively in the stored spectra from previous experiments.
FTT FTIR System
FTT has been at the forefront of supplying a turnkey solution of FTIR system in analysing toxic gases in fire effluents. The makeup of this turnkey solution comprises of an advanced FTIR analyser, heated sampling system including all the pneumatics, control/processing electronics and an industrial PC, which are mounted in a 19″ cabinet for easy accessibility and service.
FTT FTIR is an advanced FTIR gas analyser used for continuous gas monitoring in conjunction with FTT’s Cone Calorimeter, Smoke Density Chamber and Single Burning Item (SBI) for online measurements of combustion gases in fire tests.
Spectroscopic data are often complex, containing large numbers of features which often overlap. The analysis of gases in fire effluents is especially challenging due to the great number of different organic and inorganic chemicals which representative atmospheres can contain.
FTT FTIR software uses chemometrics to resolve data into meaningful and accurate information. It offers users the ability to perform chemometrics analysis on data sets. This software is designed so that untrained users can simply run preloaded models, but will also allow more advanced users to build and develop models.
FTT’s application specialists have experience of developing and implementing chemometric techniques on various projects. We can provide in depth training courses on chemometric techniques and data analysis of spectroscopic measurements, enabling users to fully benefit from this powerful software.
As any chemometric technique will only ever be as good as the calibration data it is based on, FTT FTIR is calibrated in a purpose built calibration lab using certified traceable standards. FTT FTIR is fully configurable to meet the requirements of EN 455452, ISO 19702 and IMO standards.
In addition, various process monitoring applications are also possible. Measured components and calibration ranges can be selected according to application.
FTIR Gas Analyser
The FTIR gas analyser is an integral part of the system which allows simultaneous measurement of multiple gas compounds. Typically concentrations of H2O, CO2, CO, SO2, NO, NO2, N2O, HCl, HF, NH3, etc. are continuously measured.
The analyser has a multipass gas cell which is heated to 180°C. The gas cell mirror is gold plated with protective MgF2 coating which ensures high performance even in high water vapour concentrations or corrosive gases.
The analyser also has an internal solenoid valve to allow zero gas (usually 99.999% N2) to pass to the gas cell for cell evacuation and zero background measurements. This can be set as a Normally Open (NO) valve which provides a failsafe in case of power failure to ensure the gas cell is purged and gas does not condense on the optics.
Pressure transducer is installed to monitor the pressure inside the gas cell. Fluctuations in the cell pressure will be corrected for in realtime by software.
Sampling System
The hot extractive sampling system consists of a heated sample probe, heated filter, heated sample lines and heated pump unit. The whole system is kept at 180°C to avoid condensation & subsequent washing of soluble fire gases out of the sample. Two stage particle filtration is used in order to remove particles from the sample gas. The sample pump unit includes gas connections for the FTIR gas analyser. All sample lines have a PTFE core sample line of 6mm OD, 4mm ID, together with a secondary line for calibration/span gas. End fixtures are stainless steel which is robust and provides long lifetimes.
Panel PC
The touchscreen panel PC is required to operate the analyser, to control the sampling system, to translate measured and analysed concentrations and send alarms to higher level automation and control systems. It is also used for processing and storing the sample spectra.
FTT FTIR is supplied with a suite of three analytics software.
- PASPro
- Analyser Software for Process (installed on built in touchscreen PC) PASPro is a very simple to use but comprehensive user interface with setup menus for running the FTIR and control of all system parameters. It displays real time analytical measurements for the selected gases with an optional “Pass” and “Fail” result quality indicator next to each measurement. There is an alarm window which indicates any faults with the system and an event log updates with each task the system has carried out.
- PAS Analyser Software (installed on standalone PC)
- Collected FTIR data (results and spectra) can be downloaded from the test station and transferred to another PC for further analysis using PAS which allows the analysis to be checked and interfering species to be identified. It gives the user complete flexibility over the FTIR Heated filter/valve unit, PTFE filter can be easily replaced from the front acquisition parameters and allows complex analytical models to be built. Models can be span corrected to match reference gas cylinders and linearity checks can be made for compliance with performance standards.
- Spectrum Viewer (installed on standalone PC)
- It is a standalone application for manipulating spectra and identifying species that are present. It contains no analytical or collection routines, but is a valuable tool for the analyst working with spectra, e.g. adding, subtracting, multiplying and dividing spectra, peak position locator, baseline correction, etc.
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