Monitoring Methods

DOAS stands for Differential Optical Absorption Spectroscopy. It is the fundamental principle for measuring gas concentrations in most of OPSIS’ gas analysers.

The acronym DOAS gives us good clues to what it’s about: “Spectroscopy” means the experimental investigation of the nature of light. “Absorption” indicates that we look at the “disappearance” of specific wavelengths. “Optical” limits us to study the optical wavelengths, ranging from ultraviolet via visible to infrared light. Finally, “differential” is the mathematical process applied to the recorded optical absorption spectra.

DOAS utilizes a broad band light source; in OPSIS’ case a Xenon lamp. The light is made to form a narrow and intense beam which is sent across the volume where the gas to measure resides; often referred to as the “measurement path”. The ray of light looks white to the human eye but also includes “invisible” infrared and ultraviolet wavelengths.

Given that a specific type of gaseous molecule may have narrow-band absorption features in certain wavelength regions, a fraction of the light will be absorbed along the path if such molecules are present. The higher concentration of the gas or the longer measurement path, the more absorption will occur. The remaining light arriving at the end of the path is then forwarded to a spectral analyser (the key component of the gas analyser) where the features of the absorption can be detected. Dividing the recorded spectrum by a reference (i.e. a “clean” spectrum; this division is the “D” in DOAS) and comparing it with known absorption spectra of the sought-for and other gases yields the average gas concentration along the measurement path.

Each type of gaseous molecule has its own characteristic absorption abilities in specific wavelengths, a kind of “fingerprint” for that type of molecule. This makes it possible to optically separate different gases from each other, and to use the same measurement system to detect multiple gaseous compounds. It is “just” a matter of knowing in what wavelength region the respective type of molecule has its unique fingerprint, and then look for that.

Today, DOAS is a well-established principle for gas concentration measurements, as evidenced by numerous approvals and thousands of DOAS systems deployed by OPSIS and its representatives throughout the world.

The basis of the principle used by OPSIS to identify and measure concentrations of different gases is scientifically well established: Differential Optical Absorption Spectroscopy (DOAS), which is based on Beer-Lambert’s absorption law. It states the relationship between the quantity of light absorbed and the number of molecules in the lightpath.

Because every type of molecule, every gas, has its own unique absorption spectrum properties, or “fingerprint”, it is possible to identify and determine the concentrations of several different gases in the lightpath at the same time.

DOAS is based on transferring a beam of light from a special source – a high-pressure xenon lamp – over a chosen path and then using advanced computer calculations to evaluate and analyse the light losses from molecular absorption along the path. The light from the xenon lamp is very intense, and includes both the visible spectrum and ultraviolet and infrared wavelengths.

The light is captured by a receiver and conducted through an optical fibre to the analyser. The fibre allows the analyser to be installed away from potentially aggressive environments.

The analyser includes a high-quality spectrometer, a computer and associated control circuits. The spectrometer splits the light into narrow wavelength bands using an optical grating. This can be adjusted so that an optimum range of wavelengths is detected.

The light is transformed into electrical signals. A narrow slit sweeps past the detector at high speed, and a large number of instantaneous values are built up to form a picture of the spectrum in the relevant wavelength range. This scan is repeated a hundred times a second, and the registered spectra are accumulated in the computer’s memory while awaiting evaluation.

The absorption spectrum just registered from the light path is compared with one calculated by the computer. The calculated spectrum consists of a well-balanced summation of the reference spectra for the analysis concerned.

The computer proceeds by varying the size factors for each reference spectrum until it reaches the best possible match. From this the different gas concentrations can be calculated with high accuracy

PSIS has developed an analyser for monitoring compounds specifically in the infra-red wavelength range area. The OPSIS IR technique is based on the same method for identifying and measuring concentrations of various compounds as the comprehensive OPSIS spectrometer technique.

The IR technique is based on Beer-Lambert’s absoption law, which states the relationship between the quantity of light absorbed and the number of molecules in the light path. A light source projects a light beam onto a receiver which transfers the light via an optical fibre to the analyser.

The analyser includes an interferometer, a computer and control circuit cards. The interferometer consists of a beam splitter which divides the light towards two moving mirrors. An interference pattern is formed.

By using advanced computer calculations, the interference pattern is transformed into a wavelength spectrum, corresponding to the spectrum which is measured in the OPSIS spectrometer.

The interferometer gives higher spectral resolution in the infra-red wavelength range than the spectrometer does.

The computer proceeds by varying the size factors for each reference spectrum until it reaches the best possible match. From this the different gas concentrations can be calculated with high accuracy

OPSIS laser diode gas analyser emits laser light in the near infra-red section of the wavelength spectrum. The measurement is made by rapidly scanning the laser over the absorption line in the gas absorption spectrum.

The laser operates continuously, and it is tunable, so the laser wavelength can be slightly changed. This is achieved by applying an electric voltage across the semiconductor diode. The voltage applied is precisely controlled, and varies according to a ramp function during a scan.

During a measurement, the TDL analyser averages a large quantity of scans. The measurement interval is in the order of 1-20 seconds, and the scanning rate is in the kilohertz range.

In the end of the measurement interval, the averaged spectrum enters an evaluation procedure. The result is compared through a least squares fitting procedure with the known absorbance cross section of the gas. The cross section relates to the strength of absorption in the gas, at specific wavelengths. Knowing the monitoring path length, the concentration of the gas can then be evaluated

The mass determination principle is based on the physical law whereby beta rays are attenuated as they pass through a thin layer of material.

If one defines x_f as the value of the filter’s mass density and xp as the value of the mass density of the collected particles, the following equation applies:

where K(x_f) = mass absorption coefficient and F_blank and F_collect represent the beta ray flow before and after the sampling cycle.

Using the value of the sampling surface area S (=11.95 cm², O = 3.9 cm), the mass of the suspended particles mp deposited on the filter can be computed as follows:

The function K(x_f) was determined by the instrument manufacturer and is programmed into the system. After each power-up of the instrument, the stability of the initial calibration is verified by means of two reference orifices placed in the passage of the beta rays between the source and the Geiger counter. This function can also be triggered manually and is available as a regular Auto Test feature. The result of the last “beta test” can be viewed.

OPSIS zirconia oxygen analyser consists of a zirconia sensor. It makes use of the oxygen ion conductivity of solid electrolytes composed mainly of zirconia at high temperatures.

If electrodes of platinum or similar materials are attached to both faces of a solid electrolyte and the faces are on the conditions of different oxygen partial pressures, an electrochemical reaction causes an electromotive force between both electrodes.

Microscopically, it is assumed that electrochemical reactions occur at the (three phase) interface among a solid electrolyte, electrode and oxygen.

OPSIS HG200 mercury analyser is a double beam photometer, which measures mercury (Hg0) by cold vapour atomic absorption technology (CVAA). High sensitivity (0.1 ng/Nm3) is achieved by pre-concentrating a sample on a gold trap.

The mercury analyser performs its measurements in batches. A measurement cycle consists of three activities: sampling, analysis and reconditioning.

During sampling, the flow control valve is open, and a specified volume (1-1.5 LPM) is pumped through the system. Elementary mercury in the sample stream will then be amalgamated on the gold trap.

After the sampling, the instrument enters the analysis stage. Then, the valve is shut and a critical orifice reduces the flow rate. The gold trap is heated and the mercury will be released and transported through the measurement cell where the mercury is measured. This analysis takes one minute to perform. The measurement yields an absorption spectrum from which the mass in pico-grammes of mercury in the sample is derived.

A mass flow meter registers the flow rates during the different stages. When the analysis is complete, the third stage is entered during which the gold trap is cooled in order to prepare the analyser for the next measurement cycle.

Before entering the gold trap section, the sample gas is passed through a sample-conditioning scrubber containing soda lime.

Here you can find information about the main physical characteristics and applications of a range of substances that can occur in ambient air or industrial processes.
OPSIS AB can provide monitoring solutions for all the following substances.