Some chemicals such as SO2 absorb and emit UV light. This phenomenon can be used to analyse for their presence and measure their concentration using a range of instrumentation.
NDUV analysers operate in the UV spectrum and typically employ the differential absorption technique. In the case of SO2 measurement, the analyser measures UV light absorption at a wavelength of 285 nm where the SO2 absorption band is centered. This is compared to absorption at the wavelength region of 578 nm where there is no SO2absorption.
Differential absorption NDUV instruments are common in CEMS applications and in addition to SO2, can also measure two other key pollutant molecules, NO and NO2, simultaneously without the need for a NOx converter. The NDUV technique is robust because it has low interferences. However, a limitation is that it cannot measure other pollutants like CO or CO2, for which an NDIR is more suitable. A combination of NDUV and NDIR techniques is possible for brad spectrum flue gas analysis or alternatively, an FTIR instrument would be able to measure this full suite of pollutants.
Ultraviolet fluorescence instrumentation is widely used in biotechnology for analysis of proteins and other organic molecules. For environmental monitoring and process control applications it is an established technique for SO2 measurement, for example in total sulphur measurement in hydrocarbon processing applications.
The technique relies on the absorption of UV light from a continuous or pulsed UV source at one specific wavelength, and the re-emission of the UV light at a different wavelength. Filters are used to produce a narrow waveband, for example at around 210 nm for SO2. The light emitted from the excited molecules passes through a filter to a photomultiplier detector tube. The amount of light received at the target wavelength is directly proportional to the SO2 concentration in the measurement cell, provided the sample flow rate and pressure is tightly controlled.
A limitation of UV fluorescence in CEMS applications is the 'quench effect' caused by the capture of the emitted radiation from the SO2 molecules by other molecules present in the sample gas, for example CO2, oxygen or nitrogen, which are all present in boiler flue gas stack emissions. The quenching effect varies depending on the interference molecule. It is therefore difficult to compensate for this effect when the matrix gas containing SO2 has a variable composition over time, as it will with a boiler flue gas. Despite this limitation, UV flourescence analysers can be used for emission monitoring purposes in large combustion plants if a high ratio dilution sampling system is used.
Ultraviolet fluorescence has found wide application in ambient air quality analysers for SO2. It is highly suited to this application because there is very little change in the sample composition over time.
UVF, PUVF and NDUV instruments all require calibration gas mixtures, also known as test gas or span gas, which should be specified close to the target measurement value or between 80 and 90% of the instrument full scale deflection. A zero gas, such as zero air or Nitrogen 5.0, will also be used. For the total sulphur measurement application of the PUVF spectrometer, an instrumentation grade oxidant gas such as zero air or Oxygen 4.0 grade will also be required for the pyrolysis reaction of the various sulphur compounds to SO2for analysis in the PUVF detector.
