CLIAMS:1. A system for determination of sulfur dioxide in air, the system comprising
(i) an excitation unit
(ii) a gas cell unit
(iii) a detection unit and
(iv) a gas conveyance assembly
wherein the gas conveyance assembly comprises two or more sets of polymer conduits wherein each set of polymer conduits comprises an inner conduit coaxially disposed within an outer conduit defining an annular volume between the inner peripheral surface of the outer conduit and the outer peripheral surface of the inner conduit, the inner conduit of a first set of polymer conduits having a polymeric wall selectively permeable to water vapour and the inner conduit of a second set of polymer conduits having a polymeric wall selectively permeable to aromatic hydrocarbons and the annular volume in at least one set of polymer conduits being adapted to be in selective fluid communication with the outlet nozzle of the gas cell unit.

2. The system as claimed in claim 1 wherein the first set of polymer conduits and the second set of polymer conduits are in fluid communication with each other

3. The system as claimed in claim 1 or 2 wherein the inner conduit of the first set of polymer conduits comprises a copolymer of tetrafluoroethylene and sulfonatedperfluoro(alkyl vinyl ether).

4. The system as claimed in any one of the claims 1 to 3 wherein the inner conduit of the second set of polymer conduits comprises a dimethyl siloxane polymer.

5. The system as claimed in any one of the claims 1 to 4 wherein the gas cell unit is coupled to a reflecting surface adapted to reflect radiative emissions from the gas cell unit.

6. The system as claimed in any one of the claims 1 to 5 wherein the gas cell unit is coupled to a beam dumping unit having an aperture at least partially aligned to the aperture on the excitation unit, the aperture on the beam dumping unit being adapted to receive radiation from the gas cell unit.

7. The system as claimed in any one of the claims 1 to 6 wherein the detector unit and the gas cell unit are separated by a baffle ring

8.The system as claimed in any one of the claims 6 to 7 wherein the beam dumping unit is a tubular body having a proximal end and a distal end, the proximal end being open and at least a portion of the body towards the distal endbeing tapered and curved.

9. The system as claimed in any one of the claims 6 to 8 wherein a UV photodiode detector adapted to respond to electromagnetic radiation having wavelength in the region of 190 nm to 1000nm is mounted at the distal end of the beam dumping unit.

10. The system as claimed in any one of the claims 6 to 9 wherein the beam dumping unit has the shape of a cornucopia

11. The system as claimed in any one of the claims 1 to 10 wherein the system is adapted to detect concentration of sulfur dioxide in air upto 0.5 parts per billion (ppb)

12. A system for determination of sulfur dioxide in air, the system comprising a baffle ring (7), a mirror mount (2) and a beam dumping unit (5)
,TagSPECI:FIELD OF INVENTION

The present invention relates to a system for determination of sulfur dioxide in air.

BACKGROUND

Owing to its potential hazards as an air pollutant, detection and quantitative determination of sulfur dioxide in air has received serious attention, in the recent days. Since sulfur dioxide is known to undergo fluorescence, fluorescence emission from sulfur dioxide has been analytically attempted in the past for its analysis. The radiative emission from sulfur dioxide is usually detected at right angles to the direction of excitation light, so as to reduce the background noise in the emission signal. Nevertheless, emission signal from sulfur dioxide tend to be weak against the background radiations of the excitation light that invariably enter the detector surface by manifold reflections and scattering. Further, the emissions from this gaseous analyte will also have interference from other gaseous substances in the air sample, thereby significantly affecting the sensitivity and accuracy of analysis. Though various attempts had been made in the past to improve the sensitivity and accuracy of sulfur dioxide analysis, none of the conventional fluorescence analysis systems or methods was found to provide a completely satisfactory system for determination of sulfur dioxide. Often, it requires undue experimentation to identify the interfering species and/or causes of all the interferences. There is, therefore, a need in the art for a system that allows precise and accurate determination of sulfur dioxide in air.

OBJECTS OF THE INVENTION

An object of the invention is to provide a system for accurate determination of sulfur dioxide in air.

An object of the invention is to provide a system for determination of sulfur dioxide in air using fluorescence emission of sulfur dioxide wherein the emission is free from interferences

An object of the invention is to provide a system for determination of sulfur dioxide in air using fluorescence emission of sulfur dioxide wherein radiative as well as chemical interferences are eliminated

An object of the invention is to provide a system for determination of sulfur dioxide in air using fluorescence emission of sulfur dioxide wherein the sensitivity of analysis is enhanced.

SUMMARY OF THE INVENTION

The invention provides asystem for determination of sulfur dioxide in air, the system comprising
(i) an excitation unit
(ii) a gas cell unit
(iii) a detection unit and
(iv) a gas conveyance assembly
wherein the gas conveyance assembly comprises two or more sets of polymer conduits wherein each set of polymer conduits comprises an inner conduit coaxially disposed within an outer conduit defining an annular volume between the inner peripheral surface of the outer conduit and the outer peripheral surface of the inner conduit, the inner conduit of a first set of polymer conduits having a polymeric wall selectively permeable to water vapour and the inner conduit of a second set of polymer conduits having a polymeric wall selectively permeable to aromatic hydrocarbons and the annular volume in at least one set of polymer conduits being adapted to be in selective fluid communication with the outlet nozzle of the gas cell unit.

Typically, the first set of polymer conduits and the second set of polymer conduits are in fluid communication with each other

Typically, the gas cell unit is coupled to a reflecting surface adapted to reflect radiative emissions from the gas cell unit.

Typically, the gas cell unit is coupled to a beam dumping unit having an aperture at least partially aligned to the aperture on the excitation unit, the aperture on the beam dumping unit being adapted to receive radiation from the gas cell unit.

Typically, the detector unit and the gas cell unit are separated by a baffle ring.

Typically, the beam dumping unit is a tubular body having a proximal open end and a closed distal end, at least a portion of the body being tapered and curved toward the closed end.

Typically, the beam dumping unit has the shape of a cornucopia.

Typically, the system is adapted to detect concentration of sulfur dioxide in air upto 0.5 ppb.

BRIEF DESCRIPTION OF DRAWINGS

Fig 1 is an exploded view of the system of the invention
Fig 2 is a schematic view of the gas conveyance assembly in the system of the invention
Fig 3 is a flow chart of the operation of the system of the invention
Fig 4 is a representation of the beam dumping unit
Fig 5 is the flow chart for the generation of zero air

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 is an exploded view of an embodiment of the system of the invention. Referring to Figure 1, the excitation unit 1comprises an excitation source that is typically a source of polychromatic radiation or of a monochromatic radiation having emission maxima falling in the region of electromagnetic spectrum at which sulfur dioxide absorbs significantly. Typically, electromagnetic radiation having wavelength in the region of 190-230 nm is used for excitation of sulfur dioxide molecules. Advantageously, a low pressure zinc arc lamp that emits strongly at around 214 nm is used as the excitation light source. In the absence of any gas sample in the sample cell unit 6, the whole of the excitation radiation is taken up into the beam dumping unit, 5. While in operation, sample of gas to be analysed enters the gas cell unit through the inlet valve 3. A portion of the excitation radiation is absorbed by the sulfur dioxide molecules and the unabsorbed portion traverses to the beam dumping unit 5. The beam dumping unit 5is a tubular body, having a wide opening at the proximal end, a portion of which tapers and curves to a reduced diameter towards a distal end, thus resembling shape of a cornucopia. Radiation that enters the beam dumping unit undergoes incoherent multiple internal reflections at its internal peripheral surface as it traverses towards the closed distal end of the unit, thus, effectively attenuating its intensity and preventing it from escaping the beam dumping unit. The distal end of the beam dumper is provided with a small opening having a fused silica window. A reference silicon photo diode UV detector that can detect wavelength in the range of 190nm to 1000nm is mounted at the centre of the window using an adapter. The input from this UV detector helps in compensating for any rise or fall in UV intensity which is directly proportional to the fluorescence light generated in the gas cell unit. The signal from the UV reference detector is also used as a feedback to monitor the warm-up time. Thus, the beam dumping unit plays a major role in reducing background radiative interferences as well as in improving the accuracy and sensitivity of sulfur dioxide analysis.

The detector unit 8 placed at right angles to the direction of the excitation light is so configured as to detect the radiation emitted from the sulfur dioxide molecules. Typically, the detector unit 8comprises a photomultiplier tube. The detector unit 8is separated from the gas cell unit 6by a baffle ring 7that eliminates lateral interferences from the excitation light. Often, the baffle ring is coupled, in the detector unit side, to an optical lens capable of converging the fluorescent light from the sample cell to the detecting surface of the photomultiplier tube. The baffle ring thus assists the fluorescent light free from interference from the background light in entering the detector unit. This additionally improves accuracy and sensitivity of sulfur dioxide analysis. The gas cell unit is coupled on one side to a mirror mount 2having mounted therein a mirror having a highly reflecting concave surface at least partially aligned to the detector surface. The concave mirror mounted to the mirror mount 2enables to focus the radiative emissions from the gas cell unit to the detector surface so that there is minimal loss of the emissions from the sample gas thereby improving the sensitivity of analysis.

Fig 2 is a schematic view of the gas conveyance assembly in the system of the invention. It comprises a first set of polymer conduits 9having an inner conduit 11 coaxially disposed within an outer conduit 16. The inner conduit of the first set is typically prepared from a copolymer of tetrafluoroethylene and sulfonatedperfluoro(alkyl vinyl ether). Advantageously, a polymer commercially available under the trade name Nafion® is used to prepare the inner conduit of the first set. The inner conduit 17 of the second set of polymer conduits is prepared from a dimethyl siloxane polymer. Typically, a polymer commercially available under the trade name Silastic® is used to prepare the inner conduit of the second set of polymer conduits. The inner conduit 17 of the second set is coaxially disposed within an outer conduit 12. The first and second set of polymer conduits are joined together by a connector 28. The conveyance assembly effectively eliminates two major interferences during sulfur dioxide analysis namely water vapour and aromatic hydrocarbons. Water vapour is known to quench sulfur dioxide fluorescence while the aromatic hydrocarbons add its own fluorescence to the emitted light of sulfur dioxide. By removing them effectively through two sets of coaxially arranged polymer tubes, the system of the invention provides a simple and elegant solution to a long-standing problem in sulfur dioxide analysis. After the analysis is complete, the gas sample is conveyed out of the gas cell unit 6 through the outlet valve 4into the annular volume between the outer conduit and the inner conduit of the first set 9of polymer conduits, to purge out water vapour collected therein through a gas outlet 15. The aromatic hydrocarbon vapour collected in the annular volume between and the outer and inner conduits of the second set 10of polymer conduits is purged out separately using air free from hydrocarbons prepared by passing through a charcoal scrubber.

Fig 3 is a flow chart of the operation of the system of the invention. The sample air entering the system of the invention is first filtered through a dust filter 29 wherein the dust particles in the sample air are removed. The filtered air then enters at least two sets 9, 10 of polymer conduits wherein the water vapour and aromatic hydrocarbons in the air sample are successively removed. Air free from water vapour and aromatic hydrocarbons is then made to enter the gas cell unit 6wherein the sample air is irradiated with excitation light from the excitation unit 1. The removal of water vapour and aromatic hydrocarbons successively in the first set and second set of polymer conduits is achieved by maintaining the respective annular volumes at a differential pressure (at a pressure lower than that in the respective inner conduits) with respect to the pressure in the inner conduit. This is achieved by means of a vacuum pump 30 as well as by the orifice 27. The fluorescent radiation from the gas cell unit is detected at the detecting surface of the detection unit (which consists of a photomultiplier) and the emission signal is then displayed electronically 18, 19.

Fig 4 is a representation of the beam dumping unit5. The beam dumping unit is a tubular body having an open proximal end 21 and a distal end 22 that has a reduced diameter and is curved with respect to the proximal end. The proximal end has mounting holes 20 for coupling the beam dumper with the gas cell unit 6. The distal end has a mounting holes23 having mounted therein a UV photodiode detector 24.

Practically, after prolonged operation, zero level of the system shifts (Zero drift). To counter such zero drift, zeroing is performed in the system repeatedly at regular intervals of time. In conventional systems, such zero air is supplied from cylinders or zero air generators. But in the system of the present invention, zero air is generated internally. Fig 5 is the flow chart for the generation of zero air. Zero air is generated within the system of the invention by allowing air to successively pass through a dust filter unit 29, a charcoal scrubber 25 for trapping of mainly hydrocarbons and Sulfur di-oxide, Polytetrafluoroethylene (PTFE) filter 26 for removal of dust particles greater than 4 micron and through at least two sets of polymer conduits 9, 10. The air free from hydrocarbons entering into the annular volume through the inlet tube 13 of the second set of polymer conduits purges out through a gas outlet14 the hydrocarbons in the annular volume. The system of the invention thus eliminates complexity of design and is simple, integrated and self-sufficient in operation,

The system of the invention allows precise and accurate determination of sulfur dioxide in an air sample. By efficiently removing the material interferences such as dust particles, water vapour and hydrocarbons and radiative interferences from the excitation light, the system of the invention achieves high level of accuracy and sensitivity hitherto unknown for gas analysis systems for sulfur dioxide. The system of the invention enables determination of sulfur dioxide in air with a sensitivity of upto 0.5 ppb. Apart from functional efficiency, the system of the invention is characterized by simplicity of design and ease of operation. By allowing to generate zero air for its own calibration the system of the invention eliminates the need of using cumbersome gas cylinders or zero gas generators. By using gas sample from the gas cell unit for purging out water vapour removed from the sample air, the system of the invention offers an additional advantage in terms of simplicity. Overall, the system of the invention is more efficient, robust, accurate and sensitive as compared to the conventional gas analysis systems for sulfur dioxide analysis.

The above description is illustrative only and is not limiting. The present invention is defined by the claims that follow and their full range of equivalents.