Optics: measuring and testing – By particle light scattering – With photocell detection
Patent
1998-10-05
2000-11-21
Font, Frank G.
Optics: measuring and testing
By particle light scattering
With photocell detection
356 36, G01N 2100
Patent
active
061511137
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a diffuser trapping device for use with a nebulisation system to produce a uniform atomised spray for use particularly, though not exclusively, in evaporative light scattering detectors (ELSD) for liquid chromatography.
Evaporative light scattering detectors are highly sensitive liquid chromatography detectors for non-volatile solutes dissolved in a volatile liquid stream (solvent). An ELSD operates in three stages--firstly nebulisation of the solvent occurs wherein the solvent or solvent/solute solution is atomised into a dispersion of droplets by a venturi jet operated by a jet of compressed air or an inert gas, such as nitrogen. Secondly the atomised spray passes into an evaporation chamber under the influence of the nebuliser gas flow, which may be fan assisted, that directs the atomised spray down the evaporation chamber and vents the exhaust at the rear of the instrument. The third stage is that of detection. Collimated light is passed through the instrument perpendicularly to the direction of gas flow at the base of the evaporation chamber. A light trap is positioned opposite the light source to eliminate internal reflections inside the body of the instrument. When a pure solvent is evaporated only its vapour passes through the light path and the amount of light scattered is small and constant. The present of a non-volatile solute causes a particle cloud to pass through the light path resulting in light scattering. The scattered light generates a signal response from a photomultiplier or other light sensitive device which is provided in the detection system. The quantity of light detected is dependent on both concentration and particle size distribution of the solute.
Evaporative light scattering detectors are used as concentration detectors in many liquid chromatography techniques such as gel permeation chromatography (GPC), high performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC).
Large droplets or large particles of solute that have not been fully nebulised and which may be present in the atomised spray can give excessive scattering to create a dramatic increase in background noise or noisy signal responses respectively and a decrease in the overall sensitivity of the detector. These large droplets or particles can arise from inadequate nebulisation and inefficient drying of the droplets.
By large droplets or particles it is meant those having a diameter equal to or greater than the wavelength of incident light.
With the above problems in mind we have now developed a diffuser-trapping device for use with a nebuliser system which will reduce the presence of undesirable large droplets or particles thereby reducing background noise and improving the sensitivity of the detector.
Accordingly from a first aspect the present invention provides an evaporative light scattering detector comprising a solvent nebuliser, a heated evaporation chamber and a detection chamber into which is directed a collimated light beam normal to the flow of nebulised solvent and a light sensitive device for detecting scattered light wherein a diffuser-trapping device is positioned within the evaporating chamber at a depth of between one-third and two-thirds of the height of the evaporating chamber and extends substantially across the full diameter of the chamber.
Accordingly from a second aspect the present invention comprises a diffuser-trapping device suitable for positioning within the heated area of the evaporation chamber of ELSD of the first aspect of the present invention comprising a three dimensional highly porous network of inert material preferably formed from a thermally conductive material which may have a fibrous construction.
The expression "highly porous" refers to a structure having a porosity of between 50 and 99% calculated from the void volume or free space relative to the total volume occupied by the diffuser trapping device.
The position of the diffuser trapping device depends upon the diameter of the spray exiting the nebuliser and is positione
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O'Donohue Stephen J
Warner Frank P
Font Frank G.
Manus Peter J.
Nguyen Tu T.
Polymer Laboratories Limited
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