The present invention relates generally to devices for detecting and quantifying components of a liquid sample stream, and more particularly to a nebulizer for use with a charged aerosol detection system.
Charged aerosol detection is a popular and valuable technique for the detection and quantification of substances present in a liquid sample stream, and is particularly well-suited to use in connection with liquid chromatography applications. Briefly described, a charged aerosol detection (CAD) system consists of a nebulizer for generating a spray of droplets from a liquid sample stream (for example, the effluent from a chromatographic column), a discharge source for selectively charging the nonvolatile residue particles produced by drying the droplet spray, and a collector, where the aggregate charge imparted to the particles is measured using an electrometer. The resultant signal is representative of the concentration of the nonvolatile components of the sample stream. CAD is sometimes referred to as a “universal” detection technique, as it is capable of quantifying a wide variety of nonvolatile substances with consistent response. Further details regarding the design, operation and advantages of CAD systems are set forth in U.S. Pat. No. 6,568,245 by Kaufman (“Evaporative Electrical Detector”), the disclosure of which is incorporated herein by reference.
The performance of a CAD system is closely tied to the parameters of the droplet spray produced within the nebulizer. It is generally desirable to generate a spray of droplets of uniformly small diameters, since droplets having relatively large diameters may not have adequate time to dry (i.e., to fully evaporate the solvent) and form particles of non-volatile residue prior to reaching the discharge source, which may in turn compromise the ability of the CAD system to quantify analytes contained within the droplets with high degrees of sensitivity and reproducibility. Prior art CAD systems, such as the one described in the aforementioned Kaufman patent, have commonly utilized a nebulizer in which the liquid sample stream is introduced transversely to a jet of compressed air, which is directed at a velocity sufficient to break the sample stream into droplets. Spray generation is assisted by the action of an impactor, which is located proximate to the intersection of the liquid and gas flows. The smaller droplets travel within the nebulizer under the influence of the entraining gas to the nebulizer exit, and pass thereafter to the discharge source. Larger droplets, having greater momentum, impinge on the surface of the impactor and are either broken into smaller droplets or removed to waste through a drain positioned below the impactor.
While prior art CAD system nebulizers have produced generally acceptable results, there remains a need in the art for improved nebulizer designs in order to facilitate more sensitive and reliable detection.
In accordance with certain illustrative embodiments of the invention, a nebulizer for a CAD system is provided having an emitter for generating a droplet spray, and a spray chamber having a central region into which the droplet spray is introduced. The spray chamber further includes a rear surface positioned opposite to the emitter outlet, and a partition dividing the central region from an upper region and defining a passageway through which a portion of the droplets in the droplet spray travel. The central and upper regions are arranged such that the major direction of droplet travel within the upper region is substantially reversed with respect to the major direction of ion travel in the central region. Larger droplets in the droplet spray are unable to negotiate the “hairpin turn” required to pass from the central region to the upper region and impact the rear surface of the spray chamber. The separation of the large droplets from the droplet spray via this mechanism has the advantageous effect of enabling the detector of the CAD system to sense a smaller range of particle sizes, which establishes a relatively steady electrical current at the detector. This steady baseline electrical current facilitates the identification of higher electrical currents or peaks as samples are injected and separated in the chromatographic column and subsequently introduced to the nebulizer.
In accordance with another embodiment of the invention, a method is provided for detecting analytes in a liquid sample stream. The method includes steps of introducing the liquid sample stream as a droplet spray into a spray chamber, and causing the major direction of droplet travel to be reversed within the spray chamber, such that a portion of the droplets (e.g., larger droplets) impact a spray chamber surface and are removed from the droplet spray. The remaining droplets are then conveyed to an exit of the spray chamber, and the volatile solvent fraction is evaporated (e.g., by passing the droplets through a heated evaporator tube) to produce particles of non-volatile analyte material. The particles of analyte material are then charged in a charging chamber (for example, by interacting with an ionized gas stream), and the aggregate charge imparted to the particles is subsequently measured by a collector device comprising an electrometer.
In the accompanying drawings:
Referring now to
As depicted in
It is noted that the portion of back wall 125 extending upwardly of the medial portion of central region 105 and into upper region 130 is curved, with a relatively large radius of curvature. This geometry assists in maintaining a smooth flow of gas (and the entrained droplets) into upper region 130, and avoids the creation of eddies or other turbulent flow patterns that may adversely affect stability or produce excessive deposition of the droplets or dried particles on the spray chamber walls. The portion of back wall 125 extending downwardly from the medial portion into lower region 140 is preferably gently curved as well, in order to promote the transport of accumulated liquid (resulting from the impact of the larger droplets) to the drain.
Relatively large droplets formed in the droplet spray are unable to negotiate the turn into upper region 130 due to their higher momentum, and instead impact the medial portion of back wall 125, as indicated by dotted line 330. The resultant liquid accumulated on back wall 125 flows into lower region 140 under the influence of gravity, and may be continuously or periodically removed therefrom via a drain port. The separation of the large droplets from the droplet spray via this mechanism has the advantageous effect of enabling the detector to sense a smaller range of particle sizes, which establishes a relatively steady electrical current at the detector. This steady baseline electrical current facilitates the identification of higher electrical currents or peaks as samples are injected and separated in the chromatographic column and subsequently introduced to the nebulizer.
As is known in the art, the characteristics of the spray (e.g., droplet size distribution and spray cone angle) produced by emitter 110 will vary according to a number of operational and design parameters, including the viscosity and surface tension of the liquid sample, the geometry and dimensions of the liquid and gas orifices, and the liquid and gas flow rates. For the emitter design depicted in
In certain implementations, it may be desirable to control the temperature (heat or cool) spray emitter 110 and/or spray chamber 100 in order to optimize spray characteristics, or to promote solvent evaporation. Any suitable temperature control device, such as a cartridge heater, may be used for this purpose.
It has been observed that the performance of the nebulizer and associated CAD system is influenced by the placement of the emitter nozzle tip relative to the opposing portion of back wall 125. Generally good results have been achieved, using the emitter design of
The interior of spray chamber body 100 may be sealed to the outside by means of a plate cover 155 (depicted transparently in
It should be recognized that while embodiments of the invention are described herein with reference to a conventional pneumatically-driven spray emitter, the principles of the invention (more particularly, the use of a spray chamber having a configuration and geometry that promotes effective spray generation, drying, and separation of large droplets) may be advantageously employed in connection with other types of spray emitters, such as those driven by electrostatic repulsion or the application of pulsation by a piezoelectric element.
It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.
This application is a Continuation of U.S. patent application Ser. No. 14/288,693 (now U.S. Pat. No. 9,851,333) which was filed on May 28, 2014 and which claims the priority benefit of U.S. Provisional Patent Application No. 61/828,629 for “Nebulizer for Charged Aerosol Detection (CAD) System”, filed on May 29, 2013, the disclosures of which are hereby incorporated by reference in their entirety.
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Number | Date | Country | |
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20180100840 A1 | Apr 2018 | US |
Number | Date | Country | |
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61828629 | May 2013 | US |
Number | Date | Country | |
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Parent | 14288693 | May 2014 | US |
Child | 15838081 | US |