Abstract:
The present invention is directed to a device for evaporative light scattering detection that can be used for a wide range of sample types and mobile phases. The device may be quickly converted between a single flow and a split flow configuration through the use of a retractable impactor.

Description:
BACKGROUND OF THE INVENTION 
     Evaporative light scattering detection is a method of detecting samples that have been previously separated in various chromatography methods such as, for example, High Performance Liquid Chromatography (HPLC), Gel-Permeation Chromatography (GPC), High Performance Centrifugal Partition Chromatography (HPCPC), Field Flow Fractionation (FFF), and Supercritical Fluid Chromatography (SFC). Evaporative light scattering detection is preferably used when the sample components (e.g., components to be detected) have lower volatility than the mobile phase. A wide variety of sample types can be detected in evaporative light scattering detection. Such sample types include, for example, lipids, triglycerides, surfactants, polymers, underivatized fatty and amino acids, carbohydrates and pharmaceuticals. 
     Generally, evaporative light scattering detection involves four main steps: 1) nebulization of the chromatography effluent, (which consists of the mobile phase and the sample), into an aerosol of particles, 2) evaporation of the mobile phase, 3) directing a light beam at the dry sample particles to scatter the light, and 4) detection of the scattered light. The amount of sample is determined based upon on how much light is scattered. There are two principal types of devices used in evaporative light scattering detection known in the art. In the first type (the “single flow” design), the nebulized chromatography effluent is immediately introduced into a heated drift tube where the mobile phase is evaporated. The sample particles are then flowed from the heated drift tube to an optical cell where light scattering and detection occurs. One such example of this type of device (the Alltech Model 500 ELSD) is sold by the assignee of this application, ALLTECH ASSOCIATES, INC. Details concerning the design and operating parameters for such a device are disclosed in the Operating Manual for the Alltech Model 500 ELSD, which is incorporated herein by reference. 
     In the second type of device, (the “split-flow” design), the nebulized chromatography effluent is first flowed through a nebulization chamber before entering the heated drift tube. In the nebulization chamber, the nebulized chromatography effluent is split, namely, the larger droplets are eliminated by condensation/impaction on the walls of the nebulization chamber. This condensate is drained to waste. Only the smaller nebulized droplets are subsequently flowed to the heated drift tube where the mobile phase (which is now free of the larger droplets) is more easily evaporated. Thereafter, the sample particles are flowed to the optical cell for light scattering and detection. Devices of this design type are available from, for example, POLYMER LABORATORIES, SEDERE or EUROPSEP INSTRUMENTS. 
     The above-described design types have particular advantages depending on the mobile phase and the sample type. The single flow design is preferred for use in applications involving the detection of relatively non-volatile samples in relatively volatile organic mobile phases. Moreover, because the entire sample enters the optical cell in this design, response and sensitivity is maximized. 
     However, the single flow design is not especially preferred when detecting relatively volatile samples in relatively non-volatile mobile phases (such as aqueous mobile phases). Highly aqueous mobile phases generally require higher evaporation temperatures. If the sample is volatile at these higher evaporation temperatures, sample loss is incurred during the evaporation step resulting in poorer sensitivity. By using the split-flow design (i.e., passing the chromatography effluent through a nebulization chamber to remove the larger droplets of mobile phase prior to the heated drift tube), the evaporation temperature of the mobile phase can be reduced. Thus, the mobile phase may be evaporated at a lower temperature in the drift tube, which leads to less sample loss from evaporation. In other words, by removing the larger droplets, a smaller and more uniform particle size distribution is achieved in the mobile phase, which enables evaporation of the mobile phase at lower evaporation temperatures. 
     However, for relatively non-volatile sample types in relatively volatile organic mobile phases, the split-flow design is generally less preferred because (1) loss of the relatively non-volatile sample during evaporation at lower temperatures of the relatively volatile mobile phase is not a concern (2) the relatively non-volatile sample may be lost during the splitting of the chromatography effluent in the nebulization chamber. Another problem with devices of the split-flow design is that the split ratio of the sample (i.e., the amount that goes to waste versus the amount that is ultimately detected) is affected by, among other things, the laboratory temperature. In other words, fluctuations in laboratory temperatures may lead to fluctuations in droplet size in the nebulized chromatography effluent. Thus, as ambient and/or laboratory temperatures fluctuate, the split ratio and corresponding reproducibility of sample detection may vary from run to run. 
     As is evident from the above-discussion, depending on the mobile phase and the sample type being detected, one evaporative light scattering detection design and method is advantageous over the other. However, laboratories often work with both aqueous and organic mobile phases and various sample types with different volatilities. Ideally, laboratories would have available both design types for evaporative light scattering detection. However, in order to have this benefit, the laboratory would need to purchase two separate devices, which can be expensive. It would be advantageous and constitute an improvement in the art if an evaporative light scattering detection device and system were developed which could be quickly and inexpensively converted between the single flow and split flow configuration. 
     To address this need, Applicants previously developed a device and system for easy and quick conversion between the single flow and the split flow configuration. That system and design is disclosed in co-pending application Ser. No. 08/932,262, which is assigned to the assignee of this application. The disclosure of 08/932,262 is fully incorporated herein by reference. Although the system and device disclosed in this co-pending application fulfills the above need, a disadvantage associated with that system and device is that the nebulizer must be removed, a flow splitting adaptor installed and the nebulizer replaced when converting from the single flow to the split flow configuration. The present invention avoids the need of removing the nebulizer and inserting a flow splitting adaptor when converting from the single flow to the split flow configuration. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a device for evaporative light scattering detection is provided that may be quickly and easily converted between the single flow and split flow configurations. The device comprises a nebulizer, a drift tube in flow communication with the nebulizer wherein the drift tube has a drain outlet and a retractable impactor located downstream in the direction of flow from the drain outlet, a light source positioned downstream of the drift tube and a detector. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a chematic diagram illustrating the principles of operation of an evaporative light scattering detection device. 
     FIG. 2 is a perspective view of a preferred embodiment of the device of the present invention. 
     FIG. 3 is a cross-section along line A—A of FIG. 2 with the retractable impactor in the parallel mode. 
     FIG. 4 is a cross-section along line A—A of FIG. 2 with the retractable impactor in the perpendicular mode. 
     FIG. 5 is an end perspective view of the retractable impactor. 
     FIG. 6 is top plan view of the retractable impactor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an evaporative light scattering detection device of the single flow configuration. FIG. 1 provides an overview of the principal of operation of an evaporative light scattering detection device  10 . The evaporative light scattering device  10  has a connector  12 . The connector  12  provides a fluid connection between the chromatography column (not shown) and the evaporative light scattering device  10 . The connector  12  is preferably made from stainless steel and is threadingly engaged to nebulizer bracket  16 . Chromatography effluent is flowed into the evaporative light scattering device  10  through channel  13  in connector  12 . Nebulizer bracket  16  removably attaches nebulizer  14  to drift tube assembly  18 . The nebulizer  14  contains a nebulizer needle (not shown). The drift tube assembly  18  surrounds a central heated drift tube channel  11 . Finally, a laser light source  20 , a photodetector  22  and amplifier  24  are provided. 
     In operation, and with reference to FIG. 1, the chromatography effluent is flowed through connector  12  to nebulizer  14 . The chromatography effluent is directed through the nebulizer needle (not shown). Upon exiting the nebulizer needle, the chromatography effluent is impacted by nebulizer gas to form an aerosol of droplets, preferably of generally uniform size. The nebulizing gas may include any gas that is inert to the sample such as helium, carbon dioxide, air or nitrogen, and is preferably nitrogen. 
     The nebulized chromatography effluent is then flowed through channel  11  in the drift tube assembly  18 . In channel  11 , the mobile phase is evaporated leaving behind the relatively non-volatile sample particles. The sample particles are flowed through channel  11  to the light scattering zone  19  for detection. A light source  20  emits light, which the sample particles scatter. The scattered light is then detected at the photodetector  22 . The photodetector  22  then produces a signal, which is sent to an amplifier  24  though analog outputs in the photodetector. 
     The light source  20  is preferably a Class IIIA laser product with 650 nm laser diode, 5 mW maximum power, collimating optics, and polarized. A preferred laser light source is available from COHERENT, as part no. VLM3-5L. The photodetector  22  is preferably made from a silicon photodiode. A preferred photodetector  22  is available from HAMAMATSU, as part no. S2386-8K. The photodetector  22  is preferably located at a 90-degree angle from the light source  20 . A light trap (not shown) is also preferably located at a 180-degree angle from the light source  20  to collect any light not scattered by the sample particles in the aerosol stream. After detection in the detection zone, the sample particles are flowed to waste. Preferably the exhaust is flowed to a fumed hood or other ventilation device located close to the detector to remove the detector exhaust from the laboratory. 
     In operation, the nitrogen nebulizing gas is preferably regulated from 45-80 psig with 99.9% purity or better. A stable gas flowrate and pressure are necessary for reproducible results. The gas is preferably free of contaminants, such as oil, water, particulate or any other non-volatile substances. The droplet size in the nebulized chromatography effluent may be regulated by varying the flow rate of either the chromatography effluent and/or the nebulizing gas. The lower the flow rate of the chromatography effluent, the less gas and heat necessary for nebulization and subsequent evaporation. 
     With reference to FIG. 2, a preferred embodiment of the evaporative light scattering device  100  of the present invention is illustrated. The device comprises a nebulizer  102 . The nebulizer is preferably as disclosed in co-pending application Ser. No. 08/932,262 and is available from ALLTECH ASSOCIATES, part no. 600514. Downstream in the direction of flow from the nebulizer is a tube  104 . The drift tube may preferably be constructed as set forth in co-pending application Ser. No. 08/932,262. The drift tube has two sections, a nebulizer block  106  and a mid-section  108 . The nebulizer block  106  connects the nebulizer  102  to the drift tube. The nebulizer  102  is in flow communication with the drift tube  104 . The drift tube  104  further has a drain outlet  110 , which is preferably located on the nebulizer block  106 . The drift tube also has a retractable impactor  105 , which is also preferably located on the nebulizer block  106 . The impactor  105  preferably comprises a planar, solid section  105   a  mounted on a pin  105   b . By retractable, it is meant that the planar section  105   a  may be rotated from a position where the face of the planar section is parallel to the direction of fluid flow (parallel mode) to a position in which the face of the planar section is at a 90 degree angle to the path of flow (perpendicular mode) through the drift tube  104 . When in the perpendicular mode, the planar section provides an impact surface for the nebulized mobile phase particles to strike, which leads to condensation of those particles. By directing the mobile phase against the impactor in this fashion, a portion of the mobile phase undergoes condensation and is separated from the nebulized sample particles. Moreover, because the drift tube is positioned at an angle such that the drain outlet is below the impactor relative a plane X—X through the device between the impactor and the drain outlet, condensed mobile phase is removed from the device by gravity through the drain outlet  110 . 
     Preferably, the planar section  105   a  is tapered (see FIG. 4) and is constructed from TEFLON coated stainless steel. The outer edges of the planar section  105   c  and  d  generally correspond to the contour of the inner wall  104   a  of the drift tube. The impactor pin is preferably constructed from stainless steel. In the perpendicular mode, the impactor preferably blocks about 70% of the flow path in the plane through the drift tube in which the impactor planar section  105   a  resides. In the parallel mode, the impactor preferably blocks about 18% of the flow path in the plane through the drift tube in which the impactor planar section  105   a  resides. However, because the impactor is preferably tapered (see FIG.  5 ), most of the mobile phase and sample will flow past the impactor when it is in its parallel mode. The impactor is preferably rotated between the parallel and perpendicular modes by a motor (not shown), such as a direct drive solenoid motor. However, the impactor may be rotated manually if desired. Also, rotation of the impactor may be programmed by the user and driven by logic. 
     Alternatively, the device may be constructed such that the impactor is inserted and retracted from the fluid flow channel instead of rotated between the perpendicular and parallel modes as described above. Thus, the impactor may be inserted into the flow channel of the drift tube and positioned in the perpendicular mode and removed from the flow channel all together by appropriate means. The term “retractable” is meant to include the above described impactor that may be rotated between the perpendicular and parallel modes as well as to an impactor that may be inserted into the flow channel and positioned in the perpendicular mode and removed from the flow channel all together. 
     Additionally, the shape of the impactor may be varied. The only critical aspect of the impactor is that it provide a sufficient impact surface for the nebulized mobile phase when the device is in the split flow configuration and that this impact surface may be easily removed from the path of flow for converting to the single flow configuration. 
     With further reference to FIGS. 2-4, the drift tube  104  is in flow communication with an optics block  112 . The optics block comprises a light source  114 , which is preferably a laser available from COHERENT as part VLMIII-5L, and a light trap  116 . The light trap is preferably an insulated block of black anodized aluminum of suitable curvature to funnel light to its end. The device further comprises a detector  118 , which comprises a photo diode and amplifier available from ELECTRO-OPTICAL SYSTEMS as part no. S-050-E8. The drift tube  104  and optics block  112  are in flow communication with an exhaust tube  122 . 
     In the single flow configuration, the impactor  105  is in the parallel mode (FIG.  3 ). The sample and mobile phase are introduced into the nebulizer  102  where the sample and mobile phase are nebulized. The sample and mobile phase particles are then flowed to the drift tube where the mobile phase is evaporated. The sample particles are then flowed to the optics block where the sample particles are detected. The sample particles are then flowed to the exhaust tube  122  and then to waste. 
     The device may be quickly and easily converted to the split flow configuration by placing the impactor in the perpendicular mode (FIG.  4 ). In this mode, sample and mobile phase are nebulized in the nebulizer. The nebulized sample and mobile phase strike the planar section  105   a  of the impactor, which removes a portion of the nebulized mobile phase (and possible a portion of the sample) from the flow stream by condensation. This condensate is flowed out of the drift tube through drain outlet  110 . The remaining portion of the sample and mobile phase is flowed through the drift tube where the mobile phase is evaporated. The sample particles are then flowed to the optics block where the sample is detected. 
     In a preferred aspect of the invention, the device has four zones in which the temperature is controlled independently of the other zones. Thus, the first zone is located between the nebulizer  102  and the impactor  105 . The second zone is located between the impactor  105  and the optics block  112 . The third zone is the optics block  112 . The fourth zone is the exhaust tube. The zones are heated by using heat tape embedded in the nebulizer block, drift tube, optics block and the exhaust tube as well known in the art. Moreover, each of these zones is connected to their own power source for independent temperature control. As those skilled in the art will appreciate, optimum temperature conditions for the particular sample and mobile phase are more easily achieved by dividing the device into four temperature zones. 
     According to a most preferred embodiment of the present invention, the evaporative light scattering device may have the following dimensions. The distance between the tip of the nebulizer needle to the end of the exhaust tube  122  is 23.627 inches. The distance from the tip of the nebulizer needle to the impactor  105  is 2.594 inches. The nebulizer block  106  is 3.574 inches in length. The mid-section of the drift tube  104  is about 13 inches in length. The length of the exhaust tube is 4.388 inches. The internal diameter of the drift tube  104  is about 0.87 inches. The impactor  105  preferably has the following dimensions. With reference to FIGS. 5 and 6, edges  105   f  are about 0.016 inches in length. The impactor has a thickness of about 0.091 inches along line  105   g . Bore  105   h  has an internal diameter of about 0.276 inches for receiving mounting needle  105   b . The impactor  105  further has a width of about 0.552 inches along line  105   i.    
     As those skilled in the art will appreciate, modifications may be made to the foregoing embodiments without departing from the invention, which is defined by the following claims.