Abstract:
A detector assembly includes means for determining the fluorescence characteristics, the ultraviolet absorbance characteristics and the electrical conductivity characteristics of a single sample of an eluate of a chromatographic separating column.

Description:
BACKGROUND OF THE INVENTION 
     This application is a continuation-in-part of application Ser. No. 555,182, filed Nov. 23, 1983. 
    
    
     The present invention generally relates to a detector especially useful in liquid chromatography and, in particular, relates to a detector having multiple detection functions. 
     In liquid chromatography, a sample solution is passed through a separating column which is designed to partition the sample into its constituents such that the constituents are serially eluted. The eluent of the column is analyzed to characterize the constituents as they elute. 
     Generally, the characterization of liquid chromatography eluents is performed by determining the absorption, the fluorescence or the conductivity properties of the eluent. Presently, each of these properties requires a different discrete detector mechanism. For example, to measure light absorption, the detector mechanism must direct radiation through the eluate and detect that amount of radiation which passes therethrough. To measure fluorescence, the detector must include a source of radiation to initiate the fluorescence and a sensor, usually positioned at a right angle to the initiating radiation, for receiving the fluorescence. The measurement of the conductivity of an eluate requires at least two spaced apart electrodes in contact with the eluent. 
     Due to the inherent physical configurations and component requirements, conventional liquid chromatography detectors measure only one of the above properties. Hence, in order to investigate more than a single property of an eluate it has been necessary to either arrange multiple detectors in series or perform multiple separations with the same sample but with different detectors. 
     In the first approach, i.e., the use of multiple detectors in series, the major factor inherently present which reduces the accuracy and reliability of the results is that the band dispersion of the eluate is always increased as it is passed through each detector. 
     The major difficulty in exploring the second approach is, or course, the time and expense required to perform the identical separation two, three or more different times. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is one object of the present invention to provide a detector useful in liquid chromatography which can detect a plurality of properties of an eluate. 
     This object is accomplished, at least in part, by a housing block having at least two different property characterization elements operatively associated with a single sample cell therein. 
     Other objects and advantages will become apparent to those skilled in the art from the following detailed description read in conjunction with the appended claims and the drawing attached hereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing, not drawn to scale, includes: 
     FIG. 1--which is a cross-sectional view of a detector embodying the principles of the present invention; 
     FIG. 2--which is a block diagram of a detector system employing a detector embodying the principles of the present invention; 
     FIG. 3--which is a front view of a housing block for use in the detector shown in FIG. 1; and 
     FIG. 4--which is an assembly view of the detector shown in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A detector assembly, generally indicated at 10 in the drawing and embodying the principles of the present invention, includes a housing block 12 having a sample cell 14 defined therein. The detector assembly 10 also includes a source of radiation 16, means 18 for measuring the electrical conductivity of a liquid passing through the sample cell 14, means 20 for detecting radiation passing through the sample cell 14, and means 22 for detecting fluorescence radiation occurring within the sample cell 14. The detector assembly 10 is also provided with sample inlet and outlet conduits, 24 and 26, respectively, for passing a sample fluid through the sample cell 14. 
     In a preferred embodiment, the detector assembly 10 includes the source of light radiation 16 in a first opening 28 near one end 30 of the housing block 12, a first photodetector 32 disposed inwardly of the other end 36 of the block 12, a second photodetector 38 in a third opening 40 in one sidewall 42 of the block 12, and first and second spaced apart electrodes, 44 and 46 respectively, extending through a second sidewall 48 of the block 12 and adapted to be contacted by liquid passing through the sample cell 14. 
     In addition, the housing block 12 includes a bore 50 therein for receiving the sample cell 14 and having a first optical window 52 at one end 54 of the sample cell 14 and a second optical window 56 at the other end 58 of the sample cell 14. Preferably, the second optical window 56 is shaped to disperse light passing therethrough from the sample cell 14 over more of the surface 60 of the first photodetector 32. 
     In the preferred embodiment, the first and second electrodes, 44 and 46 respectively, are hollow and serve as the inlet and outlet conduits, 24 and 26 respectively, and each is sandwiched between a pair of seals 62 which form a fluid tight seal about the ends of the sample cell 14 as well as electrically isolate the electrodes, 44 and 46. 
     As shown in FIG. 1, the second optical window 56 is securely positioned by means of a retaining washer 64 which is urged against the second window 56 by a pair of spring washers 66 held in place via a lens retaining nut 68. The first photodetector 32 is fixedly positioned within the lens retaining nut 68 by means of a detector retaining nut 70. In this fashion the spacing between the second optical window 56 and the first photodetector 32 remains constant. As a consequence, the detector measurement characteristics of the first photodetector 32, i.e., which primarily depends upon both the spacing between the second window 56 and the surface 60 of the photodetector 32 and the specific surface area of the photodetector used, remains constant for a given photodetector. 
     The detector assembly 10 is clearly advantageous since any one of a plurality of properties can be measured without interchanging the detector. Additionally, since more than one property can be measured simultaneously, all properties are measured on exactly the same sample. 
     A system, generally indicated at 72 in FIG. 2 and specifically adapted to function with the detector assembly 10, for measuring a plurality of properties includes a high voltage power supply 74 for energizing the source of light radiation 16 thereof. The first and second electrodes, 44 and 46 respectively, are connected to a measuring circuit 76 which, in the preferred embodiment, includes a Wheatstone bridge type circuit for accurately measuring the impedance between the electrodes, 44 and 46. 
     The first and second photodetectors, 32 and 38 respectively, of the detector assembly 10 are electrically connected to a signal amplifier circuit 78 which provides a representative output signal to a recording mechanism 80. Preferably, the recording mechanism 80 includes, as at least one mode thereof, a strip chart recorder for providing a chromatogram. 
     Another major advantage of the detector assembly 10 is the ease by which it can be fabricated and assembled. This advantage is more fully discussed hereinafter with respect to a specific embodiment shown in FIGS. 3 and 4. 
     A 2.5 cm by 2.5 cm by 3 cm long housing block 12 of opaque chemically resistant material, such as, for example, Delrin®, a registered trademark of the duPont Corporation, is initially machined as shown in FIG. 3. Specifically, the lamp receiving first opening 28 is formed with a diameter of about 8 mm and axially located about 6 mm from the one end 30 of the block 12. A 3 mm×1.5 mm slot 82 is formed in the third opening 40 and opens into the bore 50 which passes through the center of the block 12. Preferably, the bore 50 is about 4.5 mm in diameter. A 2 mm long internal opening 84 of about 3 mm diameter is formed between the bore 50 and the first opening 28 to form a passage through which light radiation passes into the bore 50. The third opening 40, having a diameter of about 1 cm, is formed in the one sidewall 42 generally symmetrical about the slot 82. Preferably, the third opening 40 is axially perpendicular to the bore 50. This is sized to receive the second photodetector 38, i.e., which measures the fluorescence radiation. 
     A second opening 34, for receiving the lens retaining nut 68 and the first photodetector 32 is formed in the block 12 from the other end 36 to a depth of about 8 mm. Preferably, the first opening 28 is provided with an internal thread 86, for example, 3/8-24 threads per inch, and the second opening 34 is provided with internal threads 88, for example, 1/2-20 threads per inch. In order to simplify the final assembly of the electrically conductive inlet and outlet conduits 24 and 26, respectively, a 0.8 mm slot 90 is formed in the block 12 from the other end 36 and extends thereinto to the end of the bore 50, i.e., a distance on the order of about 1.9 cm. 
     Referring now more specifically to FIG. 4, the final assembly of the detector assembly 10 is more fully discussed hereinafter. 
     As shown, the first optical window 52, in the form of a flat surfaced quartz lens, is inserted into the bore 50 from the other end 36 of the block 12. Adjacent that lens is a 0.125 mm thick polytetrafluroethelene (PTFE) seal 62 followed by the mobile phase inlet conduit 24. As an alternative, the seals 62 can also be formed from other chemically resistant materials such as that known as KAPTON®, a registered trademark of duPont Corporation. The mobile phase inlet conduit 24 includes a washer-like end 92 having a 0.75 mm diameter opening 94 therethrough. A second PTFE seal 62 is inserted and thereafter the sample cell 14 is inserted adjacent the second seal 62. Preferably, the sample cell 14 is a glass disk having an outside diameter of about 4.5 mm and an opening 96 therethrough which is 0.75 mm diameter by 2 mm long. For reasons well known in the art, such as a sample cell 14 introduces very little band dispersion and is a relatively low volume cell. Thereafter, a third PTFE seal 62 and the mobile phase outlet conduit 26, also having a washer-like end 98 and an opening 100 therethrough, are inserted and followed by a fourth PTFE seal 62. This arrangement completely seals the bore 50 and prevents fluid leakage through the slot 90. Further, this assembly 10 seals the electrodes, 44 and 46, and prevents sample leakage thereabout. Finally, the second window 56, preferably having a concave surface 102 distal the light source 16, is inserted and retained in place by retaining washer 64. In order to secure, and more exactly position the second window 56 with respect to the first photodetector 32, the second window 64, and hence the assembly within the bore 50, is retained in place by the pair of opposing spring washers 66 biased inwardly by means of the lens retaining thread 105 which mates with the internal thread 88 of the second opening 34. 
     In one particular embodiment, the sample cell 14 is opaque to the radiated light, with the consequence that light is only passed through the opening 96 into the cell 14, but transparent to the fluorescent radiation. For example, if the source of radiation 16 transmits in the approximate wavelength band of 180 nm to 280 nm and the fluorescent radiation of interest is greater than about 350 nm, such as the conditions when characterizing quinine sulfate, a borosilicate glass could be used for the sample cell 14. Of course, other light sources, such as, for example, light emitting diodes, infrared, could also be used with a corresponding selection of material for the sample cell 14. 
     Referring back to FIG. 1, the first photodetector 32 is then secured within the lens retaining nut 68 by means of a detector retaining nut 70 having openings 104 therethrough for the electrical leads 106 thereof to be connected to the amplifier circuit 78 external the block 12. In the preferred embodiment, the electrically conductive inlet and outlet conduits, 24 and 26 respectively, are formed from stainless steel tubing having an inside diameter of about 0.25 mm and an outside diameter on the order of about 0.5 mm. The washer-like disk is provided at the ends thereof to not only ensure electrical contact between the conductive conduits, 24 and 26, and the fluid flowing therethrough, but also provides a surface against which the seals 62 can create a fluid tight seal for both the sample cell 14 and the electrodes 44 and 46. 
     The detector assembly 10 described herein not only provides the chromatographic advantages of low volume and low dispersion, but additionally provides major advantages in the selection and characterization of an eluate from a liquid chromatography separating column. As will be understood from the previous discussion relating to the system 72, shown in FIG. 2, any one of three properties can be measured either individually, in pairs or all three simultaneously. Such availability of measuring possibilities on the identical amount of sample fluid provides reliable determinations and characterizations of eluents from separating columns. 
     In designing a trifunctional detector of the type disclosed, there are numerous considerations, some of which interact with each other, that must be accounted for in the final design. Ideally, the sample cell should be small in volume and preferably less than 3 μL although larger sized cells will function but not as efficiently. The cell must be sufficiently long to provide an adequate signal according to Beer&#39;s Law. As a consequence of these two criteria, the cross-sectional area of the cell must be very small. In order that the radiation intensity is sufficient to be detected as intensity is controlled by the inverse square of the distance, the radiation source must be located as close as possible to the face of the cell. It also follows for this reason that the radiation sensor should be as close as possible to the other face of the cell. As the cross-sectional area of the radiation passing through the sample is small, it is desirable to have a divergent lens to distribute the radiation passing through the sample over the surface of the radiation sensor. 
     In order for the trifunctional detector to be able to measure sample conductivity, the electrodes must be sufficiently far apart so as to permit conductivity measurement. The sample cell and the rest of the detector must also be made in a manner so that the electrodes are electrically isolated from each other. Also, since conductivity must be measured over the length of the sample cell, the electrodes must be designed so that they do not interfere with the radiation passing through the sample. 
     The cell of the trifunctional detector must also be designed so that its walls are transparent to florescent light from the sample therein. The fluorescence sensor must be located as close as possible to the cell to avoid loss of sensitivity. It is also desirable to be able to locate a filter between the cell and the fluorescence sensor to filter the incident radiation and transmit the flourescent radiation. 
     The detector must be designed in a manner permitting disassembly thereof to permit cleaning of the parts. When assembled, however, the detector must have a liquid tight seal at the typical pressure of a liquid chromatograph. 
     The present invention has been described herein in relationship to a particular specific embodiment which embodiment is exemplary only as other assemblies and configurations will become apparent to those skilled in the art from reading this description. Hence, the present invention is deemed limited only by the appended claims and the reasonable interpretation thereof.