Abstract:
The method comprising the steps of engaging the flow circuit of the detector with a reference fluid, accepting a sample in the flow circuit of the detector, sensing an attribute of the sample for determining a characteristic of the sample, changing the direction of the flow of the sample in the flow circuit, and purging the sample from the flow circuit such that the flow circuit is ready to accept another sample. Another method provides the steps of engaging the flow circuit of the detector with a reference fluid, inserting a sample in the flow circuit juxtaposed to the reference fluid, sensing an attribute of the sample for determining a characteristic of the sample, and deviating the direction of the flow of the sample from the flow circuit for purging the sample from the flow circuit such that the reference fluid is maintained in the flow circuit. 
     The apparatus comprises a detector for analyzing a sample comprising a reference cell and a sample cell such that the detector is charged with a reference fluid, a switching valve in communication with the sample cell of the detector, and one or more delay volumes and the reference cell in communication with the switching valve. The sample is juxtaposed the reference fluid for engaging the sample cell for analysis, the analyzed sample engages the switching valve for alternately diverting the analyzed sample from the flow circuit and for maintaining the detector charged with the reference fluid.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present application is a continuation-in-part application of the application of Alan Titterton, U.S. Ser. No. 11/249,839, filed Oct. 12, 2005, entitled IMPROVED MULTI-CAPILLARY VISCOMETER APPARATUS AND METHOD, now U.S. Pat. No. 7,334,457 issued Feb. 26, 2008. 

   FIELD OF THE INVENTION 
   The present invention relates to differential detectors, comparative sensors or any device that involves an analysis between two or more samples or specimen. More particularly, the present invention relates to eliminating the breakthrough peak associated with differential detectors, comparative sensors or any device that involves an analysis between two or more samples or specimen. 
   BACKGROUND OF THE INVENTION 
   Differential detectors, comparative sensors or any other device that involves an analysis between two or more samples or specimen may result in a breakthrough peak that distracts from the measurement. A multitude of detectors or sensors are susceptible to the same or similar problems. By way of example, and without limitation, viscometers, refractive index detectors, deflection detectors, reflective detectors and any combination of refractive index, ultra-violet, fluorescent, radiochemical, electrochemical, near-infra red, mass spectroscopy, nuclear magnetic resonance, and light scattering are potential instruments, detectors and sensors applicable to the present invention. 
   The refractive index of a material is the most important property of any optical system that uses refraction. The refractive index is used to calculate the focusing power of lenses, and the dispersive power of prisms, and to measure the concentrations as well as elemental analyses. Since the refractive index is a fundamental physical property of a substance, it is often used to identify a particular substance, confirm its purity, or measure its concentration. The refractive index is used to measure solids, liquids, and gases. Most commonly it is used to measure the concentration of a solute in an aqueous solution. A refractometer is the instrument used to measure refractive index. 
   The refractive index or index of refraction of a medium is a measure for how much the speed of light, or other waves such as sound waves, is reduced inside the medium. For example, typical glass has a refractive index of 1.5, which means that light travels at 1/1.5=0.67 times the speed in air or vacuum. Two common properties of glass and other transparent materials are directly related to their refractive index. First, light rays change direction when they cross the interface from air to the material, an effect that is used in lenses and glasses. Second, light reflects partially from surfaces that have a refractive index different from that of their surroundings. 
   The refractive index, n, of a medium is defined as the ratio of the phase velocity c of a wave phenomenon such as light or sound in a reference medium to the phase velocity ν p  in the medium itself: 
   
     
       
         
           n 
           = 
           
             c 
             
               v 
               p 
             
           
         
       
     
   
   The refractive index, n, is most commonly used in the context of light with a vacuum as a reference medium, although historically other reference media, e.g., air at a standardized pressure and temperature, have been common. It is usually given the symbol n. In the case of light, refractive index, n, equals:
 
 n =√{square root over (ε r μ r )},
 
   where ε r  is the material&#39;s relative permittivity, and μ r  is the material&#39;s relative permeability. For most materials, μ r  is very close to 1 at optical frequencies, therefore, n is approximately √{square root over (ε r )}. 
   The refractive index, RI, detector is the only universal detector in high-performance liquid chromatography (HPLC). HPLC is a form of column chromatography used frequently in biochemistry and analytical chemistry. It is also sometimes referred to as high-pressure liquid chromatography. HPLC is used to separate components of a mixture by using a variety of chemical interactions between the substance being analyzed (analyte) and the chromatography column. 
   The detection principle involves measuring of the change in refractive index of the column effluent passing through the flow-cell. The greater the RI difference between sample and mobile phase, the larger the imbalance will become. Thus, the sensitivity will be higher for the higher difference in RI between sample and mobile phase. On the other hand, in complex mixtures, sample components may cover a wide range of refractive index values and some may closely match that of the mobile phase, becoming invisible to the detector. The RI detector is a pure differential instrument, and any changes in the eluent composition require the rebalancing of the detector. This factor severely limits RI detector application in the analyses requiring the gradient elution, where the mobile phase composition is changed during the analysis to effect the separation. Two basic types of RI detectors are on the market today. Both require the use of a two-path cell where the sample-containing side is constantly compared with the non-sample-containing reference side. 
   The deflection detector is based on the deflection principle of refractometry. Refractometry provides that the deflection of a light beam is changed when the composition in a sample flow-cell changes in relation to the reference side as eluting sample moves through the system. When no sample is present in the cell, the light passing through both sides is focused on a photodetector, usually photoresistor. As sample elutes through one side, the changing angle of refraction moves the beam. This results in a change in the photon current falling on the detector that unbalances it. The extent of unbalance, which can be related to the sample concentration, is recorded on a recorder. 
   The advantages of this type of detector are: universal response; low sensitivity to dirt and air bubbles in the cells; and the ability to cover the entire refractive index range from 1.000 to 1.750 RI with a single, easily balanced cell. The disadvantages are: a general disability to easily remove and clean or replace the cell when filming or clogging occurs, the need to flush the sample-side intermittently, static solvent causing baseline drift and the need to refresh, replenish or recharge the reference-side. 
   Another relevant detector is a reflective detector. The reflective detector is a refractive index detector based on the Fresnel principle. In the reflective detector, the light beam is reflected from the liquid-glass interface in the detecting photocell. The introduction of sample into one cell causes light to be refracted at a different angle. The deflection of the light beam from the photoresistor causes the appearance of the electrical signal. Here, too, this difference between sample-cell signal and reference-cell signal is output to a recorder or data handling system as a peak. 
   The major advantage of the reflective detector is a very high sensitivity since the optics allow a higher concentration of signal in a particular RI range than is possible in other wide-range detectors. Other advantages include the ability to operate at extremely low flow rates with very low-volume cells, easy cell accessibility, and low cost. The disadvantages of the reflective detector are the incredible sensitivity to the flow and pressure fluctuations, and the need for changing prisms to accommodate either high or low RI solvents and the need to manually adjust the optical path when making solvent changes. 
   The refractive index of an analyte is a function of its concentration. Change in concentration is reflected as a change in the RI. A refractive index detector for liquid chromatography should be sensitive to changes as small as 10 −7  RI units corresponding to a concentration change of 1 ppm. Presence of dissolved air, changes in solvent composition, improper mixing and column bleed will contribute to baseline drift. Eluent pressure change of 15 psi will cause the change of 1×10 −6  RI unit and 1° C. temperature variation will be equivalent to the change of 600×10 −6  RI units. Thus it is obvious that both of these parameters must be closely controlled, especially temperature. To operate at high sensitivities, a RI detector must usually be thermostated (±0.01° C.), actually the using of the water bath connected to the detector head does not give required temperature stability, alternately, passive thermostabilisation with massive metallic block usually gives much better results. 
   All the above-noted apparatus, including differential detectors, comparative sensors or any device that involves an analysis between two or more samples or specimen, suffer from having a breakthrough peak that delays processing and contaminates flow paths and capillaries. Therefore, of primary concern in the present invention is the removal of the breakthrough peak, and thus, the processing delays and the contamination of the flow paths and capillaries. 
   It is, therefore, a feature of the present invention to remove the associated breakthrough peak. 
   A feature of the present invention is to improve processing efficiency by reducing processing delays. 
   Another feature of the present invention is to inherently prevent the contamination of the flow paths and the capillaries. 
   Another feature of the present invention is to reduce baseline drift by providing consistent solvent composition. 
   Another feature of the present invention is to reduce baseline drift by removing dissolved air from the measurement cells. 
   Yet another feature of the invention is to reduce baseline drift by improper mixing. 
   Still another feature of the present invention is to reduce baseline drift by preventing the effect of column bleed. 
   Another feature of the present invention is to reduce the sensitivity to the flow fluctuations. 
   Yet another feature of the present invention is to reduce the sensitivity to the pressure fluctuations. 
   Still another feature of the present invention is to easily clean the reference cell when filming or clogging occurs. 
   Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent from the description, or may be learned by practice of the invention. The features and advantages of the invention may be realized by means of the combinations and steps particularly pointed out in the appended claims. 
   SUMMARY OF THE INVENTION 
   To achieve the foregoing objects, features, and advantages and in accordance with the purpose of the invention as embodied and broadly described herein a method for purging samples from a flow circuit of a detector for analyzing samples is provided. The method comprising the steps of engaging the flow circuit of the detector with a reference fluid, accepting a sample in the flow circuit of the detector, sensing an attribute of the sample for determining a characteristic of the sample, changing the direction of the flow of the sample in the flow circuit, and purging the sample from the flow circuit such that the flow circuit is ready to accept another sample. 
   Another method provides the steps of engaging the flow circuit of the detector with a reference fluid, inserting a sample in the flow circuit juxtaposed to the reference fluid, sensing an attribute of the sample for determining a characteristic of the sample, and deviating the direction of the flow of the sample from the flow circuit for purging the sample from the flow circuit such that the reference fluid is maintained in the flow circuit. 
   The apparatus comprises a detector for analyzing a sample comprising a reference cell and a sample cell such that the detector is charged with a reference fluid, a switching valve in communication with the sample cell of the detector, and one or more delay volumes and the reference cell in communication with the switching valve. The sample is juxtaposed the reference fluid for engaging the sample cell for analysis, the analyzed sample engages the switching valve for alternately diverting the analyzed sample from the flow circuit and for maintaining the detector charged with the reference fluid. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  illustrates a schematic view of a general multi-capillary viscometer. 
       FIG. 2   a  illustrates a block diagram of one embodiment of the apparatus with two delay volume components and a fluid diverting component in orientation A. 
       FIG. 2   b  illustrates a block diagram of one embodiment of the apparatus with two delay volume components and a fluid diverting component in orientation B. 
       FIG. 3   a  is a block diagram of one embodiment of the subcircuit of the apparatus with two delay volume components and a fluid diverting component in orientation A. 
       FIG. 3   b  is a block diagram of one embodiment of the subcircuit of the apparatus with two delay volume components and a fluid diverting component in orientation B. 
       FIG. 4   a  is a block diagram of one embodiment of the subcircuit of the apparatus with a single delay volume component and a fluid diverting component in orientation A. 
       FIG. 4   b  is a block diagram of one embodiment of the subcircuit of the apparatus with a single delay volume component and a fluid diverting component in orientation B. 
       FIG. 5  is a viscometer detector used to illustrate the generation of a breakthrough peak. 
       FIG. 6  is a graph of the sample peak and the breakthrough peak by plotting Retention Volume in mL versus Viscometer DP in mV for the output of the viscometer detector illustrated in  FIG. 5 . 
       FIG. 7  is a schematic diagram illustrating the principle of differential refractive index (RI) measurement. 
       FIG. 8  is a schematic diagram illustrating the flow path for a DRI detector coupled to a GPC. 
       FIG. 9  is a schematic diagram illustrating the conventional manner of purging the solvent into the reference cell. 
       FIG. 10  is a schematic diagram illustrating a delay column used in a manner analogous to the differential viscometer to provide continuous flow of solvent to the reference compartment. 
       FIG. 11  is a schematic diagram illustrating the no-breakthrough scheme implemented for the viscometer detector will work in an analogous fashion for the RI detector. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   For a further understanding of the nature, function, and objects of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings. Detailed descriptions of embodiments of the apparatus are provided herein, as well as modes of carrying out and employing the embodiments of the present invention. It is to be understood, however, that the present apparatus may be embodied in various forms. The description provided herein relates to the common components of sample capillary, delay volume components, reference capillary and diverter valve which may form only part of a more complex circuit. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present embodiments of one form of the apparatus or method in virtually any appropriately detailed system, structure or manner. The embodiments described herein are considered illustrative of both the processes taught by the described embodiments of products and articles of manufacture yielded in accordance with the present embodiments of one form of the apparatus. The inventive device and method can be used in a multi-capillary viscometer for which the basic operating principles are disclosed in U.S. Pat. No. 4,463,598 to Haney, in U.S. Pat. Nos. 4,627,271 and 4,578,990 to Abbott, et al., and in U.S. Pat. No. 5,637,790 to de Corral. 
   In the prior art, multi-capillary viscometers are constructed and operated such that a reference flow of solvent may be maintained during a measurement by inserting a delay volume component in front of a reference capillary. The delay volume component initially contains pure solvent at the beginning of the analysis. It provides a continuing flow of solvent to a reference capillary while sample flows though a sample capillary. A limitation of such prior art multi-capillary viscometers, whether used only for viscosity measurements or in combination with other detectors in GPC or SEC analysis, is typically referred to as a breakthrough peak. The breakthrough peak is an unwanted instrument response resulting from the discharge of sample fluid from the delay volume component through a reference capillary after pressures have been detected or sensed that are useful in determining information about the viscosity of the sample. The breakthrough peak can last for a substantial period of time after the measurement process is completed because the delay volume spreads the peak, thereby delaying the measurement of the next sample. 
   The cause of the breakthrough peak effect can be appreciated by reference to  FIG. 1  accompanied with the following description.  FIG. 1  illustrates schematically a simple multi-capillary viscometer and can be described as an operating circuit. The operating circuit consists of an inlet tube  201  coupled in series to a narrower bore capillary tube  202  referred to as the sample capillary, a delay volume component  203  which has a hold-up volume of significantly large capacity, a second narrow bore capillary tube  204  referred to as the reference capillary, and finally an exit tube  205 . When a fluid such as a solvent is pumped through the circuit, a differential pressure will be generated, mainly across the capillary tubes since they have a narrower cross-sectional area than the connecting tubing and other components in the circuit. The pressure across each capillary may be monitored continuously by means of suitable transducers  206 ,  207 . A sample solution is injected into the flowing solvent stream and is carried through the circuit. Its passage through the sample capillary  202  will typically cause an increased pressure drop because of the viscosity of the sample. The increased pressure drop will be detected by the transducer  206  across the respective sample capillary  202 . However, the pressure monitored by the transducer  207  across the reference capillary  204  will remain substantially unaltered since it will still be receiving solvent eluting from the delay volume component  203 . The relative viscosity may be mathematically derived using the ratio of the pressure drop across the sample capillary  202  to the pressure drop across the reference capillary  204  as described in, for example, U.S. Pat. Nos. 4,627,271 and 4,578,990, Abbott, et al. The relative viscosity will increase as sample flows into the viscometer. 
   The volume of the delay volume component  203  is selected to be sufficient to supply the reference capillary  204  with solvent during the measurement process. Eventually, all the sample solution will emerge from the sample capillary  202  and enter the delay volume component  203 . The pressure across the sample capillary  202  will return to the baseline value and the useful part of the measurement cycle is over. However, the sample will eventually progress all the way through the delay volume component  203  and enter the reference capillary  204  where it will cause an increase in pressure, measured by the transducer  207  associated with the reference capillary  204 . This increase in pressure causes a decrease in the measured relative viscosity, which is the cause of the breakthrough peak. 
     FIG. 2   a  illustrates one embodiment of the present embodiments of one form of the apparatus comprising a fluid viscosity measurement circuit  23  having components connected together preferably using conventional tubing connectors, fittings or unions and connecting tubing. The fluid viscosity measurement circuit  23  preferably has a fluid insertion or injection tube  1  through which a fluid is inserted. Fluid viscosity measurement circuit  23  preferably has a first fluid flow circuit  60  and a second fluid flow circuit  61 . 
   In the embodiment illustrated by  FIG. 2   a , the fluid insertion or injection tube  1  attaches to a split junction  2 , which can include but is not limited to a T-bridge split junction. The split junction  2  is attached to two lengths of pipe or tubing  3 ,  3 ′. The two lengths of pipe or tubing  3 ,  3 ′ are attached to the split junction  2  so as to allow a fluid to move through the split junction  2  into the length of pipe or tubing  3  and the other length of pipe or tubing  3 ′. The length of pipe or tubing  3  is connected between the split junction  2  and a first capillary  4  in the flow circuit  60 . The first capillary  4  is preferably a conventional fluid capillary, which is preferably a tube, having a relatively small inside diameter (typically, but not limited to, the range of about 0.009″ to 0.014″) in comparison to the relatively large inside diameter of other fluid components in the circuit (typically, but not limited to 0.04″ or more for connecting tubing and fittings and 0.062″ or more for delay volumes). The first capillary  4  serves to provide a flow restriction within the fluid viscosity measurement circuit. The terms “capillaries” or “capillary” are used throughout this application in their normal and customary manner to include, for example, any structure with a cross-sectional hollow portion having a relatively small inside diameter to produce a pressure drop higher than that produced by other individual components of the fluid viscosity measurement circuit. 
   The length of pipe or tubing  3 ′ is connected between the split junction  2  and a first capillary  4 ′ of the second flow circuit  61 . The capillaries  4 ,  4 ′ are preferably conventional fluid capillaries. The first capillary  4  is preferably connected to a second split junction  5 . The first capillary  4 ′ is preferably connected to a second split junction  5 ′. The first split junction  5  and the second split junction  5 ′ are preferably connected to a transducer tube or line  24 ,  24 ′, respectively. The transducer tubes or lines  24 ,  24 ′ are preferably connected to the second split junctions  5 ,  5 ′, respectively, and to a transducer  6 . The terms “transducer” or “transducers” are used throughout this application in their normal and customary manner to include, for example, any structure or apparatus operable for sensing or measuring fluid differential pressures and, more particularly, differential pressure transducers of the type described in the Abbot, et al. and De Coral patents wherein two cavities are separated by a diaphragm which is deflected by a pressure difference in the cavities to produce an electrical signal proportional to the pressure differential. 
   The transducer  6 , and all transducers referred to herein, is preferably connected in a “dead-end” manner, such that only the inlet ports of the transducers remain open after the transducer lines and cavities are filled and purged for operation, and pressure is transmitted by static fluid in the transducer lines and cavities to the transducer diaphragm. The transducer  6  and all transducers referred to herein may also be connected in a “flow-through” manner, wherein inlet, outlet or purge ports of each cavity of the transducer are connected such that fluid flowing through one or more of the other components in the circuit also flows through the transducer cavities, and fluid pressure is transmitted to the transducer diaphragm by the fluid flowing through the transducer cavity. 
   The split junction  5  preferably connects to a fluid tube  25 . The fluid tube  25  preferably connects to a capillary  7 . The capillary  7  preferably connects to another fluid tube  26 . The fluid tube  26  preferably connects to a split junction  12 . The split junction  12  preferably connects to yet another fluid tube  32 . 
   The split junction  5 ′ in the second fluid flow circuit  61  preferably connects to a fluid tube  33 . The fluid tube  33  preferably connects to a fluid path diverter valve  8 . The fluid path diverter valve  8  preferably contains a plurality of fluid pathways  34 ,  35 . The terms “diverter valve” or “valve” are used in this application in their normal and customary manner, and by way of example, refers to any valve or structure operable to selectively direct or align the flow of fluid from one fluid pathway to another and may include, but is not limited to, a valve operable for that purpose in a prescribed or automated fashion, which may be, but is not limited to, electrical, pneumatic or timed operation or activation. The term “diverter valve” as used in this application can be, but is not limited to a 4-port, 2 position plug valve, such as Hamilton HV-86779. As shown in  FIG. 2   a , a first fluid pathway  34  is connected to a first fluid tube  36 , and a second fluid pathway  35  is connected to a second fluid tube  27 . The first fluid tube  36  is connected to a first delay volume  9 . The first delay volume  9  is connected to a downstream fluid tube  37 . The term “delay volume” is used in this application in its normal and customary manner, and for example, includes any means of delaying a fluid&#39;s arrival at another point in the fluid circuit, which may include, but is not limited to, increased volume tubing or reservoirs. A typical delay volume can include, but is not limited to a packed column. The downstream fluid tube  37  is preferably connected to a split junction  30 . A capillary  7 ′ is also connected to the split junction  30 . The capillary  7 ′ can be, but is not necessarily, substantially identical to the corresponding capillary  7 . A transducer line or tube  29  is also attached to the split junction  30 . 
   The transducer line or tube  29  is preferably connected to a transducer  10 . The transducer  10  is preferably a conventional transducer utilized in measuring fluid viscosity. The transducer  10  is preferably connected in the “dead-end” manner described above for the center transducer  6 . The transducer  10  and the center transducer  6  are preferably substantially identical. The transducer  10  is also connected to a transducer line or tube  31  such that the connections of transducer line or tube  29  and transducer line or tube  31  are on substantially opposite sides of the diaphragm associated with the transducer  10 . The transducer line or tube  31  is connected to a split junction  13 . The capillary  7 ′ is likewise connected to the split junction  13 . A fluid tube line  28  is preferably connected to the split junction  13  and to a second delay volume  9 ′. The first delay volume  9  and the second delay volume  9 ′ are preferably substantially identical. The second delay volume  9 ′ is preferably connected on its opposite end to a fluid tube  27 . The fluid tube  27  is preferably connected to the second fluid pathway  35  of the fluid path diverter valve  8 . The second fluid pathway  35  is also preferably connected on its opposite end to a fluid tube  11 . The fluid tube  11  is connected to the split junction  12 . 
     FIG. 2   b , illustrates another embodiment of an apparatus of the present invention similar to the embodiment illustrated in  FIG. 2   a . The embodiment illustrated in  FIG. 2   b  is similar to the embodiment illustrated in  FIG. 2   a  except in the embodiment in  FIG. 2   b , the fluid path diverter valve  8  is rotated to another position. Particularly, the fluid path diverter valve  8  is positioned such that the fluid pathway  34  is aligned between the respective fluid tubes  33 ,  27 , and the fluid pathway  35  is aligned between the respective fluid tubes  11 ,  36 . 
   As appreciated by one skilled in the art, the embodiment of the apparatus disclosed in  FIG. 2   a  operates in, but is not limited to, substantially the following manner. When in pre-sample testing mode, a stream of reference fluid or solvent runs through the system and follows one of a plurality of pathways. As the solvent flows into the system through the injection tube  1 , the solvent moves until it approaches the split junction  2 , at which point the fluid stream of solvent is divided to flow into both fluid tubes  3 ,  3 . The solvent then flows through the two first capillaries  4  and  4 ′ and approaches the two split junctions  5 ,  5 ′, respectively. The fluid pressure at the two split junctions  5 ,  5 ′, respectively, is transmitted by the fluid in the transducer tubes or lines  24 ,  24 ′ to opposite sides of the transducer  6 . The fluid that flows into the fluid tube  25 , adjacent to the second split junction  5 , proceeds to flow through the capillary  7 , through the fluid tube  26  and then to the split junction  12 . Alternately, the fluid that flows from the fluid tube  33 , adjacent to the second split junction  5 ′, flows into the first fluid pathway  34 . The fluid then flows into the fluid tube  36  and then into the delay volume  9 . The fluid stream then flows into the fluid tube  37 , which connects to the split junction  30 , and then into the capillary  7 ′. The fluid pressure at the split junction  30  is transmitted by the fluid in the transducer line or tube  29  to one side of the transducer  10 . The fluid entering the split junction  30  from the delay volume  9  flows through the capillary  7 ′ to the split junction  13 . The pressure of the fluid at the split junction  13  is transmitted by the fluid in transducer line or tube  31  to one side of the transducer  10 , thus enabling the transducer  10  to produce a signal substantially corresponding to the differential pressure across the capillary  7 ′ (the difference in fluid pressures upstream and downstream of the capillary  7 ′). The fluid flow, when it reaches the split junction  13 , flows into the fluid tube  28 , then into the delay volume  9 ′ and then into the fluid tube  27 . The fluid flows into the fluid pathway  35  of the fluid path diverter valve  8  and then into the fluid tube  11 . The fluid then flows into the split junction  12 . The fluids in the fluid tube  26  and the fluid tube  11  meet at the split junction  12 , then exit via the fluid tube  32  which is connected to the split junction  12 . Reference fluid may be flowed continuously through the measurement circuit to allow base line readings to be developed from the transducers  6 ,  10  with the measurement circuit full of flowing reference fluid. 
   With the fluid diverter valve  8  positioned as disclosed in  FIG. 2   b , the measurement circuit operates in a similar manner as described in  FIG. 2   a . The fluid flows from the tube  33 , into the fluid pathway  34  and then into the fluid tube  27 . The fluid then enters the delay volume  9 ′ and flows into the fluid tube  28 . The fluid then flows into the split junction  13  connecting the transducer line or tube  31  and the capillary  7 ′. The pressure of the fluid at the split junction  13  is transmitted by the fluid in the transducer line or tube  31  to one side of the transducer  10 . The fluid that flows through the capillary  7 ′ enters split junction  30  and then into the fluid tube  37 . The pressure of the fluid at the split junction  30  is transmitted by the fluid in the transducer line or tube  29  to one side of the transducer  10 . The fluid exiting the fluid tube  37  flows into the delay volume  9 . The fluid then flows from the delay volume  9  into the fluid tube  36 , which is connected to the fluid pathway  35 . The fluid flows into the fluid pathway  35 , which is connected to the fluid tube  11  and then to the split junction  12 . 
   To measure the viscosity of a sample fluid with the viscosity measurement circuit of the present invention as illustrated in  FIG. 2   a , the sample is injected or added into the reference fluid at or upstream of the inlet to the measurement circuit at the injection tube  1 . The sample then flows in solution with the reference fluid along the same fluid pathway as described above for  FIG. 2   a . The delay volume  9  delays the arrival of the sample solution at the capillary  7 ′ such that the differential pressures across the corresponding capillaries  7 ,  7 ′ may be sensed by the transducers  6 ,  10  when the capillary  7  contains sample material and the capillary  7 ′ contains only the solvent or reference fluid. Hence, the pressure drop sensed by the sample transducer  6  will be different than the pressure drop sensed at that time by the reference transducer  10  because the reference transducer  10  will still be sensing a differential pressure associated with the pure solvent in the capillary  7 ′. This enables the transducers  7 ,  10  to produce signals substantially corresponding to the pressure drops across the capillary containing pure solvent and the capillary containing the sample solution for use in determining relative viscosity, which may be mathematically related to other characterizations of the sample&#39;s properties, such as intrinsic viscosity, inherent viscosity, specific viscosity and reduced viscosity, by methods known in the art, such as is described in, for example, the Abbott, et al. patent and the De Corral patent. The transducer signals may also be used in combination with the signals produced by a refractometer or similar concentration detector, which is part of a GPC system, to determine other information about the sample&#39;s properties such as, for example, molecular weight distribution. 
   After the relative differential pressure information is obtained, the fluid path diverter valve  8  may be switched to realign the valve, in a conventional fashion, such that the flow of fluid through the reference capillary  7 ′ is reversed as illustrated in  FIG. 2   b  to move the sample fluid in a direction from the capillary  7 ′ toward the split junction  30  and the fluid path diverter valve  8 , through the fluid path diverter valve  8 , and then through the fluid tube  11  to the split junction  12 . By switching the fluid path diverter valve  8  in this manner, another relative differential pressure measurement may be taken of a new test sample flowing as per the configuration illustrated in  FIG. 2   b . Reversing the direction of flow through the capillary  7 ′ decreases or eliminates the unwanted breakthrough peak associated with the flow of the sample solution through that capillary, thus reducing the time needed for a successive analysis of another sample. 
     FIGS. 3   a  and  3   b  represent one embodiment of a subcircuit of the apparatus of the present invention in alternative positions. A second delay volume component  403 B, is added to the viscometer flowpath after the reference capillary  404 . Additionally, a fluid diverting component  411  is included to select which of the two delay volume components  403 A and  403 B is before and which is after the reference capillary  404 . Transducers may be arranged as known in the art for providing the pressure inputs to the transducer diaphragms, whereby the electrical signals provided by the transducers will be proportional to the differential pressure associated with the flow of fluid through the reference capillary and/or the sample capillary for use in the viscosity analysis. The calculation of relative viscosity may then follow that known for the 2-capillary viscometer. 
     FIG. 3   a  represents the embodiment of the subcircuit in configuration A. The fluid viscosity analysis circuit has an inlet tube  401  connected to a source of solvent flow (not shown) commonly used in the art. The inlet tube  401  connects to a capillary tube  402  known as the sample capillary, the other end of the sample capillary  402  is connected to a fluid diverting component  411  (shown as a 4-port, 2-position valve). The fluid passes through the valve  411  by way of a pathway  412 , thence onwards to the delay volume component  403 A via a tube. The other end of the delay volume component  403 A is connected to a reference capillary  404  via a tube, the other end of which is connected to the second delay volume component  403 B via a tube. The other end of the second delay volume component  403 B is connected back to the fluid diverting component  411  from which fluid in the delay volume component  403 B leaves the circuit through the pathway  413  and exits via the tube  405 . 
     FIG. 3   b  represents the embodiment of the subcircuit in configuration B. The fluid viscosity analysis circuit is substantially identical to that described above for  FIG. 3   a  except that the fluid diverting component  411  has been changed to be configured in its alternative position. Thus, the pathway from the entry point at the entry tube  401  to the exit tube  405  is: sample capillary  402 , fluid diverting component  411 , fluid pathway  412 , delay volume component  403 B, reference capillary  404 , delay volume component  403 A, fluid diverting component  411 , fluid pathway  413 , and exit tube  405 . 
   An example of a method of operation of the present invention is provided. For the purposes of clarity and to aid in understanding the method, the starting configuration is assumed to be that shown in  FIG. 3   a . The instrument is in baseline mode when the fluid pathway has only solvent passing through it. Sample solution is injected or enters the circuit by way of the inlet tube  401 . When the sample solution passes through the sample capillary  402 , there will be a change in fluid pressure sensed by an appropriately placed transducer (not shown). The simultaneous sensing of pressure across the reference capillary  404 , sensed by another appropriately placed transducer (See, for example,  FIGS. 2   a  and  2   b ), will still be that due to the passage of the solvent which is flowing from the delay volume component  403 A. Thus the relative viscosity may be computed from the transducer signals as described in, for example, the Haney, Abbott, et al. or de Coral patents. For example, when used in combination with a refractometer or similar concentration detector as the detection device of a gel permeation (or size exclusion) chromatograph, the transducer signals may also be used to determine other information about the sample&#39;s properties such as the molecular weight distribution or molecular size distribution. 
   The method of operation of the subcircuit illustrated in  FIG. 3   b  is identical to the method for  FIG. 3   a , with the following changes. The fluid diverting component  411  is configured to accept fluid from the sample capillary  402  in the pathway  412  as shown in  FIG. 3   b . The effect of this configuration of the fluid diverting component  411  is to reverse the position in the fluid pathway of the delay volume components  403 A,  403 B relative to the reference capillary  404 . Thus, the pathway of the fluid from the entry point at the inlet tube  401  becomes: the sample capillary  402 , the fluid diverting component  411 , the fluid pathway  412 , the delay volume component  403 B, the reference capillary  404 , the delay volume component  403 A, the fluid diverting component  411 , the fluid pathway  413 , and the exit tube  405 . Thus the delay volume component  403 B is in front of the reference capillary  404  and will provide the local supply of solvent to it. The delay volume component  403 A is now downstream of the reference capillary  404 . Furthermore, the direction of flow of solvent through the delay volume component  403 A is reversed and the sample solution that was passing through it is now flushed out by way of the fluid diverting component  411  via the fluid pathway  413  to the exit pipe  405  where it will take no further part in the analysis process. 
   Thus, the fluid viscosity analysis circuit may be quickly made ready to accept a further sample for analysis. The fluid pathway may stay in this configuration for the duration of the analysis of the next sample, after which the fluid diverting component  411  may again be activated to align to the alternative configuration. By this time, the delay volume component  403 A will be completely replenished with solvent. There is minimal disturbance to the fluid viscosity analysis circuit other than the activation of the fluid diverting component  411  and the consequent reversal of flow direction through a part of the circuit. Depending on the transducer connections, the reversal of flow through the reference capillary  404  may reverse the polarity of the pressure signal, but that effect can be accounted for in processing the transducer signals for a relative viscosity calculation. 
     FIGS. 4   a  and  4   b  represent a further embodiment of the apparatus containing a single delay volume component. A further difference from the embodiments in  FIGS. 3   a  and  3   b  is that the diverter  511  allows the delay volume component  503  to be connected to either the viscosity measuring circuit or to an external flushing circuit. 
     FIG. 4   a  represents another embodiment of the apparatus in a first configuration. The fluid viscosity analysis circuit has an inlet tube  501  connected to a source of solvent flow as commonly used in the industry. The inlet tube  501  connects to a capillary tube  502  known as the sample capillary, the other end of which connects to one port of a fluid diverting component  511  (shown as a 6-port, 2-position valve). The fluid pathway passes through the valve by way of a first channel  512 , thence onwards to the delay volume component  503 . The other end of the delay volume component  503  is connected back to another port of the fluid diverting component  511  and through a second channel  513  to a capillary tube  504  known as the reference capillary, the other end of which leaves the circuit by way of exit tube  505 . 
   An external fluid circuit consists of a solvent source, the purge solvent reservoir  521 , connected to a solvent pump  522 , connected to the fluid diverting component  511 , connected by way of a third channel  509  to the solvent waste vessel  523 . The solvent reservoir  521  and the solvent waste vessel  523  may be the same as those used for the main viscometer circuit or can be separate, conventional units. The solvent pump  522  is typically not activated in this mode other than at start-up to dispel any air bubbles that may be present. 
   The embodiment of the apparatus as disclosed in  FIG. 4   a  operates in, but is not limited to, substantially the following manner. The sample solution to be analyzed is injected, in the manner commonly known in the art, into the flowing stream of solvent passing through the viscometer circuit by way of the inlet tube  501  through injection. 
   When the sample solution passes through the sample capillary  502 , there will be a change in fluid pressure sensed by an appropriately placed transducer. The simultaneous analysis of pressure across the reference capillary  504  and the sample capillary  502 , analyzed by appropriately placed transducers, will still be that due to the passage of solvent which is flowing from the delay volume component  503 . The simultaneous sensing of pressure for each capillary allows the determination of relative viscosity as described in, for example, the Haney, Abbott, et al. or de Coral patents. The transducer signals may also be used in combination with the signals produced by a refractometer or similar concentration detector as the detection system of a GPC or SEC to determine other information about the sample&#39;s properties such as molecular weight distribution. 
     FIG. 4   b  represents yet another embodiment of the apparatus in yet another configuration. After the pressure readings are obtained in the analysis configuration, the fluid diverting component  511  may be switched to realign the valve, in a conventional fashion, such that the delay volume component  503  becomes part of the external circuit, while solvent continues to flow in the main viscosity analysis circuit. In this embodiment, the solvent pump  522  is activated to pump solvent through the external circuit by way of the third channel  513  of the fluid diverting component  511  and into the delay volume component  503  leaving by way of the channel  509  of the fluid diverting component  511  to the waste vessel  523  or alternative exit to waste. This way, the sample solution that was passing through the delay volume component  503  is quickly flushed out to waste before it can pass through the reference capillary  504  to cause a breakthrough peak. When the delay volume component  503  is again full of solvent, the solvent pump is switched off and the fluid diverting component  511  may be switched to realign the valve. In this way, the regenerated delay volume component  503  is restored to the analysis. 
     FIG. 5  illustrates the most common viscometer detector design.  FIG. 5  illustrates a 4-capillary bridge design invented by Dr. Max Haney. The four capillary tubes R 1 -R 4  have internal diameters of approximately 0.25 mm and are arranged in a balanced bridge configuration, analogous to the Wheatstone bridge common in electrical circuits. As above, the differential pressure transducers measure the pressure difference DP across the midpoint of the bridge and the pressure difference IP from inlet to outlet. A delay volume is inserted in the circuit before capillary R 4 , in order to provide a reference flow of solvent through R 4  during elution of, for example, a polymer sample. The requirements of the delay volume are that it must have an internal volume larger than the net elution volume of the GPC column and the flow resistance must be negligible compared to the capillary resistances. The capillary tubes R 1 -R 4  are chosen so that the flow resistances are almost equal. In this case, the DP output signal will be nearly zero and most of the pump pulsations will be cancelled out in the differential bridge measurement. 
     FIG. 6  illustrates the output of the viscometer detector illustrated in  FIG. 5 , and particularly, the breakthrough peak B. The output of the viscometer detector will respond to the viscosity of the sample as it elutes from a GPC. The first peak A corresponds to the sample as it elutes into capillaries R 1 , R 2  and R 3 , while solvent flows through capillary R 4 . The second peak B, the negative peak, illustrates the breakthrough in the delay volume. At this point in time corresponding to the breakthrough peak, the capillary R 4  associated with the delay volume contains the sample and other three capillaries R 1 , R 2  and R 3  contain solvent. The breakthrough peak is not required for the calculation and is simply an artifact of the measurement. The present invention provides an innovation that eliminates this breakthrough peak in all relevant detectors or sensors. 
     FIG. 7  is a schematic diagram illustrating the principle of differential refractive index (RI) measurement by depicting a differential refractive index detector  700 . In  FIG. 7 , a light source  702  provides a light beam  704  to a flow cell  710 . The flow cell  710  comprises a sample cell  712  and a reference cell  714 . The sample cell  712  and the reference cell  714  are separated by a cell interface  716 . The light beam  704  passing through the flow cell  710  may have a beam deflection  722 . The light beam  704 , deflected or not, passes through the flow cell  710  and engages a beam splitter  724 . The beam splitter  724  divides the beam  704  into two beams  704 A,  704 B. The two beams  704 A,  704 B engage two photodiodes  732 ,  734 , respectively. The signal from the photodiodes  732 ,  734  is processed by the differential amplifier  740  and provided to the output  750 . 
   More particularly, the glass flow cell  710  is split into two identical compartments or cells  712 ,  714 . The sample cell  712  contains the sample of interest, which is usually a dilute solution of a polymer solute in a particular solvent. The reference cell  714  contains the particular solvent. The light beam  704  shines through the cell at a 45 degree angle with respect to the interface  716  between the cells  712 ,  714  and is refracted or deflected at the interface  716  according to Snell&#39;s Law of refraction. If both cells  712 ,  714  contain only solvent then there will be no net refraction, i.e., the beam is not deflected. The beam  704  not being deflected corresponds to the baseline condition in liquid chromatography. But if one cell contains solvent and the other contains a sample, the beam  704  will be deflected in proportion to the difference in refractive index, DRI. In turn, the difference in refractive index, DRI, will be proportional to the concentration of the sample. The range of linear proportion is quite large and the differential refractive index detector  700  works for most samples and solvents. This makes the DRI detector a powerful detector in liquid chromatography, particularly for Gel Permeation Chromatography of polymers, where it is used almost universally. 
     FIG. 8  is a schematic diagram illustrating a system  800  with a DRI detector  800 B coupled to a GPC  800 A. Particularly,  FIG. 8  shows the flow path for a DRI detector  800 B coupled to a GPC  800 A. The DRI detector  800 B is shown as operated in the conventional manner. The pump  802  provides solvent  801  to the column  806  via the injector  805 . The injector  805  operates in association with the sample syringe  803  and the sample loop  804 . The sample eluting from the column  806  passes via the line  816  continuously through the sample compartment  812  and to waste  820 . However, the reference compartment  814  operates in a “static” mode. The reference cell  814  is initially charged with solvent but then the purge valve  830  is closed and the solvent remains in the reference cell  814  indefinitely. The only problem with the solvent remaining in the reference cell  814  indefinitely is that the refractive index of the static solvent continues to change due to the seals out gassing, etc. The result is drift of the baseline. Eventually, the baseline drift becomes so severe that the operator is forced to purge the reference cell  814  with new solvent and restore the baseline. 
     FIG. 9  is a schematic diagram illustrating the conventional manner of purging the solvent into the reference cell  814 . The solvent elutes from the column  806  through the conduit  816  into the sample cell  812 . Rather than being dumped to the waste  820 , the conduit  818  passes the solvent to the reference cell  814 . The solvent passes through the reference cell  814  and exits the conduit  832 , passes through the open purge valve  830 , and expels to the waste  840 . Thus, the solvent is purged through the reference cell  814 . 
     FIG. 10  is a schematic diagram illustrating a delay column  850  used in a manner analogous to the differential viscometer to provide continuous flow of solvent to the reference cell  824 . The sample is released from the column  806  and passes to the sample cell  812  via the conduit  816 . After passing through the sample cell  812 , the sample passes to the delay volume  850  via the conduit  818 A, and thereafter to the reference cell  814  via the conduit  818 B. Thus, the sample “breaksthrough” to the reference cell  814  thereby interfering with the succeeding sample injection.  FIG. 10  illustrates how the delay volume  850  can be used in a manner analogous to the differential viscometer to provide continuous flow of solvent to the reference cell  814 . Of course, the same problem observed with the differential viscometer will be observed with the refractive index detector when operated in this manner, i.e., the sample will eventually break through into the reference compartment and yield a “breakthrough peak” that interferes with the succeeding sample injection. 
     FIG. 11  is a schematic diagram illustrating the no-breakthrough differential detector of the present invention. By connecting delay columns  852 ,  854  at either end of the reference cell  814 , in operative association with a switching valve  860  in parallel with the reference cell  814 , the breakthrough peak can be eliminated by backflushing the sample. Thus, the implementation of the no-breakthrough differential detector for the viscometer detector will work in an analogous fashion for the refractive index detector, as it will for differential detectors, comparative sensors or any device that involves an analysis between two or more samples or specimen. 
     FIG. 11  illustrates the sample eluting from the column  806  via the conduit  816 . The sample enters the sample cell  812  and exists via the conduit  818  for transfer to the switching valve  860 . The switching valve  860  alternately regulates the flow of the sample to waste  840  and the solvent to the reference cell  814 . 
   It may be seen from the preceding description that a new and improved system and method for analysis of fluid properties has been provided. Although very specific examples have been described and disclosed, the embodiments of one form of the apparatus of the instant application is considered to comprise and is intended to comprise any equivalent structure and may be constructed in many different ways to function and operate in the general manner as explained hereinbefore. Accordingly, it is noted that the embodiment of the new and improved system and method described herein in detail for exemplary purposes is of course subject to many different variations in structure, design, application, form, embodiment and methodology. Because many varying and different embodiments may be made within the scope of the inventive concepts herein taught, and because many modifications may be made in the embodiments herein detailed in accordance with the descriptive requirements of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.