Patent Publication Number: US-11649263-B2

Title: Methods and apparatus for simultaneously detecting a large range of protein concentrations

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application Number PCT/US2017/063704, filed Nov. 29, 2017, which claims priority to U.S. Provisional Application No. 62/427,624, filed Nov. 29, 2016, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     A number of known techniques and instruments are suitable for separating proteins chromatographically and electrophoretically. For example, U.S. Pat. Nos. 9,304,133 and 9,400,277, each entitled “Methods and Devices for Analyte Detection,” the disclosure of each of which is hereby incorporated by reference in its entirety, describe the separation of proteins via capillary electrophoresis. 
     In some instances, it can be necessary or desirable to determine a quantity and/or concentration of analytes at one or more locations. Some known methods exist to measure a quantity and/or concentration of analytes using enhanced chemiluminescence (ECL) techniques. For example, the amount of protein captured on the inside walls of the capillary can be measured using an antibody with a horseradish peroxidase (HRP) enzyme that reacts with luminol within the capillary to produce a chemiluminescence signal that can be measured in units of photons/second and/or detections per second. 
     Traditional ECL techniques include loading a capillary with luminol and capturing one or more images of the capillary as HRP-conjugated proteins reacting with luminol. Traditional ECL techniques may be suitable in situations where each peak contains a similar quantity of protein and/or where the separation results in a single peak. Traditional ECL techniques, however, have poor dynamic range. Similarly stated, traditional ECL techniques are not suitable in situations where separation produces multiple peaks and at least two peaks contain significantly different quantities of protein. In such a situation, peaks with higher concentrations of protein will saturate and/or blind the detector, the peak with the higher concentration will consume all available luminol, and/or peaks with the lower concentrations of protein will not be detectable any of which will reduce the accuracy with which quantities and/or concentrations of protein can be determined. A need therefore exists for methods and apparatus for simultaneously detecting a large range of protein quantities and/or concentrations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of an instrument operable to measure a large range of analyte quantities and/or concentrations, according to an embodiment. 
         FIG.  2    is a flow chart of a method for measuring concentrations and/or quantities of analyte populations, according to an embodiment. 
         FIG.  3    is a plot of a simulation of light produced by an ECL technique detected over time. 
         FIGS.  4  and  5    depicts a comparison of known techniques for measuring a concentration and/or quantity of an analyte and a method of measuring a concentration and/or quantity of an analyte according to methods described herein. 
         FIG.  6    is a plot of a simulation of measurement error based on luminol consumption and mass transfer coefficient, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A particular problem in quantifying protein contained in multiple techniques using ECL techniques is the selection of an appropriate exposure time for imaging the capillary. Too short an exposure time, and peaks having a low quantity and/or concentration of protein may not be detectable and/or may not be accurately quantifiable. Too long an exposure time, and peaks having a high quantity and/or concentration of protein may saturate and/or blind the detector, wash out nearby peaks, and/or consume all available luminol. One available compromise solution is to take a number of relatively short exposures sequentially. The intensity for each peak can then be plotted versus time, and an initial intensity (e.g., as measured in photons per second) can be extrapolated using a mathematical model of the expected variation of the signal vs time. A concentration and/or quantity of protein at the peak can be determined based on the initial intensity. Such a technique, however, may produce significant statistical errors for small protein concentrations and systemic errors for large protein concentrations. 
     Some embodiments described herein relate to a method that includes separating an analyte-containing sample via electrophoresis in a capillary. The capillary is loaded with a chemiluminescence agent, such as luminol, that is configured to react with the analyte (e.g., HRP-conjugated proteins) to produce a signal indicative of a concentration and/or quantity of analyte at each location along the length of the capillary. A first image of the capillary containing the analytes and the chemiluminescence agent is captured over a first period of time. A second image of the capillary containing the analytes and the chemiluminescence agent is captured over a second period of time. The second period of time is longer than the first period of time. Similarly stated, the second image of the capillary has a longer exposure time than the first image of the capillary. A concentration and/or quantity of a first population of analytes at a first location is determined using the first image, and a concentration and/or quantity of a second population of analytes at a second location is determined using the second image. In some embodiments, the method can be used with a chromogenic detection agent instead of a chemiluminescence agent. 
     Some embodiments described herein relate to a method that includes separating a sample that contains a first population of analytes and a second population of analytes via capillary electrophoresis. Separating the sample can result in the first population of analytes migrating to a first position along the length of the capillary and the second population of analytes migrating to a second position along the length of the capillary. Images of the capillary having different exposure times are captured. A first image can be selected in which a strength of a first optical signal that is indicative of a concentration and/or quantity of the first population of analytes exceeds a predetermined threshold. An initial intensity of the first optical signal can be determined by dividing the strength of the first optical signal as detected in the first image by the exposure time of the first image. The initial intensity of the first optical signal can be used to calculate a concentration and/or quantity of the first population of analytes. A second image can be selected in which a strength of a second optical signal exceed the predetermined threshold (e.g., the same predetermined threshold). An initial intensity of the second optical signal can be determined by dividing the strength of the second optical signal as detected in the second image by the exposure time of the second image. The initial intensity of the second optical signal can be used to calculate a concentration and/or quantity of the second population of analytes. 
     Some embodiments described herein relate to an apparatus configured to effect electrophoretic separation of multiple of populations of analytes (e.g., proteins) that are disposed in a capillary. A detector is configured to capture images of a multiple locations along a length of the capillary. Similarly stated, the detector can be operable to perform full-column imaging of the capillary. A computing entity (e.g., a processor and/or a memory) is configured to select a first image captured by the detector based on an intensity of a first signal at a first location along the length of the capillary exceeding a predetermined threshold. The intensity of the first signal is indicative of a concentration and/or quantity of a first population of analytes that is located at the first location. A concentration and/or quantity of the first population of the analyte can be calculated, for example, based on the intensity of the first signal. A second image captured by the detector can be selected based on an intensity of a second signal at a second location along the length of the capillary exceeding a predetermined threshold (e.g., the same predetermined threshold). The second image can have an exposure time that is different from an exposure time of the first image. The intensity of the second signal is indicative of a concentration and/or quantity of a second population of analytes that is located at the second location. A concentration and/or quantity of the second population of the analyte can be calculated, for example, based on the intensity of the second signal and the second exposure time. 
       FIG.  1    is a schematic illustration of an instrument for measuring concentrations and/or quantities of analytes (e.g., proteins), according to an embodiment. The instrument includes a capillary  110 , electrodes  115 , and a detector  120 . The capillary  110  can bridge two reservoirs  140 . Each reservoir can be, for example, an anolyte reservoir, a catholyte reservoir, and/or a sample reservoir. 
     The instrument can be configured to draw a sample containing analytes into the capillary  110  or otherwise accept a capillary  110  containing a sample. An electric potential can be applied across the capillary  110  via the electrodes  115 , which can effect an electrophoretic separation of the sample. Similarly stated, populations of the analyte can migrate to different locations along the capillary  110  as a result of an electric potential being applied by the electrodes  115 . Populations of analytes having a similar property (e.g., mobility, isoelectric point, etc.) can form peaks or bands in the capillary  110 . 
     As discussed in further detail herein, the instrument can determine a concentration and/or quantity of an analyte in each peak. In some instances a first peak  152  at a first location of the capillary  110  can include a significantly higher quantity and/or concentration of analytes than a second peak  154  at a second location of the capillary  110 . Similarly stated, the first peak  152  can include 3 times, 5 times, 10 times, 50 times, 100 times, or more analyte than the second peak  154 . 
     The detector  120  (e.g., a camera, a charge-coupled device (CCD), a complementary metal-oxide-semiconductor (CMOS) detector, etc.) is configured to capture images of the capillary  110 , analyte within the capillary  110 , and/or photons emitted by an ECL reaction within the capillary  110 . The detector  120  can be a full column detector. Similarly stated, the detector  120  can be operable to capture an entire length of the capillary  110 , a substantial portion (e.g., at least 80%) of the length of the capillary  110 , the entire separation region of the capillary  110 , and/or to capture an image that contains at least two different locations separated along a length of the capillary (e.g., locations associated with the first peak  152  and the second peak  154 ). 
     A processor  162  and a memory  164  are communicatively coupled to the detector  120 . The processor  162  and the memory  164  can be collectively referred to as a computing entity. The processor  162  can be, for example, a general purpose processor, a Field Programmable Gate Array (FPGA), an Application Specific Integrated Circuit (ASIC), a Digital Signal Processor (DSP), and/or the like. The processor  162  can be configured to retrieve data from and/or write data to memory, e.g., the memory  164 , which can be, for example, random access memory (RAM), memory buffers, hard drives, databases, erasable programmable read only memory (EPROMs), electrically erasable programmable read only memory (EEPROMs), read only memory (ROM), flash memory, hard disks, floppy disks, cloud storage, and/or so forth. The processor  162  and memory  164  are configured to select exposure times for the detector  120 , receive images from the detector  120 , select images received from the detector  120  for analysis, and/or calculate concentrations and/or quantities of analyte populations as described in further detail herein. 
     In some embodiments described herein a chemiluminescence agent (e.g., luminol) is injected into the capillary before the capillary is imaged and/or before each image of the capillary is captured. Similarly stated, the instrument can include a luminol injector  115 . The processor  162  can be operable to cause luminol (or any other suitable chemiluminescence agent) to flow from the luminol injector  115  and through the capillary. The capillary  110  can contain HRP, which may be coated to the walls of the capillary  110 . The analyte (or populations of the analyte) can be configured to link to the HRP to form HRP-conjugated analytes. After the luminol is injected, the luminol diffuses to the wall of the capillary and reacts with the HRP-conjugated analytes, releasing photons, which can be detected by the detector  120 . Photons released by this enhanced chemiluminescence (ECL) reaction of luminol and HRP are indicative of populations of analytes. 
     A typical inner diameter (ID) of a capillary is 100 μm and a typical length is 5 cm so the luminol only diffuses a small fraction along the length of the capillary over the time the detector  120  takes a measurement (e.g., an exposure) but has many collisions with the wall of the capillary. Therefore, a local signal is indicative of a local presence of HRP-conjugated analytes, but independent of other signals or the presence of other analytes along the length of the capillary that are more than approximately 1 mm away. Similarly stated, in instances in which peak  152  is more than about 1 mm from peak  154 , the intensity of light emitted from peak  152  is independent from the intensity of light emitted from peak  154 . 
     The initial light intensity (photons/second) of an ECL reaction is proportional to an amount of protein (e.g., an analyte) at a particular location within the capillary (e.g., a peak). Accordingly, if the initial light intensity can be determined, the protein concentration and/or quantity can be determined. A number of challenges can present difficulties in accurately determining the initial light intensity, however. Because luminol is consumed by the chemiluminescence reaction, in cases in which there is a small amount of luminol available, such as in capillary-based techniques, the initial light intensity is often determined by measuring light intensity at a number of times and determining the initial light intensity by analyzing the decay curve. A detector can be used to take many exposures and fit a curve to the intensity versus time and extrapolate back to the initial time. For low concentrations of protein, it is desirable to take relatively long exposures, because the total intensity is lower and the decay of chemiluminescence slower. For high concentrations of protein, however, it is desirable to take short exposures, because the total intensity is higher and the decay of chemiluminescence faster. In many instances, however, a capillary may contain a high concentration of protein in one region, and a low concentration of protein in another, perhaps nearby, region. In such an instance, if long exposures are taken, areas of high concentration can “burn out” consuming all or most available luminol during the course of an exposure resulting in false low-concentration readings and/or masking nearby low-concentration signals. Conversely, short exposures are susceptible to be influenced by significant statistical error for low protein concentrations. A need therefore exists to for systems and methods with for accurately determining the quantity of proteins with a high dynamic range. Similarly stated, a need exists to accurately characterize a column (e.g., a capillary) which contains regions having both high concentrations of proteins and low concentrations of proteins. The instrument described above with reference to  FIG.  1    can be operable to perform methods that accurately determine the quantity and/or concentration with a high dynamic range. 
       FIG.  2    is a flow chart of a method for determining the quantity and/or concentration of analytes, according to an embodiment. The method of  FIG.  2    can be performed by the instrument described above with reference to  FIG.  1   . The method includes separating the sample, at  210 , loading the sample with luminol, at  215 , and capturing images of the capillary, at  220 . 
     In some embodiments, luminol can be loaded into the capillary a number of times, at  215 . Each time the capillary is loaded with luminol at least one exposure can be captured, at  220 . The length of exposures can vary. For example, the exposure duration can double each time the capillary is loaded with luminol, such as from 1 sec to 2 sec to 4 sec, and so forth up to 256 sec or any other suitable maximum exposure duration. Any suitable progression of exposure duration is possible. For example, each exposure can have a duration that is any integer multiple (or any other multiple) of the previous exposure duration. 
     Varying the exposures times can produce at least one exposure that is suitable for any concentration of protein that has a good signal to noise with a very small amount of signal decay (i.e., the integrated signal versus exposure duration is still linear). Similarly stated, high exposure durations can be particularly well suited for characterizing low protein concentrations, while low exposure durations can be suitable for characterizing high protein concentrations. In some instances, hardware and/or run time constraints can limit the bounds of the exposure durations. For example, extremely low concentration can be limited by the maximum exposure duration, while extremely high concentration can be limited by the minimum exposure duration. 
     Thus, at  230 , a first image for determining a concentration and/or quantity of a first population of an analyte can be selected. The first population of the analyte can be disposed at a first location along the capillary. In some embodiments, once a signal indicative of the first population of proteins is detected, the detector can continue to capture data (e.g., photons) until a pre-determined threshold is exceeded (e.g., 25,000 counts). In such an embodiment, the exposure time is determined by the time needed to reach the pre-determined threshold. In other embodiments, a series of exposure durations (e.g., 1 sec, 2, sec, 4, sec, 8 sec, etc.) can be predetermined, as discussed above. Regardless of how exposure durations are determined, the first image for determining the quantity of the first population of the analyte can be selected by identifying the image that has the shortest exposure duration in which a signal at the first location exceeds a pre-determined intensity threshold. As discussed above, the initial intensity of an enhanced chemiluminescence reaction is indicative of a quantity and/or concentration of analytes at the location. Therefore, at  235 , the quantity of the first population of analyte can be determined by dividing the intensity of the signal detected in the first image (photons or counts) by the exposure duration (seconds). 
     Selecting an image for determining a concentration and/or quantity of a population of the analyte can be repeated for each location along the capillary having a peak. For example, a second population of analytes can be disposed at a second location. A second image can be selected at  240  by identifying the image in which the signal for the second location exceeds the pre-determined intensity threshold. That signal detected in the second image can be divided by the exposure duration of the second image, and the quantity of the second population of analyte can be determined, at  245 . 
     An image can be selected in this manner for each peak or band and a concentration and/or quantity of analyte within each peak can be determined in a similar manner. Selecting an image in which a signal associated with a particular peak exceeds a predetermined threshold can mitigate the tradeoffs inherent in selecting an appropriate exposure time to which known ECL techniques are subject. For example, a very short exposure time (e.g., 1 second) can be used to determine a quantity of the peak with the highest initial intensity (and hence concentration and/or quantity). Such a short exposure may be too short to determine the quantity of other peaks in the capillary. Similarly stated, other peaks may be at or below the detection threshold of an image with a short exposure time and/or may have low signal to noise ratios. As another example, a relatively long exposure time (e.g., 64 seconds) can be used to determine a quantity of a peak with low initial intensity (and hence concentration and/or quantity). Such a long exposure may be unsuitable for determining the concentration and/or quantity of other peaks having higher concentrations and/or quantities of analytes. For example, over a 64 second measurement (exposure), high-intensity peaks may consume all locally available luminol and/or significant non-linearity of high-intensity decay may introduce relatively large measurement errors. 
     In some instances, the shortest nominal exposure duration can be selected by selecting the shortest exposure for which at least one peak is above a predetermined count threshold. The predetermined count threshold can be related to the initial luminol concentration that can be uniform throughout the capillary and independent of any specific subpopulation concentration. At this predetermined count threshold signals exceeding the predetermined threshold will typically have a suitable signal to noise ratio without significant signal decay. 
     As discussed above, multiple images of the capillary can be captured, and the images can have different durations. In some instances a series of exposures in which each exposure has the same or approximately the same exposure duration can be taken. The first series of exposures can have, for example, the shortest nominal exposure duration. For example, each exposure from the first series of exposures can have an exposure duration of 1 sec. The second series of exposures can each have, for example, an exposure duration of 2 sec, and so forth. In some instances the exposure duration of each exposure from a series of exposures can vary from the nominal exposure duration. For example, in an instance where the second series of exposures has a nominal exposure duration of 2 sec, exposures can be captured having exposure durations of 1.6 sec, 1.8 sec, 2.0 sec, 2.4 sec, and so forth. Similarly stated, the exposure duration for exposures from a series of exposures can be weighted around the nominal exposure duration for that series. In this way, the transition from one nominal exposure duration to another nominal exposure duration can be smoothed. As described in further detail herein, the light intensity can be determined by dividing the integrated signal (detected photons and/or counts) by the actual exposure duration. 
     Some embodiments described herein employ a mathematical model of luminol diffusion and reaction in a round capillary shown below that predicts the luminol concentration as a function of time (t) and radial location (r) within the capillary as well as for different ratios of reaction rate to diffusivity (mass transfer Biot number, Bi) for a capillary having a radius (b). The model shows that if the integrated signal is measured when ˜15% of the luminol has been consumed (i.e. at 15% of the maximum integrated signal), then the error in approximating the initial reaction rate (proportional to the target protein concentration) is within in narrow range (15-25%) for Bi&lt;0.5. For Bi&lt;0.1, the error is equal to % consumed luminol (reaction-limited process). Only ratios of target concentrations are meaningful (the units are arbitrary), so any fixed error percentage cancels in such a ratio. 
     
       
         
           
             
               
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     The above model and  FIG.  6   , discussed in further detail herein, demonstrate that by selecting an image for each population of an analyte (e.g., each location along a capillary) in which the signal exceeds a predetermined threshold, the decay over the exposure time for that signal can be approximated as linear. Therefore, the initial intensity can be calculated by dividing the intensity for that image by the exposure duration. 
     The actual decay over the exposure period is exponential. Therefore, the higher the predetermined threshold, the less accurate the linearizing approximation becomes. The lower the predetermined threshold, however, and the lower the signal-to-noise ratio. Selecting an appropriate predetermined threshold is therefore a balance between linearity and noise. Table 1, shown below, presents experimental data used to select an appropriate predetermined threshold. The experimental data reveals that selecting a predetermined threshold for signal intensity that is approximately 25% of the saturation level for the sensor results in a ratio of linear slope to the actual decay below 0.89, which is a suitable tradeoff. Other suitable thresholds could be between 30% of the saturation level for the sensor to 20% of the saturation level of the sensor. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Time/Lambda 
                 Rate 
                 Ratio Slope 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 0.025 
                 0.02469008797 
                 0.9876035189 
               
               
                   
                 0.05 
                 0.0487705755 
                 0.97541151 
               
               
                   
                 0.075 
                 0.07225651367  
                 0.9634201823 
               
               
                   
                 0.1 
                 0.09516258196  
                 0.9516258196 
               
               
                   
                 0.15 
                 0.1392920236 
                 0.9286134905 
               
               
                   
                 0.2 
                 0.1812692469  
                 0.9063462346 
               
               
                   
                 0.25 
                 0.2211992169  
                 0.8847968677 
               
               
                   
                 0.3 
                 0.2591817793  
                 0.8639392644 
               
               
                   
                 0.35 
                 0.2953119103  
                 0.8437483151 
               
               
                   
                 0.4 
                 0.329679954  
                 0.8241998849 
               
               
                   
                 0.5 
                 0.3934693403 
                 0.7869386806 
               
               
                   
                   
               
            
           
         
       
     
     To minimize spatial variation of linearization errors (and resulting discontinuous jumps in the intensity curve), an additional interpolation step can be employed. Signals captured in multiple images having discrete exposure times can be used. After selecting the image in which a signal is closest to the predetermined threshold (or the image with the shortest exposure time in which the signal exceeds the predetermined threshold), additional images, such as the immediately prior and/or subsequent images (e.g., images having the next-longer and/or next-shorter exposure duration) be analyzed and used to interpolate to a predicted exposure time that would yield the target threshold value. Similarly stated, a predicted exposure time to reach the predetermined threshold can be calculated. The predicted exposure time may not correspond to an exposure time of any actually captured image. In such an embodiment, calculating the initial intensity can be performed by dividing the predetermined threshold by the predicted exposure time. In some embodiments, the predicted exposure time can be calculated by linearly interpolating between a measurement that is just below the predetermined threshold and a measurement just above the predetermined threshold. 
     EXAMPLE 1 
       FIG.  3    depicts simulated signal levels using the techniques described herein.  FIG.  3    depicts three intensity levels, “low,” “medium,” and “high.” The medium intensity level has an intensity four times the intensity of the low intensity level, and the high intensity level has an intensity four times the intensity of the medium intensity level. The predetermined count threshold can be 25,000, which can be determined, for example, based on detector sensitivity and/or saturation as discussed above. Similarly stated, a count threshold of 25,000 may be 25% of the saturation level for the sensor. A nominal exposure duration of 16 sec can be selected for the low intensity signal, a nominal exposure duration of 4 sec can be selected for the medium intensity signal, and a nominal exposure duration of 1 sec can be selected for the high intensity signal. The intensities (photons/second) are determined by dividing the integrated signals by the exposure duration. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Signal Input 
                 Exposure &lt; 25,000 
                 Signal measured 
                 Intensity 
               
               
                   
               
             
            
               
                 High 
                  1 sec 
                 22,000 
                 22.000/1 = 22,000 
               
               
                 Medium 
                  4 sec 
                 22,000 
                 22.000/4 = 5,500  
               
               
                 Low 
                 16 sec 
                 22,000 
                 22.000/16 = 1,375  
               
               
                   
               
            
           
         
       
     
     EXAMPLE 2 
     Techniques described herein can increase the dynamic range of protein concentration analyses as compared to known techniques. For example,  FIGS.  4  and  5    illustrates a known method of determining protein concentration labeled “Multi image” which depict a “burnout” phenomenon that occurs when a large range of protein concentrations are processed. Burnout occurs due to high concentrations of protein consuming available luminol in the chemiluminescence reaction during the measurement (exposure) that is suitable for accurately resolving low protein concentrations. Similarly stated, the exposure duration needed to resolve the low protein concentrations is too long to detect the decay of the chemiluminescence for high protein concentrations. 
       FIGS.  4  and  5    also illustrates a “HDR” (high dynamic range) analysis using techniques described herein. The HDR analysis illustrates that both high concentrations (636 μg/mL) and low concentrations (63.6 μg/ml) of protein can be detected without the large systematic under estimation of high signal illustrated by the Original Multi image technique. 
     As described above, burnout can adversly effect known techniques for determining protein concentration. Techniques described herein may, however, benefit from burnout. For example, because luminol diffuses very slowly within a capillary, when exposure duration is incresed from short to long, the short durations can be used to accurately quantify areas with high concentrations of protein. As the exposure duration increases, areas with high protein concentrations may have depleted available luminol. As a result, a nearby low concentration signal with a lower intensity that would otherwise have been masked by the brighter signal assocaited with the high concentration (e.g., sensor blinding) may be resolvable and/or more accurately detected during a longer duration exposure captured after the high concentration has consumed locally available luminol. In instances in which high concentration peaks are relatively close to low concentration peaks, it may, therefore, be desirable to capture multiple images without refreshing luminol. 
     While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Where methods described above indicate certain events occurring in certain order, the ordering of certain events may be modified. Additionally, certain of the events may be performed concurrently in a parallel process when possible, as well as performed sequentially as described above. 
     While the embodiments have been particularly shown and described, it will be understood that various changes in form and details may be made. Although various embodiments have been described as having particular features and/or combinations of components, other embodiments are possible having a combination of any features and/or components from any of embodiments as discussed above. For example, in some embodiments, luminol can be refreshed between series of exposures, for example, by flowing luminol into a capillary between exposures and/or by continuously flowing luminol into the capillary. In some instances, flow within the capillary can be prevented while an exposure is being taken, for example, by balancing hydrostatic pressure at either end of the capillary, by eliminating electrophoretic processes during imaging, etc.