METHOD OF ANALYSING THE RESULTS OF ELECTROPHORESIS, METHOD OF SEPARATING AND DETECTING THE COMPONENTS OF A MIXTURE, AND SEPARATION APPARATUS

Abstract: Light from the pattern of spots of bands resulting from electrophoresis is detected by a two-dimensional charge coupled device (26). Visualisation of the spots or bands may be achieved by viewing in transmitted light if the material subject to electrophoresis is dyed, by fluorescent markers in the material stimulated by untraviolet, or by a scintillating material if radioactive markers are used. Preferably the charge coupled device (26) is cooled to suppress thermal dark current, improving sensitivity and dynamic range. Light can be detected from moving components during separation by electrophoresis by clocking the charge coupled device to move charge packets in synchronism with movement of the components. This means that each charge packet moves through all the elements of a charge coupled device column during the detection, resulting in automatic averaging for the detection sensitivity variations in the charge coupled device column. Further, it is thereby possible to detect each separated band or spot at the same position in the gel at different times, and thus at the same degree of resolution, instead of detecting the bands or spots at different positions at the same time.

Description: Field of invention. This invention relates to electrophoresis and concerns a method of analysing samples by use of electrophoresis, and apparatus for analysing samples by electrophoresis. Background to the invention. Electrophoresis is a separation technique in which molecules or other units are separated, e.g. for analysis or purification purposes, by application of an electric field. This causes differential migration of the units, the rate of migration of each unit depending on its charge and frictional resistance (which is related to its size and shape). The technique is used to separate mixtures e.g. of proteins or DNA fragments, with the mixture being located on a suitable carrier material such as a porous gel, typically of starch or polyacrylamide. Differential migration on application of an electric field results in either a one dimensional array of bands or a two dimensional array of spots. The resulting bands or spots are detected by a number of techniques including the following: 1. By staining with a suitable dye, rendering the bands or spots opaque and visible. 2. By use of fluorescent markers or labels which emit light when stimulated by an appropriate light source. 3. By use of radioactive markers or labels, e.g. using the technique known as autoradiography. Analysis of electrophoresis results by these techniques has serious limitations. With stained dyes viewed in transmission, the faintest spots or bands have to be detected against a bright background, while the darkest spots or bands may transmit so little light that they are impossible accurately to measure with conventional detectors such as T.V. cameras or photographic films. Flourescent marker dyes generally only produce low levels of light that make detection of the lowest spot or band levels hard. Autoradiography has all the problems associated with the handling of radioactive materials, plus poor resolution and extremely long visualisation times, of the order of days to weeks. Light sensitive charge coupled devices (CCDs) are known solid state devices which can be used for high- and low-light-level imaging. Such devices are described in, eg "Electronics Engineers Handbook" second ed. McGraw Hill, New York, 1982 at pages 11-78 to 11-82. The invention. According to one aspect of the present invention there is provided a method of analysing samples by use of electrophoresis comprising the steps of: treating the samples with fluorescent marking material so that components of the samples are fluorescently marked; applying the samples to an electrophoretic gel; running the gel to effect electrophoresis causing differential migration of different components; irradiating the gel with an U/V source to render the marked components visible; detecting the pattern of light emanating from the marked components by means of a light sensitive charge coupled device (CCD), said CCD having a two-dimensional detector array and being cooled during detection The invention also provides, in a further aspect, apparatus for analysing fluorescently marked samples by electrophoresis, comprising: a source of U/V radiation for illuminating an electrophoretic gel to which the fluorescently marked samples have been applied; means to control running of gel to effect electrophoresis, causing differential migration of different components of the samples giving rise to a pattern of light due to illumination of the gel by the U/V source; and means for detecting the pattern of light wherein the detecting means comprises a light sensitive charge coupled device (CCD) having a two-dimensional array; and means for cooling the CCD during detection. The invention is applicable to both one and two dimensional electrophoresis, with the resulting bands or spots being labelled with a fluorescent marker for viewing of emitted light. Cooling of the CCD detector system improves sensitivity and dynamic range by supressing thermal dark current. A typical system suitable for use in the invention is the CCD 2000 Imaging System produced by Astromed Limited. The invention may be carried out by completing separation by electrophoresis in conventional manner and then analysing the resulting array of bands or spots using a CCD, eg after addition of fluorescent markers. Alternatively, separation by electrophoresis may be halted, the pattern of light from the separating components detected, and electrophoretic separation resumed. As a further possibility, a frame transfer CCD can be used to detect light from moving components during separation by electrophoresis. This is achieved by transferring charge distribution across the CCD in synchronism with movement of the components, as will be described in more detail below. In this case, the components are conveniently visualised by addition of fluorescent labels prior to starting the separation process. The charge coupled device may be coated with one or more materials which absorb electromagnetic radiation of one wavelength (eg shorter than visible light) and emit electromagnetic radiation of another wavelength (eg as visible light), whereby electromagnetic radiation of wavelengths to which the charge coupled device is not sensitive can be detected indirectly. The invention enables analysis of the results of electrophoresis to be speeded up significantly as compared with conventional techniques, as well as increasing the accuracy obtainable and permitting use of smaller sample volumes than has been possible hitherto. The invention also greatly increases the range of integrated spot or band intensities contained within one array that can be handled, and allows much more accurate quantitation of the amount of protein in each separated spot or band. One preferred embodiment of apparatus in accordance with the invention will now be described, by way of example, with reference to the accompanying drawing, the single figure of which is a schematic illustration of apparatus for use in the method of analysis of the invention. Detailed description of the drawing. The illustrated apparatus comprises a gel 10 bearing a one or two dimensional array of bands or of spots produced by electrophoretic separation of a sample mixture applied thereto. The spots or bands are labelled with fluorescent markers which emit light when stimulted by shorter wavelength light 16 from ultra violet source 18. The resulting transmitted or emitted light 20 is detected by a cooled charge coupled device detector system, comprising a CCD 2000 Imaging System produced by Astromed Limited. The light first passes through a filter 22 to select the emitted light against the shorter wavelength flood light. The transmitted or emitted light is then imaged by a lens 24 onto a cooled solid-state-charge coupled device detector 26 (P8600 series CCD made by EEV Ltd) contained inside a cold box 28 cooled with liquid nitrogen or a Sterling cycle or other mechanical or electrical cooler. The CCD 26 is connected by electrical wiring 30 to a driver electronics unit 32 that provides the necessary drive waveforms and bias voltages for the CCD, and also processes the signal output by the CCD to minimise the overall system read-out noise. The driver electronics unit is driven and controlled by a host computer system 34 that allows operator control of the system through a VDU console 36, data archiving as represented at 38, e.g. on disk drive and on magnetic tape, and the display of the image of the gel by an image display unit 40. The central computer 34 also contains software that allows the distribution of the detected spots or bands on the gel to be determined together with the detailed properties of the spots and bands, such as position, shape, size, orientation and intensity. The data so obtained may be output on an associated printer 42, or archived on disk or magnetic tape for storage or to allow comparison with the spot or band distributions obtained for other gels. The above described system has a number of advantages and properties as follows: 1. A cooled CCD system has the widest dynamic range of any two dimensional imaging system, in excess of 50,000:1. In transmission mode, the high dynamic range of the system allows the detection of spots and bands that are much fainter than is possible with low dynamic range detectors such as T.V. cameras (with dynamic ranges of from 64:1 to 256:1 typically), while still allowing extremely bright spots and bands to be measured accurately. 2. The exceptionally low read-out noise (typically 6 electrons rms) together with the high quantum efficiency (greater than about 40% peak) allows the detection in fluorescence mode of spots or bands that are much fainter than are otherwise detectable. This is because it is possible to integrate the signal from the gel for many minutes or hours without adding dark signal or read-out noise at the low operation temperatures (down to about 125K). 3. The resolution on the gel obtainable may be enhanced as follows. When a scene is imaged onto a frame transfer CCD detector, a two dimensional charge distribution is established that exactly follows the light distribution falling upon it. Normally the accumulated charge distribution is read out at the end of the exposure by moving the two dimensional charge distribution across the device and reading it out one line at a time. However if, throughout the exposure, the 2-D charge distribution is transferred across the device and the light falling on the CCD is moved across the device in synchronism with the charge already accumulated then it is possible to obtain an image of arbitrary length in the direction of movement. As a signal moves along a column, each pixel in the column will contribute in turn to the signal, so that the detection sensitivity of different pixels of the column is averaged in the signal in each pixel of output. Thus the pixels of output from one column of the CCD give a column of output that is completely uniform from the detector point of view and therefore reduces the need for flat fielding the data. Additional resolution in the other direction may be obtained by stepping the CCD sideways and repeating the scan to give a set of adjacent strip scans. Movement of the charge distribution across the CCD in synchronism with movement of the light source being monitored (the electrophoretic movement) is controlled by the control computer and driver electronics. 4. Calibration chemicals (typically proteins or DNA segments) are often added to 1-D and 2-D electrophoresis gels to act as calibration standards of mass and charge. In fluorescence work it can be difficult to match the signal levels from the calibration spots or bands so that they do not swamp the faint signals to be detected. The wide spectral coverage of the CCD detector (400mm -1100mm) allows the calibration chemicals to be tagged with fluorescent dyes that fluoresce at a wavelength different enough from that of the dye used for the principal sample chemicals, for their colours to be distinguished by placing filters in front of the detector. Such a procedure allows precision calibration to be achieved of the charge and mass axes of the gel because of the exceptional geometric stability of the CCD detector. 5. The excellent geometric fidelity of the CCD system (discussed in 4. above) allows the creation of data sets that are absolutely calibrated rather than relatively calibrated as is normally the case. The procedures permit the establishment of reference databanks of the processed output of the gel/detector/computer analysis system that may be easily searched for features different to or in common with other gels or sets of gels already processed and analysed. These procedures allow gels to be run to detect differences in the gel spot or band map due to variations in the composition of the sample caused by, for example, disease or infection or the additions of pharmaceuticals (in blood serum or organ protein electrophoresis), by injury or accident (the detection of organ-specific proteins in blood serum protein electrophoresis), by contamination (in food samples, as part of food processing procedures) etc. 6. In DNA sequencing by the dideoxy (enzymatic) method, four types of DNA segments are produced (called A,C,G and T). If these are tagged with a fluorescent dye such as FITC (fluorescein isothiocyanate) and the four components run in four separate parallel 1-D tracks in an electrophoresis gel then the cooled CCD detector is able to detect segments in low concentrations because of the great sensitivity of the detector in fluorescent mode. The simultaneous detection of the four tracks side by side allows the sequencing to be performed particularly accurately. A serious problem in running DNA sequencing gels is that the resolution of the gel increases with the distance a band has moved from the sample starting point, so bands in the middle of the gel may be resolved only half as well as those at the bottom of the gel (assuming it is being used from top to bottom). Uniquely with the CCD it is possible to obtain the advantages of extreme sensitivity of a two dimensional detector while clocking the CCD in synchronisim with the moving bands while the gel is actually being run, as in the drift scan mode described in 3. above. Placing the detector near the bottom or higher resolution end of the gel to track (electrically, by clocking) the bands as they pass towards the edge of the gel allows all the bands to be detected with the same high resolution. This permits more accurate sequencing because of the higher resolution. It requires the running of a single gel (normally separate gels are used to allow the poorly-resolved top section of the gel to be properly resolved) saving time, effort and expense. The geometric and photometric precision of a cooled CCD system permits the immediate extraction of the sequence results with the minimum delay and the minimum subsequent imaging processing. 7. In some applications the almost complete lack of sensitivity to wavelengths less than 4000 Angstroms is a great advantage (such as when using a UV light source to generate flourescence in the visible wavelength range). In others extended blue and UV response would be helpful when the light to be detected as part of the procedures described herein is of wavelength less than 4000 Angstroms. Enhanced blue and UV sensitivity may be obtained by coating the CCD with a thin (few microns thick) layer of a mixture of laser dyes in a solid matrix. The dye layer absorbs short wavelength radiation and re-emits it in the visible region of the spectrum where part of this re-emission is detected by the CCD. Each laser dye absorbs a photon and re-emits it with a wavelength only a few hundred Angstroms greater, so a cocktail of laser dyes is required to shift an incident photon by greater wavelength differences. Efficiencies approaching 50% of the visible or red detective quantum efficiency of the device are achieved in practice. Care has to be taken to select laser dyes that are not affected by high ambient lighting conditions if they are to be easy to use. The use of laser dyes as outlined above gives a detector that has high quantum efficiency over a very wide wavelength range.

Claims: A method of analysing samples by use of electrophoresis comprising the steps of: treating the samples with fluorescent marking material so that components of the samples are fluorescently marked; applying the samples to an electrophoretic gel; running the gel to effect; electrophoresis causing differential migration of different components; irradiating the gel with a U/V source to render the marked components visible; detecting the pattern of light emanating from the marked components by means of a light sensitive charge coupled device (CCD), said CCD having a two-dimensional detector array and being cooled during detection. A method according to claim 1, in which the samples are treated with fluorescent marking material prior to application to the gel. A method according to claim 1 or 2, in which the charge coupled device is a frame transfer CCD and the pattern of light is detected during electrophoretic movement, charge packets being shifted through the CCD in the same direction as and in synchronism with movement of the pattern of light falling on the CCD due to the electrophoretic movement. A method according to claim 3, in which the CCD is used to detect bands or spots contained in the pattern of light when they are brought to a predetermined location by the electrophoresis. A method according to claim 1 or 2, in which the running of the gel to effect separation by electrophoresis is halted, the pattern of light from separated components is detected, and running of the gel is then resumed. A method according to any one of the preceding claims, in which the charge coupled device is cooled during operation to substantially below ambient temperature. A method according to claim 6, in which the charge coupled device is cooled to down to about 125K. A method according to any one of the preceding claims, in which the samples are marked with a fluorescent material which emits electromagnetic radiation at a first wavelength and a calibration chemical is mixed with the material, the calibration chemical being marked with a fluorescent material which emits electromagnetic radiation at a second wavelength, and the wavelengths detected by the charge coupled device are varied to enable the calibration chemical to be distinguished from the material subject to electrophoresis. A method according to any one of the preceding claims, in which the output from the charge coupled device is compared with the results of previous electrophoretic separations stored in a databank. A method according to any one of the preceding claims, in which the charge coupled device is coated with one or more materials which absorb electromagnetic radiation at one wavelength and emit electromagnetic radiation at another wavelength. A method according to any one of the preceding claims, in which the electrophoresis is performed on four parallel one-dimensional tracks, a respective DNA segment (A, C, G or T), being subject to electrophoresis in each track. Apparatus for analysing fluorescently marked samples by electrophoresis, comprising: a source of U/V radiation for illuminating an electrophoretic gel to which the fluorescently marked samples have been applied; means to control running of gel to effect electrophoresis, causing differential migration of different components of the samples giving rise to a pattern of light due to illumination of the gel by the U/V source; and means for detecting the pattern of light; wherein the detecting means comprises a light sensitive charge coupled device (CCD) having a two-dimensional detector array; and means for cooling the CCD during detection. Apparatus according to claim 12, in which two-dimensional electrophoresis can be performed. Apparatus according to claim 12 or 13, in which the charge coupled device is a frame transfer CCD. Apparatus according to claim 14, having driving circuits for the charge coupled device adapted to drive the charge coupled device in synchronism with movement of the light falling thereon due to electrophoretic movement. Apparatus according to any one of claims 12 to 14, in which the cooling means can reduce the temperature to down to about 125K. Apparatus according to any one of claims 12 to 16, in which means is provided to vary the wavelength of light detected by the charge coupled device. Apparatus according to any one of claims 12 to 17, including a databank of results of previous electrophoresis runs and electronic means for comparing the output of the charge coupled device with the content of the databank. Apparatus according to any one of claims 12 to 18, in which the charge coupled device is coated with one or more materials which absorb electromagnetic radiation of one wavelength and emit electromagnetic radiation of another wavelength.

Inventors: ASTROMED LIMITED
Applicants: ASTROMED LIMITED
Application year: 1986
Main classification: G01N27
IPC codes: G01N21, G01N27