Abstract:
A method and apparatus are provided for the position sensitive detection of radioactivity in a fluid stream, particularly in the effluent fluid stream from a gas or liquid chromatographic instrument. The invention represents a significant advance in efficiency and cost reduction compared with current efforts.

Description:
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     The U.S. Government has rights in this invention pursuant to contract number DE-AC05-84OR21400 between Lockheed Martin Energy Research Corporation and the Department of Energy. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method and apparatus for the position sensitive detection and analysis of radioactivity in a fluid stream, and more particularly to such detection and analysis in gas and liquid chromatography. 
     Chromatography has been an important research tool for many years in a wide variety of applications. The principles of chromatography are well known to those of ordinary skill in the art. Essentially, a sample containing a mixture of constituent components, referred to herein as analytes, is introduced into a fluid stream. In a simple type of chromatographic apparatus, the fluid stream is brought into contact with a resin bed. The resin has been chemically or physically treated so as to selectively retard the passage of the analytes. The result is that the sample mixture is collimated, that is, separated into its discrete analytes. The fluid stream with the collimated sample then flows out, as an effluent stream, from the collimating means. The collimated effluent fluid stream is typically passed through a detector or analyzer which determines the presence, amount, and or type of analyte. The presence of a particular analyte can be depicted graphically as a peak on a chart. The timing and size of each peak provides significant information regarding the analyte. 
     While chromatographic techniques and technology have enjoyed great advances generally, one aspect of chromatography has not. This area is the detection and analysis of radioactive and radiolabelled analytes. Indeed, the current state of the art in this area has not changed greatly since the early 1970&#39;s except for the fairly peripheral advancements in the use of computers and integrated circuits. 
     The current method of radioactivity detection in gas chromatography is referred to as gas proportional counting (GPC). Typical of the state of the art in this regard is the original design of the Packard Model 894 counter used for GPC. In this instrument the effluent stream from the chromatograph contains or has added to it a quenched gas. The stream is then passed into a counting tube. In the counting tube is mounted a high voltage wire. Radioactive decay events, such as the emission of a beta particle or a gamma ray, create ions in the gas stream, which in turn is electrically detected via the wire. The events can be detected and counted to determine the presence and approximate amount of the radioactive analyte in the tube. 
     This technology has significant limitations. The wire in the counting tube is sensitive to background noise such as ambient radiation and radiation from internally deposited debris. The apparatus can be “tuned,” that is, adjusted so as to increase efficiency, but is then sensitive to voltage and gas fluxes that may produce false readings. Typically, then, the instruments are not tuned in order to increase operational ease at the expense of analytical efficiency. 
     The resolution capability of this type of instrument also lags significantly behind the resolution capability of modern gas chromatography. Modern gas chromatography can separate and resolve many analyte peaks per minute. The state of the art GPC, however, can only recognize a single peak for all decay events occurring within the entire volume and length of the counting tube. Peak detection for radioactivity is thus limited to less than one per minute, making it impossible to correlate the radioactivity peak with the analyte peaks. Moreover, due to low counting efficiencies, quenching, detuning, and high background, detecting smaller radioactive peaks (for example, less than 250 disintegrations per minute (dpm)) is difficult. Thus a small but analytically important radiolabelled analyte may be completely missed or ignored by even modern GPC detectors. 
     Problems exist with radioactive assays performed with liquid chromatography also. In order to detect radioactive analytes, the liquid effluent stream from a liquid chromatograph must include or be mixed with liquid scintillation fluid. Decay events excite the liquid scintillation fluid to produce light, which can then be detected and measured. This process is expensive and inefficient. Relatively large amounts of scintillation fluid must be used and then safely disposed. Sample sizes typically must be larger, increasing the trouble and expense of obtaining the sample analyte. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a method and apparatus for position sensitive detection of radioactivity in a fluid stream. 
     It is a further object of this invention to provide a highly efficient and relatively inexpensive method and apparatus for detecting the position of a radioactive decay event in a fluid stream. 
     It is a further object of this invention to provide a method and apparatus that can be used in conjunction with standard liquid or gas chromatography techniques to obtain position sensitive data regarding radiolabelled analytes in the chromatographic effluent stream. 
     This invention is a method and apparatus that provides for the highly efficient, yet relatively inexpensive, detection and analysis of radioactive or radiolabelled material in a fluid stream. More particularly, it can be used in chromatographic analysis to detect and measure radioactive or radiolabelled analytes in the effluent fluid stream from the chromatograph, providing higher and faster peak resolution while requiring a smaller sample size of the analytes in question. The invention is useful with both types of fluid (liquid or gas) chromatography. The method and apparatus have even broader application in any system having a fluid stream where position sensitive detection of radioactivity is desired. 
     In one embodiment of the invention, the apparatus comprises a channel through which the effluent fluid carrying the collimated analytes passes. A scintillant material, reactive to radioactive decay events such as the emission of a beta particle, is placed along the channel, at or near it, so as to be exposed to radioactive decay events occurring in the fluid. The channel and the scintillant can take any desired shape or geometry as is known to those of ordinary skill in the art. Detectors, for example in the case of a light-emitting scintillant, photomultiplier tubes, are placed so as to detect the effect on the scintillant of a decay event. Each detector will produce an output voltage, or output signal, upon detecting light emitted from the scintillant indicative of a radioactive decay event. The output signals of the detectors can then be analyzed by conventional means to generate data indicative of the number, position, and other characteristics of the decay event. The data can then be displayed in any conventional manner, such as in the form of graphs, histograms, and the like. The data can also be processed in conjunction with other data from the chromatograph, such as is obtained by the conventional chromatograph detector. 
     While there are many known ways to place detectors with respect to a scintillant, in one embodiment of this invention a detector is placed at either end of the scintillant. The output signals from the paired detectors can be transmitted to an analytically means. The analytical means can compare the output signals of the pair of detectors by a comparator of conventional design to determine the time differential between the respective output signals produced by a single event. The time differential provides an indication of the position of the event along the scintillant, and hence an indication of the position of the radioactive analyte within the channel and the fluid stream. An accumulator, or counter, can utilize the output of either the photomultiplier tubes or the comparator to generate data indicative of the total number of events detected. The accumulator can determine the number of events as a matter of time or volume as desired. The output from the comparator and/or the accumulator can also be input to a processor also receiving data from the chromatograph&#39;s conventional detector and/or a flow meter, whereby the radioactive peaks can be compared to the conventional analyte peaks to derive additional data. 
     In an alternative embodiment of the invention, a plurality of detectors can be placed along the channel rather than at the ends thereof. The output signals of the detectors are transmitted to the analytical means which in this embodiment would provide an indication of the position of the decay event based on which detector provided the output signal. Accumulator means and processor means would provide data as described above. 
     The channel and scintillant can be coated or covered so as to reduce the amount of background noise caused by ambient events and light. 
     In one embodiment of this invention, the scintillant is a solid, such as plastic, and the channel itself is made in or of the solid scintillant. 
     In another embodiment of the invention, the effluent stream can be divided, e.g., by a manifold, and allowed to flow through more than one channel, each channel having or being a scintillant and having detectors. The output signals from the detectors can be analyzed by comparators, accumulators, and/or processors and the data for each channel synchronized with that from other channels for analytical purposes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic depiction of the channel, scintillant, and detector assembly along the fluid stream, wherein the detectors are placed at either end of the channel. 
     FIG. 2 is a schematic depiction of the channel, scintillant and detector assembly along the fluid stream, wherein a plurality of detectors is positioned along the channel. 
     FIG. 3 is a schematic depiction of one means for analyzing the detector output signals by comparing, accumulating, and processing the output signals of the detectors. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A schematic illustration of one possible arrangement of the channel, the scintillant, and the detectors is shown in FIG.  1 . This illustration is equally applicable to either gas or liquid fluid applications. A channel  10  is provided through which the effluent stream carrying the collimated analytes is directed. The fluid stream flows through a tube or conduit  50  to enter the channel  10  at a point  60  in the direction of the arrow as shown. The source of the fluid stream in conduit  50  may be a chromatograph. The stream flows out of channel  10  through outflow conduit  50 ′, leaving channel  10  at a point  60 ′, as schematically shown by the arrow. The direction of flow is, of course, arbitrary: it is only necessary that the fluid flow through the channel. The direction of flow and the geometry of channel  10  will depend on a variety of known factors. Outflow conduit  50 ′ may be a continuation of conduit  50 , or may be a separate conduit piece connected to channel  10  at point  60 ′. 
     Channel  10  can be made of any conventional material transparent to radioactive decay events. It can be a portion of conduit  60 ,  60 ′, or of either, or made separately. It is desirable that channel  10  have the smallest practical diameter so as to minimize the chance that an event will be masked by absorption by the fluid and/or by the analyte itself. 
     A scintillant  20 , is provided alongside channel  10 . Scintillant  20  can take any of several forms. In one embodiment of the invention, scintillant  20  is a solid, such as a solid plastic. In another, scintillant  20  can be a contained liquid, such as a fluid contained within a closed tube. The material from which the tube is made in the latter embodiment should be transparent to decay events. 
     Scintillant  20  can be of any desired length or form consistent with its purpose. The length of scintillant  20  may be a necessary datum for precise evaluation of event occurrences. Scintillant  20  can also be made of any material that will produce an effect in response to an event. Typically a scintillant produces light emissions in response to a decay event, but any scintillant may be chosen so long as it produces a detectable emission or signal in response to a decay event. Other examples of useful scintillants are so-called mediated scintillants wherein a decay event creates an effect in one substance which in turn causes a second, associated substance to produce light or some other detectable emission. 
     The choices of length, form, and material of scintillant  20  can be chosen depending on known factors, including for example the type of analyte to be detected, the geometry of channel  10 , and the like. The factors determining these parameters are well known to those of ordinary skill in the art. 
     Channel  10  and scintillant  20  can take any desired shape and/or path geometry. Additionally, in one embodiment of this invention, channel  10  is formed directly from scintillant  20  in which case channel  10  is both the channel and the scintillant . The material from which scintillant  20  is made can be formed into a desired channel, or a channel can be made, for example, by drilling through the scintillant material. 
     In the schematic illustration of FIG. 1, the means for detecting the emissions of scintillant  20  in response to a decay event are shown at  30  and  30 ′. Because a scintillant typically emits light in response to a decay event, scintillant  20  will likely (but need not) be of that type. Detector means  30 ,  30 ′ will therefore typically be photomultiplier tubes (PMs). PMs are well known in the art, and react to light by generating an output signal. In its simpler form, the output signal is an output voltage proportionate to the amount of light detected. More modern instrumentation may enable the output signal to be digital or be digitized by known means. In the schematic representation of FIG. 1, output signal connectors are shown respectively at  35 ,  35 ′. Connectors  35 ,  35 ′ transmit the respective output signals to analytical means described below with respect to FIG.  3 . 
     FIG. 2 is a schematic representation of another embodiment of the invention. The numbering scheme of FIG. 1 is retained because the nature of the components is not changed. In the apparatus illustrated in FIG. 2, the detectors  30 ,  30 ′ are placed along channel  10  instead of at the ends thereof as shown in FIG.  1 . Each detector is placed so as to be reactive to decay events occurring within the stream. There may be a plurality of additional detectors (not shown) depending on the length and geometry of channel  10 . In this alternative embodiment, each detector will detect decay events within only a portion of channel  10 . An output signal from one of the detectors will provide an indication of the position of the decay event simply by knowing which detector sensed the decay event. 
     FIG. 3 is a schematic representation of one analytical means for analyzing the data represented by the output signals from detectors  30 ,  30 ′. Detectors  30 ,  30 ′ may be positioned as illustrated in either FIG. 1 or FIG. 2, and there may be additional detectors. Detectors  30 ,  30 ′ are connected by connectors  35 ,  35 ′, respectively so as to send the output signals thereof to a means for comparing the respective output signals. The output signals from detectors  30 ,  30 ′ may be connected to respective constant fraction discriminators (CFD)  110  and  110 ′. CFDs  110  and  110 ′ function to convert the detector output signals to timing signals. CFD  110  functions identically to CFD  110 ′. CFD  110  processes the output signal from detector  30  to determine whether the decay event was strong enough to cross a predetermined threshold. If so, CFD  110  generates a time signal indicative of the time the decay event was detected. CFD  110 ′ will detect the same decay event and send a similar signal. The timing signal outputs of CFDs  110  and  110 ′ are transmitted to a picosecond time analyzer (PSTA)  130 . PSTA  130  is a conventional instrument capable of interpreting the converted output signals to indications of the positions of the decay events detected by detectors  30 ,  30 ′. The output of PSTA  130  can then be used to record, display, or otherwise manipulate the data. PSTA  130  will also include accumulator means, such as conventional electronic counters, to generate data representative of how many decay events were detected, and to associate such data with the position indicating data. The data can then be displayed, for example, as conventional peaks. 
     The output of PSTA  130  can also be transmitted to a processor means  160 . Processor means  160  can be any suitable processor known to those of ordinary skill in the art, for example, a computer with appropriate software. Processor means  160  can further manipulate the detector data, for example, to compare the data regarding decay events with the data generated by a conventional chromatography detector (CCD)  150 . CCD  150  is intended to include any known apparatus used, for example, to detect, quantify, display, record, or otherwise manipulate data representative of the analytes in a given sample. Such conventional detectors are typically an integral part of the chromatograph. 
     An older, alternative analytical means for analyzing the output signals from detectors  30 ,  30 ′ consists of connecting CFDs  110 ,  110 ′ to a time-to-amplitude converter (TAC). A TAC is a comparator means which compares the output from CFDs  110  and  110 ′. This comparison provides a signal indicative of the position of the decay event. The TAC is connected to a multi-channel analyzer (MCA) which can provide an accumulator means to provide an indication of the number of decay events and to associate this number with the position of the decay events. This data can also be recorded, displayed, or otherwise manipulated as indicated above. Likewise, the data may be transmitted to a processor means such as processor  160  shown in FIG.  3 . 
     The analytical means shown in FIG. 3 is constructed from commercially available instruments and electronics. There may be several individual elements performing the indicated functions, or the entire analytical means may be incorporated into a single device. 
     The foregoing description of the claimed invention is of necessity simplified and by way of example only. For example, it may prove useful to interpose various electronic filters and amplifiers between detectors  30  and  30 ′ and CFDs  110  and  110 ′, respectively, to refine the output signals of the detectors. Also, there are different varieties of commercially available PSTAs or equivalents thereto, or ones that use the output signals to determine position in a different manner. 
     An alternative to having a single channel through which the fluid stream is directed is to split the stream into a plurality of channels. Each channel would have associated detectors. The channels should be set in parallel, either physically or by manipulation of the respective generated data so as to preserve the collimation of the sample. Providing a plurality of a channels means that each separate channel can be very narrow and the scintillant can be very close to the analyte entrained in the stream. This can be useful where the sample, and hence the radioactivity, is very small or where the fluid in which the analyte is entrained tends to absorb or block the emissions from radioactive decay events. 
     The nature of the chromatograph, the nature and type of analytes and fluid streams, and simple design choice may lead to several modifications of the disclosed device. There are several different types of instruments commercially available to process the data from decay events. All of these modifications and substitutions are considered within the scope of the novel invention disclosed herein and embodied in the claims below.