On-line radioisotope detectors

The availability of an on-line radioisotope detector for CE is especially appealing for several reasons. First, state-of-the-art radiation detection technology offers extremely high sensitivity. Second, radioisotope detection affords unrivaled selectivity because only radiolabeled sample components yield a response at the detector. Third, the radiolabeled molecule possesses the same chemical properties as the un-labeled molecule, thereby permitting tracer studies. Fourth, radioisotope detection can be directly calibrated to provide a measurement of absolute concentration of the labeled species. Finally, a capillary electrophoresis system in which radioactivity detection is coupled with more conventional detectors adds extra versatility to this new separation technique. 

We report here the design and characterization of three simple, on-line radioisotope detectors for capillary electrophoresis. The first detector utilizes a commercially available semiconductor device responding directly to 7 rays or particles that pass through the walls of the fused silica separation channel. A similar semiconductor detector for 7-emitting radiopharmaceuticals separated by HPLC was reported by Needham and Delaney (XI). The second detector utilizes a commercially available plastic scintillator material that completely surrounds (360 ) the detection region of the separation channel. Light emitted by the plastic scintillator is collected and focused onto the photocathode of a cooled photomultiplier tube. Alternatively, a third detection scheme utilizes a disk fashioned from commercially available plastic scintillator material positioned between two-room temperature photomultiplier tubes operated in the coincidence counting mode. 

Three simple, on-line radioisotope detectors for capillary electrophoresis were described and characterized for the analysis of 43P-labeled analytes. The minimum limit of detection for these systems was shown to be strongly dependent upon the conditions under which the analysis is performed. For standard CE separations performed at a relatively high (constant) voltage, the minimum limit of detection was found to be in the low nanocurie (injected sample... 

The large gain in sensitivity afforded by on-line radioisotope detection in comparison with the more commonly used UV-absorbance detector is illustrated in Figure 7. In this example, a UV-absorbance detector, monitoring at 254 nm, was positioned 8.5 cm downstream from a CdTe radioisotope detector, and 2P-labeled ATP was injected at a concentration of approximately 5 x 10 M. Under these conditions, ATP is detected with an excellent signal-to-noise ratio by the radioisotope detector but is completely undetectable by UV absorbance. 

Nassar AEF, Bjorge SM, Lee DY. On-line liquid chromatography-accurate radioisotope counting coupled with a radioactivity detector and mass spectrometer for metabolite identification in drug discovery and development. AnaZyft caZ Chemistry 75, 785-790, 2003. 

Beta-emitters such as tritium (H-3) or carbon-14 (C-14) are the most commonly used radioisotopes in drug metabolism, agricultural metabolism, and toxicology studies. In HPLC, the radiochemical detector can be off-line or on-line. Off-line detection requires coupling the chromatograph to a fraction collector. The collected fractions are combined with a suitable liquid scintillation cock-tail and then counted by a liquid scintillation counter. This method allows for... 

For the case of radioisotope tracer experiments, nonintrusive methods are used to get the outlet concentration of tracer by utilizing collimated scintillation detectors. Radioisotope tracers have many advantages such as on-line detection, high detection sensibility, and availability in different compatible forms over conventional tracers.This method can also help in troubleshooting and checking the performance of industrial TBR under operational conditions. 

Figure 4,11 Pulse shapes observed with a proportional counter and spectrometer electronics. The detector is responding to Ag and Mn K lines from radioisotopes, (a) The charge-sensitive preamplifier output, (b) The output from a semigaussian shaping ampli-fler with a -fxs shaping time constant, (a) and (b) are multiple traces from an oscilloscope, (c) The energy (pulse height) spectrum obtained by analyzing the amplifier output on a multichannel pulse height analyzer. (Reprinted by courtesy of EG G ORTEC.)...

The time distribution of the x-ray photons incident on the detector in an x-ray fluorescence spectrometer is described by Poisson statistics, providing that the half-life of the radioisotope excitation source is long compared to the data accumulation period, or providing that the x-ray tube output is truly constant. This means that both the x-ray tube current and voltage must be constant, with negligible drift or line frequency ripple. The arrival of photons at the detector is random in time as illustrated in Fig. 4.40. 

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