Detection isotopic counting

To measure gas and water vapor permeability, a film sample is mounted between two chambers of a permeability cell. One chamber holds the gas or vapor to be used as the permeant. The permeant then diffuses through the film into a second chamber, where a detection method such as infrared spectroscopy, a manometric, gravimetric, or coulometric method isotopic counting or gas-liquid chromatography provides a quantitative measurement (2). Die measurement depends on the specific permeant and the sensitivity required. 

A small number of methods allow for simultaneous determination of FSH and LH in a single assay tube. Doublelabel RiAs using Co-labeled LH and I-labeled FSH as tracers are available as commercial kits. Following separation, each radioisotope is determined in the bound fraction by dual isotope counting or by repeat counting. A simultaneous immunofluorometric assay of LH and FSH, based on the use of the fluorescent lanthanides Eu " " and Tb, has also been described each is detected with a time-resolved fluorometer. ... 

Syntheses of several F-18 fluorosugars have now been developed to the point at which they can easily be prepared on a small scale where radiation exposure is not a serious issue (say, up to 1 mCi) in any laboratory situated within several hours of a suitable nuclear reactor (52.) or accelerator. Molecular and biological scientists can, therefore, consider working with this isotope. One advantage is that F-18 is easily detected and counted quantitatively, in any kind of sample, without the tedious preparation needed for C-14 or tritium the time saved more than compensates for the need to prepare the F-18 tracer every day. Another is the extremely high specific radioactivity obtainable, which allows a tracer state (i.e., no perturbation of the system under study) to be maintained, and which permits slow processes and binding sites present in low concentrations to be investigated. 

The method used for detection depends on the type of label used. Isotopic counting is employed for radioisotopes, colorimetry for enzyme assays, luminescence and fluorescence measurements can be achieved by means of photomultiplier tubes, while turbidimetry or nephelometry is used for particle enhanced assays. 

FIGURE 21.4 The detector electronics must be able to switch fast enough to detect isotope ratios that require both pulse and analog counting modes (data copyright 2003-2007, all rights reserved, PerkinElmer Inc.). 

An important property of the MOT is the ability to catch atoms whose optical frequencies are shifted from the laser frequency by only a few natural linewidths. This property has been applied for ultrasensitive isotope trace analysis. Chen et al. (1999) developed the technique in order to detect a counted number of atoms of the radioactive isotopes Kr and Kr, with abundances 10 and 10 relative to the stable isotope Kr. The technique was called atom trap trace analysis (ATTA). At present, only the technique of accelerator mass spectrometry (AMS) has a detection sensitivity comparable to that of ATTA. Unlike the AMS technique based on a high-power cyclotron, the ATTA technique is much simpler and does not require a special operational environment. In the experiments by Chen et al. (1999), krypton gas was injected into a DC discharge volume, where the atoms were excited to a metastable level. 2D transverse laser cooling was used to collimate the atomic beam, and the Zee-man slowing technique was used to load the atoms into the MOT. With the specific laser frequency chosen for trapping the Kr or Kr isotope, only the chosen isotope could be trapped by the MOT. The experiment was able to detect a single trapped atom of an isotope, which remained in the MOT for about a second. 

Gas-flow counting is a method for detecting and quantitating radioisotopes on paper chromatography strips and thin-layer plates. Emissions are measured by interaction with an electrified wire in an inert gas atmosphere. AH isotopes are detectable however, tritium is detected at very low (- 1%) efficiency. 

Although performance varies with the isotopes for which they are intended, and with the balance in the design between resolution and efficiency, the overall sensitivity of a y-camera collimator is on the order of 5000 counts/(MBqmin) (several hundred counts/(/iCi-min)). In terms of photons detected per photon emitted, this is equivalent to about 2 x lO ". In other words, about two photons out of 10,000 emitted arrives at the crystal. This necessitates exposure times that range from several minutes to the better part of an hour. Fortunately, the large number of photons available from a modest injected radioactive dose more than offsets the poor detector sensitivity. The camera s abiUty to resolve small objects, however, is ultimately limited by the collimator inefficiency. 

Detection systems. Prior to the past decade, most instruments used for uranium-series analysis were single-collector instruments, for which ion beams of the various isotopes are cycled onto a single low-intensity detector, usually with electronics operating in pulse counting mode (Chen et al. 1986 Edwards et al. 1987 Bard et al. 1990 Goldstein et al. 1989 Volpe et al. 1991 Pickett et al. 1994), in order to measure the low-intensity ion beams of °Th, Pa, Pa, Ra and Ra. Daly detectors and... 

Linked Systems Isotopic Enrichment. The power of advanced measurement techniques can often be extended by linking them with other techniques. Such is the case for the detection of trace quantities of natural radionuclides and isotope enrichment. We have already found this extremely valuable for gas proportional counting of 37Ar and accelerator atom counting of 14C [8,9]. The first nuclide was enriched by means of thermal diffusion (Ar) the... 

Different isotopes are detected because the differing count of neutrons that characterizes atoms of different isotopes produces ions of different mass-to-charge ratios in the mass spectrometer. Their relative abundances are related to the intensity of the respective signals in the mass spectrum. 

---