Gamma counters

The counting equipment used to establish quantitation is radioassay and radioimmunoassays may be either a gamma counter or a liquid scintillation counter depending upon the isotope being used. Most assays use as a label and require a... 

If only one counter can be purchased, then a gamma counter is the instrument of choice since most assays are now performed with gamma emitting isotopes. Table I lists the isotopes in common usage for competitive protein binding assays. 

To determine the optimal reaction time for a particular radioiodination, 5 pi aliquots of the reaction medium can be removed every 30 seconds, diluted 1 20,000 with 20 mM Tris, 1 mM ethylenediaminetetraacetic acid (EDTA), pH 7.4, containing 0.5 mg/ml BSA as a carrier protein. Finally, precipitate a small amount of the diluted aliquot with trichloroacetic acid (TCA, 60 percent), centrifuge to recover the pellet, wash the pellet once with TCA, and measure the amount of radioactivity in the pellet and supernatant using a gamma counter. The reaction period representing optimal radiolabel incorporation should be used for subsequent radioiodinations. 

Isotope Methods. The isotopes of calcium have relatively short half-lives and are readily counted using liquid scintillation or gamma counters as appropriate to the nuclide. Calcium isotopes may be quantitated in the excreta, blood, tissues or in the whole body. This has made them useful for many nutritional metabolic studies. However, because of safety concerns, radioactive isotopes are cumbersome to work with and many researchers are unwilling to administer them to human beings. This has limited the use of isotopes to those studies in which alternate methods are not available or are imprecise. Methodologies for stable isotopes of calcium, which may be safely used in human being, are becoming available for use in metabolism studies. These will be practical alternatives to radioactive isotopes in the future. 

To assay specifically the afferent arm of the DTH response, the proliferation of the popliteal lymph node cells to SRBCs can also be measured (White et al., 1985). As described, mice treated with the test article are sensitized to SRBCs by inoculation of SRBCs into the hind footpad. However, 1.5 h later they are challenged intraperitoneally with FUdR and 2h later they are administered [125I]IUdR intravenously (instead of 125I-labeled HSA). Mice are sacrificed 24 h after challenge and both popliteal lymph nodes are removed and counted in a gamma counter. 

Gamma counters are a type of ectrttillatlfri counter add measure gamma radiation by detectlngthe emission of light Induced in a scintiBsnt... 

Figure 5.4 A gamma counter. The scintillations from a crystal of fluor caused by gamma radiation are detected by a photomultiplier.

Motor-driven blade homogenizer, low-speed refrigerated centrifuge, ultracentrifiige, spectrophotometer, and gamma counter. 

The examples for radioactive labeling by phosphorus-32 ( P) and iodine-125 ( 1) in this chapter were chosen for two reasons on one hand, they are relatively easy to do, and on the other hand, the measurement of radioactivity is simple. is counted in water in a liquid scintillation counter by measuring the Cerenkov radiation and is measured in a gamma counter. Both isotopes may be detected also by autoradiography. A further advantage of both isotopes is their short half-life, which eases the disposal of nuclear waste. 

Procedure Measure radioactivity in a gamma counter Result Amount of radioactivity is inversely proportional to the concentration of hormone in the sample. Wy yyv ... 

The ELISA techniques offer advantages of longer shelf life of the labelled reagents and elimination of the use of radioisotopes which require expensive scintillation/gamma counters and special disposal needs (38, 56). They are also more sensitive than the RIA s. ranging from pg/ml to pg/ml depending on the size of the molecules, affinity of the antibody and the assay format used (48). 

Gamma counter and y counter tubes (75 x 12 mm, 5-mL tubes) (Sarstedt, Leicester, UK). 

Put the different samples to count in a gamma-counter in the following order Tio, S1( S2, S3, blood cell pellet, tube 500, duplicate tubes 100, and duplicate tubes of skin sites 1, 2, and so on. 

All samples to be counted in the gamma counter should have approximately similar geometry. In our automatic 10-head counter (Canberra Packard Ltd., Pangboume, Berks, UK), we use 2 mL screw-capped Eppendorf tubes (filled to... 

We use tubes 100 for determination of plasma levels of intravenously injected 125I-albumin in experiments in which plasma protein accumulation is measured in the same skin sites as inIn-eosinophils (using cross channel correction for the spillover of one isotope into the energy window used to measure the other isotope). Therefore to comply with the tube order programmed into our gamma counter, tubes 100 (or blank positions, when measurement of plasma protein accumulation is not required) are always utilized. 

Walker and Levy (1989) used implantable pellets of propylthiouracil to induce thyroid dysfunction in rats. In the modification based on uptake of labelled iodine, as standard dose of radio-iodine 131-1 is injected in each animal, and the amount of radioactivity in the thyroid gland is determined in a gamma counter. 

---