Isotopes 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. 

The effecf of fhe source of Ca on fhe magnifude of Ca-Fe interactions in vivo was assessed in rodents (Smith, 1988), using a whole body radioisotopic retention test as an endpoint to determine true iron bioavailability (i.e., Fe that is absorbed and utilized). A single 50 gg liquid dose of Fe-labeled FeCla was administered by oral gavage to rats at a Ca Fe ratio of 60 1 and 120 1 fo replicate a human iron intake of 15 mg/day and a Ca intake of 800 mg/day or 1600 mg/day, respectively. Ca sources included CaCOa, Ca Phosphate (CaP), bone meal, and Ca hydroxyapatite (CaHA), while the control dose contained no Ca and was normalized to represent 100% Fe retention for comparison purposes. Isotope counts were performed immediafely after dosing (to measure 100% retention) and subsequent counts over 6 days were divided by the 100% count to estimate Fe retention. For CaCOa, Fe retention was 68% at a Ca Fe ratio of 60 1, and only declined a furfher 2% when the ratio was increased to 120 1. Fe retention values for ofher forms of Ca at a 60 1 Ca Fe ratio were as follows 77% for bone meal, 89% for CaP, and 99% for CaHA. Fe retention decreased in response to the higher Ca Fe ratio of 120 1 (i.e., Fe retention in the presence of bone meal, CaHA, and CaP was 49%, 72%, and 78%, respecfively). This is indicative of a dose-response effect of Ca on Fe retention. This sfudy also underscored fhe importance of the source of Ca in relation fo Fe refenfion. 

By choosing suitable reaction conditions, it is possible to use the system as a very sensitive assay technique. In the presence of a fixed amount of binding protein, the amount of steroid will determine the ratio of unbound steroid to steroid-protein complex at equilibrium. By adding a tracer amount of labelled steroid to the system, a simple means of determining this ratio is available, and hence, by reference to a standard curve, the amount of steroid can be found. In practice, this usually requires separation of the unbounded [St] and bound [St Prot] fractions, and then isotope counting of one or other fraction. Separation of unbound and bound steroid may be... 

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. ... 

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. 

The AMS is used to measure rare isotopes in samples that include air, water, ice, soil, minerals, meteorites, biological tissue, and archeological artifacts. Sample sizes generally are in the mg range to produce adequate rare isotope count rates for analysis. 

Dowell and Weiss, 1976). Other isotopes counted by LSC have recently been listed by Gibson (1976), Bransome and O Conner (1978), and Soini (1978). 

Fig. 2. HFLC Identification of acyl-ACF by HFLC. The first and second stage reactions are described in the legend of Fig. 1. An aliquot from the second Stage reaction was mixed with authentic [ ]-16 0-ACF and analyzed by HFLC. Radioactivity in the HFLC fractions was measured by liquid scintillation spectrometry using a dual Isotope counting mode.

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