Specific activity radioisotopes

More confident attribution usually is possible when radionuclides in a sample can have only limited origin from, say, atmospheric fallout and one or two nuclear facilities. For tritium, I, or uranium in the environment, for example, the specific activity (radioisotope/stable isotope ratio) can indicate the origin. Certain activation products can be attributed to specific nuclear medicine or reactor facilities. 

Many applications ia tracer technology require products of high specific activity, ie, compounds having a high degree of substitution of specific atoms with radioisotopes. For many labeled compounds nearly 100% labeling can be achieved at one or more locations ia a molecule usiag... 

An important consideration for all radiopharmaceuticals and especially radiolabeled biologically active molecules is specific activity. There are two types of specific activity radionucHdic and biological. RadionucHdic specific activity refers to the ratio of the number of atoms of a particular radioisotope to the total number of atoms of the element. For Tc, the radionuchdic specific activity is the number of Tc atoms to the total number of Tc and Tc atoms. Because all isotopes of an element ate chemically identical, a low specific activity may lead to a low yield in the synthesis of a radiopharmaceutical if a significant proportion of the reagents is consumed by the undesited isotopes. 

Radioisotope-labeled nitrosamines have proven valuable in development of analytical methods and for demonstrating efficiency of recovery of nitrosamines from tobacco products and smoke (37-39). The very high specific activity required for low part-per-billion determinations has discouraged most analysts from using this approach. Unless a radiochromatographic detector with adequate sensitivity is available, samples must be counted independently of the final chromatographic determination, and one of the advantages of internal standardization, correction for variation in volume injected, is lost. 

The radioligand should also have a high specific activity so that very small quantities of bound ligand can be accurately measured. The specific activity, simply defined as the amount of radioactivity, expressed in becquerels (Bq) or curies (Ci) per mole of ligand, is dependent on the half-life of the isotope used and on the number of radioactive atoms incorporated into the ligand molecule. A radioisotope with a short half-life decays rapidly so that many disintegrations occur in unit time,... 

One of the first decisions to be made when designing an experiment is the method of detection to be used with a particular solute. If radiolabeled material is available, a simple method of analysis is to count the radiolabel appearing in the receiver compartment as a function of time. While convenient, this can be a dangerous practice. Depending upon the type of radioisotope, its position in the molecule, and its specific activity, radiolabeled compounds can be subject to a variety of chemical and solution-catalyzed degradation pathways. If the stock solution contains a significant amount of radioactive impurities or generates them as a result of solution instability, then the possibility for preferential transport of... 

A mixture is being assayed by radioisotope dilution analysis. 10 mg of the labelled analyte (0.51 pCi mg-1) was added. 1.5 mg of the pure analyte was separated and its specific activity measured and found to be 0.042 pCi mg1. What was the amount of analyte in the original sample ... 

Specific activity of a radiopharmaceutical may be defined as the amount of radioactivity per unit mass of a radioisotope or a labeled compound. For example, if 100 mg l3lI-labeled albumin contains 150 mCi l3lI radioactivity, its specific activity would be 150/100, i.e., 1.5 mCi/mg. Specific activity is usually expressed in units such as Ci/g, mCi/mg, or MBq/mg. It is also expressed in terms of the radioactivity per mole of a labeled compound, e.g., mCi/mole, MBq/mole, mCi/pmole, or MBq/p,mole. Specific activity is usually provided on the product label. 

Carrier-free radiochemical is a radionuclide that is not contaminated with a stable or radioactive nuclide of the same element. The no carrier added (NCA) designation applies to most of the elements to which the term carrier free is indicated. The specific activity of a carrier-free radioisotope can be calculated by the following formula ... 

The specific activity of the original radioisotope with a total activity (A) is ... 

In these types of isotopic analysis, the same compound as that to be measured is used (element or molecule) where one of the atoms in it has been replaced by a radioisotope to allow radioactivity measurements. A small, precisely known quantity of the labelled compound, called the tracer, is added to the sample and, after homogenisation, an aliquot of the spiked sample is isolated by a fractionation technique such as recrystallisation or chromatography. The specific activity of the tracer is measured before and after fractionation. 

In other words, 1 MBq of tritium contains about 3 ng of tritium. Thus, an important feature of radionuclides becomes apparent—we routinely work with extremely small quantities of material. Pure samples of radioisotopes are called carrier free. Unless a radionuclide is in a carrier-free state, it is mixed homogeneously with the stable nuclides of the same element. It is, therefore, desirable to have a simple expression to show the relative abundances of the radioisotope and the stable isotopes. This specification is readily accomplished by using the concept of specific activity, which refers to the amount of radioactivity per given mass or other similar units of the total sample. The SI unit of specific activity is Bq/kg. Specific activity can also be expressed in terms of the disintegration rate (Bq or dpm), or... 

The sensitivities of the label-based approach depend mainly on the specific activity of the labels linked to the oligonucleotide probe. Radioisotopic [48], fluorescent [50] and enzymatic [38,40,47] labels have been commonly used. Besides the above labels, NPs have attractive properties to act as DNA tags [18,51]. The fact that NPs present an excellent biocompatibility with biomolecules and display unique structural, electronic, magnetic, optical and catalytic properties have made them a very attractive material to be used as label [52,53]. 

The CFSTRs used in the TCE desorption experiments were contaminated with [14C] TCE. 500 pCi of[14C] TCE (specific activity equal to 6.2 mCi/mmol) was obtained from Sigma Chemical Co. and mixed with nonradioactive TCE to yield a net volume of 6 mL of neat liquid. The resultant chemical and radiochemical purity of the radioisotope was greater than 98 percent. A specific volume of [l4C] TCE was injected into each CFSTR. 

This is essential for nutritional studies and is often desirable when labelling cultures with certan radioisotopes, so that (a) the specific activity of the precursor may be accurately known, and (b) the specific activity may be the maximum possible. Thus, it is common... 

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