Isotopes: Definition, Chemical Properties and Importance

In this article we will discuss about:- 1. Definition of Isotopes 2. Chemical Properties of Radioactive Substances 3. Half-Life 4. Methods of Assay 5. Biochemical and Diagnostic Importance.

Contents:

1. Definition of Isotopes
2. Chemical Properties of Radioactive Substances
3. Half-Life of Isotopes
4. Methods of Assay
5. Biochemical and Diagnostic Importance of Isotopes

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1. Definition of Isotopes:

Isotopes may be defined as atoms having the same atomic number but different atomic weights. They are the subspecies of the same chemical element and occupy the same position in the peri­odic table, but have different physical properties.

The atomic constitution of three isotopes of hydrogen are illustrated:

H¹ or ordinary hydrogen consists of a nucleus containing a proton (charge +1, mass 1) around which revolves one electron (charge -1).

Heavy hydrogen contains an additional nu­clear particle, a neutron (charge 0, mass 1), H³ con­tains two neutrons. There are two classes of isotopes. The “sta­ble” isotopes which have no distinguishing characteristics other than their mass. They are obtained from natural sources by fractional procedure.

“Radioactive” isotopes not only differ in mass but they are also characterized by unstable nuclei. This causes them to decompose spontaneously emitting radiation in the form of waves. They oc­cur in nature in traces.

They are prepared for ex­perimental use by bombardment in the cyclotron or the atomic pile. The concentration of radioac­tive isotopes is expressed in terms of intensities of radiation emitted. The un-stability of these isotopes is commonly expressed as their “half-life”—the time required to decrease to half its initial value.

Preparation of Labelled Material:

It can be prepared by Grignard reaction as:

2. Chemical Properties of Radioactive Substances:

The artificial isotopes of a chemical element have the same properties as the natural ones and also a similar metabolic activity. Quantitative dissimilari­ties may occur since differences in the mass num­bers may result in different mass in the diffusion and dissociation constant.

The more the relative atomic weight of the isotope differs from that of natural element the greater is this “isotope effect”. For the same reason, the greater the number of iso- topic atoms a molecule contains, the more will its metabolic turnover differ from that of ordinary molecule.

The replacement of a stable by a radioactive atom in the molecule of a chemical compound is known as “tagging” or “labelling”—resulting in isotope as a ‘tracer’. Biosynthesis by living organ­isms and enzymatic synthesis in vitro are the least harmful methods of tagging organic substances.

Compounds of very high specific activity tagged with carrier free short-lived radioactive sub­stances should not contain more than one radioac­tive atom per molecule.

The disadvantage of high specific activities is that organic compounds may be decomposed by their own radiation. Radio substances are designated “carrier free” when they are not contaminated by any other iso­tope of the same element.

Most of the radioactive substances commercially available are not abso­lutely carrier free owing to the unavoidable adsorp­tion phenomena occurring at the surfaces of pipettes and other glassware at the extremely low concen­trations used in their preparations.

Apart from this apparatus, the solvents often contain traces of the stable isotopes. For this reason, the specific activ­ity of many carrier free radio substances falls dur­ing storage.

3. Half-Life of Isotopes:

The time taken for half the atoms in a quantity of a radioactive substance to disintegrate is known as the half-life. The concept of the half-life is also applicable to the biological disappearance of a substance tagged with the radioactive substance in a biologi­cal system.

This disappearance, at least initially, is often not a uniformly exponential process. The effective half-life which is always shorter than both the biological (T_b) and physical (T_1/2) half-life’s, can be calculated from the following for­mulae:

This formula is valid only for uniform system often it suffices to estimate T_eff by graphic lines.

4. Methods of Assay:

a. Radioactive isotopes can be determined by means of Geiger-Muller counter.

b. Particles or rays from the decay of the iso­tope enter an ionisation chamber and in­teract with the gas to form ions which causes a discharge of current by positive and negative plates.

c. The current is amplified and registered au­tomatically in a counting device which registers the total number of counts for any desired period of time.

d. Isotope concentration can then be ex­pressed as number of counts per unit time per unit weight of the sample.

5. Biochemical and Diagnostic Importance of Isotopes:

Biochemical importance:

a. I¹³¹ in the form Nal¹³¹ is administered in the body to study the thyroid physiology. About 1/3 of the iodine ingested is taken up by the thyroid and 2/3 is excreted by the kidney in normal human adults.

b. Plasma volume is measured by using I¹³¹ labelled serum albumin and erythrocyte volume is measured by using Cr⁵¹ labelled erythrocytes. Total body water is meas­ured by using I¹³¹ labelled iodo -antipyrin.

c. Absorption of fat is studied by using I¹³¹– oleic acid, absorption of iron by using Fe⁵⁹ ferrous salts and intestinal protein loss by intravenous injection of Cr⁵¹ labelled pro­tein.

d. The life span of RBC has been determined by tagging Cr⁵¹ to RBC.

e. Radioactive isotopes are widely applied in the study of the intermediary metabo­lism, almost every phase of metabolism, e.g., TCA cycle, amino acid metabolism, protein biosynthesis, nucleic acid synthe­sis, fatty acid synthesis, biosynthesis of haem and cholesterol etc. have been stud­ied by using compounds containing C¹⁴, N¹⁵, H², H³, P³², S³⁵ etc.

Mineral metabo­lism has also been studied by using Ca⁴⁵, Fe⁵⁹, 1¹³¹, Na²⁴, K⁴² and CI³⁶ etc..