PTT 202 Organic Chemistry for

BiotechnologyLecture 5: Radioisotopes

zulkarnainidris@unimap.edu.myZulkarnain Mohamed Idris

Semester 1 2013/2014

Introduction Isotopes: atoms that have same atomic number

but have different masses. Radioisotopes: radionuclides that spontaneously

and continuously emit characteristic types radiation.

Half life (t½): is the time required for a quantity to fall to half its value as measured at the beginning of the time period (radioactive decay).

Positron: or antielectron is the counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron. When a low-energy positron collides with a low-energy electron, resulting in the production of two or more gamma ray photons (generated when proton is converted to neutron due to radioactive decay).

Introduction

Nature of radioactivity Atomic nucleus is made up of protons and

neutrons. # of protons=atomic number Total # of Protons +Neutrons=atomic mass Proton: is a nuclear particle with mass of 1 and

positive charge of 1. Neutron: is a nuclear particle with a mass of 1

and no charge. The stability of atom depends upon the critical

balance between repulsive and attractive forces involving protons and neutrons. For lighter elements, the ratio (N:P)=(1:1), and for heavier elements, the ratio (N:P)=(1.5:1).

The element with (N:P) ≠ (1:1) or (1.5:1) is said to a radioactive

C614C6

12

Carbon-14 (isotope of carbon)

Types of radioactivityAlpha particle:

If nucleus too heavy and its atomic number exceeds 82, reverts to stable arrangement by releasing both neutrons and protons (emission of alpha particle).

contains 2 protons + 2 neutrons + a helium nucleus ().

Relatively large particles and emitted with a limited # of energy levels.

Attract electrons from atoms of material through which they pass, causing ionization.

Short in range, present very little hazard as an external source of radiation, but their effects within living cells or tissues can be serious.

Types of radioactivityBeta radiation:

A nucleus that has an excess of neutrons will undergo neutron to proton transition that requires the loss of an electron and result the atomic number increases by one: emitted a high speed electron known as negatron (β-).

A nuclei that contain an excess of protons undergo proton to neutron transition with the emission of positively charged beta particle (β+) known as a positron and with the reduction of atomic number by one.

An alternative mechanism to positron emission involves the process known as electron capture (EC). Nucleus captures an orbital electron from inner shell to restore the N:P ratio and as result emits X-ray due to electron transition from higher to lower orbital.

Types of radioactivityGamma radiation:

A nucleus that has N:P ratio in the stable range is still possible for it to be an unstable energetic state and to emit protons as electromagnetic radiation of extremely short wavelength known as gamma rays.

Does not involve the change in the mass or charge, and as result cannot cause direct ionization effects, but energy associated with them is absorbed by an atoms causing electron to be ejected, which in turn produces secondary ionization effects.

Types of radioactivityBeta radiation:

Shows a range of energy levels and only small proportion of the emissions may be of this energy.

Shows max. range in a particular absorbing material with many emissions penetrating considerably less.

Penetration distances are about ten times greater than for alpha particles, values of 5-10 mm in aluminium being common.

Because they are light: easily deflected by other atoms and less effective as ionizing agents than alpha particles.

Decay The emission of radiation by atom results in a

change in the nature of the atom or decayed. The rate at which the quantity of an isotope decays

is proportional to the number of unstable atoms present and a graph of activity against time results in typical exponential curve.

The rate of decay is provided by the half-life (t1/2) that can be determined by the following equation:

Decay

Example: The emission of radiation by an isotope results in change in the nature of the isotope, which is said to have decayed. Consider iodine-131 and its radioactivity data is given in Table 5.1.

Table 5.1: Radioactivity of iodine-131Time (days) Radioactivity (%) 0 100 4 70.8 8 50.1 12 35.5 16 25.1 20 17.8 24 12.6 28 8.9 32 6.3 36 4.5

0 4 8 12 16 20 24 28 32 360

102030405060708090

100f(x) = 99.91333404403 exp( − 0.086266706617 x )R² = 0.999994928854617

Time (days)

Nt (

%)

Solution: (i) Plot the logarithm of the radioactivity of

the iodine-131(Nt) against time (t).

0 4 8 12 16 20 24 28 32 360

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2f(x) = − 0.0374651546557146 x + 1.99962345134221R² = 0.999994928854617

Time (days)

log

10

Nt

Solution: (ii) Solve the half-life (t1/2) for iodine-131. = 0.3010/ gradient since gradient of the line is equal to 0.0375, = 0.3010/ 0.0375 = 8.0267 ≈ 8.03 days

Unit of Radioactivity The curie unit (Ci) is based on the activity of

1g of pure radium-226, which undergoes 3.7 x 1010 transformation per second.

It is defined as the quantity of a radioactive isotope which gives 3.7 x 1010 disintegrations per second.

SI unit= barquerel (Bq), where 1 Ci = 3.7 x 1010 Bq.

Specific activity of an isotope indicates the activity per unit mass or volume and is quoted as becquerels per gram (Bq g-1)or milicuries per gram (mCi g-1)

Safety Not only high emitter radioisotopes are

dangerous, but the weak emitters with long half-lives also can cause serious hazard over a period of time if incorporated into the body (e.g. tritium and carbon-14).

General principles of good practice when working with radioisotopes as following:

Detection and measurement of radioactivity

The detection of radiation depends on the ionization effects (either direct or indirect).

3 methods most frequently of used:1. Ionization of gases2. Excitation of liquids or solids3. Induction of chemical change

Radioactivity is measured as either the number of individual emissions in a unit time (differential measurement) or the total cumulative effect of all emissions in a given time (integral measurement).

Any critical measurement of radioactivity demands that a background count (blank) must be done to eliminate the reading of emissions that are not due to the sample.

Geiger counters

Geiger counters The ionizing effect of radiation on a gas may

be detected if potential difference is applied across two electrodes.

The electrons displaced as a result of ionization will be attracted to the anode and current will flow between the electrodes.

Gieger-Muller tube consists of a hollow metal cylinder (the cathode) within a glass envelope and a wire (the anode). The tube is filled with the counting gas often a mixtures of neon and argon under relative low pressure.

Radiation entering the tube causes ionization of the detector gas, as a result, a pulse of current flows between electrodes together with the emission of UV radiation.

Scintillation counters

Scintillation counters Some substances, known as flours or scintillants

respond to the ionizing effects of alpha and beta particles by emitting flashes of light (or scintillations).

They do not respond directly to gamma rays, but they do respond to the secondary ionization effects that gamma rays produced.

Crystals of sodium iodide containing a small amount of thallous iodide are very efficient detectors of gamma radiation and instruments based on this design are called gamma counter.

The crystal is positioned on the top of a photomultiplier which can detect the flashes of light and both are covered with light—proof material that is thin enough to allow gamma radiation to pass into the crystal.

When a sample is placed on the crystal, the pulses of current generated and counted electronically.

Autoradiography Autoradiography produces an image formed

by a substance’s own radioactivity when exposed to a photographic film.

Useful for demonstrating the location of radioactive isotopes in tissues or chromatograms (investigation of biological processes).

The sample is placed on a photographic film, protected from light and allowed to remain contact long enough for an adequate exposure. The exposure time is dependent upon the intensity of the radiation.

The technique is a convenient way of monitoring the amount of radiation to which worker has been exposed (dosimetry)

Biochemical uses of isotopesTracers:

Labelled or tagged radiactive isotopes to monitor the metabolism of compounds that contain the isotopes.

Applications: studies metabolism in whole animals or organ to sensitive quantitative and qualitative analytical techniques such radioimmunoassay (e.g. phosphorus-32 is used in work with nucleic acids, particularly in DNA sequencing and hybridization techniques).

This method involves the synthesis of the molecule either in vivo or in vitro and the use of enzyme that permits the isotope to be introduced in a particular position in the molecule.

Biochemical uses of isotopesTracers:

The choice of isotope for tracer studies need to consider not only the radiochemical properties but also the effects that they might have both biochemically and analytically.

The isotope should have a half-life that is long enough for the analysis to be completed without any significant fall in its activity.