Radiochemistry Flashcards
What causes radioactivity?
Spontaneous nuclear disintegration
Unstable isotopes
What does nuclear stability depend on?
*ratio neutrinos to protons
*gives rise to spontaneous decay (radioactivity) with particle emission
What are a (alpha) particles?
*nucleus of helium
*emitted by elements MW>140
*2 neutrons and 2 protons
*heavy
*slow moving
What is the most commonly used radioisotopes in biochem?
b emitters
What two forms do B emitters exist in?
*electron - b-
*positron- b+
During decay a B particle is emitted- what other particle is emitted?
*antineutrino- almost massless particle- carries away from energy from decay
How can anti neutrinos be stopped?
Small atomic number materials eg plastics
What are y emitters (gamma)?
A photon of electromagnetic radiation
Usually secondary process after B emission
What kinetics order is spontaneous disintegration?
First order
What is the equation for radioactive disintegration? (Half life)
=0.693/y
What are the units of radioactivity?
*Ci (curie)- large- usually use millicuries mCi and microcuries (uCi)
*Bq (becquerel) - small
What are the units of radiation?
*roentgen (coulomb/kg)
*rad (gray)
*rem (sievert)
What is the gray (rad) unit used for?
Amount of energy transferred by radiation to an object
What two methods are used to detect radioactivity?
*liquid scintillation counter- best counting efficiency
*greiger-muller tube -high energy B emitters (Small ones struggle to enter tube)- counting efficiency not as good as LSC
(Both used for B emitters only)
What does a recording instrument register?
Counters per min
What is the equation for instrument efficiency?
%efficiency= observed cpm - background cpm / actual disintegration per min x 100
What three components are used in liquid scintillation?
*radioactive material
*organic solvent
*one or more organic FLUORESCENT SUBSTANCES
How is the scintillation counter process carried out?
B particle collides with solvent particle
B + S > B (less energy) + S
S reacted with primary Fluor F1
S* + F1 > S + F1*
F1 molecules emit photons at higher wavelength- often not enough so secondary Fluor added
F1* + F2 > F1 + F2*
F2* > F2 + hv2 (light)
What is an example of fluors?
*PPO
*POPOP
What are the components of a scintillator counter?
*PMT x2- photomultiplier tubes- two used due to tube damage from thermal noise
*pulse summation
*coincidence gate- pick up info from two PMT’s
*pulse height analyser- allows counting of pulses with DEFINED energy limits
*scaler
*printer
What does channel 1 of scintillator detect?
All 3H
What does channel 2 of scintillator detect?
14C
What is the problem with quenching during scintillation?
*causes underestimation of activity of sample
*variable quenching causes similar sample to be counted differently
What is chemical quenching?
Chemicals that interfere with energy transfer to fluors- capturing it and releasing it as heat
What is colour quenching?
Unwanted dyes absorb in violet blue region act as filter- reducing intensity of photon
How can quenching be corrected?
*use of internal standard
*channels ratio
*external standardisation
What is an example of a quenching agent?
Chloroform
