Molti lavori di geochimica abientale misurano Sr e Pb con Quadrupole ICP-MS (circa 150k€) detto anche “ICP-Massa”:Da non confondere con MC-ICPMS (cica 650k€)
• No double focusing• No magnet• No multicollector• Bad precision!!! (un ordine di grandezza peggio)
The quadrupole mass analyzer (or spectrometer) (QMS). It consists of four cylindrical rods, set parallel to each other. The quadrupole is the component of the instrument responsible for filtering sample ions, based on their mass-to-charge ratio (m/z). Ions are separated in a quadrupole based on the stability of their trajectories in the oscillating electric fields that are applied to the rods.
Quadrupole ICP-MS
Three mass fragments enter into the quadrupole vacuum chamber. The voltage of the rods is set so that only the pink mass fragment passes completely through the quadrupole rod array and into the detector. The green and blue fragments are unstable at this voltage combination and their path eventually brings them into contact with the rods so that they never reach the detector.
Quadrupole ICP-MS :
Detector à Secodary Electron Multiplier
• Lo strumento fa una scansione leggendo sequenzialmente tutti gli elementi
• Ottimo per misurare le abbondanze degli elementi: per ogni elemento di interesse si passano una serie di soluizioni di standard a concentrazioni note e si fa una retta di taratura per quell’elemento dalla quale si ricalcola la concentrazione dell’elemento nel campione “unknown”.
• Si possono misurare anche rapporti isotopici (meglio se tra stesso elemento) ma si deve considerare• Interferenze (87Rb su 87Sr)• Risoluzione dei picchi (e.g. 207Pb e 208Pb si sovrappongono?)• Elementi diversi possono avere trasmissioni diverse
ALTRE TECNICHE ANALITICHE
• 87Sr/86Sr ; 143Nd/144Nd; 20xPb/20yPb ; (230Th/238U) à Mass spectrometers (TIMS o MC-ICPMS)
• Isotopi antropogenici, molto poco abbondanti e con tempi di dimezzamento molto brevi (e.g. Pu; 90Sr ; 137Cs; 210Pb; 236U etc) à si misura direttamente la Radiazione, quindi il tasso di decadimento e da quello si calcola la quantità di moli presenti.• α counters• β counters• γ counters
Bq = Becquerel = decay/seconds
Porcelli & Baskaran (2011)
Porcelli & Baskaran (2011)
• Alpha particle energies are discrete, leading to a single peak in the alpha emission spectrum.• The alpha particle energies of many radionuclides are close together and may be difficult to resolve using a solid state detector.• Sample purity for the radionuclide of interest is very important to successful analysis.• The radiochemist can adjust conditions to get the optimum purity and peak resolution in the alpha particle spectrum.
Good energy resolu<on
• È necessario purificare il campione prima della misura (anche per β counting)
• È necessario depositare il campione possibilmente in uno strato di spessore infinitesimo (anche per β counting)
• È necessario aggiungere uno spike a concentrazione (à ritmo di decadimento) noto per calcolare la counting efficiency (in pratica si misura per ID)
Porcelli & Baskaran (2011)
Porcelli & Baskaran (2011)
Coun%ng efficiency = par%cle counted/par%cles emi7ed
N = 0.0001/2.2 x10-‐13 = 4.55 x 107 atomi àdiviso per N avogadro (6.022x1023) à 8 x 10-‐17 moli à 0.08 fmol
λ (yr-‐1) = ln(2) / T1/2
T1/2 (yr) à T1/2 (sec) T= 1 x105(yr1) à λ = 2.2 x 10-‐13
(sec-‐1)
0.01mBq = 0.0001Bq
Porcelli & Baskaran (2011)
Coun%ng efficiency = par%cle counted/par%cles emi7ed
Semiconductor dectectors
• Ionizing radiation is measured by the number of charge carriers set free , by the radiation, in the detector material which is arranged between two electrodes. Ionizing radiation produces free electrons and holes.
• The number of electron-hole pairs is proportional to the energy of the radiation to the semiconductor.
• As a result, a number of electrons are transferred from the valence band to the conduction band, and an equal number of holes are created in the valence band.
• Under the influence of an electric field, electrons and holes travel to the electrodes, where they result in a pulse that can be measured in an outer circuit.
1-‐10 days of coun<ng!
Porcelli & Baskaran (2011)
Porcelli & Baskaran (2011)
• Geiger counter à No risoluzione energetica (anche perché le β hanno energie variabile) à purificazione campione
Porcelli & Baskaran (2011)
• Gas Ionisation dectors (e.g. Geyger-Muller counter)• Liquid scintillators counters (LSC)
Gaseous ionization detectors are radiation detection instruments used in particle physics to detect the presence of ionising particles, and in radiation protection applications to measure ionizing radiation.
If a particle has enough energy to ionize a gas atom or molecule, the resulting electrons and ions cause a current flow which can be measured.
The three basic types of gaseous ionization detectors are:ionization chambersproportional countersGeiger-Müller tubes (GM counters)
Depending on the applied voltage and signal multiplication
Ioniza<on chambers operate at a low electric field strength, selected such that no gas mul%plica%on takes place. The ion current is generated by the crea<on of Ion pairs. Ion chambers are preferred for high radia%on dose rates because they have no "dead <me"; a phenomenon which affects the accuracy of the Geiger Muller tube at high dose rates. +ve = posi<ve ions -‐ve = nega<ve ions (electrons)
Proportional counters operate at a slightly higher voltage, selected such that discrete avalanches are generated. Each ion pair produces a single avalanche so that an output current pulse is generated which is proportional to the energy deposited by the radiation. This is in the "proportional counting" region.
GM tubes operate at high voltage, so to creates an avalanche. The current pulses produced by the ionising events are passed to processing electronics which can derive a visual display of count rate or radia<on dose, and usually in the case of hand-‐held instruments, an audio device producing clicks.
The Geiger-Müller tube is filled with an inert gas (He, Ne, Ar) low pressure, which briefly conducts electrical charge when a particle or photon of incident radiation makes the gas conductive by ionization. The ionization current is greatly amplified within the tube by the Townsend avalanche effect to produce an easily measured detection pulse.
LIQUID SCINTILLATION COUNTERS
A scintillation counter is an instrument for detecting and measuring ionizing radiation.It consists of a scintillator which generates photons of light in response to incident radiation, a sensitive photomultiplier tube which converts the light to an electrical signal, and the necessary electronics to process the photomultiplier tube output
Porcelli & Baskaran (2011)
Esempio di decadimento β+
L’ 14O (Z=8) emette un positrone β+ per formare il radionuclide figlio 14N (Z = 7) secondo la reazione:
148O 14
7N + β+ + Q + ν
Fig. 13.8. Schema per il decadimento β+ del 14O in 14N. L’energia emessa Q è pari a 5.12 MeV, sebbene si possa seguire due vie di decadimento con il raggiungimento di due stadi isomeri a diversa energia, per poi rilasciare l’ eccesso di energia tramite emissione di 2 raggi γ da: G. Faure (1987) Principles of Isotope Geology. J. Wiley &
Sons
T1/2 = 70.6 s
Esempio di decadimento α
Il 222Rn emette un singolo set di particelle α per formare il radionuclide figlio 218Po secondo la reazione:
222Rn 218Po + α + Q
Fig. 13.5. Schema per il decadimento α del 222Rn in 218Po. L’energia emessa Q è pari a 5.4897 MeV, sebbene rimane un eccesso di energia (0.51 MeV) che sarà emessa tramite radiazione γ per il passaggio dall’isomero eccitato allo stato di base del 218Po. da: G. Faure (1987) Principles of Isotope Geology. J. Wiley &
Sons
T1/2 = 3.8 days
Porcelli & Baskaran (2011)