USYD CHEM 2404 Notes.by A.Gupta.
1 of 37 SEMESTER 1 CHEM 2403 A.GUPTANeutron Activation Analysis General Principle: Incident neutron hits target nucleus, which creates an intermediate radionuclide, which then decays producing radiation. By using the constituent radiation and decay products we can find out composition of sample. Explained: *An incident neutron hits the target nucleus. - prompt ray emitted * Compound nucleus formed, then intermediate radionuclide formed - particle emitted - delayed ray emitted *The constituents of the sample are identified by the decay products, i.e. the rays emitted of the new compound isotope. NAA creates an elemental fingerprint of the sample.
Advantages of NAA: - Simultaneous detection for most elements - Highly sensitive for most elements Little to none sample manipulation required, sample intact (nondestructive) Highly selective as many parameters can be changed - Fast and efficient Disadvantages of NAA: - Neutron source required - Not all elements can be detected - Unknown chemical structure only elemental makeup known (up to 20 at a time). Parameters of NAA: Irradiation time, decay time, counting time. Procedure: Sample is typically in mg to g scale. Inside polyethylene or quartz container. Radiated in groups in a reactor. Various parameters can be set such as
- type of radiation, energy, neutron flux hence all are performed with respect to astandard. - sample is then removed, allowed to cool and spectrum is then recorded. Decay can be detected by ionisation detectors, or scintillation counters. rays interact with matter in various ways the photoelectric effect and Compton scattering. The Photoelectron Effect
2 of 37 SEMESTER 1 CHEM 2403 A.GUPTAIncident ray annihilated, and e- is ejected. The highest probability of the electron being ejected is when is low energy (< MeV) and the atom is heavy.
Compton Scattering Only some of the energy is lost with interaction with the electron, the products formed are low energy and e-. Highest probability is when is medium energy and the atom is heavy.
Detector Efficiency Only a fraction of emitted radiation is actually detected. With higher resolution it is possible to discriminate between similar energies of radiation.
Ionisation Detectors *Chamber with electrons and filled with gas at 0.1 1 atm *Detects the presence of , radiation and X-rays, not effective with
3 of 37 SEMESTER 1 CHEM 2403 A.GUPTA
*Ions are produced when the gas inside is ionised by the incoming radiation, and these ions move to electrodes, which generates a current. *Voltage at 200-300V so as to collect all ion pairs, but low enough to avoid secondary ionisation *Magnitude of current energy of radiation Scintillation Detectors *Incident radiation causes ionisation and atom excitation. *De-excitation produces emission of light *e.g. band gap of pure NaI is large hence p(electron transition) is low, and any hv is not visible *It is then doped with Ti, providing accessible energy levels Requirements: -Material reacts to radiation -High light output and linear energy response -Short delay between excitation and emission -Easy machining Suitability: -X-rays, and high energy electrons - rays have good efficiencies because, crystals are high density, contain a high charge element and large crystals can be grown -NOT good for fast counting, long recovery time (230ns), hence low resolution See LN for more types of scintillation detectors notably Ge, Li, Si.
Activation Processes i-NAA ii-Charged Particle Activation Analysis iii-Instrumental Photon Activation Analysis (IPAA)
4 of 37 SEMESTER 1 CHEM 2403 A.GUPTANAA Classification of Neutron Activation Analysis ! Origin of rays *PGNAA (Prompt Gamma Ray NAA), measures emitted by the intermediate nuclide *NAA, measures the delayed emitted by final nuclide PGNAA - detected as theyre produced. That is the emitted from the intermediate radionuclide. *This is more difficult than NAA as it must be done on site. Used to distinguish prompt from other (e.g. background) radiation. *Used for elements with high neutron capture -elements that decay rapidly -elements that produce STABLE isotopes -elements with weak decay intensity ! Separation Process *Is there any chemical separation ? *If NO INAA *If YES -Before irradiation : Chemical Neutron Activation Analysis (CNAA) -After irradiation : Radiochemical Neutron Activation Analysis (RNAA) CNAA example : Vanadium in Marine Animals *Sample wet ash/HNO3 at 65C cation exchange column final sample NAA RNAA example: see LN Further classification; neutron energies. *Thermal (~0.025 eV) *Epithermal (~0.1 1eV), resonance neutrons (1 1000ev) *Fast (>0.5 meV) *14 MeV fast (14-MeV INAA, no chemical separation) CPAA - Charged Particle Activation Analysis Analyte activated by charged particles, e.g. protons, deuterons, particles w/ energies in MeV Less sources of these particles, hence used less often than NAA. Prompt rays detected by PIGE, PIPPS, GRALE Particle Induced Gamma Ray Emission Analysis Particle Induced Prompt Photon Spectroscopy Gamma Ray Analysis of Light Elements IPAA - Instrumental Photon Activation Analysis
5 of 37 SEMESTER 1 CHEM 2403 A.GUPTA
Interference in Activation Analysis NAA Effectiveness can be decreased by Interference reactions -Primary, Secondary, 2nd order interference, -ray spectral interference. Primary
See LN for other sources of interference, and explanation.
Projectile and Gunshot Residues
6 of 37 SEMESTER 1 CHEM 2403 A.GUPTALocards Principle: No clean contact. There is always exchange of contaminants and materials when two bodies interact. Small Arms Round Schematic
NB: On discharge, propellant explodes, the projectile is forced down a barrel. Barrel rifled to add spin to bullet If bullet larger than diamter of barrell, then creates, scrateches, etc. Fragments clips off
Residues - Powder from bullet residues from propellant deposited onto shooter - Used to analyse whether death was suicide, identify shooter, identify bullet fragments Typical propellant is black poweder. 15C: 75KNO3: 10S Primer also has composition, explosive (lead styphnate), oxidiser(BaNO3), fuel (Sb2S3 )
Projectile and Gunshot Residues Detection: Testing for nitrates. Nitrate chemical assay for most propellants (nitro-cellulose).
7 of 37 SEMESTER 1 CHEM 2403 A.GUPTASpot test
Bullet Leads: Also have unique and identifyable composition. -Soft; 99% Pb, 1ppm < [Sb] < 1500 ppm -Hard; 95-99% Pb, 0.4% < [Sb] < 4% -Metal Jacket; Cu-Zn alloy. 90% Cu/10% Zn or 95%Cu/ 5% Zn Spot test for Pb; area swabbed with soln of Sodium Rhodizonate. Acid wash to extract insoluble Pb. If present free particles turn reddish-purple. Firearms Discharge Residue (DFR) Gunshot Residue (GS) Cartridge Discharge Resuidue (CDR)
Spectroscopy General Principle AAS: Atomic absorption, electrons of the atoms of the sample are promoted to higher energy states, by absorbing a characteristic of energy, i.e. specific . Specific to a particular electron transition in a particular element. Absorption of sample measured over 100s of . Dimensionless value of absorbance at corresponds linearly with prevalence of element in sample.
8 of 37 SEMESTER 1 CHEM 2403 A.GUPTA
General Principle AES: Atoms in sample are excited by flame and/or atomization. Each element in sample emits excess energy at a characteristic which falls in visible the spectrum (i.e. 400-800nm). Each element has characteristic emission spectrum which may have many . Intensity of emission corresponds linearly with [element].
Beer-Lambert Law: Transmittance T = I / I0 (where 0 < T < 1)
9 of 37 SEMESTER 1 CHEM 2403 A.GUPTA
Absorbance A = log10 (I / I0 ) = -log10 (T)
Deviations from Beer-Lambert: -Applies well in dilute solutions, derivation may be due to chemical/ instrumental error or fault. Chemical -Interations between solute and solvent -Refractive index of solvent, (i.e. changes) -Concentration of certain species dependant on chemical equilibrum e.g. acid dissociation -Complex equilibria of metal ions, ligand dependant -The presence of a dimer/ monomer equilibrium will cause and absorbption spectra to change -The presence of intefering species in sample
Instrumental -Less of a problem, generally. -Radiation is not entirely monochromatic - spread of wavelengths is used, okay at ABSmax -Can be a problem on the side of an absorption changes rapidly with , hard to accurately determine [sample] -Stray radiation, mismatched cells, air bubbles in solution (affects path length) all affect result. -Better to calibrate against standard samples rather than rely on values. Interpolate, instead of extrapolating.
Sensitive to all most all elements (>13 Al), rapid analysis, several elements can analysed together, equipment relatively cheap and portable, sample can be g to g range.
10 of 37 SEMESTER 1 CHEM 2403 A.GUPTAHowever, no speciation information, no compounds identified only elements. Electrons in shells, principle quantum number (1), incoming energy (,,) is absorbed and knocks out core (K) shell electron. Ejected electron goes into continuum, atom ionised.
As electron is lost from core and not valence shell (L, M), resulting ion is an excited state. Excited ion relaxes to ground state as electrons in valence band fill hole in core. The excess energy of a L or M shell electron is lost as x-ray radiation energy.
Auger Effect: - The emitted x-ray as a result of above, may not escape the atom and instead an outer shell electron is ejected. - Auger spectroscopy characterizes elements by energies of ejected electrons -
X-Ray Fluorescence: NB:
11 of 37 SEMESTER 1 CHEM 2403