Chem2404 Notes Entire Semester

  • Published on
    13-Apr-2015

  • View
    254

  • Download
    3

Embed Size (px)

DESCRIPTION

USYD CHEM 2404 Notes.by A.Gupta.

Transcript

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

Shotgun Cartridge

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.

X-Ray Fluorescence:

-

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 A.GUPTA

- X-ray fluorescence; resonance transition. Electron on continuum has a wave function defined by De Broglie = h/mv - Transition probability maximised when phase and frequency of bound and free electrons are the same. The intensity of each emission is dependent on; - Probabilty of atom ionisation - Probability that core shell will be filled - Probability that flourescence photon will escape or be absorbed by ion (Auger Effect) -

XRF Sources: -Most common source of incident photons is x-ray tube. -Cathode is heated to emit electrons which are accelerated towards anode -Continuous x-ray spectrum emitted as electron trajectories bent by W (tungsten) nucleus.

X-Ray Fluorescence: Choice of emission

12 of 37 SEMESTER 1 CHEM 2403 A.GUPTA

AAS: Advantages : - High sensitivity, ppm levels of analyte usually, ppb sometimes possible - Rapid analysis - Simple anayses

13 of 37 SEMESTER 1 CHEM 2403 A.GUPTADisadvantages: -Not as accurate as many wet chemical methds -Instumentation and equipment is expensive -No information given about speciation (i.e. molecular and chemical structure is unknown) Flame Emission AAS Flame Photometry:

The sample is aspirated into a flame, and them atomised at flame temperature, the atoms then; - Absorb energy from the flame, to reach excited atomic electronic state. From this state, they may decay from this excited state which results in a spectrum of lines, (i.e. a characteristic EMISSION spectrum.) This is then recorded, and is known as flame emission spectrometry. -OR, they may absorb energy from a specific appropriate cathode light source with a specific energy and , to reach the same excited state. The output radiation is measured and the difference (i.e. loss in intensity) is proportional to the amount of sample. This is the Atomic ABSORPTION spectrum. i.e. similar techniques -FE has no light source, atomization / excitation all done by flame. Emission spectrum measured measurement from excited state atoms decaying to ground -AAS has a light source, atomization by flame, excitation by light, measurement is from ground state to excited state atoms. Difference between the two techniques is the relative population of atoms in ground and excited states. Sensitivity of AAS is prop to number of ground state atoms. Flame Photometer : Relatively inexpensive instrumentation, uses photoelectric detector and filters, NOT a monochromator, and detects only the presence of radiation and not wavelength specific. Used to determine presence of Na, K, Li, Ca, Ba etc etc Filters can be used as selectors -Interference filters are used if there are a limited number of known

14 of 37 SEMESTER 1 CHEM 2403 A.GUPTA

-Energy throuput is more important than resolution -If cost is more important than flexibility

NB : Note that, different fuel mixtures and composition result in different temperatures.

Atomic Absorption Spectroscopy:

-Sample is aspirated into flame, but most atoms are in ground electronic state. Some are thermally excited -Ground state atoms absorb characteristic energy from radiation source or cathode lamp made of element -loss of intensity of incident light is measured. Atomic Absorption Spectroscopy: -Width of absoprtion line very narrow, for high res you need a very sharp line, use hollow cathode lamp

15 of 37 SEMESTER 1 CHEM 2403 A.GUPTA

-Ne or Ar gas ionised by high voltage. Positive Ne+ or Ar+ ions accelerated towards cathode -Metal ions sputtered into gas phase in excited electronic states, emit radiation to return to ground state. -Emits MONOCHROMATIC light, De Beers law applies, hence [atomic vapour] [analyte]

Background correction, there is a chopper which periodically blocks any radiation reaching the sample from the lamp. The difference in the lamp + flame signals and flame only signals allows the flame only signal to be subtracted.

Atomic Absorption Spectroscopy: Other forms of AAS Graphite Furnace: -Sample placed in electrically heated (300K) graphite tube in light path. All of sample introduced to light path -Higher sensitivity, flameless, responsible for decline of NAA

16 of 37 SEMESTER 1 CHEM 2403 A.GUPTA-Comparable sensitivity to NAA, lower instrumental and operating costs

Flameless AAS: See lecture notes Analytical Methods: - Construct a standard curve using known concentrations of analyte -OR -Add known quantities of desired element to analyte and measure the corresponding increase in signal, then extrapolate back to concentration of unknown. (This is STANDARD ADDITION) Emission Spectroscopy: -Similar to AAS but there is NO light source. Flame promotes atoms into excited state, then decay to ground state produces radiation which recorded. This technique relies on promotion of atoms to excited state. 99% of the atoms are still in ground state, and only 1% are excited, therefore only a small % of analyte gives an emission signal. Normal AAS has superior detection limits due to more % of atoms being excited, at elements that absorb at low .

Carbon Monoxide Poisoning: -Between 1979 and 1988 average of 1100 Carbon Monoxide (CO) deaths per year. -Estimated 10,000 people in the US every year seek medical help due to CO...