Instrumental Analysis

moseley’s law
relationship between atomic # and wavelengths (E) of the emitted x-ray photons. *Equation*
fluorescence yield
Efficiency of the emission of x-ray photons. Yield is higher in the order of K series> L series > M series (higher for more inner orbital transitions)
Competition between x-ray fluorescnece and auger process keeeps fluorescence yield low.
Higher for heavier elements.
XRF instrumentation
crystal spins in relation to beam, maintaining angle between crystal at specimen at 2x angle between crystal and detector.
counting progresses until sufficient counts for precise results. source is polychromatic tube.
scanning range allows the complete spectrum to be acquired. the detector rotates at twice the speed of the crystal to maintain 2x angle.
crystal not needed (don’t need to disperse emitted e-). source is radioactive material or polychromatic tube. Si(Li) detector. Source is closer to detecter: increase in E reaching detector, weaker source can be used. Increased sensitivity, no moving parts, all wavelengths measured simultaneously.
X-ray tube
Continuum source. x-ray can be generated with wide range E. Electrons are accelerated from a heated filament to a metal where x-rays are generated and deflected out window. Extremely inefficient. Energy of x-rays proportional to voltage used. heater controls xray intensity.
continuum and line spectra from electron beam sources
-comtinuum radiation from an electron beam source results from collissions between the e- of the beam, and atoms of target material.
-line spectra result from electronic transitions that involve the innermost atomic orbitals
duane hunt law
energy of photon is equal to the differences in kinetic energies before and after collission. max photon E when E of e = 0, representing the instant deaccel of e- to zero.
dispersion for WDXRF instruments
1. spacing between layers of atoms must approx. = wavelength of radiation
2. centers must be spaced in highly regular way
Gas filled detector as X-ray transducer
1. ionization chamber (not used for xrays)
2. gieger tube
3. proportional counter
how gas filled x-ray detector works
x-ray enters detector and interacts with gas inside to produce a cloud of ionization. # e- reaching anode is proportional to total # formed by single photon.
Geiger tube
amplifycation is high and independent of the type and E of radiation. slower response.
Proportional counter
amplifycation is lower and depends on the E of radiation. faster repsonse. widely used. used with pulse height analyzer.
X-ray detectors operated as photon counters. Photon counting.
arrival of individual photons are accurately transduced and recorded for low intensity beams.
Photons generate e-, which accumulate on C and give pulses. (this is how individual pulses produced as quanta of radiation are absorbed and counted by transducers)
advantages of photon counting over analog signal processing
1. improved signal to noise ratio
2. Sensitivity to low radiation levels
3. improved precision for a given measurement time
Proportional counter
Number of photoelectrons is proportional to the energy of the photon and then the pulse height.
Multichannel pulse heigh analyzer
1000 separate channels. Each channel detects a height with a different V window. Can tell # of photons with particular energy.
Scintillation counter
x-ray loses energy to scintillator. Flashes of light are transmitted to photocathode of a photomultiplier. # of fluorescence photons is proportional to the x-ray energy. electrons at photomultiplier tube (amplified)*photoelectric effect to convery photons into electrons.Converts height of peak into E of x-ray photons.
EDXRF instrument
1. proportional Si(Li) detector gives a distribution of voltage pulses proportional to the spectrum of X-ray photons.
2. Multichannel analyzer is used to locate the voltage pulses into discrete intervals. Consecutive output of MCA intervals allows complete spectrum to be displayed.
Si(Li) Detector
e- used to separate charge in detector. Improves resolution using semiconductor.
UV-Spec single beam
need blank to measure Po (incident radiant power), then measure sample, then do correction
UV-spec double beam
reference and sample irradiated simultaneously. Advantage that fluctuations in source intensity are cancelec and is electronic drift.
Deuterium lamp; continuum source.
energy of the radiation (difference between quantized E levels of excited molecule and the kinetic E of atoms) can vary continuously over the same range.
Tungsten and tungsten/halogen lamps
1. most common source of vis and near IR radiation. includes small amt of iodine prolongs lamp life (sublimes with metal and causes it to be redeposited)
dark current
small current that exists in radiation tansducer in the absence of radiation. *noise*
multichannel spectrometer
detect entire spectral range simultaneously and quickly produce spectrum. Speed and long term reliability.
effects of instrumental noise on spectrometric analyses
monochromators for wavelength selection
two monochromators needed- one for each spectrim (emission and excitation)
difference btw. excitation and emission spectra FLuorescence
Emission- excitation wavelength held constant against a varying emission wavelength
Excitation- emission wavelength held constant while excitation wavelengths are scanned. Excitation looks like absorption.
has filters for wavlength selection (can only use if you know your fluorphore/ emission/ excitatino properties) *restricted to a few wavelengths. *microscope: choose filter to pass specified stain.
measures absorption, chemical nature and molecular struc.
rotational/vibrational IR differences
rotational- far IR
vibrational- mid IR
IR gas spectrum
series of closely spaced lines
1. due to excitation of rotational motion during a vibrational transition
2, well defined lines are observed because both vibrational and rotational levels are quantized as char. by vibrational and rotational quantum #s.
IR broadening in liquid and solid samples
because molecular rotation is highly restricted in solids and intramolecular collissions and interactions are more frequent than period of rotation in liquids.
group/ finger print regions
force constant
calculated from IR spectra- represent stiffness of the bond. affected by balance of nuclear repulsions, electron repulsions, and electron-nuclear repulsions, NOT mass.
anharmonic oscillator model for molecular vibrations is more realistic because
1. coloumbic repulsion between nuclei, potential energy raises more rapidly than the harnomic oscillator predicts
2. dissociation of atoms take place at high enough interatomic distances.
3. change in E becomes smaller at higher quantum #s
IR selection rules
change in dipole during vibration in order for a molecule to absorb infrared
2. fundamental absorption occurs with deltav= -/+1 and e=hv(m)
3. change E becomes smaller at higher quantum numbers and the selection rule is not rigorously followed.
# of vibration modes
3N-6 nonlinear
3N-5 linear
less experimental IR bands found when
1. dipole is same in normal mode
2. energies of two or more vibrational modes are nearly Idential
3. absorption intensity too weak or beyond instrument range
more experimental nodes found when
1. overtone bands observed
2. two vibrational bands are excited to give a combination band
vibrational coupling
strong coupling when two vibrationa have an atom in common
between two bending vibrations with a commmon bond between them
strongest when energies are equal
no coupling if groups are separated by 2+ bonds
whole spectrum measured at once- no dispersion. Uses interferometer. mirror position precisely monitored.
IR transducers
1. photon- individual photons change electrical properties
2. thermal- absorption of photons leads to temperature change
photoconductive detectors
semiconductor that decreases resistance when photons are absorbed
pyroelectric transducers
capacitor that can be charges by an influx of heat
pyroelectric material
spontaneous t dependent polarization
intereferometer from a laser fringe reference system provides sampling interval information to enable precise signal sampling, precise signal averaging based on mirror postion. Then cycles are superimposed to give noise corrected spectrum.
monitor mirror position from -1 to 1. give timing of sample based on frequency. superimpose spectra from cycles to get noise reduced total spectrum. *cycles must be lined up precisely*
how to illuminate with Ir: decay of evanescent wave (penetrates to a certain depth based on material)
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