Instrumental Analysis Test Questions – Flashcards
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| EDXRF |
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| 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. |
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| X-ray tube |
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| 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. |
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| continuum and line spectra from electron beam sources |
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| -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 |
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| duane hunt law |
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| 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. |
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| dispersion for WDXRF instruments |
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| 1. spacing between layers of atoms must approx. = wavelength of radiation 2. centers must be spaced in highly regular way |
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| Gas filled detector as X-ray transducer |
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| 1. ionization chamber (not used for xrays) 2. gieger tube 3. proportional counter |
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| how gas filled x-ray detector works |
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| 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. |
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| Geiger tube |
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| amplifycation is high and independent of the type and E of radiation. slower response. |
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| Proportional counter |
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| amplifycation is lower and depends on the E of radiation. faster repsonse. widely used. used with pulse height analyzer. |
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| X-ray detectors operated as photon counters. Photon counting. |
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| 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) |
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| advantages of photon counting over analog signal processing |
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| 1. improved signal to noise ratio 2. Sensitivity to low radiation levels 3. improved precision for a given measurement time |
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| Proportional counter |
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| Number of photoelectrons is proportional to the energy of the photon and then the pulse height. |
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| Multichannel pulse heigh analyzer |
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| 1000 separate channels. Each channel detects a height with a different V window. Can tell # of photons with particular energy. |
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| Scintillation counter |
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| 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. |
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| EDXRF instrument |
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| 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. |
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| Si(Li) Detector |
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| e- used to separate charge in detector. Improves resolution using semiconductor. |
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| UV-Spec single beam |
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| need blank to measure Po (incident radiant power), then measure sample, then do correction |
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| UV-spec double beam |
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| reference and sample irradiated simultaneously. Advantage that fluctuations in source intensity are cancelec and is electronic drift. |
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| Deuterium lamp; continuum source. |
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| 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. |
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| Tungsten and tungsten/halogen lamps |
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| 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) |
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| dark current |
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| small current that exists in radiation tansducer in the absence of radiation. *noise* |
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| multichannel spectrometer |
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| detect entire spectral range simultaneously and quickly produce spectrum. Speed and long term reliability. |
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| effects of instrumental noise on spectrometric analyses |
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| spectrofluorometer |
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| monochromators for wavelength selection two monochromators needed- one for each spectrim (emission and excitation) |
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| difference btw. excitation and emission spectra FLuorescence |
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| Emission- excitation wavelength held constant against a varying emission wavelength Excitation- emission wavelength held constant while excitation wavelengths are scanned. Excitation looks like absorption. |
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| fluorometer |
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| 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. |
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| IR |
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| measures absorption, chemical nature and molecular struc. |
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| rotational/vibrational IR differences |
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| rotational- far IR vibrational- mid IR |
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| IR gas spectrum |
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| 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. |
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| IR broadening in liquid and solid samples |
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| because molecular rotation is highly restricted in solids and intramolecular collissions and interactions are more frequent than period of rotation in liquids. |
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| group/ finger print regions |
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| force constant |
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| calculated from IR spectra- represent stiffness of the bond. affected by balance of nuclear repulsions, electron repulsions, and electron-nuclear repulsions, NOT mass. |
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| anharmonic oscillator model for molecular vibrations is more realistic because |
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| 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 |
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| IR selection rules |
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| 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. |
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| # of vibration modes |
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| 3N-6 nonlinear 3N-5 linear |
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| less experimental IR bands found when |
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| 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 |
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| more experimental nodes found when |
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| 1. overtone bands observed 2. two vibrational bands are excited to give a combination band |
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| vibrational coupling |
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| 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 |
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| FTIR |
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| whole spectrum measured at once- no dispersion. Uses interferometer. mirror position precisely monitored. |
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| IR transducers |
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| 1. photon- individual photons change electrical properties 2. thermal- absorption of photons leads to temperature change |
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| photoconductive detectors |
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| semiconductor that decreases resistance when photons are absorbed |
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| pyroelectric transducers |
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| capacitor that can be charges by an influx of heat |
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| pyroelectric material |
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| spontaneous t dependent polarization |
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| FTIR |
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| 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. |
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| FTIR |
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| 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* |
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| ATR |
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| how to illuminate with Ir: decay of evanescent wave (penetrates to a certain depth based on material) |
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