241 exam 3
how is an emission experiment done w/ luminescence setup
ONE excitation wavelength used luminescence observed 90 degrees to incident light
Be able to understand the particle/wave duality characteristics of electromagnetic radiation. Which phenomenon and equations support each?
Particle: photoelectric effect Wave: interference, diffraction, polarization Both: reflection and refraction
Know the order of energies and wavelengths for the basic types of EM radiation.
cosmic gamma x ray UV vis infrared micro radio
Know advantages of double-beam spectrophometer over the single-beam instrument.
dynamic solutions reduces fluctuations in sensitivity and source intensity scatter and reflections reduced
Describe what happens when a molecule absorbs light?
gain energy from the photon and electron is excited from the ground state to an excited state
What are some typical sources used in IR spectrophotometric techniques?
globar nichrome wire nernst wire
calibration curve definition
graph of signal vs. concentration for a series of standards to generate a best fit line
disadvantages single beam
have to rezero w reference every time drifting error -- electronic components warm up and change in sensitivity over time blank and sample cuvette repositioning not always the same can't monitor dynamic changes
sources of infrared light
heat sources, passing current through a filament material globar nichrome wire nerst glower
when does beer's law fail? when does it work best?
high concentrations because the proximity of particles influences how they absorb beer's law works best for very dilute solutions, < or equal to 0.1 M, monochromatic solutions
Why are emission spectrophotometric techniques generally more sensitive than absorbance techniques?
light against dark vs light against light
key features of luminescence instrument
light source excitation monochromator sample cell emission monochromator detector
Discuss how groove spacing on a grating relates to dispersion and resolution?
smaller groove spacing/more grooves in grating= greater resolution and dispersion (at a cost of signal/noise ratio)
molar absorptivity depends on
wavelength and analyte
standard addition use
when matrix effects interfere with signal of absorbance (complex mixtures, swamp water)
What is a Fourier Transformation? What types of instruments is it used in? What is the big advantage of an FT instrument? What major discovery a few decades back made the use of FT instrumentation more feasible?
FT: breaking down of a curve into sin and cosine graphs, used in computers Discovery: LIGO experiment?
why do the lambda zeroes of absorbance and emission not overlap completely
Franck-Codon principle lamba 0 for excitation/abssorbance higher than lambda 0 emission bc the molecule will return to S0 with S1 geometry and solvation, which is an unstable configuration
Be able to draw the instrumental layout for both an absorption spectrophotometer and for an emission spectrophotometer? What are the similarities and critical differences?
IDK
NaCl and KBr cuvets used for....
IR range
What is a Michelson Interferometer and what is it's only moving part? What is it used for?
MI detector using FT technique to produce an interferogram which gives you the signal of all wavelengths at once without scanning mirrors or gratings its only moving part is the mirror that is struck by the source light
phosphorescence: transition, example, radiational
T to S T1 to T0 Yes
sources of visible light for spectrophotometric instruments
Tungsten lamp
how does hollow cathode lamp help linewidth problem
cathode made of element whose emission lines we want cations strike cathode, and the atoms are vaporized and emit photons radiation emitted = frequency absorbed by analyte monochromator will choose one wavelength from the lamp and rejects emission from the flame
What is Beer's Law and be able describe the relationship between Abs and T?
describes relationship b/w absorbance and concentration abs and T are logarithmically related
Discuss the difference between electronic excited states, vibrational excited states, and rotational excited states?
electronic excited states: electron promoted to a higher energy state vibrational excited state: oscillations of atoms in molecules increase energy rotational excited state: movement of atoms in different planes in molecules increases energy
energy transitions for UV VIS
electronic transition of valence electrons vibrational rotational
What advantages are gained from employing a beam chopper in a flame AA instrument?
factors out background noise
polyethylene cuvet used for....
far IR range
What is the signal-to-noise ratio in a spectroscopic signal and how can it be improved?
improved by signal averaging, using FT technique, beam-choppers
advantages of double beam
minimization of fluctuations in source intensity and detector sensitivity scatter and reflections are reduced as error sources can monitor dynamically changing rxn mixtures
spectrophotometric titration
use absorbance to determine stoichiometric endpoint of a rxn
energy transitions for infrared
vibrational and rotational
glass cuvets used for....
visible range
What are three main types of noise and how can each be reduced or dealt with?
white/static noise: improved by cooling the system or changing set up "drifting", or 1/f noise: usually do to incorrect set-up, can be improved by beam chopping and "lock in" amplifiers which pulsates light at a frequency that cancels out the 1/f line noise: interference at discrete frequencies, like 60Hz. reduced by beam chopping, electrical shielding, and grounding the shielding and instrument to same ground point
relaxations: transition, radiational
within a state no
calibration curve use
determine an unknown concentration from absorbance or emission measurements
sources of ultraviolet light for spectrophotometric instruments
deuterium arc
how would you adjust your solution to remain within desired absorbance values
dilute solution longer path length
Define and differentiate between the terms dispersion and resolution for grating monochromators?
dispersion: how much the wavelengths can separate from each other in angular space resolution: how well you can differentiate between two wavelengths that are close to each other
Explain the "linewidth" problem encountered in atomic absorption (AA) spectroscopy if conventional source/monochromator setups are used? what is the solution?
line source of radiation must be narrower than the linewidth of the absorbing sample solution: hollow cathode lamp
Describe the sources of error which dominate spectrophotometric measurements at low and high abs?
low absorbance: too much light gets through, can't distinguish transmittance of sample from reference high absorbance: too little light gets through, intensity is hard to measure
Average peak width is 15mm. What is the best slit width you should choose?
3 mm; 1/5 of absorbance width
Choose the best answer: if a spectrometer will be used to measure absorbance in the ultraviolet region of the spectrum, the best choice for a source would be a: A. deuterium lamp B. quartz cuvette C. glass cuvette D. photomultiplier tube E. tungsten lamp F. globar source G. vacuum photodiode H. prism
A. deuterium lamp
Choose the best answer: If a substance you wish to measure is fluorescent, then fluorescence will be a much more sensitive method than absorbance for measuring extremely low concentrations of this substance because: A .fluorescence involves measuring the difference between a very low level of light and virtually zero light, while absorbance involves measuring a very small decrease in a relatively high level of light intensity. B. fluorescence usually involves the emission of infrared light which is easier to detect with high sensitivity than ultraviolet light. C. absorbance measurements require more complex instrumentation, making the likelihood of noise (uncertainty) in the measurement greater D. fluorescence involves relaxation from an excited singlet electronic state while absorption does not involve such a process. E. absorbance measurements involve exposure of the sample to greater levels of ultraviolet radiation, increasing the probability of photodecomposition of the trace substance you are trying to measure. F. strong absorbance (high molar absorptivity) is a rare property in chemical compounds, so fluorescence is inherently more sensitive.
A. fluorescence involves measuring the difference between a very low level of light and virtually zero light, while absorbance involves measuring a very small decrease in a relatively high level of light intensity.
In a lab with a lot of money to spend, which would be the main reason to purchase an absorbance spectrophotometer prior to purchasing a fluorescence spectrophotometer? A.more universal detector B. greater sensitivity C. easier to use D. greater selectivity E. NONE of the answers are correct
A. more universal detector
When a fluorescent compound is excited, which of the following is true? A. the wavelengths emitted by fluorescence are always longer the wavelengths absorbed in the excitation. B. the wavelengths emitted by fluorescence are always shorter the wavelengths absorbed in the excitation. C. the wavelengths emitted by fluorescence are the same (and overlap) the wavelengths absorbed in the excitation. D. None of the answers above are true
A. the wavelengths emitted by fluorescence are always longer the wavelengths absorbed in the excitation.
Which analytical method is useful when studying samples dissolved in complex mixes (with matrix affects) by absorbance or luminescence techniques? A. calibration curves B. internal standards C. standard addition D. serial dilution E. none of the above
C. standard addition
In a standard additions method workup what information from the linear regression is most closely related to the unknown concentration? (used to determine it) A. the slope of the std. addn. line B. the y-intercept of the std. addn. line C. the x-intercept of the std. addn. line D. 1/slope of the std. addn. line E. none of the above.
C. the x-intercept, gives you diluted concentration
Which of the following types of EM radiation has the largest wavenumber? A. radio B. microwave C. red light D. green light E. X-ray
E. X-ray
When an electron is excited from So to S1 in an atom during a molecular absorbance, what accounts for the broadness of the peaks on the absorbance spectrum? A. radiationless decay B. collisions with solvent molecules C. the presence of vibrational energy states D. the presence of rotational energy states E. both C and D
E. both C and D
T/F: fluorescence has a longer lifetime
FALSE: phosphorescence has a longer lifetime
How come the two lambda 0's are not equal in Figure 18-19? Read about it on those pages
Franck-Codon principle: solvation effects cause the lamba 0 of the emission energy to be less than the lambda 0 of excitation energy
Know both the order of energies and wavelengths and the approximate wavelength numbers for the colors in visible EM spectrum
ROYGBIV longest wavelength to shortest wavelength
fluorescence: transition, example, radiational
S high to S low S1 to S0 Yes
absorbance: transition, example, radiational?
S low to S high S0 to S1 Yes radiational
internal conversion: transition, example, radiational
S to S S1 to S0 no
Relationship between S/N ratio and light throughput?
S/N increases w/ light throughput
which measurements are more sensitive: luminescence or absorbance and why
luminescence, bc you're directly measuring emission of light and not comparing it to anything else light against dark absorbance is light against less light
Fused silica/quartz cuvet used for....
UV/Vis range
What are some common SOURCES used in spectrophotometry and what are characteristics of each?
Vis light - Tungsten UV - D arc Infrared -- globar, nichrome wire, nernst glower; work by passing current through filament materials Lasers - light amplification by stimulated emission of radiation Wavelength selectors: prisms and grating monochromators
List and briefly describe the three different types of instrument noise encountered in spectroscopic techniques. Briefly discuss a way each might be reduced.
wavelength: use lambda max absorption bandwidth: wide enough for light power to reach detector but narrow enough to distinguish peaks stray light beam chopper
What are the difference between Fluorescence and Phosphorescence? both practically and in terms of the energy level description. What are some "real world" examples?
fluorescence = shorter lifetime, transition from S1 to S0, higher in energy phosphorescence = longer lifetime, transition from T1 to S0, lower in energy
List the advantages and disadvantages of using the following in atomic spectroscopy techniques for the introduction of sample: conventional flame, inductively coupled plasma (ICP), and graphite furnace.
conventional flame: good bc easy. bad bc low signal and broad radiation. ICP: hotter, more efficient flame created by RF coils. high, consistent heat, efficient at atomization, used mainly for emission Graphite furnace: more sensitive than a regular flame, used for wet, biological samples
Know advantages and disadvantages of Abs and Fluorescence techniques.`
fluorescence more sensitive, easier to see absorbance happens easier
Calculate the energy of a photon with a frequency of 573.8 nm. A. 3.46 x 10−37 J B. 3.80 x 10−40 J C. 3.46 x 10−28 J D. 3.80 x 10−31 J E. 3.46 x 10−19 J
E. 346 * 10^-19
parameters to adjust absorbance to
0.4 < A < 0.9
Almost all monochromators used in spectrophotometric instruments today separate light based on what principle? A. refraction B. diffraction C. index of refraction D. focusing E. light filtering
B. diffraction
When a molecule absorbs IR radiation, what type(s) of energy transition(s) occur(s)? A. electronic and vibrational transitions only B. vibrational and rotational transitions only C. rotational transitions only D. electronic, vibrational and rotational transitions E. nuclear spin transitions only F. vibrational transitions only G. electronic transitions only
B. vibrational and rotational
Which of the following types of EM radiation would be of the highest frequency? A. green light B. radio C. infrared D. blue light E. microwave
D. blue light
Which of the following processes would best be described as a radiationless process? A. absorbance B. fluorescence C. phosphorescence D. internal conversion E. chemiluminescence
D. internal conversion
When an electron excitation (n --> π*) occurs in a molecule and the spin of the excited electron flips in the process, what type of excited state results? A. So B. singlet excited state (S1) C. doublet excited state (D1) D. triplet excited state (T1) E. phosphorescence
D. triplet excited state
A triplet state is... A. the condition when a molecule has two electrons simultaneously in two different excited electronic states. B. an highly excited vibrational state of the ground electronic state C. an electronic excited state containing two electrons. D. an extremely rare situation in which three electrons share the same molecular orbital. E. an electronic excited state lower in energy than the ground singlet electronic state F. an highly excited vibrational state of the second excited electronic state. G. an electronic excited state in which the electron spin is no longer paired with the electron remaining in the ground state.
G. an electronic excited state in which the electron spin is no longer paired with the electron remaining in the ground state.
What does the acronym LASER stand for? Discuss how a LASER works? What are the advantages and disadvantages of it?
Light Amplification by Stimulated Emission of Radiation works by having a population inversion where all the electrons in an excited state drop to the ground state simultaneously and emit light advantages: high intensity, monochromatic, coherent, polarized, fluorescence disadvantages: monochromatic, expensive, high maintenance
Discuss the various types of detectors used in spectrophotometric techniques and give the specific advantages of each? Phototube Photomultiplier Tube Photodiode CCD IR detectors
Phototube: signal is the current produced when the photocathode is struck by photons Photomultiplier tube: v sensitive, emitted electrons strike a second surface called dynodes, and are accelerated Photodiode: mobile negative or positive charges in silicon, solid state component, generates when struck by hv. VERY FAST detector and sees all wavelengths simultaneously CCD: highest S/N ratio, electrons stored in pixels and moved to the next register IR detectors: thermocouple, ferroelectric, photoconductive
intersystem crossing: transition, example, radiational
T to S or S to T T1 to S0 No
T/F: fluorescence measured more often than phosphorescence
TRUE because fluorescence has a shorter lifetime meaning you could actually see things -- a lot of time could pass before you see something phosphoresce
Discuss how slit width in a spectroscopic instrument relates to sensitivity, noise, and resolution?
bigger slit width = greater sensitivity / signal, but also increases noise and decreases resolution
standard addition definition
determine an unknown concentration by adding "spikes" of known amts of same analyte
energy transitions for x-ray
electronic transitions of core electrons
emission spectrum graph
emission intensity versus wavelength of emission
excitation spectrum graph
emission intensity vs wavelength of excitation
What are the differences between the absorption and the emission spectra for a compound?
emission wavelengths, like fluorescence and phosphorescence, are longer, aka less energetic, than absorbance wavelengths look like mirror images of each other
describe triplet spin state
excited e to same spin it was before
describe singlet spin state
excited e to the opposite spin it was before
Discuss the major difference between how absorbance techniques and fluorescence techniques depend on source intensity. How do each technique depend on concentration of sample and path length.
fluorescence, emission: low concentration, intensity proportional to concentration absorbance: concentration doesn't affect bc its a ratio can increase intensity of emitted radiation in fluorescence by increasing concentration or light source increasing light source won't affect absorbance measurements in Beer's law bc ratio
when does Beer's law fail
highly concentrated solutions where there are multiple forms of a species present
energy transitions for radio waves
nuclear spin state
Why do "atomic" spectroscopic techniques show very narrow linewidths when compared with that of molecular techniques?
only e- level transitions, no rotational or vibrational energies
What are two different types of monochromators and which one is most commonly used today?
prisms: works by changing the position of it and measuring refracted light grating: works off diffraction of light, the dispersion of wavelengths upon reflection from a grooved surface grating most commonly used
What is true regarding the resolution of a monochromator?
resolution increases are you increase more grooves per mm
energy transitions for microwaves
rotational transitions
# scans signal-averaged FTR spec to increase signal by a factor of 7?
set 7 = sqrt rt N, thats your answer
What is "signal averaging" and how does it relate to FT technique?
signal averaging = averaging of spectra. want high number. increases w/ more signals. FT can have massive signal averaging bc it can do many scans in minimum time
What is the difference between a Singlet and Triplet excited state?
singlet: flips to opposite spin triplet: flips to same spin
What is a disadvantage of fluorescence techniques?
v selective wavelength
