Spectroscopy
Explain the differences between atomic spectra and molecular spectra.
Atomic - determines the type of atom in the sample - line spectra - electronic transition in an element Molecular - determines structure of molecules - vibrational, rotational, & electronic transition in a molecule
What is the dual nature of light?
Light behaves a wave w/frequency (interference, diffraction, & reflection) and a particle (discrete energy that can be absorbed & emitted)
What is the difference between continuum and line sources?
Line sources emit a narrow band of radiation with high selectivity and sensitivity and reduce spectral interference Continuum sources are typically used for background correction, to eliminate the matrix.
If the color orange is absorbed, what is observed?
blue
If the color red-orange is absorbed, what is observed?
blue-green
What is the transitions of x-rays?
bond-breaking & electronic transition of core e-
How does interference filters work?
filters operate by transmitting a selected wavelength region with high efficiency while rejecting, through reflection and destructive interference, all other wavelengths.
What is the order of the EMS regions?
gamma rays, x-rays, UV, visible, infrared, microwave, radio
What kind of cuvette material is used in UV-vis spec?
glass, cyclic olefin copolymer, and polystyrene & methacrylate
If the color red is absorbed, what is observed?
green
If the color violet is absorbed, what is observed?
green-yellow
vibrational relaxation
is the process by which a molecule loses its excess vibrational energy without emitting radiation
What are the disadvantages to using a photomultiplier tube?
stray light is also amplified and multiple wavelengths/entire spectrum cannot be measured
What are two types of line sources?
low-pressure mercury lamp emission and fluorescent lamp.
The longer the wavelength...
lower energy
The promotion of nonbonding e- to antibonding pi orbital
lowest energy electronic transition
Explain the excited state .
molecule absorbs photon (increase in energy)
Explain the ground state.
molecule emits photon (decrease in energy)
what is internal conversion?
molecule in S1 lvl could enter highly excited vibrational lvl of S0 w/same energy as S1. Then, molecule can relax back to ground state & transfer energy to other molecules through collisions. (Same spin multiplicity)
In fluorescence spectroscopy, what is the function of the emission monochromator?
monitor luminescence by minimizing the intensity of excitation light reaching the detector and select one wavelength at a time to observe
What do we mean by a photon of light of a given energy?
photon energy is the energy carried by a single photon
Lowest energy orbital is the...
pi antibonding orbital (pi)
What is the chromophore?
the part of the molecule responsible for light absorption
How does a photomultiplier detect light?
uses a series of dynodes to achieve gain
What is intersystem crossing?
where molecule crosses from S1 to excited vibrational level T1
What is the range of transmittance?
0 to 1
A charge coupled device analyzes region from ______ to ______ nm using ______ array of ____________________ ___________.
400 - 1050 nm; 2-D; photosensitive pixels.
What are the disadvantages to using a single beam spectrophotometer?
- The sample and reference must be placed alternatively in the beam. - have to run reference at each wavelength to measure multiple wavelengths - can't measure absorbance as a function of time because intensity and detector response tend to slowly drift
What are the advantages to using a double beam spectrophotometer?
-Po and P are frequently & automatically compared. -Provides automatic correction for changes of source intensity and detector response w/time and wavelength.
What are the disadvantages to using a double beam spectrophotometer?
-higher cost -lower sensitivity because throughput of light is poorer (complex optics) - low reliability bc of its great complexity
What are the conditions where Beer's law applies?
1) monochromatic radiation 2) solutions is not too concentrated 3) homo solution 4)absorbing species not participating in concentration dependent equilibrium
What are the two types of electronic states (hint: spins)
1) singlet 2) Triplet
What is the equation of a wavenumber?
1/wavelength (units: cm^-1)
When given an absorbance value for a blank sample cuvette and the absorbance of the actual sample what must you do to get the right absorbance?
A corrected = A(sample) - A(blank)
What can happen if an atomic species absorbs certain EM radiation as compared to Molecular species?
Atomic species absorb can go to an excited state and jump back to lower level radiating energy When molecular species absorb EM radiation it provides transition between rotational levels
what are the types of luminescence?
Bio, Chemi, Fluoro, Phosphoro
What type of cuvette material is used in UV?
Cyclic olefin copolymer
Order the detectors from least to most sensitive.
DAD, CCD, PMT
In a UV-Vis absorbance experiment, transmittance (T) is defined as P/Po (or Pout/Pin). Thinking about this and the block diagram below, why is it a problem if the absorbance is too low?
Difficult to distinguish b/w P and Po. This is because the irradiance going into the cell (Po) does not decrease because it is not being absorbed, so the irradiance detected (P) will be close to Po.
Increasing intensity does or does not change absorbance?
Does not
What is the transitions of UV-Vis rays?
Electronic transitions of valence, vibrational & rotational
Why is emitted light lower in energy that absorbed light?
Emitted light must undergo vibrational relaxation which causes energy to be dispersed as KE through vibrations.
T of F; vibrational relaxation decreases temp.
False
T or F; In absorbance measurements, you want scattering to occur.
False
Explain what occurs in the refractive index.
Frequency stays the same but wavelength changes. It is greater than 1 for most visible light because it travels slower through matter than water, glass, etc.
What regions on the EM spectrum use a xenon arc lamp? What electrical discharge does it have?
For fluorescence and has an electrical discharge of Xe gas
What are the trends in the EMS?
From L to R, frequency & energy decreases but wavelength increases
What type spectroscopy is used for IR?
IR spec
What is one error that has to do with the cuvette surface that can cause inaccuracies?
If the cuvette is reflective, contains scratches or smudges
What are inner filter effects?
In highly concentrated solutions the excitation beam is reduced causing the surface of the sample facing the excitation beam to fluoresces strongly. Whereas the center experiences low fluorescence that affects signal detected. Intensity of fluorescence decreases as concentration increases.
Explain the charge used in a charged coupled device.
It is a photogenerated charde with a buildup proportional to intensity of radiation.
Why is fluorescence more sensitive than absorbance?
It is measured against a black continuum that allows emitted light to be seen clearly.
What are the advantages of using a single beam spectrophotometer?
Its is cost-effective and has a better performance because of its high energy throughput since the beam isn't split (high sensitivity of detection)
What is luminescence?
Materials absorb energy and emit visible light
What is the purpose of the reagent blank?
Measure the absorbance of the solution matrix without the analyte present
what is the purpose of the solvent blank?
Measure the absorbance of the solvents in the solution without the analyte present.
What is the difference between monochromatic and polychromatic light?
Monochromatic light is light composed of a single component, but polychromatic light consists of multiple colors at diff wavelengths & frequencies.
To what extent does a photomultiplier tube amplify a photon?
No charge build up. 10^6 electrons
Which is more sensitive to light, PMT or DAD?
PMT
What are the two types of blanks used?
Reagent and Solvent
what is the difference between refraction and diffraction?
Refraction is the change in direction of waves that occurs when waves travel from one medium to another. Diffraction is the bending of waves around obstacles and openings.
Describe Fluorescence.
S1 --> S0 + hv (gives off light) shorter wavelength, thus more energetic (10^-9 - 10^-4)
When using the Internal standard, the identity of the standard is:
Same or different relative to the analyte
Why is a 90 degree cell geometry used in the fluorescence spectroscopy?
Since it has a high sensitivity, one has to minimize the intensity of excitation light reaching the detector.
Describe phosphorescence.
T1 --> S0 + hv longer wavelength, less energetic (10^-4 - 10^2)
what is transmittance?
The amount of light that passes through the sample and comes out the other side.
If a blue cupric solution halves the amount of red laser light reaching a solar cell ( a light sensitive detector), what will happen if a second identical solution is added?
The detector will read 1/4 of the original value
How do you identify inner filter effects?
The excitation intensity on a graph is not linearly consistent and the part of the molecule reabsorbs (self-absorbs)
When using the standard addition method, the identity of the standard is:
The same as the analyte
What regions on the EM spectrum use a deuterium arc lamp? What electrical discharge does it have?
UV radiation, electrical discharge of D2
What regions does a photomultiplier tube detect?
UV-Vis Region
What type spectroscopy is used for UV-Vis?
UV-Vis spec
What regions does a diode array detector detect? How does it do this?
UV-Vis to near IR. Does it by a linear array that consists of adjacent photosensitive diodes.
What regions on the EM spectrum use a tungsten light bulb?
UV-Vis, Infrared, and UV
What is a CCD typically used for? What does it measure?
Used for spectroscopy and for imaging purposes. Measures multiple wavelengths at one and very fast. *low noise*
Explain the electric charge aspect of a diode array detector.
Uses a photogenerated charge, where charge buildup is proportional to intensity of radiation
What is the order of colors in the visible spectrum?
VBGYOR
When is a quartz-tungsten halogen lamp used?
When measuring visible light.
Instrumental consideration: What happens in the absorbance is too high in a spectrophotometer?
barely any light will reach the detector, which makes it difficult to perform an analysis.
What are the 2 equations that represent the duality of light?
c = (wavelength)v E = hv
Light traveling outside of a vacuum is represented by what equation
c/n
as absorbance increases transmittance ...
decreases
How do you fix measurements outside linear region?
dilute the sample to bring reading w/in linear region
Which detectors is commonly used in chromatography? which ones?
diode array detector in CE and HPLC
UV-Vis transition states
electronic, vibrational, and rotational
What two radiative processes occur in the jablonski diagram?
fluorescence and phosphorescence
What is molar absorptivity?
how strongly a species absorbs a particular wavelength of radiation being shined on it.
A globar lamp is typically used for the ________ region.
infrared
What type spectroscopy is used for microwave?
microwave spec
What does vibrational relaxation, internal conversion and intersystem crossing have in common?
nonradiative processes
what is the transition of radio?
nuclear and electronic spin
What type spectroscopy is used for radio?
nuclear magnetic resonance spec (NMR)
What is the spin of singlet states?
opposed spins
If the color blue is absorbed, what is observed?
orange
What is the spin of the triplet state?
parallel spins
If the color green is absorbed, what is observed?
purple-red
If the color blue-green is absorbed, what is observed?
red
Microwave transition state
rotational
What is the transitions of microwave?
rotational
In fluorescence spectroscopy, what is the function of the excitation monochromator?
select excitation wavelength and direct it towards the sample
The highest energy occupied orbital is ...
the nonbonding orbital (n)
What is the transitions of infrared rays?
vibrational & rotational
Infrared transition states
vibrational and rotational
If the color yellow-green is absorbed, what is observed?
violet
If the color yellow is absorbed, what is observed?
violet-blue
What type spectroscopy is used for x-ray?
x-ray photoelectron spectroscopy
If the color violet-blue is absorbed, what is observed?
yellow
