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analytical > Optical > Flashcards

Optical Flashcards

1
Q

Whare are chromophores?

A

Functional groups which give rise to observed transitions

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2
Q

UV-VIS

Why does spectrum broadening happen?

A
  • large number of overlapping vibronic transitions
  • transitions broadened by collisions with solvent
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3
Q

Beer Lambert Law

What are the fundamental limitations?

3 points

A
  1. Invalid at high concentrations
  2. Refractive index depends on concentration
  3. Extinction coefficient depends on refractive index
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4
Q

Why is Beer-Lamberts law invalid at high concentrations?

A

Aggregation of molecules affects extinction coefficicient

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5
Q

Beer-Lambert Law

What type of deviation does high sample concentration lead to?

A

Positive

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6
Q

Beer-Lambert Law

What are instrumental limitations?

2 points

A
  1. Stray radiation reaching detector without passing through sample
  2. Polychromatic radiation, as BL assumes monochromatic radiation
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7
Q

Beer-Lambert Law

What type of deviation does stray radiation lead to?

A

Negative

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8
Q

Define bandwidth

A

The full-width of the radiation at half of its maximum intensity

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9
Q

Beer-Lambert Law

Why do you measure absorbance at the top of the absorbance peak

A

BL assumes monochromatic radiation, spectrometer has finite bandwidth. Effect is minimised if the value of epsilon is constant over bandwidth (top of the abs peak)

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10
Q

Describe the procedure for quantitative analysis using Beer-Lamber Law

4 points

A
  1. Measure absorbance and apply BL law
  2. Construct a calibration curve
  3. Linear regression of absorbance versus concentration to give calibration equation
  4. Substitute sample’s absorbance into calibration equation to give analyte concentration
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11
Q

How is qualitative analysis carried out using UV-VIS?

A
  1. Use characteristic absorption spectrum of chromophore to identify compound
  2. Compare to the library of digital compounds
  3. Use of derivative spectra can improve discrimination between similar spectra
  4. UV-vis spectra tend to be broad and unstructured – not “fingerprints”
  5. Use PCA for multi-component mixtures
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12
Q

Define Luminescence

A

Emission of light by electronically excited molecules

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13
Q

Define Fluorescence

A

Molecule excited by absorption of light

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14
Q

Compare and contrast Fluorescence and Phosphorescence

A

F: spin allowed and very short lived
P: Spin forbiddena and long lived

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15
Q

What is a singlet electronic state?

A

When all the electron spins are paired in the molecular electronic state and the electronic energy levels do not split when the molecule is exposed into a magnetic field

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16
Q

What is a triplet electronic state?

A

Two electrons in different molecular orbitals have parallel spins

17
Q

Fluorescence

What is vibrational relaxation?

A
  • Collision with solvent molecules -rapid loss of vibrational energy to solvent as heat.
  • Moving down one energy level within the same state
18
Q

Fluorescence

What is Excitation (absorption)?

A

Promotion of electrons into S1/2 etc into the same energy level

19
Q

Fluorescence

What is fluorescence?

A

Transition from lowest vibrational level(s) of S1 state to S0

20
Q

Fluoresence

What is internal conversion?

A

Conversion of electronicenergy to vibrational energy S1 -> S1

21
Q

What is Kasha’s Rule?

A

Fluorescence always from lowest vibrational levels of S1 state to any level of S0

22
Q

Fluorescnce

What is intersystem crossing?

A

Transition between states of different multiplicity (S1 to T1/ T1 to S0)

23
Q

Fluorescence

Fate of radiactive decay

3 points

A
  1. Radiative
  2. Non radiative
  3. Intramoleculay decay
24
Q

Fluorescence

What is radiative decay?

3 points

A
  1. Energy lost by emission of a photon
  2. Fluorescence S1 to S0 spin allowed
  3. Phosphorescence T1 to S0 spin forbidden
25
Q

What is on the axes of Stern-Volmer plot?

A

F0/F vs [Q]

26
Q

What is the gradient and intercept of Stern-Volmer plot?

A

grad: K_SV (Stern-Volmer constant)
intercept: 1

27
Q

Fluorescence

What is the innter filter effect?

2 points

A
  1. If concentration is too high maximum fluorescnce happens in the middle of the sample
  2. This results in a distorted spectrum, as intensity in the middle of cuvette is reduced.
28
Q

Fluorescence

How can the inner effect be prevented?

A

Absorbance should be < 0.1 at excitation wavelength

29
Q

Fluorescence

What is spectrum reabsorption?

3 points

A
  • If absorption and emission spectra overlap, and concentration is too high fluorescence is reabsorbed as it passes through solution
  • Fluorescence intensity is reduced
  • Emission spectrum distorted
30
Q

What is a fluorophore?

A

Fluorescent molecule or group

31
Q
  • What groups do flurophores contain?
  • Which transition does it allow?
A
  1. Aromatic groups
  2. pi to pi*
32
Q

List advantages of fluorescence over UV-VIS

5 points

A
  • High detection sensitivity
  • High sensitivity
  • Use as probe
  • Sensitive to molecular environment
  • Sensitive to molecular interactions
33
Q

Which 3 characteristic parameters are responsible for high sensitivity of fluorescence over UV-VIS

A
  1. Excitation wavelength
  2. Emission wavelength
  3. Lifetime
34
Q

List disadvantages of fluorescence over UV-VIS

5 points

A
  • Intensity of fluorescence is not a direct measure of concentration
  • Intensity depends on experimental & instrumental factors
  • Intensity depends on quantum yield
  • Requires careful production of calibration curves
  • Ratiometric measurement advantageous
35
Q

What are the experimental and instramental factors that affect fluorescent intensity?

4 points

A
  • Intensity of excitation source
  • Efficiency of collection of fluorescence
  • Inner filter effect
  • Reabsorption
36
Q

Fluorescence

What is quantum yield affected by?

2 points

A
  1. Sample conditions (pH, temp, quenching)
  2. Intermolecular interactions
37
Q

IR

How does a FTIR work?

4 points

A
  1. Transmittance is measured (like UV-VIS)
  2. An interferometer is used instead of monochromator
  3. All IR frequencies incident on detector simultaneously
  4. Fast spectrum acquisition
38
Q

IR

How does an interferometer work?

A
  • Interferometer produced signal which has all the infrared frequencies encoded into it
  • Fourier transform converts intensity versus time to intensity versus frequency

analytical (5 decks)

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