INSTRUMENTAL ANALYSIS ACS LATEST EXAM TEST
QUESTIONS AND ANSWERS SURE A+
✔✔Fluorescence: Stokes Shift - ✔✔The emitted photon has lower energy (longer
wavelength) than the absorbed photon due to vibrational relaxation in the excited state.
✔✔Resonance Fluorescence - ✔✔Resonance Fluorescence: Emitted photon has the
same wavelength as the absorbed photon.
✔✔Fluorescence Spectrum: - ✔✔Change in energy during
fluorescence emission is less
than that for absorption;
therefore emission is at a
higher wavelength than
absorption
• Because fluorescence may
return a molecule to several
vibrational energy levels,
emission is over a range of
wavelengths
✔✔Fluorescence: Quantum Yield - ✔✔-Quantum Yield: the fraction of excited state
molecules that return to ground state by fluorescence.
• Range from 0-1
• The fluorescence intensity is proportional to the radiation absorbed
- it is defined as the quantitative value of fluorescence efficiencies
✔✔Phosphorescence - ✔✔Mechanism: Emission of a photon from an excited triplet
state to a singlet ground state (spin-forbidden transition involving intersystem crossing).
Timescale: Longer-lived emission compared to fluorescence, ranging from milliseconds
to seconds or even longer.
(from c to a)
✔✔Phosphorescence: Intersystem Crossing (ISC) and Vibrational Relaxation -
✔✔Intersystem Crossing (ISC): A non-radiative transition from an excited singlet state
to a triplet state.
Vibrational Relaxation: Loss of excess vibrational energy within an electronic state.
✔✔. Factors Affecting Luminescence - ✔✔-Molecular Structure: Rigidity, presence of
conjugated systems, and aromaticity can enhance fluorescence. Heavy atoms can
enhance intersystem crossing and phosphorescence.
- Concentration: At high concentrations, self-absorption and quenching can occur,
reducing luminescence intensity.
- Temperature: Higher temps decrease luminescence intensity due to increased non-
radiative decay pathways (collisional quenching).
, - Solvent Effects: Viscosity, polarity, and the presence of heavy atoms in the solvent can
influence luminescence.
- pH: The luminescence of some molecules is pH-dependent.
- Quenching: Processes that decrease the fluorescence quantum yield (e.g., collisional
quenching, energy transfer).
✔✔Instrumentation for Molecular Luminescence: Excitation Source - ✔✔- Typically a
lamp (e.g., xenon arc lamp) or a laser that provides the necessary wavelengths to excite
the sample.
✔✔Variables that Affect Fluorescence - ✔✔Quantum Yield
Temperature effects QY; as the temperature increases; QY
decreases due to collisions between molecules
• Viscous solvents also effect QY due to collision of molecules
• pH can affect fluorescence
✔✔Variables that Affect Phosphorescence - ✔✔Because of the longer lifetime of
phosphorescence the quantum
yield is very small
• To increase QY you may lower temperature, use a more viscous
solvent or put sample on a solid substrate
✔✔Why use Fluorescence or
Phosphorescence? p1 (scale, accuracy, precision) - ✔✔They're used because they can
detect very small amounts and are more selective than regular absorption.
1. Scale (Detection ability)
If the molecule glows really well (high quantum yield), you can detect EXTREMELY tiny
amounts.
👉 Think: way more sensitive than UV-Vis.
- Results are usually pretty close to the true value (1-5% error). Phosphorescence can
be slightly more accurate.
-Fluorescence measurements are very consistent (low variation). Phosphorescence is a
bit less consistent.
✔✔Why use Fluorescence or
Phosphorescence? p2 (selectivity, sensitivity - ✔✔- Not every molecule glows → so
there's less interference.
👉 You can also choose:
what wavelength excites it
what wavelength you detect
So you can avoid other compounds messing up your result.
-in Fluorescence You get a stronger signal because:
High quantum yield → more light emitted
Pick best excitation wavelength → absorbs more energy
Measure more emitted light → bigger signal (up to 5-30× better)
QUESTIONS AND ANSWERS SURE A+
✔✔Fluorescence: Stokes Shift - ✔✔The emitted photon has lower energy (longer
wavelength) than the absorbed photon due to vibrational relaxation in the excited state.
✔✔Resonance Fluorescence - ✔✔Resonance Fluorescence: Emitted photon has the
same wavelength as the absorbed photon.
✔✔Fluorescence Spectrum: - ✔✔Change in energy during
fluorescence emission is less
than that for absorption;
therefore emission is at a
higher wavelength than
absorption
• Because fluorescence may
return a molecule to several
vibrational energy levels,
emission is over a range of
wavelengths
✔✔Fluorescence: Quantum Yield - ✔✔-Quantum Yield: the fraction of excited state
molecules that return to ground state by fluorescence.
• Range from 0-1
• The fluorescence intensity is proportional to the radiation absorbed
- it is defined as the quantitative value of fluorescence efficiencies
✔✔Phosphorescence - ✔✔Mechanism: Emission of a photon from an excited triplet
state to a singlet ground state (spin-forbidden transition involving intersystem crossing).
Timescale: Longer-lived emission compared to fluorescence, ranging from milliseconds
to seconds or even longer.
(from c to a)
✔✔Phosphorescence: Intersystem Crossing (ISC) and Vibrational Relaxation -
✔✔Intersystem Crossing (ISC): A non-radiative transition from an excited singlet state
to a triplet state.
Vibrational Relaxation: Loss of excess vibrational energy within an electronic state.
✔✔. Factors Affecting Luminescence - ✔✔-Molecular Structure: Rigidity, presence of
conjugated systems, and aromaticity can enhance fluorescence. Heavy atoms can
enhance intersystem crossing and phosphorescence.
- Concentration: At high concentrations, self-absorption and quenching can occur,
reducing luminescence intensity.
- Temperature: Higher temps decrease luminescence intensity due to increased non-
radiative decay pathways (collisional quenching).
, - Solvent Effects: Viscosity, polarity, and the presence of heavy atoms in the solvent can
influence luminescence.
- pH: The luminescence of some molecules is pH-dependent.
- Quenching: Processes that decrease the fluorescence quantum yield (e.g., collisional
quenching, energy transfer).
✔✔Instrumentation for Molecular Luminescence: Excitation Source - ✔✔- Typically a
lamp (e.g., xenon arc lamp) or a laser that provides the necessary wavelengths to excite
the sample.
✔✔Variables that Affect Fluorescence - ✔✔Quantum Yield
Temperature effects QY; as the temperature increases; QY
decreases due to collisions between molecules
• Viscous solvents also effect QY due to collision of molecules
• pH can affect fluorescence
✔✔Variables that Affect Phosphorescence - ✔✔Because of the longer lifetime of
phosphorescence the quantum
yield is very small
• To increase QY you may lower temperature, use a more viscous
solvent or put sample on a solid substrate
✔✔Why use Fluorescence or
Phosphorescence? p1 (scale, accuracy, precision) - ✔✔They're used because they can
detect very small amounts and are more selective than regular absorption.
1. Scale (Detection ability)
If the molecule glows really well (high quantum yield), you can detect EXTREMELY tiny
amounts.
👉 Think: way more sensitive than UV-Vis.
- Results are usually pretty close to the true value (1-5% error). Phosphorescence can
be slightly more accurate.
-Fluorescence measurements are very consistent (low variation). Phosphorescence is a
bit less consistent.
✔✔Why use Fluorescence or
Phosphorescence? p2 (selectivity, sensitivity - ✔✔- Not every molecule glows → so
there's less interference.
👉 You can also choose:
what wavelength excites it
what wavelength you detect
So you can avoid other compounds messing up your result.
-in Fluorescence You get a stronger signal because:
High quantum yield → more light emitted
Pick best excitation wavelength → absorbs more energy
Measure more emitted light → bigger signal (up to 5-30× better)
