Spectrophotometry: Absorption and Scattering

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Questions and Answers

What is the significance of the molar absorption coefficient in the context of bulk matter absorption?

The molar absorption coefficient describes the combined absorption properties of a mole of molecules, providing insights into how a substance absorbs light as a whole.

How is the absorption cross-section defined in spectrophotometry?

The absorption cross-section is defined as the ratio of the power absorbed by a molecule to the incident power per unit area.

Explain the relationship between the absorption coefficient and the probability of transitions between molecular states.

The absorption coefficient indicates the likelihood of a transition from one molecular state to another for a photon of specific frequency.

What role does the refractive index play in molecular scattering?

The refractive index affects the angle and intensity of scattered light, influencing how light interacts with the molecules in a medium. Signup and view all the answers

Why is it necessary to sum the absorption cross-sections of molecules in a mole when discussing absorption in biological media?

Summing the absorption cross-sections provides a comprehensive view of how bulk biological media absorb light, which may contain various types of absorbers. Signup and view all the answers

How does elastic scattering differ in biological tissues compared to other materials?

Elastic scattering is much more significant in biological tissues at visible and NIR wavelengths compared to other materials. Signup and view all the answers

What happens to a molecule when the frequency of light does not excite any resonance?

When the frequency does not excite resonance, the oscillating dipole radiates an electromagnetic wave, reradiating a photon in a random direction. Signup and view all the answers

Explain why light appears to travel slower in glass than in a vacuum.

Light seems to travel slower in glass because it is repeatedly scattered, resulting in a phase shift and cumulative delays. Signup and view all the answers

What is the relationship between the speed of light in a medium and its refractive index?

The speed of light in a medium is given by the formula $c/n$, where $c$ is the speed of light in a vacuum and $n$ is the refractive index. Signup and view all the answers

How do scattered waves affect the original incident wave?

Scattered waves add together to the original wave, resulting in a wave that is phase shifted from the original. Signup and view all the answers

What role does the concept of an oscillating dipole play in molecular scattering?

An oscillating dipole acts as a radiating source, emitting electromagnetic radiation when subjected to incident light. Signup and view all the answers

Why is it important to consider scattering when studying light in biological tissues?

Scattering is crucial as it affects how light interacts with biological tissues, influencing imaging and treatment techniques. Signup and view all the answers

What is the total molar absorption coefficient of a material composed of multiple chromophores?

It is the sum of the molar absorption coefficients weighted by their concentrations. Signup and view all the answers

How does the intensity of a continuous plane wave of light behave as it travels through a purely absorbing medium?

The intensity decreases in the direction of propagation as the light is absorbed. Signup and view all the answers

What does the term 'spectrophotometry' refer to?

It refers to the measurement of optical absorption spectra. Signup and view all the answers

In a mixture of chromophores, how can the concentrations of each chromophore be estimated?

By making measurements of absorbance at multiple wavelengths. Signup and view all the answers

What defines elastic scattering of photons?

In elastic scattering, energy is conserved and the scattered light has the same frequency as the incident light. Signup and view all the answers

What occurs during inelastic scattering of photons?

Energy is lost or gained, resulting in scattered light having a different frequency from the incident light. Signup and view all the answers

Why is skin often opaque in terms of optical scattering?

In biological tissue, scattering is often a dominant characteristic, preventing light from passing through easily. Signup and view all the answers

What is the significance of measuring absorbance at different wavelengths in spectroscopic analysis?

It allows for the identification and quantification of various chromophores in a sample. Signup and view all the answers

Study Notes

Absorption and Scattering

Absorption coefficient is a key factor in spectrophotometry.
The absorption cross-section defines the likelihood of a transition between states.
The absorption cross-section is the ratio of power absorbed by a molecule to the incident power per unit area.
The absorption cross-section has units of area.
Absorption coefficient is also represented as an area.
The molar absorption coefficient measures the sum of the absorption cross-sections of all the molecules in one mole of a substance.
The molar absorption coefficient relates to the Avogadro constant NA = 6.022×10⁻²³
Absorption coefficient as a probability is the probability that a photon will be absorbed while traveling a distance, represented by µₐδz.
Beer's Law: Φ = Φ₀ exp(-µₐz)
Spectrophotometry measures optical absorption spectra.
The detected light intensity can be expressed as Iₗ(λ) = I₀(λ) exp(-µₐ(λ)d)
µₐ(λ) = -1/d ln(Iₗ(λ)/I₀(λ))
α(λ) = -1/Cd ln(Iₗ(λ)/I₀(λ))
A = -log₁₀(Iₗ/I₀) = - log₁₀(exp(-aCd)) = log₁₀(e) aCd.
T = Iₗ/I₀, A = -log₁₀(T) = eCd
(Extinction Coefficient) e
Spectroscopic Inversion is a technique used to estimate concentrations of constituent by measuring multiple wavelengths.
Sample containing several known chromophores allows estimations of constituent concentrations using measurements from multiple wavelengths.
A(λ₁) = eₐ(λ₁)Cₐd + e(λ₁)C₈d + eₕ(λ₁)Cₕd
A(λ₂) = eₐ(λ₂)Cₐd + e(λ₂)C₈d + eₕ(λ₂)Cₕd
A(λ₃) = eₐ(λ₃)Cₐd + e(λ₃)C₈d + eₕ(λ₃)Cₕd

Optical Scattering

Scattering in biological tissues is often a dominant characteristic, making them opaque.
Photon scattering is divided into elastic and inelastic types.
Elastic scattering conserves energy, and the scattered light has the same frequency as the incident light.
Inelastic scattering involves energy gain or loss, resulting in scattered light with a different frequency compared to the incident light.
Elastic scattering is more substantial in most biological tissues at visible and near-infrared (NIR) wavelengths.
The frequency not exciting any resonance will cause the oscillating dipole to radiate an EM wave.
A photon with equal energy will be re-radiated in a random direction.
Light travels at speed c in a vacuum.

Refractive Index

Light travels at c/v through a medium with a refractive index.
The emergent wave moves through the medium at c/n speed.
The apparent change in speed causes refraction, bending the light as it enters a medium with a different refractive index.
Refractive indices vary spatially, resulting in microscopic and macroscopic scattering.
The key factor determining whether a refractive index fluctuation will scatter strongly is the relationship between the scatterer's size and the light's wavelength.
The reduced scattering coefficient incorporates scattering coefficient and anisotropy: µs' = µs(1 - g) [cm⁻¹]
The mean free path is the average distance a moving particle (like an atom, molecule, or photon) travels between impacts.
The purpose of µs' is to describe photon diffusion in a random walk with a step size of 1/µs' [cm], where each step incorporates isotropic scattering.
g = <cos θ>

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Description

Explore the principles of absorption and scattering in spectrophotometry. This quiz covers important concepts such as the absorption coefficient, Beer's Law, and the relationship between molar absorption coefficients and Avogadro's constant. Test your knowledge of how light interacts with matter and the mathematical expressions involved.