Fundamentals of Spectrophotometry

Questions and Answers

What type of radiation is primarily responsible for stimulating vibrations in molecules?

Visible radiation

Infrared radiation (correct)

Ultraviolet radiation

Microwave radiation

What occurs to a molecule when it absorbs a photon of light?

The molecule loses its electron.

The molecule is promoted to a lower energy state.

The molecule's temperature decreases.

The molecule is promoted to a more energetic excited state. (correct)

In terms of absorbance values, new instruments exhibit the most precision at what absorbance range?

A < 0.1

A ≈ 0.1 to 0.3

A ≈ 0.3 to 2 (correct)

A > 2

What structural change occurs in formaldehyde when it is in its excited state?

The molecule remains planar but has increased vibrational energy. Signup and view all the answers

What type of spectroscopy is suitable for measuring solid samples?

Diffused reflectance spectroscopy Signup and view all the answers

What happens to the energy of a molecule when it emits a photon?

The molecule's energy decreases by an amount equal to the energy of the photon. Signup and view all the answers

Which of the following best describes the role of ultraviolet radiation in molecular interactions?

It promotes electrons to higher energy orbitals. Signup and view all the answers

What is a common potential issue when measuring serum iron levels using spectrophotometry?

Interference of Cu+ ions Signup and view all the answers

What does infrared spectroscopy primarily study?

The interaction of matter with electromagnetic radiation in the infrared region Signup and view all the answers

Which statement about the vibrational states of molecules is accurate?

Vibrational states correspond to quantized energy levels Signup and view all the answers

How does molecular orbital theory relate to absorption spectra?

Molecular orbitals determine the allowed electronic transitions that result in absorption spectra Signup and view all the answers

What is the relationship between energy transitions and the absorption of light in spectroscopy?

Light absorption corresponds to transitions between different energy levels in molecules Signup and view all the answers

Which of the following correctly describes a characteristic of absorption spectra?

They can indicate the energies required for electronic transitions Signup and view all the answers

Which property of light allows spectroscopy to differentiate between types of atoms?

The wavelength of light absorbed by different atoms Signup and view all the answers

What role do vibrational and rotational states play in infrared spectroscopy?

They represent quantized transitions that can be probed by infrared light Signup and view all the answers

The concept of a spectrum has evolved to encompass which of the following?

The entire electromagnetic spectrum Signup and view all the answers

What occurs when a molecule absorbs radiation in relation to its electronic state?

It becomes promoted to an excited electronic state. Signup and view all the answers

Why do the emission and absorption spectra roughly appear as mirror images of each other?

Emission occurs at lower energy due to molecular geometry changes. Signup and view all the answers

What is the main measurement taken during the generation of an excitation spectrum?

The emitted light at a specific wavelength. Signup and view all the answers

According to the principles of emission spectroscopy, what does doubling the incident irradiance (P0) affect?

It doubles the emission intensity up to a certain point. Signup and view all the answers

Which principle is true regarding absorbance in relation to incident irradiance?

Absorbance is a ratio of two intensities and remains constant when P0 is doubled. Signup and view all the answers

In terms of luminescence intensity, what relationship exists at low concentrations?

Emission intensity is proportional to analyte concentration. Signup and view all the answers

What kind of scattering must be accounted for in every analysis involving luminescence?

Blank sample scattering. Signup and view all the answers

Which statement is true regarding the geometry of a molecule in its ground state compared to its excited state?

Excited state geometry may differ due to electronic repulsion. Signup and view all the answers

Study Notes

Interaction of Light with Molecules

Microwave radiation causes molecular rotation upon absorption.
Infrared radiation induces vibrations in molecules.
Visible and ultraviolet radiation elevate electrons to higher energy levels.

Absorbance Measurement

Common cuvettes are used for visible and ultraviolet spectroscopy.
Flow cells enable continuous solution movement for temperature regulation.
Modern instruments provide high precision in absorbance levels between 0.3 to 2.

Light Absorption and Emission

Absorption of a photon promotes a molecule to an excited state, while emission reduces energy equivalently.
The geometry of formaldehyde changes upon excitation; it is planar in its ground state.

Absorption vs. Emission Spectra

Absorption and emission spectra are similar but do not perfectly overlap; emission occurs at slightly lower energy.
The electronic ground state of a molecule influences its behavior when absorbing radiation.

Excitation and Emission Spectra

Excitation spectrum displays emission intensity as a function of varied excitation wavelength.
The excitation spectrum resembles an absorption spectrum; high absorbance leads to increased emission.

Scattering and Luminescence

At low concentrations, emission intensity is directly proportional to the concentration of the analyte.
Light scattering must be controlled and blank samples should be analyzed to ensure accuracy.