Espectrometria de absorção molecular - Parte 1: Fundamentos

Summary

In this lecture, Cassiana, a researcher in analytical chemistry at the University of São Paulo, introduces molecular absorption in the ultraviolet-visible region. She explains the Beer-Lambert law, which correlates light absorption with analyte concentration, enabling quantitative analysis. Cassiana discusses absorbance, transmittance, and how they are related to the amount of analyte. She covers the factors influencing absorption, including concentration, molecular nature, and optical path length. The lecture also explores absorption spectra and energy transitions in molecules, helping students understand why absorption profiles appear as bands rather than lines. The session concludes with an invitation to follow future lectures and updates on research.

Takeaways

• 😀 The focus of the lesson is on molecular absorption of radiation in the ultraviolet-visible region, particularly explaining the Beer-Lambert Law.
• 😀 The Beer-Lambert Law is an equation that correlates radiation absorption with the concentration of an analyte, making quantitative analysis possible.
• 😀 Molecular absorption refers to the measurement of electromagnetic radiation transmittance or absorbance by molecules in solutions.
• 😀 When white light (containing all visible wavelengths) passes through a permanganate solution, it absorbs light in the green region (530 nm) and transmits violet light.
• 😀 Transmittance is the fraction of incident light that passes through a solution, and it varies between 0 (when all light is absorbed) and 1 (when no light is absorbed).
• 😀 Absorbance is the log-transformed inverse of transmittance and provides a more direct measure of absorption, allowing easier analysis of the relationship between concentration and absorption.
• 😀 The probability of photon absorption depends on the concentration, chemical nature of the species, and instrumental parameters such as optical path length.
• 😀 The Beer-Lambert Law states that absorbance is proportional to the molar absorptivity, path length, and analyte concentration.
• 😀 Molar absorptivity varies with the wavelength of light, so the absorbance of a solution also depends on the specific wavelength used in the measurement.
• 😀 Absorption spectra show peaks at specific wavelengths where substances absorb light, which can help identify the optimal wavelengths for analysis of different substances.
• 😀 The absorption profile is not a single line but a band, due to the various transitions that occur in the molecule when it absorbs radiation, including rotational, vibrational, and electronic transitions.

Q & A

What is molecular absorption?

Molecular absorption refers to the measurement of the absorbance or transmittance of electromagnetic radiation in the ultraviolet-visible region by molecules in solutions. The absorbed radiation is used to analyze the concentration of the analyte.

What is the Beer-Lambert Law and what does it describe?

The Beer-Lambert Law describes the relationship between the absorption of light by a substance and the concentration of the substance in a solution. It states that absorbance is directly proportional to the concentration of the analyte, the path length of the light, and the molar absorptivity.

What factors affect the absorption of a photon?

The absorption of a photon depends on factors such as the concentration of the analyte, the chemical nature of the substance, and instrumental parameters such as the optical path length.

What is absorbance, and how is it calculated?

Absorbance is a measure of how much light is absorbed by a solution. It is calculated using the formula: Absorbance = -log10(Transmitance), where transmitance is the ratio of transmitted radiation to incident radiation.

Why is absorbance considered a better parameter than transmittance?

Absorbance is a more useful parameter than transmittance because it is directly related to the concentration of the analyte. Transmittance is inversely proportional to the amount of substance, making it less straightforward to interpret.

What is the relationship between concentration and absorbance?

The concentration of an analyte is directly proportional to the absorbance, as described by the Beer-Lambert Law. As the concentration increases, absorbance increases, allowing quantitative analysis.

How does the molar absorptivity relate to absorption?

Molar absorptivity, or molar absorption coefficient, is a constant that represents the ability of a substance to absorb light at a particular wavelength. It varies with the wavelength of radiation, and its product with the path length and concentration gives the absorbance.

What is a spectrum of absorption?

A spectrum of absorption shows the wavelengths of light absorbed by a substance. It typically appears as a series of absorption peaks or bands, reflecting the energy transitions within the molecules, such as electronic, vibrational, and rotational transitions.

Why do absorption bands appear instead of sharp lines?

Absorption bands appear instead of sharp lines because energy transitions in molecules involve not only electronic changes but also rotational and vibrational transitions. These additional transitions create a range of energies, resulting in broader absorption features.

How does the path length affect the absorption process?

The optical path length is directly proportional to the amount of light absorbed. A longer path length allows more interaction between the light and the analyte, leading to higher absorption.