Beer's Law and Absorbance

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Questions and Answers

What is absorbance often denoted as?

Concentration of a solution

Optical density (correct)

Molar absorptivity

Radiant power

What is the relationship between the concentration of a solution and the amount of light it absorbs according to Beer's Law?

Indirectly proportional to the concentration

Directly proportional to the square of the concentration

Directly proportional to the concentration (correct)

Inversely proportional to the concentration

What is the unit of absorptivity when the concentration is expressed in moles per liter and the cell length is in cm?

L mol-1 cm-1 (correct)

g L-1 cm-1

mol L-1 cm-1

L g-1 cm-1

What is one of the limitations of Beer's Law?

Path length is not constant Signup and view all the answers

What type of radiation is required for Beer's Law to be observed?

Monochromatic radiation Signup and view all the answers

What is one of the main components of optical instruments?

Sample containers Signup and view all the answers

What is the equation that relates absorbance to concentration and path length according to Beer's Law?

A = εbc Signup and view all the answers

What is the proportionality constant in Beer's Law?

a Signup and view all the answers

What is the primary characteristic of an arc?

High current and stable discharge Signup and view all the answers

What is the typical application of Glow Discharge Emission Spectroscopy (GD-ES)?

Depth profiling of solid samples Signup and view all the answers

What is the typical pressure range for a glow discharge?

1 to 10 torr Signup and view all the answers

What is the process by which neutral sample atoms are ejected in a glow discharge?

Sputtering Signup and view all the answers

What is the typical voltage range for a glow discharge?

100 to 1000 V Signup and view all the answers

What is the main purpose of atomic emission spectroscopy?

To measure the elemental composition of a sample Signup and view all the answers

What is the purpose of modulating the output of the source in atomic absorption instruments?

To eliminate the effect of flame emission Signup and view all the answers

What is the principle behind the operation of a hollow cathode lamp?

Electrical discharge through an inert gas Signup and view all the answers

What is the primary composition of the atomic vapor produced in a glow discharge?

A mixture of atoms and ions Signup and view all the answers

In FES, what is the source of heat that excites the atoms?

Flame's heat Signup and view all the answers

What is the function of the ICP in ICP-OES?

To excite the atoms Signup and view all the answers

What is the primary function of a glow discharge device?

Both sample introduction and sample atomization simultaneously Signup and view all the answers

What is the purpose of the monochromator in atomic absorption instruments?

To remove much of the emitted radiation from the flame Signup and view all the answers

What is the characteristic of the light emitted by the excited atoms in a hollow cathode lamp?

It has a specific wavelength characteristic of the element present in the lamp Signup and view all the answers

What is the term for the instrumental setup used to disperse the emitted light in ICP-OES?

Czerney-Turner Grating Signup and view all the answers

What is the typical gas used in a glow discharge?

Argon Signup and view all the answers

What is the role of the inert gas in a hollow cathode lamp?

It carries the electrical discharge Signup and view all the answers

What is the purpose of the high-voltage electrical discharge in arc spark emission spectroscopy?

To excite the atoms Signup and view all the answers

What is the difference between 'arc' and 'spark' in arc spark emission spectroscopy?

Arc and spark are often used interchangeably, but they refer to different phenomena Signup and view all the answers

What is the function of the detector in atomic absorption instruments?

It converts the radiation from the source into an electrical response Signup and view all the answers

Which technique uses an inductively coupled plasma as the excitation source?

Inductively Coupled Plasma Emission Spectroscopy Signup and view all the answers

What is the purpose of the cathode in a hollow cathode lamp?

It is made of the metal element to be analyzed Signup and view all the answers

What is the common feature among all atomic emission spectroscopy techniques?

Emittance of light at characteristic wavelengths Signup and view all the answers

What is the effect of relaxation of the excited species in atomic absorption spectrophotometry?

It produces a low-intensity glow Signup and view all the answers

What is the primary function of the burner in an atomic absorption spectrophotometer?

To convert the sample solution into a fine aerosol of vaporized atoms Signup and view all the answers

What type of mechanisms are typically used in a nebulizer to break the liquid into small droplets?

Pneumatic and ultrasonic mechanisms Signup and view all the answers

What is the purpose of the mixing chamber in an atomic absorption spectrophotometer?

To mix the nebulized sample with a fuel gas and an oxidant gas Signup and view all the answers

What is critical for achieving optimal atomization and signal intensity in an atomic absorption spectrophotometer?

The operation of the burner, including the flow rates of the sample solution, fuel gas, and oxidant gas Signup and view all the answers

What is the result of proper optimization of the burner operation in an atomic absorption spectrophotometer?

Efficient atomization of the sample and enhanced sensitivity and accuracy of the analysis Signup and view all the answers

What is the purpose of the atomization process in an atomic absorption spectrophotometer?

To enable the absorption of light by the analyte atoms in the gas phase Signup and view all the answers

Study Notes

Absorbance and Beer's Law

Absorbance (A) is a logarithmic measure of the ratio of the radiant power of incident light to the radiant power of light transmitted through a sample.
Beer's Law, also known as the Beer-Lambert Law, describes the relationship between the concentration of a solution and the amount of light it absorbs.
Absorption is directly proportional to the path length through the medium and the concentration of the absorbing species: A = abc
a is a proportionality constant, with units dependent on the units of b (path length) and c (concentration).
When concentration is expressed in moles per liter and cell length is in cm, absorptivity is called molar absorptivity (ε): A = εbc
ε has units of L mol-1 cm-1
Beer's Law applies to mediums containing more than one kind of absorbing substance.

Limitations of Beer's Law

Real limitations: assumes a linear relationship between absorbance and concentration over a limited range of concentrations.
Chemical deviations: analyte dissociates, associates, or reacts with solvent to produce a product with a different absorption spectrum.
Instrumental deviations: requires truly monochromatic radiation.
Path length: assumes a constant path length for light passing through the sample.
Temperature and pressure effects.
Non-homogeneous samples.

Main Components of Optical Instruments

Sources of radiation
Wavelength selectors
Sample containers
Radiation transducers
Signal processors and readouts

Atomic Emission Spectroscopy (AES)

Analyzes elemental composition of a sample by measuring characteristic wavelengths of light emitted by atoms when excited.
Main types:
- Flame Emission Spectroscopy (FES)
- Inductively Coupled Plasma Emission Spectroscopy (ICP-OES)
- Arc Spark Emission Spectroscopy
- Glow Discharge Emission Spectroscopy (GD-ES)

Flame Emission Spectroscopy (FES)

Sample is introduced into a flame where it is vaporized and atomized.
Atoms in the vaporized sample are excited by the flame's heat.
Excited atoms return to their ground state, emitting light at characteristic wavelengths.
Emitted light is analyzed to determine elemental composition of the sample.

Inductively Coupled Plasma Emission Spectroscopy (ICP-OES)

Sample is introduced into an inductively coupled plasma (ICP) where it is vaporized and atomized at high temperatures.
High temperature of the plasma excites the atoms, causing them to emit light at characteristic wavelengths.
Emitted light is dispersed by a spectrometer and detected to identify elemental composition of the sample.

Arc Spark Emission Spectroscopy

Generates a high-voltage electrical discharge (arc) between two electrodes, one made of the sample material.
Intense heat of the arc vaporizes and excites the atoms in the sample.
Excited atoms return to their ground state, emitting light at characteristic wavelengths.
Emitted light is analyzed to identify elements present in the sample.

Glow Discharge Emission Spectroscopy (GD-ES)

High voltage is applied to a sample in a low-pressure gas atmosphere, creating a glow discharge.
Glow discharge excites the atoms in the sample, causing them to emit light at characteristic wavelengths.
Emitted light is analyzed to determine elemental composition of the sample.
GD-ES is often used for depth profiling of solid samples.

Atomic Absorption Spectrophotometry

Involves elimination of interferences caused by emission of radiation by the flame using a modulated power source.
Hollow cathode lamp (HCL) is a type of light source commonly used in AAS, operating based on the principle of electrical discharge through a gas.
Burner plays a crucial role in atomizing the sample and generating a vapor phase of the analyte element for analysis.
Nebulizer generates a fine spray or aerosol of the sample solution, which is mixed with fuel gas and oxidant gas in the mixing chamber.

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Description

This quiz covers the principles of Beer's Law and absorbance, including their applications in spectroscopy and the relationship between concentration and light absorption.