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Atomic Absorption, Lecture notes of Chemistry

Chemistry

Transition involves promoting an electron from a ground state to a higher empty atomic state orbital, this state is referred to as the excited state.

Typology: Lecture notes

2022/2023

Uploaded on 03/01/2023

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Chapter 8 - Atomic Spectroscopy

Chapter 9 – Atomic Absorption

Excellent series of methods for determining the elemental composition in

environmental samples, foods and drinks, potable water, biological fluids,

and materials.

Notice high resolution!

A(λ) = ε(λ)bC

Partial preview of the text

Download Atomic Absorption and more Lecture notes Chemistry in PDF only on Docsity!

Chapter 8 - Atomic Spectroscopy

Chapter 9 – Atomic Absorption

Excellent series of methods for determining the elemental composition in environmental samples, foods and drinks, potable water, biological fluids, and materials.

Notice high resolution!

A( λ ) = ε ( λ )bC

Atomic Absorption Spectrophotometer

Sample – aerosol mist, desolvation, atomization – atoms in gas phase! Flame – sample holder – creates atoms to absorb wavelengths from lamp. Radiation source – hollow cathode lamp, emission lines for the element being analyzed

High resolution

Origins of Atomic Spectra

Spectroscopy of atoms or ions do not involve vibrational or rotational transitions. Transition involves promoting an electron from a ground state to a higher empty atomic state orbital, this state is referred to as the excited state.

Shown to the right is the three absorption and emission lines for Na. Atomic p- orbitals are in fact split into two energy levels for the multiple spins of the electron. The energy level is so small however that a single line observed. A high resolution would show the line as a doublet.

Chemical Problem

The first excited state of Ca is reached by absorption of 422.7 nm light. Calculate the energy difference (kJ/mole) between the ground and excited states.

E = h = hc/ (6.62 x 10-34^ J-s)(3.00 x 10^8 m/s) E = (^) (422.7 nm)(1.00 x 10-9 (^) m/nm) = 4.69 x 10 -19 (^) J/photon

(4.69 x 10-19^ J/photon)(6.02 x 10^23 photons/mol) = 2.83 x 10^5 J/photon

(2.83 x 10^5 J/photon) (1 kJ/1000 J) = 283 kJ/mol

Excitation Wavelengths and Detection Limits

These are wavelengths with relatively large ε(λ ) values so signals are good to use for quantitation.

Chemical Problem

Calculate the emission wavelength (nm) of excited atoms that lie 3.371 x 10-19^ J per molecule above the ground state.

E = hc/ or  = hc/E

(6.62 x 10-34^ J-s)(3.00 x 10^8 m/s) 3.371 x 10-19^ J

= 5.89 x 10-7^ m

(5.89 x 10-7^ m) (1) 1.00 x 10-9^ m/nm

= 589 nm

Visible light!!

The Uncertainty Effect

Spectral lines always have finite widths because the

lifetimes of one or both of the transitions states are finite,

which leads to uncertainties in the transition times.

 • t > 1

Lifetime of the ground state is long but the lifetime of the

excited state is brief, 10-8^ s.

If one wants to know  with high accuracy, then the time

of the measurement, t, must be very long!

Line widths due to uncertainty broadening are sometimes

called natural line widths, and are about 10-4^ Å.

Doppler Broadening

Detector Detector

Wavelength of radiation emitted or absorbed by rapidly moving atom decreases if motion is toward the detector and increases if motion is away from the detector.
10 -2^ to 10-1^ Å Situation is the same for an absorbing atom moving toward or away from the source.

/o = /c

 = velocity of an emitting and moving atom

Doppler Broadening - When molecules are moving towards a detector or away from a detector the frequency will be offset by the net speed the radiation hits the detector. This is also known as the Doppler effect and the true frequency will ether be red shifted (if the chemical is moving away from the detector) or blue shifted (if the chemical is moving towards the detector)

Atomization Process

Temperature effects are significant

Nj/No = Pj/Po exp(-Ej/kT)

The process by which a sample is converted into atomic

vapor is called atomization.

sample

N 2

Nebulization

Aerosol particles

Heat and volatilization

Atomic vapor (these atoms absorb or emit light (EMR)

Nature of the Sample in AAS

Neutral atoms in the gas phase are desired!!

Process of sample introduction into the flame where absorption

occurs. It is the “sample”holder”.

Sample Holder is the Flame

Atoms have no vibrations and rotations (energy

levels associated with molecules), therefore,

spectral bands are more narrow so high resolution

instrument

Neutral atom in gas phase

Hollow Cathode Lamp (Line Source)

A lamp with a matching cathode material is required for each element.

Atomic radiation emitted by the lamp has the same

frequencies at that absorbed by atoms in the flame

or furnace.

Gaseous metal atoms sputtered from cathode by impacting Ar+

in an excited state. They release this “extra energy” by emitting

photons and return to ground state.

Flame = Sample Holder

1. Nebulization

2. Desolvation

3. Atomization

Atomization is the

process of breaking

analyte into gaseous

atoms, which are then

measured by their

absorption or emission

of radiation.

Acetylene – air

(2400 -2700 K)

Graphite Tube Furnace

Furnaces offer increased sensitivity (significantly

lower detection limits) and require less sample than a

flame.

1 – 100 μ L sample volume

All sample is atomized and all atoms remain in optical path for several seconds.

Atomic Absorption, Lecture notes of Chemistry

Related documents

Partial preview of the text

Download Atomic Absorption and more Lecture notes Chemistry in PDF only on Docsity!

Chapter 8 - Atomic Spectroscopy

Chapter 9 – Atomic Absorption

A( λ ) = ε ( λ )bC

Atomic Absorption Spectrophotometer

Origins of Atomic Spectra

Chemical Problem

Excitation Wavelengths and Detection Limits

Chemical Problem

The Uncertainty Effect

lifetimes of one or both of the transitions states are finite,

which leads to uncertainties in the transition times.

 • t > 1

excited state is brief, 10-8^ s.

of the measurement, t, must be very long!

called natural line widths, and are about 10-4^ Å.

Doppler Broadening

/o = /c

Atomization Process

Nj/No = Pj/Po exp(-Ej/kT)

vapor is called atomization.

N 2

Nature of the Sample in AAS

Process of sample introduction into the flame where absorption

occurs. It is the “sample”holder”.

Sample Holder is the Flame

Atoms have no vibrations and rotations (energy

levels associated with molecules), therefore,

spectral bands are more narrow so high resolution

instrument

Hollow Cathode Lamp (Line Source)

A lamp with a matching cathode material is required for each element.

Atomic radiation emitted by the lamp has the same

frequencies at that absorbed by atoms in the flame

or furnace.

Gaseous metal atoms sputtered from cathode by impacting Ar+

in an excited state. They release this “extra energy” by emitting

photons and return to ground state.

Flame = Sample Holder

1. Nebulization

2. Desolvation

3. Atomization

Atomization is the

process of breaking

analyte into gaseous

atoms, which are then

measured by their

absorption or emission

of radiation.

Graphite Tube Furnace

Furnaces offer increased sensitivity (significantly

lower detection limits) and require less sample than a

flame.