Atomic Absorption Spectroscopy

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Atomic Absorption Spectroscopy

Prof Mark A. BuntineSchool of Chemistry

Dr Vicky BarnettUniversity Senior College

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“This material has been developed as a part of the Australian School Innovation in Science, Technology and Mathematics Project funded by the Australian Government Department of Education, Science and Training as a part of the Boosting Innovation in Science, Technology and Mathematics Teaching (BISTMT) Programme.”


Professor Mark A. BuntineBadger Room [email protected]

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•AAS is commonly used for metal analysis

•A solution of a metal compound is sprayed into a flame and vaporises

•The metal atoms absorb light of a specific frequency, and the amount of light absorbed is a direct measure of the number of atoms of the metal in the solution

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Atomic Absorption Spectroscopy:An Aussie Invention

•Developed by Alan Walsh (below) of the CSIRO in early 1950s.

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Electromagnetic RadiationSinusoidally oscillating electric (E) and magnetic (M) fields.

Electric & magnetic fields are orthogonal to each other.

Electronic spectroscopy concerns interaction of theelectric field (E) with matter.

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The Electromagnetic Spectrum

• Names of the regions are historical.• There is no abrupt or fundamental change in

going from one region to the next.• Visible light represents only a very small

fraction of the electromagnetic spectrum.

1020 1018 1016 1014 1012 108

-rays X-rays UV IR Micro-wave

Frequency (Hz)

Wavelength (m)10-11 10-8 10-6 10-3

Visible

400 500 600 700 800 nm

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The Visible Spectrum < 400 nm, UV

400 nm < < 700 nm, VIS > 700 nm, IR

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The Electromagnetic Spectrum

• Remember that we are dealing with light.• It is convenient to think of light as

particles (photons).• Relationship between energy and

frequency is:

E h

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Energy & Frequency• Note that energy and frequency are

directly proportional.• Consequence: higher frequency radiation

is more energetic.

E.g. X-ray radiation ( = 1018 Hz): 4.0 x 106 kJ/mol IR radiation ( = 1013 Hz): 39.9 kJ/mol

(h = 6.626 x 10-34 J.s)

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Energy & Wavelength• Given that frequency and wavelength

are related: =c/• Energy and wavelength are inversely

proportional• Consequence: longer wavelength

radiation is less energetic

eg.-ray radiation ( = 10-11 m):1.2 x 107 kJ/mol Orange light ( = 600 nm): 199.4 kJ/mol

(h = 6.626 x 10-34 J.s c = 2.998 x 108 m/s)

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Absorption of Light• When a molecule absorbs a photon, the

energy of the molecule increases.

• Microwave radiation stimulates rotations• Infrared radiation stimulates vibrations• UV/VIS radiation stimulates electronic

transitions• X-rays break chemical bonds and ionize

molecules

Groundstate

Excitedstate

photon

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Absorption of Light• When light is absorbed by a sample,

the radiant power P (energy per unit time per unit area) of the beam of light decreases.

• The energy absorbed may stimulate rotation, vibration or electronic transition depending on the wavelength of the incident light.

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• Uses absorption of light to measure the concentration of gas-phase atoms.

• Since samples are usually liquids or solids, the analyte atoms must be vapourised in a flame (or graphite furnace).

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Absorption and Emission

Ground State

Excited States

Absorption Emission MultipleTransitions

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Absorption and Emission

Ground State

Excited States

Absorption Emission

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Atomic Absorption• When atoms absorb light, the

incoming energy excites an electron to a higher energy level.

• Electronic transitions are usually observed in the visible or ultraviolet regions of the electromagnetic spectrum.

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Atomic Absorption Spectrum

• An “absorption spectrum” is the absorption of light as a function of wavelength.

• The spectrum of an atom depends on its energy level structure.

• Absorption spectra are useful for identifying species.

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Atomic Absorption/Emission/Fluorescence Spectroscopy

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• The analyte concentration is determined from the amount of absorption.

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• The analyte concentration is determined from the amount of absorption.

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• Emission lamp produces light frequencies unique to the element under investigation

• When focussed through the flame these frequencies are readily absorbed by the test element

• The ‘excited’ atoms are unstable- energy is emitted in all directions – hence the intensity of the focussed beam that hits the detector plate is diminished

• The degree of absorbance indicates the amount of element present


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• It is possible to measure the concentration of an absorbing species in a sample by applying the Beer-Lambert Law:

Abs log IIo

Abscb

= extinction coefficient

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• But what if is unknown?• Concentration measurements can be

made from a working curve after calibrating the instrument with standards of known concentration.

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AAS - Calibration Curve

• The instrument is calibrated before use by testing the absorbance with solutions of known concentration.

• Consider that you wanted to test the sodium content of bottled water.

• The following data was collected using solutions of sodium chloride of known concentration

Concentration (ppm)

2 4 6 8

Absorbance 0.18

0.38

0.52

0.76

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Calibration Curve for Sodium

Concentration (ppm)

Absorbance

2 4 6 8

0.2

0.4

0.6

0.8

1.0

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Use of Calibration curve to determine sodium concentration {sample

absorbance = 0.65}

Concentration (ppm)

Absorbance

2 4 6 8

0.2

0.4

0.6

0.8

1.0

Concentration

Na+ = 7.3ppm

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• Instrumentation

• Light Sources

• Atomisation

• Detection Methods

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Light Sources• Hollow-Cathode Lamps (most common).

• Lasers (more specialised).

• Hollow-cathode lamps can be used to detect one or several atomic species simultaneously. Lasers, while more sensitive, have the disadvantage that they can detect only one element at a time.

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Hollow-Cathode Lamps• Hollow-cathode lamps are a type of

discharge lamp that produce narrow emission from atomic species.

• They get their name from the cup-shaped cathode, which is made from the element(s) of interest.

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Hollow-Cathode Lamps

• The electric discharge ionises rare gas(Ne or Ar usually) atoms, which in turn, are accelerated into the cathode and sputter metal atoms into the gas phase.

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Hollow-Cathode Lamps• The gas-phase metal atoms collide

with other atoms (or electrons) and are excited to higher energy levels. The excited atoms decay by emitting light.

• The emitted wavelengths are characteristic for each atom.

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MM

MM**

M + e MM + e M**

M + ArM + Ar** M M**

M

M*

MM** M + M + hh

collision-inducedexcitation

spontaneousemission

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Hollow-Cathode Spectrum

Harris Fig. 21-3:Steel hollow-cathode

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Atomisation• Atomic Absorption Spectroscopy (AAS)

requires that the analyte atoms be in the gas phase.

• Vapourisation is usually performed by:– Flames– Furnaces– Plasmas

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Flame Atomisation• Flame AAS can only analyse solutions.

• A slot-type burner is used to increase the absorption path length (recall Beer-Lambert Law).

• Solutions are aspirated with the gas flow into a nebulising/mixing chamber to form small droplets prior to entering the flame.

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Flame Atomisation

Harris Fig 21-4(a)

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Flame Atomisation• Degree of atomisation is temperature

dependent.

• Vary flame temperature by fuel/oxidant mixture.

Fuel Oxidant Temperature (K)Acetylene Air 2,400 - 2,700Acetylene Nitrous Oxide 2,900 - 3,100Acetylene Oxygen 3,300 - 3,400Hydrogen Air 2,300 - 2,400Hydrogen Oxygen 2,800 - 3,000Cyanogen Oxygen 4,800

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Furnaces• Improved sensitivity over flame sources.

• (Hence) less sample is required.

• Generally, the same temp range as flames.

• More difficult to use, but with operator skill at the atomisation step, more precise measurements can be obtained.

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Furnaces

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Furnaces

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Inductively Coupled Plasmas• Enables much higher temperatures to be

achieved. Uses Argon gas to generate the plasma.

• Temps ~ 6,000-10,000 K.• Used for emission expts rather than

absorption expts due to the higher sensitivity and elevated temperatures.

• Atoms are generated in excited states and spontaneously emit light.

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Inductively Coupled Plasmas• Steps Involved:

– RF induction coil wrapped around a gas jacket.

•Spark ionises the Ar gas.

•RF field traps & accelerates the free electrons, which collide with other atoms and initiate a chain reaction of ionisation.

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Detection• Photomultiplier Tube (PMT).

pp 472-473 (Ch. 20) Harris

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Photomultiplier Tubes• Useful in low intensity applications.

• Few photons strike the photocathode.

• Electrons emitted and amplified by dynode chain.

• Many electrons strike the anode.

Atomic Absorption Spectroscopy

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