SpectroscopySpectroscopy

Chapter 7Chapter 7

The electromagnetic The electromagnetic spectrumspectrum

► All forms of spectroscopy use a part of the All forms of spectroscopy use a part of the electromagnetic spectrum to give us information electromagnetic spectrum to give us information about the materials around us.about the materials around us.

► Electromagnetic radiation interacts with atoms and Electromagnetic radiation interacts with atoms and molecules. molecules.

► The nature of this interaction depends upon the The nature of this interaction depends upon the energy of the electromagnetic radiation.energy of the electromagnetic radiation.

SpectroscopySpectroscopy

►Various spectroscopic techniques Various spectroscopic techniques provide us with information about:provide us with information about: The type of atom of molecule that is The type of atom of molecule that is

presentpresent How much of a particular atom or How much of a particular atom or

molecule is presentmolecule is present The structure and bonding of the The structure and bonding of the

molecule.molecule.

SpectroscopySpectroscopy

►Spectroscopy techniques utilise the fact Spectroscopy techniques utilise the fact that:that: Atoms or molecules absorb and emit Atoms or molecules absorb and emit

electromagnetic radiation of specific energieselectromagnetic radiation of specific energies Atoms and molecules undergo a change Atoms and molecules undergo a change

when they absorb electromagnetic radiationwhen they absorb electromagnetic radiation Different parts of the electromagnetic Different parts of the electromagnetic

spectrum affect different parts of the atom or spectrum affect different parts of the atom or moleculemolecule

EnergyEnergy

► In each of the spectroscopic In each of the spectroscopic techniques we will look at this chapter, techniques we will look at this chapter, the atom or molecule absorbs a the atom or molecule absorbs a specific quantum of energy which specific quantum of energy which causes the atom or molecule to move causes the atom or molecule to move to a higher energy level. to a higher energy level.

►With atoms we look at the movement With atoms we look at the movement of electrons to higher energy levels.of electrons to higher energy levels.

EnergyEnergy

►With molecules, as well as the With molecules, as well as the movement of electrons to high energy movement of electrons to high energy levelslevels

►We observe the movement of molecules We observe the movement of molecules to higherto higher Vibrational Vibrational Rotational Rotational Nuclear spin Nuclear spin

Energy levelsEnergy levels

Analysis of AtomsAnalysis of Atoms

►The following 3 techniques that use The following 3 techniques that use radiation from the visible region of the radiation from the visible region of the electromagnetic spectrum to give us electromagnetic spectrum to give us information about the elements information about the elements present in a sample.present in a sample. Flame testsFlame tests Emission SpectroscopyEmission Spectroscopy Absorption absorption spectroscopyAbsorption absorption spectroscopy

Flame TestsFlame Tests

►A simple form of qualitative analysisA simple form of qualitative analysis► Identifies certain atoms in a Identifies certain atoms in a

compound.compound.►Atoms of different elements have Atoms of different elements have

different electron arrangements and different electron arrangements and hence different capacities to absorb hence different capacities to absorb and emit electromagnetic radiationand emit electromagnetic radiation

How Flame Tests WorkHow Flame Tests Work

►When atoms become excited, by heating, they When atoms become excited, by heating, they absorb energy as electrons move to higher absorb energy as electrons move to higher energy levels, further from the nucleus.energy levels, further from the nucleus.

► Excited atoms emit energy as electrons return to Excited atoms emit energy as electrons return to lower energy levels, closer to the nucleus. This lower energy levels, closer to the nucleus. This energy is emitted in the form of photons of light.energy is emitted in the form of photons of light.

► These photons correspond to a particular These photons correspond to a particular wavelength.wavelength.

► The unique electron arrangement of each metal The unique electron arrangement of each metal means it produces a unique flame colour.means it produces a unique flame colour.

Atomic Emission Atomic Emission SpectroscopySpectroscopy

►Turn to page 80Turn to page 80►As a class we are going to read.As a class we are going to read.►Also don’t forget ROYGBIVAlso don’t forget ROYGBIV►Then it is your turnThen it is your turn►Page 81Page 81►Question 3 Question 3 ►Page 106Page 106►Question 20Question 20

Atomic Absorption Atomic Absorption SpectroscopySpectroscopy

►Atomic Spectroscopy is:Atomic Spectroscopy is: One of the most widely used modern One of the most widely used modern

instrumental techniquesinstrumental techniques An Australian inventionAn Australian invention Very versatile, being capable of detecting Very versatile, being capable of detecting

over 70 elementsover 70 elements Extremely sensitive, detecting Extremely sensitive, detecting

concentrations of elements as part per concentrations of elements as part per million (ppm) levels or, in some cases, part million (ppm) levels or, in some cases, part per billion (ppb)per billion (ppb)

AASAAS

►AAS can be used to:AAS can be used to: Measure the amount of mercury in fishMeasure the amount of mercury in fish Detect toxic metals such as copper in Detect toxic metals such as copper in

bloodblood Test urine and blood to detect an excess Test urine and blood to detect an excess

or deficiency of metalsor deficiency of metals Environmental samplingEnvironmental sampling Soil samplingSoil sampling

How does AAS workHow does AAS work

►Atoms will absorb light if the energy of Atoms will absorb light if the energy of the light is exactly that required to the light is exactly that required to promote an electron from its normal promote an electron from its normal energy level to a higher energy levelenergy level to a higher energy level

►Each element has its own absorption Each element has its own absorption spectrumspectrum

►Each element to be analysed requires Each element to be analysed requires its own light source that will emit light its own light source that will emit light of the correct wavelengthof the correct wavelength

AASAAS

How does AAS work?How does AAS work?

► A solution of the sample to be analysed is sprayed A solution of the sample to be analysed is sprayed into a flameinto a flame

► It is converted into atomic vapourIt is converted into atomic vapour► Light containing the chosen wavelength is passed Light containing the chosen wavelength is passed

through the flamethrough the flame► Atoms of the element being analysed that are Atoms of the element being analysed that are

present absorb some of the radiationpresent absorb some of the radiation► The light beam is then passed through a filter The light beam is then passed through a filter

(monochromator) to select for the light of the (monochromator) to select for the light of the chosen wavelengthchosen wavelength

► Its intensity is measured by an elecrontronic Its intensity is measured by an elecrontronic detectordetector

Interpreting atomic absorption Interpreting atomic absorption spectraspectra

►Worked Example 7.3 on page 83Worked Example 7.3 on page 83

Your TurnYour Turn

►Page 84Page 84►Question 5Question 5►Page 106Page 106►Question 23 and 24Question 23 and 24

►Makes use of the fact that many Makes use of the fact that many substances absorb light of substances absorb light of characteristic wavelengthscharacteristic wavelengths

►The wavelengths of the light absorbed The wavelengths of the light absorbed by compounds can be useful for their by compounds can be useful for their identificationidentification

UV-Visible SpectroscopyUV-Visible Spectroscopy

UV-Visible SpectroscopyUV-Visible Spectroscopy

► Whereas Flame Emission and AAS require that Whereas Flame Emission and AAS require that the sample being analysed is sprayed into a the sample being analysed is sprayed into a flame, UV-visible spectroscopy involves the flame, UV-visible spectroscopy involves the sample, in aqueous solution, being placed in a sample, in aqueous solution, being placed in a glass holder.glass holder.

► Ultraviolet or visible light at a Ultraviolet or visible light at a wavelength wavelength strongly absorbed by the speciesstrongly absorbed by the species being being analysed for, is passed through the solution analysed for, is passed through the solution and and the amount of light absorbed is the amount of light absorbed is directly related to the amount of that directly related to the amount of that species presentspecies present in the sample in the sample

►When a substance absorbs visible light, it When a substance absorbs visible light, it appears coloured.appears coloured.

► The colour observed is the compliment of The colour observed is the compliment of the absorbed colour because this is what the absorbed colour because this is what remains to reach our eyes.remains to reach our eyes.

UV-Visible SpectroscopyUV-Visible Spectroscopy

Visible spectrumOf chlorophyll

► Although it can be useful for qualitative analysis. Although it can be useful for qualitative analysis. UV-visible spectroscopy is usually used for UV-visible spectroscopy is usually used for determining concentration of a substance in a determining concentration of a substance in a samplesample

► The procedure involves recording the spectrum The procedure involves recording the spectrum of the pure substance and selecting a of the pure substance and selecting a wavelength at which the substance absorbs wavelength at which the substance absorbs strongly but other components of the sample do strongly but other components of the sample do not.not.

► The absorbance of the sample is then measured The absorbance of the sample is then measured at this wavelength and compared to the at this wavelength and compared to the absorbance of a series of standard solutions.absorbance of a series of standard solutions.

UV-Visible Spectroscopy UV-Visible Spectroscopy ApplicationsApplications

UsesUses

►Clinical analysis, such as the haemoglobin Clinical analysis, such as the haemoglobin content and sugar levels in bloodcontent and sugar levels in blood

►Determining the amount of coloured dyes Determining the amount of coloured dyes in plasticsin plastics

► In qualitative analysis of DNA and In qualitative analysis of DNA and proteins in the field of molecular biologyproteins in the field of molecular biology

►Determining the levels of nutrients, Determining the levels of nutrients, additives and contaminants in water and additives and contaminants in water and foods.foods.

UV-Visible SpectroscopyUV-Visible Spectroscopy

►The substance to be analysed is place in The substance to be analysed is place in a special vial. a special vial.

►A reference cell must be used which A reference cell must be used which contains pure solvent. This is used to contains pure solvent. This is used to compensate from any reflection, compensate from any reflection, scattering or absorbance of the light by scattering or absorbance of the light by the solvent.the solvent.

Interpreting ultraviolet-visible Interpreting ultraviolet-visible spectraspectra

►To interpret spectra’s from UV-visible To interpret spectra’s from UV-visible spectrometry the absorbance by a spectrometry the absorbance by a series of standard solutions is series of standard solutions is measured and a calibration graph is measured and a calibration graph is drawn.drawn.

►The graph shows absorbance on the y-The graph shows absorbance on the y-axis and concentration on the x-axisaxis and concentration on the x-axis

► If the absorbance of an unknown If the absorbance of an unknown solution is known we can use this to solution is known we can use this to find the concentration.find the concentration.

Worked Example 7.4Worked Example 7.4

►Page 87Page 87

Your TurnYour Turn

►Page 88Page 88►Questions 6-8 and 12Questions 6-8 and 12►Pg 106-107Pg 106-107►Question 23, 24 and 26Question 23, 24 and 26