Spectroscopy Chapter 7

The electromagnetic spectrum All forms of spectroscopy use a part of the electromagnetic spectrum to give us information about the materials around us. Electromagnetic radiation interacts with atoms and molecules causing a change. The nature of this interaction depends upon the energy of the electromagnetic radiation.

Spectroscopy Various spectroscopic techniques provide us with information about: The type of atom of molecule that is present How much of a particular atom or molecule is present The structure and bonding of the molecule.

Spectroscopy Spectroscopy techniques utilise the fact that: Atoms or molecules absorb and emit electromagnetic radiation of specific energies Atoms and molecules undergo a change when they absorb electromagnetic radiation Different parts of the electromagnetic spectrum affect different parts of the atom or molecule

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

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

Energy With molecules, as well as the movement of electrons to high energy levels We observe the movement of molecules to higher Vibrational Rotational Nuclear spin Energy levels

Flame Tests A simple form of qualitative analysis Identifies certain atoms in a compound. Atoms of different elements have different electron arrangements and hence different capacities to absorb and emit electromagnetic radiation

How Flame Tests Work When atoms become excited, by heating, they absorb energy as electrons move to higher energy levels, further from the nucleus. Excited atoms emit energy as electrons return to lower energy levels, closer to the nucleus. This energy is emitted in the form of photons of light. These photons correspond to a particular wavelength. The unique electron arrangement of each metal means it produces a unique flame colour.

Atomic Absorption Spectroscopy Atomic Spectroscopy is: One of the most widely used modern instrumental techniques An Australian invention Very versatile, being capable of detecting over 70 elements Extremely sensitive, detecting concentrations of elements as part per million (ppm) levels or, in some cases, part per billion (ppb)

AAS AAS can be used to: Measure the amount of mercury in fish Detect toxic metals such as copper in blood Test urine and blood to detect an excess or deficiency of metals Environmental sampling Soil sampling

How does AAS work Atoms will absorb light if the energy of the light is exactly that required to promote an electron from its normal energy level to a higher energy level Each element has its own absorption spectrum Each element to be analysed requires its own light source that will emit light of the correct wavelength

AAS

How does AAS work? A solution of the sample to be analysed is sprayed into a flame It is converted into atomic vapour Light containing the chosen wavelength is passed through the flame Atoms of the element being analysed that are present absorb some of the radiation The light beam is then passed through a filter (monochromator) to select for the light of the chosen wavelength Its intensity is measured by an elecrontronic detector

Interpreting atomic absorption spectra Worked Example 7.3 on page 83

Your Turn Page 84 Question 5 Page 106 Question 23 and 24

UV-Visible Spectroscopy Makes use of the fact that many substances absorb light of characteristic wavelengths The wavelengths of the light absorbed by compounds can be useful for their identification The observed colour of a sample is complementary to the absorbed colour.

UV-Visible Spectroscopy Whereas Flame Emission and AAS require that the sample being analysed is sprayed into a flame, UV-visible spectroscopy involves the sample, in aqueous solution, being placed in a glass holder. Ultraviolet or visible light at a wavelength strongly absorbed by the species being analysed for, is passed through the solution and the amount of light absorbed is directly related to the amount of that species present in the sample

UV-Visible Spectroscopy When a substance absorbs visible light, it appears coloured. The colour observed is the compliment of the absorbed colour because this is what remains to reach our eyes. Visible spectrum Of chlorophyll

UV-Visible Spectroscopy Applications Although it can be useful for qualitative analysis. UV-visible spectroscopy is usually used for determining concentration of a substance in a sample The procedure involves recording the spectrum of the pure substance and selecting a wavelength at which the substance absorbs strongly but other components of the sample do not. The absorbance of the sample is then measured at this wavelength and compared to the absorbance of a series of standard solutions.

Uses Clinical analysis, such as the haemoglobin content and sugar levels in blood Determining the amount of coloured dyes in plastics In qualitative analysis of DNA and proteins in the field of molecular biology Determining the levels of nutrients, additives and contaminants in water and foods.

UV-Visible Spectroscopy The substance to be analysed is place in a special vial. A reference cell must be used which contains pure solvent. This is used to compensate from any reflection, scattering or absorbance of the light by the solvent.

Interpreting ultraviolet-visible spectra To interpret spectra’s from UV-visible spectrometry the absorbance by a series of standard solutions is measured and a calibration graph is drawn. The graph shows absorbance on the y-axis and concentration on the x-axis If the absorbance of an unknown solution is known we can use this to find the concentration.

Worked Example 7.4 Page 87

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