Chapter 7: Spectroscopy

Spectroscopy is the study of the interaction of light with substances. The identity of the substance, and its concentration, can be determined by the substance’s response to a beam of radiation.

Spectroscopic techniques give information about:

The type of atom / molecule present (qualitative)

How much of an atom / molecule is present (quantitative)

The structure and bonding of the molecule.

Spectroscopy work on the basis that: Atoms / molecules absorb and emit electromagnetic radiation of specific energies Atoms / molecules change slightly when they absorb radiation Different parts of the spectrum affect different parts of the atom / molecule.

All forms of spectroscopy use part of the electromagnetic spectrum to give information about materials. Radiation from each portion of the electromagnetic spectrum has a specific ____________, ___________ and _________ associated with it.

When atoms / molecules absorb a specific quantum of energy, they move to a higher energy level.

Atoms – we look at the electrons moving to higher energy levelsMolecules – we also look at the molecule’s vibrational, rotational and nuclear spin energy levels.

Analysis of atoms- 1

3 techniques that use radiation from the visible region of the electromagnetic spectrum are flame tests, atomic emission spectroscopy and atomic absorption spectroscopy

Flame TestsSome metals (usually from the Transition Metal series) produce particular colours when they are heated. These colours can be used to determine if the metal is present in a sample by inserting the sample into a non-luminous flame and comparing the colours observed to a known list.

Why are colours produced?Electrons are located in shells (often called energy levels) around the nucleus. They are held there by electrostatic attraction and they are most stable when all the electrons are in the lowest energy levels available (ground state).

Electrons however can jump from a lower energy level to a higher energy level (further from the nucleus) if they absorb the amount of energy that corresponds exactly to the difference in energy between the lower energy level and the next shell or energy level. The atom is considered to be ‘excited’ and unstable in this state and the electron drops back to the lowest energy level available. When this happens the extra energy is emitted (released) as a photon of light. The energy emitted is equal to the difference in energy between the higher energy level and the lower energy level to which it returns. The light has a specific wavelength and colour.

Diagram of electron jumps (Fig 7.7)

Flame tests can only give limited qualitative information about the likely elements present. Only a few elements give a coloured flame, and the colours of some are alike. In impure samples, fainter colours can be masked by a stronger one.

Atomic Emission is produced by heating a sample (in a hotter flame to ensure sufficient energy is available to excite electrons), passing the light produced through a prism where it is split into colours and recording the information produced. AES is a superior method of analysis to flame tests as in AES, more elements produce emission spectra in the hotter flame, and a much smaller quantity of an element needs to be present in order for it to be detected. Using AES, the amount of the element present in a sample can be accurately determined, even in the presence of larger amounts of other elements that would mask the flame colour of the element to the naked eye.

Draw Fig. 7.8

AES records the energy emitted as excited electrons return to their ground state. This is produced as coloured bars on a white or black background. (Limited to Group 1 and 2 metals only). Electrons in an atom can be excited to a number of higher energy levels so light emitted is likely to be a mixture of several different colours. Each line in the spectrum corresponds to radiation of a specific wavelength, frequency and energy equal to the difference in energy of the electron energy levels.

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The amount of energy released, hence the wavelength or colour of the light emitted, is unique to the metal atom used. In this way, the wavelengths identify the metal and the intensity is proportional to the concentration.

Flame – Sample is atomised in flame, light is emitted. The greater intensity of the light, the higher the concentrationMonochromator – isolates a particular wavelength of lightPhotomultiplier – converts light beam into an electric currentAmplifier and Recorder – registers the current produced.

As a consequence of the presence of different numbers of protons in the nuclei of atoms of different elements, the energies of electrons in the shells of the differing atoms are not the same. Different amounts of energy are released when an electron moves from a higher energy level to a lower level in one atom compared with when the same process occurs in another atom. These different energies are seen as light of different colours.

Atomic Absorption Spectroscopy (AAS) records the energy absorbed when electrons are excited. A solution is converted to a vapour in a flame. Light of a particular wavelength (energy level) is passed through the flame and atoms can absorb some of this energy. The light then passes through the monochromators (filter) to select the light of the chosen wavelength. Its intensity id measured by a detector. This can then be recorded as black bars on a coloured spectrum background or as a printed fact sheet. The amount of light absorbed indicates the quantity of the element present in the original sample.

AAS is much more versatile than AES and can detect over 70 elements It can accurately detect trace elements of up to parts per billion AAS is one of the most widely used of modern instrumental techniques Examples of uses: analysis of toxic metals in food and drink; urine and blood analysis;

testing for air pollution.

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