Spectroscopy Uses emission and absorption of light by electrons moving between ground and excited state configuration, hence electronic configuration is important.

Different types of Spectroscopy Flame tests Atomic Emission Atomic Absorption Colourimetry UV – visible Nuclear Magnetic Resonance (NMR) IR Spectrometry Unit 2 – yr 11 Unit 3 – yr 12

Flame tests Electrons can be excited to a number of higher energy levels and the light that is observed is often a mixture of different wavelengths and therefore energies. Note: electrons can only go up to a particular shell if they have absorbed the correct amount of energy (they cannot go half way). Samples of ionic compounds are heated in a Bunsen burner flame, and emit a characteristic flame colour.

What is happening at an atomic level? When excited, electrons are promoted from their ground shell to a shell that is further away from the nucleus (of higher energy and is unstable). As the electron fall back to a shell closer to the nucleus the extra energy that is no longer needed is emitted as a specific wavelength (colour of light)

The electromagnetic spectrum Within the visible spectrum, particular wavelengths correspond to particular colours

Disadvantages of Flame Tests: Some metals do not produce a coloured flame.(eg Be and Mg do not colour a flame because their emissions do not lie within the visible region of the electromagnetic spectrum) Some metals have similar coloured flames, (eg. Ca, Sr both produce a red flame) Cannot detect a mixture of compounds Cannot determine amount (ie qualitative only – not quantitative) Sample is destroyed.

Atomic Emission Spectrometry AES   A spectroscope can be used so the light that is produced, once electrons have been excited and return to lower energy levels, is passed through a prism and broken up into its constituent wavelengths. This produces an atomic emission spectrum.

An emission spectrum is achieved by: The wavelengths of light emitted by a flame test are passed through a prism. Light is broken up into its constituent wavelengths. The coloured bands are presented as an emission spectrum. Coloured lines on a black background are produced.

Emission Spectra Emission spectra are characteristic for each element and can be used as a qualitative analysis technique. Emission spectra appear as discrete coloured lines on a black background, corresponding to the specific energy emissions given out as electrons return to the ground state.

Sample AES spectrums. What is the unknown?

Limitations of Atomic Emission Spectroscopy:   Few elements can be excited by even the hottest laboratory flame. Mainly used for Group I & II elements.

Atomic Absorption Spectrometry - AAS   This is a parallel technique to atomic emission but instead of looking at what light is emitted after excitation, we look at what light is absorbed during excitation.

An absorption spectra is achieved by: White light is passed through an atomized, gaseous sample. Some wavelengths of light are absorbed. These correspond to wavelengths of light of the correct energy to excite electrons in the sample. The wavelengths not absorbed are passed through a prism. Light is broken up into its constituent wavelengths. A spectrum of black lines on a coloured background are produced.

AES vs AAS The lines on an absorption spectrum of a particular element will have corresponding lines at exactly the same wavelengths on the emission spectrum for the same element. However, the emission spectrum may have extra lines compared with the absorption spectrum.

How an AAS works Hollow cathode Lamp Burner Monochromator and slit Detector Emits light of the correct wavelength (ie – the metal being tested for) Sample is atomized. Some light is absorbed by sample. Creates spectrum and selects wavelength for analysis. Measures intensity of light that is not absorbed.

AAS Atomic absorption spectrometry can also be used in a quantitative manner to determine the concentration of a particular element in the sample. The extent of the absorption depends on the relative quantity of the element in the sample, allowing the concentration of the element in the sample to be determined.

AAS This is done by comparing the absorption of the sample with a standard calibration curve for the particular element being analysed. A standard calibration curve is obtained by spraying samples of solutions of known concentration into the flame, recording their absorbance then plotting this information on a graph.

AAS Although atomic absorption spectroscopy is only able to detect one element at a time, a mixed sample could be analysed by testing one element at a time, using the appropriate hollow cathode lamp.

Advantages of AAS: It is very sensitive, detecting concentrations at mg/L and µg/L. A wider range of elements can be detected (68). It is more accurate than AES. It can be used both quantitatively and qualitatively. AAS – Using standard curves problem sheet