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Study Notes: What’s This Thing Called ‘Spectroscopy’?

Spectroscopy is to do with the light an object absorbs or emits and its relationship to the make-up of the object.

Atoms, ions and molecules emit or absorb light at certain wavelengths that are unique to the particular atom, ion or molecule. Examination of the light coming from an object can enable a picture to be built up of the components that make up the object. Spectroscopy, for instance, has been harnessed by astronomers to determine the composition of the objects in the universe as well as other valuable information.

Astronomers pass the light from a star through a spectrograph which is an instrument that separates the light into its various wavelengths, producing a characteristic spectrum. This process is similar to the way white light can be divided into its component colours (ie wavelengths) upon travelling through a prism. By examining the spectrum for the absence or presence of certain wavelengths, astronomers can determine which chemical elements are present in the star as well as information such as its temperature and density. A shining star

Spectroscopy in chemical analysis works in the same way. Put very simply, light is shone on a sample and the light that leaves the sample is examined to determine chemical composition or concentration.

A ray of light moves through a beaker containing a blue coloured solution. The emerging ray has lost some intensity. The result is that the solution contains copper ions.

Analytical chemists commonly measure absorbance (the amount of light ‘soaked up’) or, alternatively, transmittance (the amount of light that passes straight through). Light that is absorbed and subsequently emitted can also be measured and is known as emission.

Light covering a wide range of wavelengths (or energies) can be passed through a sample. When we know the wavelengths that have been absorbed, we know what chemical entities are present in the sample. If the sample is radiated with a wavelength that is not absorbed, nothing will have been learnt.

Spectroscopy involves specific wavelengths. The energy associated with the wavelengths corresponds to the atoms, ions or molecules in the sample moving to certain excited states. More on this subject later. Chemical equation showing a copper atom being excited by light & proclaiming 'I'm excited!'

That different atoms, ions and molecules absorb light at different wavelengths of the electromagnetic spectrum illustrates the various forms of spectroscopy. For example, there is infrared (IR) and ultraviolet (UV) spectroscopy.

In essence, spectroscopy concerns the study of the interaction of electromagnetic radiation with atoms, ions and molecules. Spectroscopic instruments in analytical chemistry use the information from these interactions to identify components in a sample and measure their concentrations.

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