Ultraviolet visible spectrometry tells us about electronic transitions in atoms and molecules.
Spectra are produced when electrons in molecules or atoms move from one electronic energy level
to another of higher energy. In doing so they absorb energy
equal to the gap between the two levels. Compounds that absorb in the visible
region such as some transition metal compounds and organic dyes are colored.
Those that absorb only in the ultraviolet region are colourless.
Inside a UV visible spectrometer
there are usually two light sources, one giving out visible light and one ultraviolet.
This one is a tungsten lamp
like a car headlamp bulb for the visible region.
And this deuterium lamp gives out ultraviolet.
This mirror directs light from the appropriate source into the monochromator.
This contains a diffraction grating
that acts rather like the playing surface of a CD to split the light
into its constituent wavelengths. Different wavelengths correspond to different colours.
Red is about 700 nanometers and blue around 400 nanometers.
Wavelengths shorter than about 350 nanometers are called ultraviolet.
Shorter wavelength light has higher energy.
The source produces white light that includes all wavelengths, all colours.
The instrument scans through the spectrum sending different wavelengths of light
through the sample in sequence. This is done by the grating which rotates.
A single wavelength passes into the modulator
which consists of a rotor with mirrors on it.
This chops the light into two beams. One beam passes through the sample cell
while the other passes through the reference cell.
So the instrument is referred to as a double beam instrument.
Both sample and reference beams
are directed by mirrors onto a detector. This compares their intensities
and send a signal proportional to the ratio of their intensities
to the computer that controls the instrument. The logarithm of
this ratio gives a quantity called absorbance which is a measure
how much light is being absorbed by the sample at that particular wavelength.
Ultraviolet visible spectra are
usually run on solutions. Light does not normally pass through solid samples.
Here we will run the spectrum of a green food dye.
To run the spectrum we place some of the solvent in a sample cuvette to act as a blank, a reference.
The cuvettes may be made of glass or plastic if only the visible region of the spectrum is required.
Quartz cuvettes are needed for work in the ultraviolet range
because glass and plastics absorb UV light.
We then place a solution of the sample in a second cuvette. The blank
and the sample are placed in the sample holders. The lid is closed
to prevent light from the laboratory interfering with spectrum.
The operator types details such as the wavelength range required
and scanning speed into the computer that controls the instrument.
He also zeros the instrument at a point where the sample does not absorb
and then starts the scan. The spectrum appears on screen,
the data is saved on the computer and a hard copy can be printed.
The horizontal axis is normally wavelength, and the vertical one absorbance
which is a measure the amount of light absorbed by the sample.
The peaks on the spectrum are the wavelengths of light that are absorbed by the sample
and the troughs are where light passes through.
So the dye absorbs orange and blue light and lets through green.
So it appears green in colour.