tells us about electronic transitions in atoms and molecules

Spectra

are produced when it electons 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 inorganic

dyes are colored. Those that

absorb only in the ultraviolet region are colorless.

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

teuterium 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 it's 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 high-energy.

The source produces white light

that includes all wavelengths all colors. 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 cuvette 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

and where light passes through. So the dye

absorbs orange and blue light and lets through green.

So it appears green in color