Ultraviolet-Visible Spectrophotometer

PHRM 309

The typical ultraviolet-visible spectrophotometer

consists of light source, monochromator, sample

cell and detector.

Ultraviolet-visible spectrophotometer

Conventional Spectrophotometer

Schematic of a conventional single-beam spectrophotometer

Conventional Spectrophotometer

Optical system of a double-beam spectrophotometer

In single-beam UV-VIS absorption spectroscopy,

obtaining a spectrum requires manually measuring

the transmittance of the sample and solvent at

each wavelength.

The double-beam design greatly simplifies this

process by measuring the transmittance of the

sample and solvent simultaneously.

Deuterium lamp: emits electromagnetic radiations

in the ultraviolet region of the spectrum.

Light source

Quartz halogen or Tungsten lamp: emits

electromagnetic radiations in the visible region of

the spectrum.

All spectrometers require a light source. More than

one type of source can be used in the same

instrument which automatically swap lamps when

scanning between the UV and visible regions:

Light sources

 UV Spectrophotometer

1. Hydrogen Gas Lamp

2. Mercury Lamp

Visible Spectrophotometer

1. Tungsten Lamp

In a typical double-beam spectrophotometer, the

light emanating from the light source is split into

two beams, the sample beam and the reference

beam.

When there is no sample cell in the reference

beam, the detected light is taken to be equal to the

intensity of light entering the sample (I0).

The reference beam commonly used to correct the

reading of the sample. A blank sample is usually

placed in reference beam which is most commonly

the solvent in which the sample is dissolved.

Its role is to spread the beam of light into its

component wavelength by means of a prism or

grating which are further selected by slit.

The monochromator is rotated so that a range of

wavelengths is passed through the sample as the

instrument scans across the spectrum.

Monochromator

The sample cell must be constructed of a material

that is transparent to the electromagnetic ration

are being using in the experiment.

Cell materialFor spectra in the visible range of the spectrum:

glass or plastic

For spectra in the ultraviolet range of the spectrum:

quartz

Cell sizeThe most common size of the cell is 1cm (width)

which hold 5-6ml of solution.

Sample cell

Sample cells

UV Spectrophotometer

Quartz (crystalline silica)

 Visible Spectrophotometer

Glass

A typical sample cell (commonly called a cuvette )

The light that passes through the sample cell

reaches the detector, which records the intensity of

the transmitted light.

Standard UV/VIS detectors are photomultipliers and

silicon diodes.

Silicon diodes are smaller and cheaper, whereas

photomultipliers have a higher sensitivity.

Most research instruments are based on

photomultipliers.

Detector