8. Structure of a spectrophotometer (3)
Components of spectrophotometer
Two kinds of lamps, a Deuterium for measurement in the ultraviolet range and a tungsten lamp for measurement in the visible and near-infrared ranges, are used as the light sources of a spectrophotometer.
Types and properties of light source
|Property||A continuous spectrum of 300 - 3,000 nm is emitted.||A continuous spectrum of 168 - 500 nm, with maximum energy at 250 nm, is emitted.|
|Wavelength range||340 - 1,100 nm||185 - 360 nm|
A spectroscope plays a role in selecting a monochromatic light from a light source (white light).
Spectroscopes include Filter type, Prism type, and Grating (diffraction grating) type.
Types and properties of dispersion
|Properties||A single wavelength can be extracted with a filter. A filter is also used in combination with diffraction grating for filtering out stray light.||A spectrum of 175 - 2,700 nm can be dispersed. The degree of dispersion varies with the wavelength.||Dispersion is homogeneous at the entire wavelength, and a wide-range wavelength can be obtained with a diffraction grating. In addition, a constant spectrum featuring a constant slit breadth can be obtained.|
|Types and materials|
|Colored glass filter Interference filter||Crystal or fused quartz||Plane diffraction grating Concave diffraction grating|
A container that contains a sample is usually called "cell"; two types are available, glass and quartz cells. Since light in the ultraviolet range with a wavelength of 340 nm or less hardly passes through a glass cell, it is used for measurement in the visible range of 340 nm or more.
On the other hand, although a quartz cell allows passage of light in the entire wavelength in the ultraviolet and visible ranges, it is mainly used for the measurement in the ultraviolet range due to its high price.
A detector plays a role in changing the light transmitted from a sample into an electric signal.
Optical semiconductor, Types of photomultiplier, etc. are available.
High speed, high sensitivity, and a low noise are features over the wavelength from Ultraviolet to Near infrared. A photo cell sensitive only in the visible range is used. A Si photodiode is a representative one.
This combines a photoelectric tube and an amplifier (approximately 10-fold amplification) sensitive in both the ultraviolet and visible regions, and the sensitivity can be changed sharply by increasing the applied voltage.