Chapter 25: Instruments for Optical Spectrometry

By: Kenji Lucas

Most spectroscopic instruments in the UV/visible and IR regions are made up of five components:

(1) a stable source of radiant energy.

(2) a wavelength selector to isolate a limited region of the spectrum for measurement;

(3) one or more sample containers.

(4) a radiation detector, to convert radiant energy to a measurable electrical signal.

(5) a signal-processing and readout unit consisting of electronic hardware and in modern instruments a computer.

Optical Materials

The cells, windows, lenses, mirrors, and wavelength-selecting elements in an optical spectroscopic instrument must transmit radiation in the wavelength region being investigated.

Spectroscopic Sources

Spectroscopic sources are of two types: continuum sources, which emit radiation that changes in intensity only slowly as a function of wavelength, and line sources, which emit a limited number of spectral lines, each of which spans a very narrow wavelength range.

Continuum Sources in the Ultraviolet/Visible Region

The most widely used continuum sources:

Wavelength Selectors

Spectroscopic instruments in the UV and visible regions are usually equipped with one or more devices to restrict the radiation being measured to a narrow band that is absorbed or emitted by the analyte. Many instruments use a monochromator or a filter to isolate the desired wavelength band so that only the band of interest is detected and measured. Others use a spectrograph to spread out, or disperse, the wavelengths so that they can be detected with a multichannel detector.

Monochromators and Polychromators

The wavelength range passed by a monochromator, called the spectral bandpass or effective bandwidth, can be less than 1 nm for moderately expensive instruments to greater than 20 nm for inexpensive systems.

Gratings

Most gratings in modern monochromators are replica gratings, which are obtained by making castings of a master grating. The latter consists of a hard, optically flat, polished surface on which have been ruled with a suitably shaped diamond tool a large number of parallel and closely spaced grooves.

Types of Gratings:

Echelette Gratings – One of the most common types of reflection gratings Concave Gratings – A concave grating permits the design of a mono- chromator without auxiliary collimating and focusing mirrors or lenses because the concave surface both disperses the radiation and focuses it on the exit slit.

Holographic Gratings – One of the products that emerged from laser technology is an optical (rather than mechanical) technique for forming gratings on plane or concave glass surfaces.

Radiation Filter

Filters operate by blocking or absorbing all but a restricted band of radiation.

Two types of filter in spectrometry:

Interference Filter – Interference filters are used with ultraviolet and visible radiation, as well as with wavelengths as long as about 14 mm in the infrared region.

Absorption Filter – Absorption filters, which are generally less expensive and more rugged than interference filters, are limited in use to the visible region.

Detecting and Measuring Radiant Energy

To obtain spectroscopic information, the radiant power transmitted, fluoresced or emitted, must be detected in some manner and converted into a measurable quantity. A detector is a device that identifies, records, or indicates a change in one of the variables in its environment such as pressure, temperature, or electromagnetic radiation. A transducer converts nonelectrical quantities, such as light intensity, pH, mass, and temperature, into electrical signals that can be subsequently amplified, manipulated, and finally converted into numbers proportional to the magnitude of the original quantity.

Photon Detectors

Widely used types of photon detectors include phototubes, photomultiplier tubes, silicon photodiodes, photodiode arrays, and charge-transfer devices such as charge- coupled and charge-injection devices. Detects and converts transmitted light into photo electric energy.

Signal Processors and Readout Devices

A signal processor is an electronic device that may amplify the electrical signal from the detector. Displays output of the detector system.

Cells or Cuvettes