Understanding Spectrophotometry: Principles and Applications
Spectrophotometry
Absorption and Emission of Radiant Energy
The widespread use of spectrophotometry is due to several factors:
- The broad range of radiant energy wavelengths and their diverse interactions with matter.
- The availability of increasingly precise measuring instruments.
- The inherent advantages of the method.
Spectrophotometric methods are crucial in most industrial, clinical research, and teaching laboratories.
Nomenclature
Standardized nomenclature for photometry development led to the formation of a spectroscopy nomenclature applied by the Society of Applied Spectroscopy and the American Society for Testing Materials (ASTM). The definitions, symbols, and names used are recommended by these organizations. Interrelated concepts should have similar names, achieved by adding suffixes to the root word expressing the basic concept. Some common suffixes include:
- -OR: Refers to an apparatus or mechanism (e.g., reflector, comparator). Sometimes also indicates resistance (e.g., filament).
- -ANCE: Signifies a property of the device or body (e.g., transmittance, absorbance, capacitance).
- -ITY: Indicates a property of a substance (e.g., density, solubility, conductivity, absorptivity, emissivity).
- -METER: A measuring instrument (e.g., ammeter, calorimeter, densitometer, spectrophotometer).
- -SCOPY: An optical device or apparatus for vision (e.g., microscope, telescope, spectroscope).
-GRAPH
An apparatus for recording observations (e.g., polarograph, spectrograph).
-GRAM
The record produced by an instrument (e.g., polarogram, spectrogram).
Nature of Radiant Energy
Radiant energy is defined as energy transmitted in the form of electromagnetic radiation. Substances may emit this energy under conditions of high excitation, such as those produced by high temperatures or electric discharge. This energy can be absorbed, reflected, transmitted, or refracted by many substances in different states of aggregation (solid, liquid, gas, or solution) if the incident radiation has an appropriate wavelength.
Electromagnetic radiation exhibits both diffraction and refraction properties. Radiation behaves as a wave, although it does not require a physical medium for propagation. In emission and absorption phenomena, electromagnetic radiation also exhibits particle properties, called photons.
The electromagnetic spectrum spans over 20 orders of magnitude of wavelengths, so its performances are usually done on a logarithmic scale.
Frequency
Frequency is the number of waves passing a fixed point per unit time (e.g., cycles per second (cps) or megacycles per second (MHz)).
10 MHz = 106 cycles.
The frequency and wavelength are related to the speed of light in a vacuum. Another convenient unit, particularly in infrared spectrometry, is the number of waves per centimeter.