The Determination Of An Analyte’S Concentration Based On Its Absorption Of Ultraviolet Or Visible Radiation
Every chemical compound absorbs, transmits, or reflects light (electromagnetic radiation) over a certain range of wavelength. Spectroscopy is study of the interactions of radiation and matter, it has played a vital role in the development of modern atomic theory, the most powerful tool available for the study of atomic and molecular structures and also used in the analysis of wide range of samples both organic and inorganic. Measurements based on light and other forms of electromagnetic radiation are widely used throughout analytical chemistry (Skoog, et al., 2013). Spectroscopic techniques employ light to interact with matter and thus probe certain features of a sample to learn about its consistency or structure (Hofmann, 2010). Spectrophotometry is a method to measure how much a chemical substance absorbs light by measuring the intensity of light as a beam of light passes through sample solution. The basic principle is that each compound absorbs or transmits light over a certain range of wavelength Spectroscopic methods can be classified according to the region of the electromagnetic spectrum used or produced in the measurement. The gamma-ray, X-ray, ultraviolet (UV), visible, infrared (IR), microwave, and radio-frequency (RF) regions have been used. UV-Visible Spectrophotometry is one of the most frequently analytical techniques employed in analysis. It involves measuring the amount of ultraviolet or visible radiation absorbed by a substance in solution. Advantages of this spectroscopic method are that it is easily adapted to the field analysis of samples using a filter photometer, low time and labour consumption, the precision of the method is also excellent. (Masoom, et al., 2013).
Instrument which measures the ratio or function of ratio of the intensity of two beams of light in the UV-Visible region is called Ultraviolet-Visible Spectrophotometer. In qualitative analysis, organic compounds can be identified by use of spectrophotometer, if any recorded data is available, and quantitative spectrophotometric analysis is used to ascertain the quantity of molecular species absorbing the radiation. Spectrophotometric technique is simple, rapid, moderately specific and applicable to small quantities of compounds (Skoog, et al., 2013).
Most organic molecules and functional groups are transparent in the ultraviolet and visible region of electromagnetic spectrum, region where wavelength ranges from 190nm to 800 nm (Pavia, et al., 2001).
1.2 What is an Electromagnetic Radiation?
Electromagnetic radiation is a form of energy that is transmitted through space at enormous velocities and can also be described as a wave with properties of wavelength, frequency, velocity, and amplitude. Electromagnetic radiation in the uv/visible region can be called light. In contrast to sound waves, light requires no transmitting medium; thus, it can travel readily through a vacuum. Light also travels nearly a million times faster than sound (Skoog, et al., 2013).The behaviour of electromagnetic radiation is described by the properties of both waves and particles. The optical properties of electromagnetic radiation, such as diffraction, are explained best by describing light as a wave.
Many of the interactions between electromagnetic radiation and matter, such as absorption and emission, however, are better described by treating light as a particle, or photon. The exact nature of electromagnetic radiation remains unclear, as it has since the development of quantum mechanics in the first quarter of the twentieth century (Harvey, 2000).
Fig 1: Electromagnetic spectrum
1.3 Properties of Electromagnetic Radiation
The interaction of electromagnetic radiation with matter is a quantum phenomenon and dependent upon both the properties of the radiation and the appropriate structural parts of the samples involved. These dual views of radiation as particles and waves are not mutually exclusive but complementary and energy of a photon is directly proportional to its frequency. Similarly, this duality applies to streams of electrons, protons, and other elementary particles, which can produce interference and diffraction effects that are typically associated with wave behaviour.
The particle nature of Light
The wave model fails to account for phenomena associated with the absorption and emission of radiant energy. For these processes, electromagnetic radiation can be treated as discrete packets of energy or particles called photons or quanta. We relate the energy of a single photon to its wavelength, frequency, and wave number by
where, h is the planck constant (h= 6.63 x 10-34 js)
is the frequency of the radiation.
c is the speed of light (c=2.998 x 108 m/s)
In dealing with phenomena such as reflection, refraction, interference, and diffraction, electromagnetic radiation is conveniently modelled as waves consisting of an electric and a perpendicular magnetic vector, each one oscillating in plane at right angles to the direction of propagation.
The electric field for a single frequency wave oscillates sinusoidally in space and time. The electric field is represented as a vector whose length is proportional to the field strength (Hofmann, 2010).
Magnetic field x
Fig 2: Plane-polarized electromagnetic radiation showing the electric field, the magnetic field, and the direction of propagation.
field 0 A
-Time or distance
Fig 3: Electric field oscillation of plane-polarized electromagnetic radiation
The Amplitude of an electromagnetic wave is a vector quantity that provides a measure of the electric or magnetic field strength at a maximum in the wave.
The Period of an electromagnetic wave is the time in seconds for successive maxima or minima to pass a point in space.
The Frequency (v) of an electromagnetic wave is the number of oscillations that occur in one second. It is expressed in hertz (Hz), which corresponds to one cycle per second, that is, 1 Hz =1 s-1. The frequency of a beam of electromagnetic radiation does not change as it passes through different media. It is noteworthy that a wave of high frequency has a shorter wavelength and vice versa.
Wavelength: The distance between any two successive crest or troughs of an electromagnetic wave. It is denoted by the Greek letter (lambda) and expressed in centimetres or metres (essentials).
Wave number (): It describes the number of completed wave cycles per distance. It is the reciprocal of wavelength and is frequently used to characterize infrared radiation, with the units given in reciprocal centimetre cm-1 (Hofmann, 2010).