Ultraviolet-visible spectroscopy or ultraviolet-visible spectrophotometry (UV-Vis or UV/Vis) refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible spectral region. Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation. Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm. The visible spectrum is the portion of the electromagnetic spectrum that is visible to (and can be detected by) the human eye, in the range of 390 to 750 nm. This means that UV spectrophotometry uses light in the visible and nearby (near-UV and near-infrared (NIR)) ranges.
The absorption or reflectance in the visible range directly affects the perceived color of the chemicals involved. The color of chemicals is a physical property of chemicals that in most cases comes from the excitation of electrons due to absorption of energy performed by the chemical. Excitation is an elevation in energy level above an arbitrary baseline energy state. In this region of the electromagnetic spectrum, molecules undergo electronic transitions. The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. Molecular electronic transitions take place when electrons in a molecule are excited from one energy level to a higher energy level.
What does the UV Spectrometer measure?
The instrument used in ultraviolet-visible spectroscopy is called a UV/Vis spectrophotometer. It measures the intensity of light passing through a sample (I), and compares it to the intensity of light before it passes through the sample (Io). The ratio I / Iois called the “transmittance”, and is usually expressed as a percentage (%T). The absorbance, A, is based on the transmittance:
A = − log (T% / 100%)
The UV-visible spectrophotometercan also be configured to measure reflectance. In this case, the spectrophotometer measures the intensity of light reflected from a sample (I), and compares it to the intensity of light reflected from a reference material (Io)(such as a white tile). The ratio I / Io is called the “reflectance”, and is usually expressed as a percentage (%R).
What are the Basic Parts of a UV Spectrophotometer?
The basic parts of a spectrophotometer are a light source, a holder for the sample, a diffraction grating in a monochromator or a prism to separate the different wavelengths of light, and a detector.
A diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions.
A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input.
The radiation source is often a Tungsten filament (300-2500 nm), a deuterium arc lamp, which is continuous over the ultraviolet region (190-400 nm), Xenon arc lamps, which is continuous from 160-2,000 nm; or more recently, light emitting diodes (LED) for the visible wavelengths.
A deuterium arc lamp (or simply deuterium lamp) is a low-pressure gas-discharge light source often used in spectroscopy when a continuous spectrum in the ultraviolet region is needed.
Gas-discharge lamps are a family of artificial light sources that generate light by sending an electrical discharge through an ionized gas. A xenon arc lamp is an artificial light source.
The detector is typically a photomultiplier tube, a photodiode, a photodiode array or a charge-coupled device (CCD). A photo multiplier tube is a type of gas-filled or vacuum tube that is extremely sensitive to light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. A photodiode is a type of photo detector capable of converting light into either current or voltage, depending upon the mode of operation. A charge-coupled device (CCD) is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated.
What are the different types of UV Spectrophotmeter?
In a single beamUV spectrophotometer, all of the light passes through the sample cell. Io must be measured by removing the sample. This was the earliest design, but is still in common use in both teaching and industrial labs.
In a double-beamUV spectrophotometer, the light is split into two beams before it reaches the sample. One beam is used as the reference; the other beam passes through the sample. The reference beam intensity is taken as 100% Transmission (or 0 Absorbance), and the measurement displayed is the ratio of the two beam intensities.
What are the Samples used in UV/Vis Spectrophotometer?
Samples for UV/Vis spectrophotometer are most often liquids, although the absorbance of gases and even of solids can also be measured. Samples are typically placed in a transparent cell, known as a cuvette. Cuvettes are typically rectangular in shape; commonly with an internal width of 1 cm. Cuvettes are made of high quality fused silica or quartz glass because these are transparent throughout the UV, visible and near infrared regions.
What are the Applications of UV/Vis spectroscopy?
A UV/Vis spectrophotometer may be used as a detector for HPLC which is the High-performance liquid chromatography (or high-pressure liquid chromatography, HPLC) is a chromatographic technique that can separate a mixture of compounds and is used in biochemistry and analytical chemistry to identify, quantify and purify the individual components of the mixture.
UV/Vis spectroscopy is routinely used in analytical chemistry for the quantitative determination of different analytes, such as transition metal ions, highly conjugated organic compounds, and biological macromolecules. Determination is usually carried out in solutions. Transition metals are elements which belong to groups 3 to 12 on the periodic table. Conjugated system is a system with delocalized electrons (electrons in a molecule or solid metal that are not associated with a single atom) in compounds with alternating single and multiple bonds.