UV and Visible Spectrometers

UV and visible spectrometers measure the amount of ultraviolet and visible light transmitted or absorbed by a sample placed in the spectrometer. The wavelength at which a chemical absorbs light is a function of its electronic structure and the intensity of the light absorption is related to the amount of the chemical between the light source and the detector, so a UV / Visible spectrum can be used to identify some chemical species.

UV and visible spectrometers may come with one of three common photometric design systems. Scanning single beam systems can be one of two types of instrument, either a single wavelength light source or a selector for a single wavelength. The light from the lamp is dispersed before reaching the sample cell.  A scanning double beam instrument measures both the transmittance of the sample and solvent at each wavelength simultaneously and computes the ratio. The light from the lamp is dispersed before reaching the sample cell.  All wavelengths pass through the sample and the dispersing element is between the sample and the array detector. This allows the detector to simultaneously record the transmitted light power at multiple wavelengths.  Commonly UV spectrometers and visible spectrometers will have a monochromator, which is used to separate the wavelengths of the light source before the light passes through the sample and enters the detector.

Common detector types found in UV spectrometers and visible spectrometers include photomultiplier tubes, diode arrays and charge-coupled devices (CCDs).  A photomultiplier tube, useful for light detection of very weak signals, is a photo emissive device in which the absorption of a photon results in the emission of an electron. These detectors work by amplifying the electrons generated by a photo cathode exposed to a photon flux.  A diode array detects which light is absorbed and which is reflected.  A CCD chip, is an array of light-sensitive elements. Which are, in fact, some small electronic capacitors. The electrons generated by the light charge these capacitors. In fact, each light element (commonly called photon), that reaches the CCD array's atoms, displaces some electrons, which are providing a current source. These current sources, are localized in small, delimited areas (the capacitors), called pixels.

Important parameters in specifying UV and visible spectrometers are wavelength range, resolution and accuracy.  Wavelength range is the dispersion of the grating across the linear array; also expressed as the "size" of the spectra on the array. The spectral resolution is the width of an analytical peak at half its height expressed in nanometers. A measure of the instruments ability to separate two overlapping peaks. At the point of resolution, two peaks of equal height are said to be resolved if you can see a dip between the tops of the two peaks separate two overlapping peaks. At the point of resolution, two peaks of equal height are said to be resolved if you can see a dip between the tops of the two peaks. The wavelength accuracy is the agreement between a mean measured value and a true or accepted value as quantified by error. Related to determinate errors and quantified by bias. Detector sample volume and sample flow rate range are also important specifications pertaining to UV and visible spectrometers.

Many UV spectrometers and visible spectrometers will give measurement values for transmittance, absorbance and reflectance.  Transmittance is the ratio of the radiant power transmitted by a sample to the radiant power transmitted by a blank in an equivalent cell or by some other means of compensation for solvent absorption, reflection losses, etc.  Absorbance is a measure of concentration of material present: negative log (base 10) of Transmittance [- log 1/ T] of product of extinction coefficient, path length, and concentration, written as A = e bc.  Reflectance is the ratio of the radiant energy reflected by a body to the energy incident on it. Spectral reflectance is the reflectance measured within a specific wavelength interval.

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