Ultraviolet-visible spectroscopy

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Ultraviolet-visible spectroscopy or ultraviolet-visible spectrophotometry (UV/ VIS) involves the spectroscopy of photons and spectrophotometry. It uses light in the visible and adjacent near ultraviolet (UV) and near infrared (NIR) ranges. In this region of energy space molecules undergo electronic transitions.

Contents

1 Applications
2 Beer-Lambert Law
3 Ultraviolet-visible spectrophotometer
4 Ultraviolet-visible spectrum
5 Types of spectrophotometers
6 See also
7 External links

Applications

UV/Vis spectroscopy is routinely used in the quantitative determination of solutions of transition metals and highly conjugated organic compounds. It is possible to do so because transition metals are often colored because of the possibility of d-d electronic transitions within the metal atoms. Organic molecules, especially those with a high degree of conjugation also absorb light in the UV or visible regions of the electromagnetic spectrum.

The solvents for these determinations are often water for water soluble compounds, or ethanol for organic-soluble compounds due to the low UV-cutoff.

The Beer-Lambert law states that within small ranges, the concentration of the desired compound varies linearly with the absorbance. Thus UV/VIS spectroscopy can determine the concentration of an unknown solution, based on reference molar extinction coefficients or more accurately, using a calibration curve.

While charge transfer complexes also give rise to colors, the colors are often too intense to be used for quantitative measurement.

Beer-Lambert Law

The method is most often used in a quantitative way to determine concentrations of an absorbing species in solution, using the Beer-Lambert law:

[A =\ ] −[\log_(I/I_0) = \epsilon\cdot c\cdot L],

where A is the measured absorbance, [I_0] is the intensity of the incident light at a given wavelength, [I] is the transmitted intensity, L the pathlength through the sample, and c the concentration of the absorbing species. For each species and wavelength, ε is a constant known as the molar absorptivity or extinction coefficient. This constant is a fundamental molecular property in a given solvent, at a particular temperature and pressure, and has units of [1/M*cm] or often [AU/M*cm].

The absorbance and extinction ε are sometimes defined in terms of the natural logarithm instead of the base-10 logarithm.

The Beer-Lambert Law is useful for characterizing many compounds but does not hold as a universal relationship for the concentration and absorption of all substances. A 2nd order polynomial relationship between absorption and concentration is sometimes encountered for very large, complex molecules such as organic dyes (Xylenol Orange or Neutral Red, for example).

Ultraviolet-visible spectrophotometer

The instrument used in Ultraviolet-visible spectroscopy is called a ultraviolet-visible spectrophotometer. To obtain absorption information, a sample is placed in the spectrophotometer and ultraviolet or visible light at a certain wavelength, or range of wavelengths, is transmitted through the sample. The spectrophotometer measures how much of the light is absorbed by the sample. The intensity of light before going into a certain sample is symbolized by [I_0]. The intensity of light remaining after it has gone through the sample is symbolized by [I]. The fraction of light transmittance is ([I/I_0]), which is usually expressed as a percent Transmittance (%T). From this information, the absorbance of the sample is determined for that wavelength or as a function for a range of wavelengths. Sophisticated UV/ Vis spectrophotometers often do this automatically.

Although the samples could be solid, or even gaseous, they are usually liquid. A transparent cell, often called a cuvette, is used to hold a liquid sample in the spectrophotometer. The pathlength L through the sample is then the width of the cell through which the light passes through. Simple, economical spectrophotometers may use cuvettes shaped like cylindrical test tubes, but more sophisticated ones use rectangular cuvettes, commonly 1 cm in width. For just visible spectroscopy, ordinary glass cuvettes may be used, but ultraviolet spectroscopy requires special cuvettes made of a ultaviolet-transparent material such as quartz.

Ultraviolet-visible spectrum

An ultraviolet-visible spectrum is essentially a graph of light absorbance versus wavelength in a range of ultraviolet or visible regions. Such a spectrum can often be produced by a more sophisticated spectrophotometer. Wavelength is often represented by the symbol λ. Similarly, for a given substance, a standard graph of extinction coefficient ε vs. wavelength λ may be made or used if one is already available. Such a standard graph would be effectively "concentration-corrected" and thus independent of concentration. For the given substance, the wavelength at which maximum absorption in the spectrum occurs is called λ_max, pronounced "Lambda-max".

Woodward-Fieser rules are a set of empirical observations which can be used to predict λ_max, the wavelength of the most intense UV/Vis absorption, for conjugated organic compounds such as dienes and ketones.

Types of spectrophotometers

In a single-beam ultraviolet-visible spectrophotometer, the light only passes through the sample. In a double-beam ultraviolet-visible spectrophotometer, the light passes through a beam chopper which alternately directs the beam through the sample or a reference cell several times per second.

External links

[The Science of Spectroscopy] - supported by NASA. Spectroscopy education wiki and films - introduction to light, its uses in NASA, space science, astronomy, medicine & health, environmental research, and consumer products.

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