Ultraviolet-visible spectroscopy

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Categories: Spectroscopy | Scientific techniques

Ultraviolet-visible spectroscopy or ultraviolet-visible spectrophotometry (UV/ VIS) involves the spectroscopy of photons in the UV-visible region. It uses light in the visible and adjacent near ultraviolet (UV) and near infrared (NIR) ranges. In this region of the electromagnetic spectrum, molecules undergo electronic transitions. This technique is complementary to fluorescence spectroscopy.

1 Applications
2 Beer-Lambert law
3 Ultraviolet-visible spectrophotometer
4 Ultraviolet-visible spectrum
5 See also
6 External links

Applications

UV/Vis spectroscopy is routinely used in the quantitative determination of solutions of transition metal ions and highly conjugated organic compounds.

Solutions of transition metal ions can be coloured (i.e., absorb visible light) because d electrons within the metal atoms can be excited from one electronic state to another. The colour of metal ion solutions is strongly affected by the presence of other species, such as certain anions or ligands. For instance, the colour of a dilute solution of copper sulphate is a very light blue; adding ammonia intensifies the colour and changes the wavelength of maximum absorption (Failed to parse (Missing texvc executable; please see math/README to configure.): \lambda_{max}

Organic compounds, 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. (Organic solvents may have significant UV absorption; not all solvents are suitable for use in UV spectroscopy. Ethanol absorbs very weakly at most wavelengths.) Solvent polarity and pH can effect the absorption spectrum of an organic compound. Tyrosine, for example, increases in absorption maxima and molar extinction coefficient when pH increases from 6 to 13 or when solvent polarity decreases.
While charge transfer complexes also give rise to colours, the colours are often too intense to be used for quantitative measurement.

The Beer-Lambert law states that the absorbance of a solution is directly proportional to the solution's concentration. Thus UV/VIS spectroscopy can be used to determine the concentration of a solution. It is necessary to know how quickly the absorbance changes with concentration. This can be taken from references (tables of molar extinction coefficients), or more accurately, determined from a calibration curve.

A UV/Vis spectrophotometer may be used as a detector for HPLC. The presence of an analyte gives a response which can be assumed to be proportional to the concentration. For accurate results, the instrument's response to the analyte in the unknown should be compared with the response to a standard; this is very similar to the use of calibration curves. The response (e.g., peak height) for a particular concentration is known as the response factor.

Beer-Lambert law

Main article: 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:

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−Failed to parse (Missing texvc executable; please see math/README to configure.): \log_{10}(I/I_0) = \epsilon\cdot c\cdot L

where A is the measured absorbance, Failed to parse (Missing texvc executable; please see math/README to configure.): I_0

is the intensity of the incident light at a given wavelength, Failed to parse (Missing texvc executable; please see math/README to configure.): 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 Failed to parse (Missing texvc executable; please see math/README to configure.): 1/M*cm
or often Failed to parse (Missing texvc executable; please see math/README to configure.): 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

See also: Spectrophotometry

The instrument used in ultraviolet-visible spectroscopy is called a UV/vis spectrophotometer. It measures the intensity of light passing through a sample (Failed to parse (Missing texvc executable; please see math/README to configure.): I ), and compares it to the intensity of light before it passes through the sample (Failed to parse (Missing texvc executable; please see math/README to configure.): I_o ). The ratio Failed to parse (Missing texvc executable; please see math/README to configure.): I/I_o

is called the transmittance, and is usually expressed as a percentage (%T). The absorbance, Failed to parse (Missing texvc executable; please see math/README to configure.): A

, is based on the transmittance:

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The basic parts of a spectrophotometer are a light source (often an incandescent bulb for the visible wavelengths, or a deuterium arc lamp in the ultraviolet), a holder for the sample, a diffraction grating or monochromator to separate the different wavelengths of light, and a detector. The detector is typically a photodiode or a CCD. Photodiodes are used with monochromators, which filter the light so that only light of a single wavelength reaches the detector. Diffraction gratings are used with CCDs, which collects light of different wavelengths on different pixels.

Image:UV-vis.png

Diagram of a single-beam UV/vis spectrophotometer.

A spectrophotometer can be either single beam or double beam. In a single beam instrument (such as the Spectronic 20), all of the light passes through the sample cell. Failed to parse (Missing texvc executable; please see math/README to configure.): I_o

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-beam instrument, 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. Some double-beam instruments have two detectors (photodiodes), and the sample and reference beam are measured at the same time. In other instruments, the two beams pass through a beam chopper, which blocks one beam at a time. The detector alternates between measuring the sample beam and the reference beam.

Samples for UV/Vis spectrophotometry 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. (This width becomes the path length, Failed to parse (Missing texvc executable; please see math/README to configure.): L , in the Beer-Lambert law.) Test tubes can also be used as cuvettes in some instruments. The best cuvettes are made of high quality quartz, although glass or plastic cuvettes are common. (Glass and most plastics absorb in the UV, which limits their usefulness to visible wavelengths.)

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 directly by a more sophisticated spectrophotometer, or the data can be collected one wavelength at a time by simpler instruments. Wavelength is often represented by the symbol λ. Similarly, for a given substance, a standard graph of the 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 transmitance in the spectrum occurs is called λ_max, pronounced "Lambda-max".

The Woodward-Fieser rules 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.

This spectrum can be used qualitatively to identify components in a sample as each component has their own unique absorbance spectrum (like a fingerprint).

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.
Algorithms used in Spectroscopy