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  • Ultraviolet visible spectrometry

  • tells us about electronic transitions in atoms and molecules

  • Spectra

  • are produced when it electons in molecules or atoms move from one electronic energy

  • level

  • to another of higher energy. In doing so they absorb energy

  • equal to the gap between the two levels. Compounds that absorb in the visible

  • region such as some transition metal compounds inorganic

  • dyes are colored. Those that

  • absorb only in the ultraviolet region are colorless.

  • Inside a UV visible spectrometer

  • there are usually two light sources, one giving out visible light

  • and one ultraviolet. This one is a tungsten lamp

  • like a car headlamp bulb for the visible region. And this

  • teuterium lamp gives out ultraviolet. This mirror

  • directs light from the appropriate source into the monochromator.

  • This contains a diffraction grating

  • that acts rather like the playing surface of a CD to split the light

  • into it's constituent wavelengths. Different wavelengths correspond to

  • different colours. Red

  • is about 700 nanometers and blue around 400 nanometers.

  • Wavelengths shorter

  • than about 350 nanometers are called ultraviolet.

  • Shorter wavelength light has high-energy.

  • The source produces white light

  • that includes all wavelengths all colors. The

  • instrument scans through the spectrum sending different wavelengths of light

  • through the sample

  • in sequence. This is done by the grating which rotates.

  • A single wavelength passes into the modulator

  • which consists of a rotor with mirrors on it.

  • This chops the light into two beams. One beam passes through the sample cell

  • while the other passes through the reference cell.

  • So the instrument is referred to as a double beam

  • instrument. Both sample and reference beams

  • are directed by mirrors onto a detector. This compares their intensities

  • and send a signal proportional to the ratio of their intensities

  • to the computer that controls the instrument. The logarithm of

  • this ratio gives a quantity called absorbance which is a measure

  • how much light is being absorbed by the sample at that particular wavelength.

  • Ultraviolet visible spectra are

  • usually run on solutions light does not normally pass through solid samples.

  • Here we will run the spectrum of a green food dye.

  • To run the spectrum we place some of the solvent in a sample cuvette to act as a

  • blank,

  • a reference.

  • The cuvette may be made of glass or plastic if only the visible region of

  • the spectrum is required.

  • Quartz cuvettes are needed for work in the ultraviolet range

  • because glass and plastics absorb UV light.

  • We then place a solution of the sample in a second cuvette. The blank

  • and the sample are placed in the sample holders. The lid is closed

  • to prevent light from the laboratory interfering with spectrum.

  • The operator types details such as the wavelength range required

  • and scanning speed into the computer that controls the instrument.

  • He also zeros the instrument at a point where the sample does not absorb

  • and then starts the scan. The spectrum appears on screen,

  • the data is saved on the computer and a hard copy can be printed.

  • The horizontal axis is normally wavelength

  • and the vertical one absorbance which is a measure the amount of light absorbed by

  • the sample.

  • The peaks on the spectrum

  • are the wavelengths of light that are absorbed by the sample and the troughs

  • and where light passes through. So the dye

  • absorbs orange and blue light and lets through green.

  • So it appears green in color

Ultraviolet visible spectrometry

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B2 中上級

紫外・可視分光法(UV-Vis (Ultraviolet/Visible Spectroscopy (UV-Vis))

  • 59 6
    Cheng-Hong Liu に公開 2021 年 01 月 14 日
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