Ultra Violet- Visible- Near Infrared spectrometer

Ultra Violet- Visible- Near Infrared spectrometer

A gemmologist commonly uses hand-held or desk model spectroscopes to observe the absorption features of a gemstone, in the visible region 400 - 700 nm. However, human eyes are not capable to observe very minute features, which are missed to be seen and hence, may affect the conclusions and overall results. Therefore, spectrometers or spectrophotometers may be used, which are capable to analyse the sample in detail and registers the absorption features in the form of a graph. These spectrometers not only analyses the sample within 400 - 700 nm range, but much beyond this, from 175 nm in the ultra violet region up to 3300 nm in near-infra red region. This however depends on the type and grade of instrument and accordingly, the spectrometers can be termed as UV-Vis or UV-Vis-NIR spectrometers. 

In UV-Vis-NIR spectrometry, the scale or position of absorptions is expressed in wavelength nanometers (nm) as against the IR or Raman spectrometers, using wavelength inverse (cm-1). The UV-Vis-NIR spectrometry is based on selective filtering of wavelengths through the sample. The absorbed energies or wavelengths are displayed in the form of a graph and expressed in comparison with the incident light i.e. original light emitted from the source.
In general, double beam spectrometers are used, where a beam splitter divides the incoming light beam into two beams, one for the sample and another for a reference (known as reference beam). A typical spectrophotometer contains a light source, which may be deuterium and tungsten or a xenon lamp; monochromator, which is basically a wavelength selector and selects the required wavelength from many wavelengths emitted from the source, as per the application; and detectors, which are usually photomultiplier tubes used to measure the dispersed components of the beam after it passes through the sample. 

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