Spectrometers & Spectroscopy
Spectroscopy is the study of the interaction of electromagnetic radiation in all its forms with matter. This interaction might give rise to electronic excitations, (e.g. UV), molecular vibrations (e.g. IR) or nuclear spin orientations (e.g. NMR).
When a light or other radiation falls upon certain material liquid, gas or solid, a part of it gets absorbed by the material. This absorption causes the atoms, the molecules, and the bonds between them to vibrate at the same range of frequencies as the incident radiation. As a result we either see illuminance or a change in polarization or change in dipole moment. This entirely depends upon the type of radiation we are using.
Spectroscopy is the study of changes that occur in a sample due to absorption of the radiation. Spectrometers use these changes to identify and evaluate the sample. When a beam of white light strikes a triangular prism it is separated into its various components (ROYGBIV). This is known as a spectrum. The optical system which allows production and viewing of the spectrum is called a spectroscope or spectrometer. There are many other forms of light which are not visible to the human eye and spectroscopy is extended to cover all of these.
How does a spectrometer work:
A spectroscopic instrument or spectrometer generally consists of an entrance slit, collimator, a dispersive element, such as a grating or prism, focusing optics and a detector. In a monochromator system there is normally also an exit slit, and only a narrow portion of the spectrum is projected on a single one-element detector. In monochromators the entrance and exit slits are in a fixed position and can be changed in width. Rotating the grating scans the spectrum.
The basic function of a spectrometer is to take in light, break it into its spectral components, digitize the signal as a function of wavelength, and display it through a computer. The first step in this process is to direct light through a fiber optic cable into the spectrometer through a narrow aperture known as an entrance slit. The slit vignettes the light as it enters the spectrometer. In most spectrometers, the divergent light is then collimated by a concave mirror and directed onto a grating. The grating then disperses the spectral components of the light at varying angles, which are then focused by a second concave mirror and reflected on to the detector. Alternatively, a concave holographic grating can be used to perform all three of these functions simultaneously. This alternative has various advantages and disadvantages, which will be discussed in more detail later on.
Once the light is imaged onto the detector the photons are then converted into electrons which are digitized and read out through a USB (or serial port) to a computer. The software then interpolates the signal based on the number of pixels in the detector and the linear dispersion of the diffraction grating to create a calibration that enables the data to be plotted as a function of wavelength over the given spectral range. This data can then be used and manipulated for countless spectroscopic applications.