Knowledge base: a short introduction to spectrophotometers

1. What is a spectrophotometer?

A spectrophotometer is an instrument which measures the transmitted or reflected radiation energy as a function of wavelength or frequency.A typical spectrophotometer consists of two main components:

- a monochromater or spectrometer which is used to produce a narrow band radiation source for the measurement,

- a photometer which is used for the energy/intensity measurement of the beam.

A monochromater is an instrument which separates a wide band radiation. Optical gratings, prism or the combination of two prisms, are normally used for this purpose. The radiation is normally focused first, then goes through a slit, a collimating optical system, and illuminates the grating. The reflected or transmitted beam is separated by wavelength spatially, which enables wavelength selection through another narrow slit. The grating, collimation systems have different configurations, which ensure path of the output beam to be relatively stable.

The collimated narrow band beam from the monochromater is then used for the measurement. The importance of a plane wave is to simplify the light path length in the media. The beam passes through the sample under interest. The transmitted intensity is then collected by the detector. Between two measurements, a relative ratio can be acquired. The relative ratio is converted into the absorptance, if the other measurement is well controlled.

Baseline is widely used in the spectrophotometry. Baseline is introduced because the spectral distribution of the radiation source might fluctuate from time to time, moreover, the spectral distribution is unlikely to be uniform (flat).

A spectrophotometer belongs to one of the two main classes: a single beam or a double beam spectrophotometer. This classification depends on whether the beam is split. A double beam spectrophotometer is more complex, however, it is more likely to be immunized to the temporal fluctuation of radiation source.

2. What is a spectrophotometer used for?

A spectrophotometer is an instrument to carry spectrophotometric analysis or spectrophotometry. The analysis itself is normally a pure physical process; however, it is more likely to be seen in a chemical, biological, environmental, quality control and medical researches or even food industries.

The instrument is also used to study solutions and liquids. a container called the covette which is used, which is manufactured of high performance optical glasses. Normally a few covettes are provided with the purchase of the instrument. The absorptive property of the covettes can be accounted into the baseline by inserting the covette with the solvent into the path in a single beam spectrophotometer.In a double beam spectrophotometer, the absorptive property of the covettes can be accounted into the baseline in the reference path.

3. General specifications and tips to select a proper product for your research.

A typical spectrophotometer includes following specifications that you might pay attention to when you choose a product:

  • spectral range, wavelength range or wavenumber range:
  • Shows the spectrum that the monochromater/spectrometer can provide and the detector is capable to detect. The unit for the wavelength or spectral range should be in nm or μm, while the unit for wave number (denoted by k and k=2×p/l) range should be inverse of the length unit. Check if it satisfies your requirement.
  • spectral bandwidth:
  • Explains how well the output beam from the monochromater/spectrometer tends to be monochromatic. Normally, narrower spectral bandwidth is better.
  • wavelength accuracy:
  • Indicates how close the wavelength of the beam from the monochromater/spectrometer is to the value it claims to be.
  • wavelength reproducibility or wavelength repeatability:
  • Indicates how well the instrument can reproduce a narrow band beam for measurement.
  • optical system:
  • This is normally used to specify what kind of optical system is in the monochromater; e.g, configuration, grating, etc.
  • Photometric Range:
  • Shows photometric measurement range, normally in Abs. For absorption in transmitted measurement mode, T = Itransmitted/I0/I×100; and Abs = - log10(T/100) = log10(I0/I)
  • Photometric Accuracy:
  • How accurate the measurement will be, in Abs or Transmittance percentage
  • Photometric Reproducibility or photometric repeatability:
  • Shows how well the measurement value can be reproduced, normally shown in Abs.
  • Size, weight, power supply requirement, etc