Components of a Spectrophotometer

Light Source

• Must be able to produce light within the range that will be used to excite the sample

Continuous Sources

• Produce light in a wide range of wavelengths using blackbody radiation
• Each wavelength will have a different relative intensity that must be accounted for in reading the background
• Multiple lamps may be used to get complete coverage of the range to be studied
  • E.g. Tungsten lamp (Vis) + Deuterium lamp (UV)

Line Sources

• Excitation of a specific element without blackbody will produce a line spectra
• The spectra emitted must match the sample to be analysed
  • E.g. Ca lamp to analyse Ca content

Lasers

• Produce directional light within a specific range of wavelengths
• High power
• Very expensive to fine tune a laser to desired specifications

Sample Holder

• Must be transparent to the light being analysed
• Must be chemically inert so as to not react with the sample
• Must be mechanically strong, so as to ensure a long life
  • Expensive and weak is unsustainable

Monochromator

Prisms

• Split light based on the varying refractive indexes of different wavelengths
• Must be transparent in the wavelength to be used

Diffraction gratings

• Implement many fine scratches on a surface to diffract light of different wavelengths at different angles
• Much cheaper and ore reliable than prisms
• The width of the grooves determines the wavelengths diffracted
  • 50\(\mu\)m apart for Far IR
  • 166nm apart for UV-Vis
• Can be made of plastic to reduce cost further

Controlling the light source

• The light needs to be collimated (made parallel) and focused appropriately to select for individual wavelengths
• The Reflection grating will scatter the light, and the light will be focused again by a concave mirror
• The exit slit will determine the selected wavelength
  • The narrower the exit slit, the greater the precision of the instrument, however the less light will be passed through the sample
  • A Larger slit will result in a higher SNR

Photodetectors

• Use two principles:
  1. External photoelectric effect: electrons become free from the metal surface by energy absorption obtained by streams of incident photons
  2. Internal photoelectric effect: free charge carriers are generated by absorption of incident photons in semiconductor junctions
• Choice off photodetector depends on the wavelengths being studied

Photomultiplier

• Light falls on a photosensitive alloy which causes electrons to be fired towards secondary electrodes called dynodes
• This causes them to gain more energy and release more electrons (4-50 per dynode)
• There is a cascade of dynodes so the process is amplified to create a much larger signal than a single photon would otherwise produce
• The signal is then detected by an anode and sent to a computer to plot the results

Photodiode Array

• Consist of an array of silicon based photodetectors
• As light hits the substrate, electrons are released, which cause a change in current required to power the substrate. The change in current is detected and plotted
• The array is set up in a way that each diode will detect for a specific wavelength range
• Much cheaper to produce yet are less accurate
• Have lower resolution that photomultipliers but can measure multiple wavelengths at the same time
• Contains no moving parts, which makes them more durable