Editorial Feature

An Introduction to Optical Spectrometry (Spectrophotometry)

How does a spectrophotometer work? - Video Credit: BioNetwork | YouTube

Spectrophotometers, like all spectrometers, are devices used to discover the components which make up a material. This is done by measuring the radiant energy, also known as electromagnetic radiation, or light that is either transmitted through the material or reflected by the material. This radiant energy is plotted against the wavelength of the radiant energy.

Graphs such as these have already been carried for many elements and materials, providing scientists with a large data set to reference. Scientists take their analysis of the materials spectrophotometry and compare their graphs to the graphs of known materials in order to deduce the composition of the material being studied. By comparing a materials’ spectrophotmetric analysis using this procedure, scientists are able to determine the elements that a material is composed of.

Spectrometers vs. Spectrophotometers

This analysis however is not the same for all spectrometers and the procedure outlined is more specific to spectrophotometers. There are some key differences between the way spectrometers function and the way spectrophotometers function.

Spectrometers measure which wavelengths a material absorbs and which wavelengths it reflects, whilst a spectrophotometer measures the amount of light or the intensity of light a material absorbs or reflects at a specific wavelength. As a result, spectrophotometers can provide more information about the material being studied than a spectrometer.

All of this analysis and spectrophotometry is carried out using a complex system of components working together to analyze the material. Essentially, a light source is fired on to a monochrometer, then the sample being studied before hitting a detector.

Light Source

The light source is arguably the most important component of any spectrophotometer. As such a number of characteristics are being considered when a manufacturer is choosing a light source for a spectrophotometer.

A light source should be bright across many wavelengths so that the results obtained from the spectrophotometer can be easily linked to a known material’s spectrophotometer results. The light source also needs to be incredibly stable, in terms of both brightness and the lifespan of the light source, so that the light source won’t be creating false readings due to its own variation.

The final important consideration for a good spectrophotometer light source is its cost. If a material within the light source is so rare that it greatly increases the cost of the light source, it is very unlikely that it will be used in a spectrophotometer.

Tungsten Halogen Lamps as Spectrophotometer Light Sources

Tungsten halogen lamps work in a similar way to how the average household incandescent light bulb works. A filament within the lamp, composed of tungsten, is supplied an electrical current which heats it and emits light in both the visible spectrum and the near infrared spectrum.

The tungsten filament begins to evaporate from this heat but the small amount of halogen within the inert gas of the lamp bulb causes the tungsten to return to the filament. The effect of the halogen returning the tungsten to the filament is that the filament is able to maintain its brightness, whilst still having a long lifespan. This makes tungsten halogen lamps one of the most common light sources for spectrophotometers.

Monochrometer

The light produced by the light source, such as a tungsten halogen lamp, is then focused and directed by a concave mirror towards the monochrometer. Monochrometers serve to split up the white light from the lamp into its component colors.

In the past, monochrometers were prisms but in more recent years diffraction gratings have been used. These diffraction gratings are normally ruled gratings which are made by etching parallel grooves into the surface of the material.

These grooves serve to split up the white light into its component colors, as the interference caused by the grooves causes different wavelengths of light to reflect away in different directions. The now separated wavelengths propagate towards the sample compartment.

Sample Compartment

The sample compartment serves to hold the sample in the path of the monochromatic light produced by the light source and split by the monochrometer. There are two major groups of sample compartments: single beam and double beam.

A single beam spectrophotometer contains only one sample and only one beam of light passes through that sample. A double beam spectrophotometer contains slots for two samples to be placed and one light beam passes through each sample.

A two beam spectrophotometer has the beam of light from the monochrometer split into two beams before it enters into the sample compartment. This two beam setup is more useful when trying to compare the unknown sample with a known element or reference sample. A two beam setup is often used with one of the sample compartments left as blank.

The measurements obtained from this empty blank compartment will show any inaccuracies the machine may have so that the analysis of the sample will be proven to be more accurate. This calibrating measurement can also be done in a single beam spectrophotometer by measuring an empty sample compartment before measuring the sample.

Detector

Once the light beams have passed through the samples, they will continue onwards to the detector. There are a huge number of different detectors that are used in spectrophotometers as a detectors’ effectiveness varies with the wavelengths being used in the spectrophotometric analysis. Two common detectors for ultraviolet and visible regions are photomultipliers and silicon photodiodes.

Photomultipliers have a sensitivity to light that other detectors cannot match. This is achieved by using a photoelectric surface. Photoelectric surfaces release photoelectrons when subjected to light and these photoelectrons cause further electron emissions that ultimately make the detector incredibly sensitive to light.

This high level of sensitivity is important when a light source isn’t as powerful or when precise measurements are needed. The effectiveness and sensitivity of photomultipliers as a detector is decided by what photoelectric material is used in the detector.

Silicon photodiodes, when compared to photomultipliers, are far less expensive to make. Using the internal photoelectric effect, by which the electrical properties of a semiconductor change when exposed to electromagnetic radiation (light), silicon photodiodes function as detectors. A photomultiplier on the other hand uses the external photoelectric effect where electrons are freed from the surface when energy is absorbed from light.

References and Further Reading

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