How Does a Spectrophotometer Work

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How Does a Spectrophotometer Work?

Brian LewIST 8A

Winter 2007

February 2, 2007

I. IntroductionWhat is a Spectrophotometer?II. Components of the SpectrophotometerIII. How a Spectrophotometer Works

A. The Light PathB. The Charge-Coupled Device (CCD)C. The Interpreter

IV. Different Types of SpectrophotometersA. Single Beam vs. Double BeamB. Visible LightC. Ultraviolet LightD. Infrared Light

V. Uses of a Spectrophotometer

Abstract

A spectrophotometer is a device that measures light intensity as a function of

wavelength. It does this by diffracting the light beam into a spectrum of wavelengths,

detecting the intensities with a charge-coupled device, and displaying the results as a

graph. There are different types of spectrophotometers for different purposes.

I. Introduction What is a Spectrophotometer?

A spectrophotometer is a device to measure light intensity at different

wavelengths. It produces light with a light source, and after the light passes through a

subject, the light is diffracted into a spectrum which is detected by a sensor and

interpreted into results we can use.

The output of a spectrophotometer is usually a graph of light intensity versus

wavelength. The data collected to generate this graph can typically be saved as a table of


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wavelengths and intensities. The yvalues of the graph can be represented as either

transmittance or absorbance.

II. Components of the Spectrophotometer

There are four main parts of a spectrophotometer: the light source, subject,

detector, and interpreter. Some examples of light sources are visible, infra red, and

ultraviolet light. The light created by the light source passes through the subject where

some light is usually absorbed, is received by the sensor, and is interpreted into an output

such as a graph.

III. How a Spectrophotometer Works

A. The Light path

In the Ocean Optics USB2000 Spectrophotometer, light is produced by a violet

LED-boosted tungsten lamp. The light passes through the sample (usually a solution in a

cuvet) and enters the spectrophotometer through a slit. The narrow slit disperses the

light, spreading it out. The light reflects off of a concave collimating mirror and is

reflected to a dispersion grating. The dispersion grating reflects the light and also

disperses it towards a second concave mirror. This focusing mirror focuses the light onto

a detector.


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Figure 1. Schematic Optical Path of the Ocean Optics USB2000 Spectrophotometer

Figure 2. Drawing of Ocean Optics Spectrophotometer


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B. The Charge-Coupled Device (CCD)

At the end of the light path is the detector. In most spectrophotometers, it is a

linear charge-coupled device (CCD). A CCD is a type of image sensor that detects light.

It is an integrated circuit made up of an array (or in this case, a linear arrangement) of

linked/coupled light-sensitive capacitors. The light-sensitive capacitors detect the

intensity of light received and convert it into an electrical signal.

The linear CCD detector corresponds to the range of wavelengths on a hand held

spectrophotometer. Each pixel on the CCD represents a specific wavelength of light, and

the more photons absorbed, the more electrical signal generated. Therefore, the electrical

signal output by the CCD at each pixel is proportional to the light intensity at each

corresponding wavelength.

C. The Interpreter

Spectrophotometers can have their own display for output, but it is more common

for them to be connected to a computer where software manipulates the data and displays

it in a usable for, like a graph of transmittance or absorbance versus wavelength.

Figure 3. Sample Output Graph of a Spectrophotometer


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IV. Different Types of Spectrophotometers

A. Single Beam vs. Double Beam

There are two classes of spectrophotometers: single and double beam. The single

beam spectrophotometer was the first invented, and all the light passes through the

sample. In this case, to measure the intensity of the incident light, the sample must be

removed so all the light can pass through. This type is cheaper because there are less

parts and the system is less complicated. Later, the double beam spectrophotometer was

invented. In this type, the light source is split into two separate beams before it reaches

the sample. One beam is used for reference and the other passes through the sample.

This is advantageous because the reference reading and sample reading can be taken at

the same time. In some double beam spectrophotometers, there are two detectors and the

sample and reference beams can be measured simultaneously. Other double beam

spectrophotometers that have only one detector use a beam chopper. This device inside

blocks one beam at a time and the detector alternates between measuring the sample and

reference beams.

B. Visible Light

The visible region of light is about 400-700nm. Visible region

spectrophotometers vary in accuracy. Some have CCD detectors with enough pixels to

take reading every 10nm, while others can take several reading per nanometer. These

spectrophotometers can use incandescent, halogen, LED, or a combination of these

sources. For example, the Ocean Optics USB2000 Spectrophotometer uses an LED

boosted tungsten bulb.


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Figure 5. Ocean Optics USB4000 Spectrophotometer

Figure 6. Spectrum of a Halogen lamp, using Ocean Optics USB400


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C. Ultraviolet Light

UV spectroscopy is most commonly used for liquids, but can also be used for

gases and even solids. Samples are placed in a cuvette, a small rectangular container,

usually 1cm in width. These are can be made of plastic, glass, or quartz (listed in

increasing expense). Plastic and glass absorb UV, so they can only be used for visible

light spectroscopy.


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Figure 4. Spectrum of Ethyl Alcohol

D. Infrared Light

Infrared spectroscopy is used to study molecules and the vibrations associated

with their structures. Different chemical structures vibrate in different ways in response

to different wavelengths, due to the varying energies associated with each wavelength.

For example, mid-range infrared tends to cause rotational vibrations, while the near

infrared (higher energy) tends to cause whole molecule harmonic vibrations like

stretching, and rocking.

V. Uses of a Spectrophotometer

Spectrophotometers are directly used to measure light intensity at different

wavelengths, and this can be represented as percent of incident light transmitted or

absorbed. Using this information and comparing it to other data obtained or known,

spectroscopy can be used as a tool. One example is comparing spectra to determine


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concentrations of a solute in solution. This can be done by recording

transmittance/absorbance at a specific wavelength (a wavelength that the solute absorbs)

and known concentration. Then analysis of a solution of unknown concentration can be

compared to the known data, and be interpolation the concentration can be calculated.

This can even be done with solutions containing multiple solutes, however is it most

accurate when the different solutes absorb different wavelengths.

Spectrometers that do not have a light source, but generate spectra based on the

incoming light can be used in a similar way to identify light sources. The spectra graph

obtained from an unknown light source (or mixture of sources) can be compared to a

database of graphs for different known light sources to identify the unknown light source.

Another application of the spectrophotometer is to determine the equilibrium

constant of a reaction involving ions, which takes place in aqueous solution. Starting a

solution containing only one reactant, the spectrum is measured. Then small, measured

amount of the other reactant is added and after each addition, the spectrum is measured

again. This method works best if there is a known wavelength that the product absorbs.

Then, as more products are formed from adding more reactant, more light will be

absorbed. When the solution becomes saturated and the reaction reaches net equilibrium,

the increase in light absorption will level out, indication equilibrium.

Bibliography

Balch, Alan. Chemistry 2BH Laboratory Manual. Davis, CA: University of California,

Davis, 1999.

Charge-coupled device. Wikipedia. 5 Feb. 2007. 10 Feb. 2007


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.

Infrared spectroscopy. 9 Feb. 2007. 10 Feb. 2007

.

Spectrophotometry. Wikipedia. 8 Feb. 2007. 10 Feb. 2007

.

Oxtoby, D. W.; Gillis, H.P.; and Nachtrieb, Norman H. Principles of Modern Chemistry.

Fifth edition. South Melbourne, Australia: Thomson Learning, 2002.

Ultraviolet-visible spectroscopy. Wikipedia. 2 Feb. 2007. 10 Feb. 2007

.

How Does a Spectrophotometer Work

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