what is absorbance spectroscopy?
Light is directed at an object, passes through, and is detected on the other side. Some of the light is absorbed, reflected and transmitted. The detector, on the other side of the object, measures how much of that light is absorbed. The detector can also measure which electromagnetic wavebands have been absorbed and which have been transmitted. This, in its most simple sense, is absorbance spectroscopy, also known as absorption spectroscopy.
Why is absorbance spectroscopy useful?
Specific applications for absorbance spectroscopy are outlined further in this article. In the meantime, it is useful to know the general methodological principles and why absorbance spectroscopy is useful analytical tool.
A sample of liquid or gas will contain a number of chemical elements, compounds and molecules. Individual chemical elements and compounds have a unique fingerprint when a known light source is passed through the element and detected on the other side. These spectral signatures can inform scientists which elements and compounds are present in a sample.
An example of a spectral signature is given below. Certain absorption peaks along the electromagnetic spectrum are present for different elements. These absorption peaks are unique to that particular element. For example, the element hydrogen, in visible light, has absorption peaks at 410nm, 434nm, 486nm and 656nm. By comparing measurements against a database of known spectral signatures, a scientist can quickly determine which compounds and elements are present in their sample. Here is an example database that was consulted for the element hydrogen:
Kramida, A., Ralchenko, Yu., Reader, J., and NIST ASD Team (2014). NIST Atomic Spectra Database (ver. 5.2), [Online]. Available: http://physics.nist.gov/asd [2015, September 27]. National Institute of Standards and Technology, Gaithersburg, MD.
Elements, compounds and molecules have absorption peaks at various points along the electromagnetic spectrum and, when plotted, is known as an absorption spectrum or a spectral absorption line identification graph. Below are example spectral absorption line identification graphs for hydrogen (H) and mercury (Hg):
Below are outlined a few example applications where absorption spectroscopy is used. This list is not exhaustive, and the technique is used across many additional fields of science.
Blood and Haemoglobin
Aborbance spectroscopy has been used to determine total haemoglobin as well as oxygenated and deoxygenated haemoglobin in red blood cells. Absorption spectra along the electromagnetic wavelength differs with red blood cells and this information can provide details about physiological state. For example, at wavelengths 550nm and 570nm, the absorption spectra for oxy and deoxy haemoglobin are equal and values at these specific wavelengths provides information on total haemoglobin. Between 610nm and 630nm, absorption spectra for oxygenated haemoglobin is virtually zero whereas there is a small amount of absorption for deoxygenated haemoglobin. Therefore, measurements between 610nm and 630nm provide information on changes in deoxygenated haemoglobin and oxygen extraction.
Image source: www.lmtb.de
Water Quality and Toxins
Absorbance spectroscopy is commonly used to analyse liquid water samples for environmental assessment and management. For example, a sample is taken from groundwater, a river, ocean, or some other water body. The sample is then placed in a cuvette and this cuvette is placed in the VLS-1000 Spectral Analysis System. A halogen lamp light source passes a uniform spectrum of electromagnetic light through the cuvette sample and the VLS-1000 Spectrometer detects wavebands between 300 and 1100nm. Chemical toxins with unique spectral signatures can then be determined.
As with the example given above in this article on our sun’s light passing through a leaf and being measured by a spectrometer, celestial bodies and other suns, emitting light, pass through various astronomical objects and can be measured with spectrometers on Earth or above Earth’s atmosphere. The measurements can then be used to derive information about the chemical composition of exoplanets, as well as temperature, density, mass, distance, luminosity, and relative motion using Doppler shift measurements.
The Hubble Space Telescope is effectively one giant spectrometer. Scientists have been using spectrometer data from the Hubble Space Telescope to determine a large amount of information on the universe. Watch the You Tube video below to learn more about absorbance spectroscopy and the Hubble Space Telescope (video source: NASA):