Atomic Spectroscopy is based upon the ability of atoms to absorb or emit light. In atomic absorption spectrophotometry the atoms are heated enough in a flame or graphite tube to free them from solvents and disrupt the formation of salts, but not enough to pump electrons to an excited electronic state. The free atoms, with electrons in the ground state, are ready to absorb light of just the right energy to promote electrons to an excited electronic state. (Atoms, having no vibrational modes, absorb and emit very narrow bands of light.) The greater the concentration of that atom in the sample, the less light reaches the detector; so it is relatively straight forward to determine concentration of a particular atom in a sample by measuring how much light it absorbs.
Instrumentation used to carry out atomic absorption spectrophotometry requires a source of light that matches the narrow bands of light that a particular atom absorbs (a hollow cathode lamp), a flame or graphite furnace to heat the sample, a monochromator to select the wavelength of light, and a photodetector.
In atomic emission spectrophotometry the purpose of heating the sample is not only to free the atoms from the solvent and formation of salts, but to provide enough energy to pump electrons into excited electronic energy levels. As these electrons fall back to lower energy levels they emit light at specific wavelengths. The more light that is detected at that wavelength, the more of that element must be in the sample. Instrumentation to carry out atomic emission would not need a lamp. Inductively coupled plasmas exist at extremely high temperatures (around the temperature of the surface of the sun), and are used in atomic emission spectroscopy.
Varian 220 FS Atomic Spectrophotometer
This Atomic Spectrophotometer operates in both atomic absorption and emission modes. The FS in the name stands for Fast Scanning; which means that this instrument can take a measurement for one element then quickly scan to the next wavelength to collect a measurement for another element, then the next, etc. Which means that the instrument can collect data on approximately ten elements in the same time that it would take to measure one element. It has 4 lamp sockets, and can automatically switch between these four lamps using a moveable mirror. The mirror and the monochromator wavelength is controlled by software as is the data collection. So the instrument can be fully automated.
Perkin Elmer 5000 Atomic Spectrometer
The PE 5000 was a popular AA back in the mid to late 1980s, and there are a lot of these spectrometers still in operation. They are very large instruments that are not interfaced to a PC, but have microprocessor control of most instrument functions from the front keypad. The model we have also has a graphite furnace attachment with an autosampler. This spectrometer is in excellent working condition and provides us with flexibility when several groups of students are working on AA at the same time.
Applications of Atomic Absorption and Emission Spectrophotometry
In Analytical Chemistry courses, students are asked to propose an analysis using atomic spectroscopy and execute it. Some of the analyses performed over the past few years have included:
Determination Mn and
Fe in cat hair and cat food to determine a link
Analysis of Ni and Cu in tea leaves
Determination of Ca and Mg in coal and coal ash
Analysis of a bullet from an antique anti-vampire kit to determine the purity
of silver
Determine the elemental composition of a statue
Antimony and silver in bullet lead
Quantitation of Fe in a few green vegetables
Water hardness
Effectiveness of soils as metal traps
Analysis of lead in paint
Determination of zinc, sodium and potassium in urine
Analysis of soil for Zn around a superfund site
Other broad areas where
atomic spectroscopy is used to quantify elements include:
Agriculture
Biological
Industrial
Food
Forensics
Geochemistry
Petroleum
Environmental Analysis
etc.
Links to Atomic Spectroscopy Resources
Vendors of Atomic Spectrophotometers
Agilent
Perkin Elmer
Aurora Instruments
Shimadzu
Atomic Spectroscopy Resources
Journal
of Analytical Atomic Spectroscopy
Spectrochimica Acta Part B: Atomic Spectroscopy
AA
Tutorial
http://www.gmu.edu/departments/SRIF/tutorial/aas/aas.htm