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Chemistry of a nation on file
For many decades, samples of geologic materials (rocks, soils, sediments, waters, and others) have been chemically analyzed. The geochemical data collected from these and other scientific programs and projects provide the basis of a growing national geochemical data base. A part of the data base contains chemical analyses of stream sediments from hundreds of thousands of drainage basins throughout the United States. These analyses represent the chemistry of surface materials in these basins and may be used in many applications concerning health, the environment, and natural resources. (Scroll up a few screens to see the map of lead in stream sediments of Colorado.)
This map shows the coverage of analytical data generated during theNational Uranium Resource Evaluationprogram that is stored in the National Geochemical Data Base.
10,000 element determinations a day
One of the principal methods of analyzing samples that shows up frequently in the National Geochemistry Data Base is inductively coupled plasma-atomic emission spectrometry (ICP-AES). This method provides a rapid and precise means of monitoring up to 50 elements simultaneously for minor- and trace- levels. The ICP-AES technique is widely regarded as the most versatile analytical technique in the chemistry laboratory.
When the sample solution is introduced into the spectrometer, it becomes atomized into a mist-like cloud. This mist is carried into the argon plasma with a stream of argon gas. The plasma (ionized argon) produces temperatures close to 7,000 C, which thermally excites the outer-shell electrons of the elements in the sample.
In an inductively coupled plasma-atomic emission spectrometer the (1) aqueous sample is pumped and (2) atomized with argon gas into the (3) hot plasma. The sample is excited, emitting light wavelengths characteristic of its elements. (4) A mirror reflects the light through the (5) entrance slit of the spectrometer onto a (6) grating that separates the element wavelengths onto (7) photomultiplier detectors.
The relaxation of the excited electrons as they return to the ground state is accompanied by the emission of photons of light with an energy characteristic of the element. Because the sample contains a mixture of elements, a spectrum of light wavelengths are emitted simultaneously. Just as rain breaks sunlight into a rainbow, the spectrometer uses a grating to disperse the light, separating the particular element emissions and directing each to a dedicated photomultiplier tube detector. The more intense this light is, the more concentrated the element. A computer converts the electronic signal from the photomultiplier tubes into concentrations. The determination portion of the process takes approximately 2 minutes to complete. In 1 day a chemist using the ICP-AES can analyze 200 samples for a total of 10,000 elemental determinations. Return to this point in index.
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