|MadSci Network: Chemistry|
Nick - Atomic emission spectroscopy is based on the phenomenon that when atome in a gaseous state (like in a flame) are heated the outer electrons are placed in a more energetic state. At some point in time (very rapidly) that energy is lost as a photon of visible light and the electron goes back to its initial (ground) state. A typical home example of that is the yellow flame you see when a bunen burner with a properly adjusted flame (transparent blue flame) is tapped and there is a burst of yellow seen in the flame. That yellow is from sodium (wavelength 5890 Angstrom) that is commonly found in the environment. An atomic emission spectrometer has a heat source (gas burner, graphite furnace, carbon eletrodes, or induction coupled plasma) that excited the sample. The sample is typically dissolved in liquid to form a solution. The resulting solution is then transported into the heat source. For my example, a gas burner with an aspirator that sucks the sample into the burner and then impacted the sample stream against a glass bead to form a mist of the sample. A small portion of the sample (5 to 15 percent)is transported into the flame and is analyzed. The light is given off by the element of interest is focused using lenses onto the entrance slit of a monochromator. A monochromator is a device that filters out all of the light except the color (Wavelength) that I wish to measure. At the exit of the monochromator, a detector of some type is used such as a photomultiplier tube, photodiode, photodiode arrays, or a charged coupled device (CCD) that converts the light into electrical energy. The amount of electrical energy is measured and recorded by a device like a voltmeter, a chart recorder, or a computer. By keeping all variables constant like the temperature of the flame, the amount of sample going into the flame, if I vary the concentration of the material I want to measure, I will get a larger amount of electrical energy out of the detector if I have more of the analyte I'm trying to determine. With five or so standard solutions with known concentrations, I can develop a linear relationship between the amount of light given off in the flame and the concentration of my standards. I then can aspirate an unknown sample and calculated the concentration of the element I am interested in based on the amount of light given off. One point-when I say light given off I am refering to light of a particular wavelength (color) that is characteristic for the element of interest. I will attempt to forward more info and websites to you as my work schedule permits this week. Feel free to e-mail me directly if I can help you further.
Try the links in the MadSci Library for more information on Chemistry.