Did You Know: Inductively coupled plasma atomic emission spectrometry

Why read? This one is a bit more difficult to understand than some of my other Did You Know postings.  But it is very interesting if you can work through it.  The appeal of this essay is the engineering of this scientific instrument: the ingenious exploitation of multiple fundamental physical principles of electricity, light, and atoms, to produce a precise and useful result, could aptly be called magic, and the engineers, wizards.  See the diagram below.

Article Inductively coupled plasma atomic emission spectroscopy is a method for determining the elemental constituents of a sample.  The instrument is normally just referred to as an ICP, which you might think would be confusing, but I guess most scientists have never heard of the Insane Clown Posse.  There are different forms of this instrument, but I’ll describe the one we had and used at Evergreen (The Evergreen State College in Olympia, Washington), which is used to determine which elements are present in an aqueous solution and the quantities of those elements.

This instrument embodies a few very cool physical principles on which I’ve expounded a bit in previous Did You Know essays: electromagnetism;plasma; and atomic emission (you can look for underlined terms in previous essays for further examples or background on concepts—see keywords list at bottom).  What impresses me most about this instrument, and many other scientific instruments, is the creative engineering used to exploit subtle physical principles in ingenious combinations.  The most overtly impressive feature of the ICP is the plasma flame, which has a temperature of up to 10,000 degrees Kelvin (Kelvin units are the same as Celsius except 0°K is -273°C).  This is hotter than the surface of the sun, which is less than 6,000 K.

In short, the process of determining the elements in an aqueous solution is this: The solution is injected into the machine and aspirated—broken apart into an aerosol of fine particles—into a plasma flame.  The elements in the solution, when exposed to the high energy of the plasma, are excited and emit light.  Each element emits light of a wavelength (color) specific to that element.  A photometer counts the number of photons of whichever wavelength you prescribe and gives you a value, which is used to determine how much of that element is present in the solution.  (see diagram in my pics folder “Did You Know”).

Now the mechanics of the thing (the cool part!).

Plasma is the state of having 1% or more of the electrons in the substance disassociated from any atoms.  This is an extremely high energy state as the electrons are free to wreak havoc on any molecules that contact the plasma.

Induction of the plasma:  Argon gas flows through a vertical tube—the flame will be formed from the gas at the top of the tube.  Radio waves are generated in rings around the tube.  A radio wave is, like light, an electromagnetic wave.  The electromagnetic field extends beyond the rings.  A spark knocks a few electrons off of the argon gas atoms.  Electrons are negatively charged, so they get caught in the electromagnetic loop circulating around the gas tube.  These electrons smash into atoms of the gas knocking more electrons loose until more than one percent of all the electrons are zipping around smashing into atoms.  The result is a blue flame between 5,000° and 10,000° K—blue because that is the color emitted by excited argon atoms.

When atoms are excited, their electrons jump to a higher energy state.  This is an unstable state and the electrons always drop back down to their ground state.  When they drop down, the excess energy is emitted as light.  The wavelength, or frequency, of light emitted is proportional to the difference in energy between the excited state and the ground state.  The different elements all have different structures and the degrees to which the nucleus pulls the electrons inwards from the outer levels varies.  Therefore for each element the electrons will be excited to different degrees when exposed to the argon plasma.  So each element will release a signature wavelength.

If you want to look for arsenic in your sample, you set the machine to detect the wavelength specific to arsenic.  This setting adjusts a diffraction grating, which allows only light of the desired wavelength to pass through.  Behind that diffraction grating is a photometer.  When your sample is injected into the machine, then aspirated into the flame, the atoms emit light and the photometer counts the number of photons that pass through the diffraction grating, thus only counting photons emitted by arsenic.  In this way you can determine how much arsenic is in your sample.

For my Marine Biology course at Evergreen I worked with three other students to analyze marine sediments for two toxic heavy metals, arsenic and antimony.  We were trying to determine the spatial pattern of deposition of those metals from the smokestack of a metals smelter in Tacoma.  We actually found no distinct spatial pattern, but we did find measurable levels of antimony in all the sediments tested.  Toxic heavy metals in marine sediments could poison deposit-feeding clams (clams that feed on nutrients and microbes on the surfaces of grains of sand) and the organisms that eat them.

Source Mostly from memory.  In order to get our “driver’s license” on any of the scientific instruments at Evergreen, we had to demonstrate a mastery of the use of the instrument as well as the theory behind it.  I double-checked the process of flame-induction and the temperatures of the flame and the sun’s surface on the web.

Keywords:      electromagnetism; plasma; atomic emission; ground state