Spectroscopy

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Spectroscopy is a term to general describe an analysis of sample that requires and uses the properties of light to detect and/or quantitate the level of material in the sample.[1] For the Beaver Run Project, we used a particular type of spectroscopy called Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), which is an instrumental separation technique that allows us to identify and quantify levels of many different metal constituents from a single water sample.

The Basics of ICP-OES

The ICP-OES is a type of spectrometer – an instrument that detects elements in a sample by using light. Spectrometers and spectrophotometers rely on the principles of quantum mechanics and the properties of light and energy as they related to atoms.

Sample Introduction


Each sample and standard is held in a plastic centrifuge tube in an autosampler tray attached to the instrument. When ready, each sample is introduced into the instrument by a small straw that "sucks up" the sample via a peristaltic pump. The sample then passed into an aspirator, which mixes the aqueous sample with gases to form a very fine mist. Most of the liquid actually goes to a waste container, but a tiny amount of the spray is passed into the ionization chamber.

Excitation via Plasma Torch


Each droplet of mist that enters the next stage passes through a plasma torch. The torch is a quartz tube surrounded by an electro-conductive coil that generates a radio frequency. The combination of ignited gas mixture (argon, nitrogen, and air) and the electrical field create a plasma flame with temperatures as high as 8,000 – 10,000 kelvin (14,900 – 18,500 °F). When the sample droplet passes through this torch, all liquid is evaporated and any metal atom present will be excited – meaning that the energy of some of its electrons will dramatically increase.

Detection by Light Emission


When an atom undergoes excitation from a ground energy state to an excited energy state, the rules of quantum mechanics come into play. Basically, the electron energy levels in an atom can be thought of as rungs on an eccentric-looking ladder – with enough energy, an electron can move up one or more steps on this ladder. And just as with a physical ladder in your house, you can only put your foot in specific (or discrete) steps (rungs).

What this means for us is that each atom on the Periodic Table has a different and unique "electron ladder" – in other words, each atom has different and unique energy levels where the electrons can go when excited. When electrons are excited to a higher energy level, they immediately get rid of that excess energy in order to relax back to the ground state (lowest energy). One of the ways in which electrons get rid of extra energy is to release it as light.

Light is really nothing more than energy of specific wavelength (or frequency) – when we see different colors of the rainbow, we're just seeing different energy levels of light. Red has the lowest energy of the visible light spectrum and violet has the highest. Nonvisible light (aka electromagnetic radiation) ranges from AM/FM radio waves to gamma (cosmic) radiation, and visible light is bordered by infrared (heat) radiation on the "red side" of the rainbow and ultraviolet (UV) radiation on the "violet side" of the rainbow.

Therefore, since each element has its own unique "electron energy ladder", each element will emit only a few distinct "colors" of light when the electrons are excited and relaxed. By using a series of optics and photodiode array detectors, we can identify each metal or element in a complex mixture by its unique light fingerprint. In order to identify and quantify each anion in our mixture, we rely on a calibration curve.

See the general method for how we use ICP-OES to analyze samples.

Notes

  1. The term spectroscopy is used to describe the field of study that uses light to analyze samples, where spectrometry is the practical application (e.g., instrument) used to provide an answer.



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