|  | Emission Emission occurs when excited electrons of an atom -due to thermal energy (heat of a flame)- fall back from a higher energy level (excited state) to a lower energy level (ground state). Light is emitted when the electrons return to the ground state. Each element emits light at a characteristic wavelength. The emission intensity is proportional to the element concentration. |  |

|  | Absorption The excitation of an electron from the ground state E 0 to the first excited state E 1 by radiation with just the right amount of photon energy (hf). This is the process of energy transfer: the photon loses its energy (and disappears), whilst the electron acquires it. Or: radiation is taken up (= absorbed) by matter. Therefore the process is called absorption. |  |

|  | Proton The nucleus of an atom contains protons and neutrons (not shown in picture). So protons are subatomic particles with a positive electric charge (+). |  |

|  | Electron An electron is a subatomic particle that has a negative electric charge (-). |  |

|  | Energy level An atom is assumed to consist of a heavy, positively charged nucleus, surrounded by a number of (negative) electrons. The entire atom is electrically neutral. The electrons move round the nucleus within given orbits which have a fixed energy level (black oval lines; quantized). In ground state there is minimal energy. Due to absorption of energy atoms can become in an exited state. Also atom can return from the exited state into the ground state (emission). |  |

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*Traditionally also other methods were used for the analysis of specific inorganic compounds such as phosphate, nitrate and ammonium. But those are not covered in this case.

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learn the difference between emission and absorption

Determination of specific element content

The most common general technique to measure elements is spectrometry. Within spectrometry all kinds of techniques can be used, like atomic absorption spectrometry (AAS), atomic emission spectrometry (AES) and spectrophotometry (UV/VIS).

General principle of atomic spectrometry

The principle of atomic spectrometry is that every chemical element is capable of absorption and emission of radiation (e.g. light) at specific wavelengths. Absorption and emission of radiation are caused by electronic transitions of atoms.

The picture below explains the difference between atomic absorption and atomic emission. Please move with your mouse over the boxes in the picture below to understand the absorption and emission processes. This will help you to understand the AAS and AES methods.