Atomic Absorption Spectrophotometer, AAS is used to measure the concentration of a specific element, in a given sample. In an AAS, the sample is atomized and a beam of electromagnetic radiation emitted from a light source passes through the vaporized sample. Some of the radiation is absorbed by the atoms in the sample; the amount of light absorbed is a function of the concentration of the element of interest.
An AAS consists of five basic functional parts; a light source which is usually a Hollow Cathode Lamp, an atomizer section for atomizing the sample which could be a flame or an electrothermal or a cold vapor/hydride generation system, a monochromator for selecting the analysis wavelength for the target element, a detector for measuring the amount of light absorbed and a recorder for recording the output from the detector.
Light source: The Hollow Cathode Lamp is perhaps the most commonly used light source in an AAS. The Hollow Cathode lamp produces narrow emissions from atomic species. The Hollow Cathode Lamp consists of a cup-shaped cathode which is made from element of interest and an Anode which is usually made from tungsten and placed in a hollow tube filled with an inert gas such as Ar or Ne. Application of a high potential difference across the electrodes causes a discharge of electrons. These gaseous ions bombard the cathode and eject metal atoms from the cathode in a process called sputtering. Some sputtered atoms are in excited state and emit radiation (light) characteristic of the metal as they fall back to the ground state.
Atomizer: This consists basically of the burner for flame analysis or a graphite furnace for electrothermal analysis or cold vapor or a hydride system, the nebulizer introduces the sample as a fine mist and the spray chamber to create a good mix for both air and the fuel gas.
Monochromator: This is used to isolate a single atomic resonance line from the spectrum of lines emitted from the interaction of the sample with the resonance light from hollow cathode lamp. Essentially it is an adjustable filter that selects a specific narrow region of the spectrum for transmission to the detector and excludes all wavelengths outside this region.
Detector: It converts light coming from a monochromator into an amplified electrical signal which can be recorded. The amount of light getting to the detector is a function of the concentration of the element of interest in the sample being analyzed. The type of detector found in AAS is the photomultiplier tube – the principle of operation is the emission of electrons upon exposure to radiation.
The Recorder, which today, is usually a computer system and a suitable software, receives the analog signals from the detector and converts it to readable responses.
Authored by: Ayodeji Ogunlowo; a Service Engineer at Applied Analytical Systems Litd., 8, Atunwa Street, Ikeja, Lagos, Nigeria. Phone (234) 802 2320 862, email: [email protected] website www.aasnig.com