Chapter 10Atomic Emission Spectroscopy.Flame Emission and Atomic Absorption SpectroscopyCHE. 331Chapter 10Atomic Emission Spectroscopy.History and Theory of Atomic Absorption SpectroscopyAs the name implies, atomic absorption is the absorption of light by free atoms. An atomicabsorption spectrophotometer is an instrument that uses this principle to analyze the concentrationof metals in solution. The versatility of atomic absorption an analytical technique (Instrumental technique) has led to the development of commercial instruments. In all, a total of 68 metals can be analyzed.Advantages of AA· Determination of 68 metals· Ability to make ppb determinations on major components of a sample· Precision of measurements by flame are better than 1% rsd. There are few other instrumental methods that offer this precision so easily.· AA analysis is subject to little interference.· Most interference that occurs have been well studied and documented.· Sample preparation is simple (often involving only dissolution in an acid)· Instrument easy to tune and operateKirchoff and Bunsen's ExperimentBetween 1859 to 1861, Gustav Kirchoff (Prussian physicist), with his colleague RobertTunsen, a German chemist, at the University of Heidelberg demonstrated that every element gives off a characteristic color when heated in incandescence. The apparatus used for their classic experiment is shown here. Applying this new research tool, they discovered the element cesium and rubidium.Kirchoff - Absorbance & Emission LineKirchoff and Bunsen not only identified various characteristic spectra, but theyestablished the relationship between the emission spectra and the absorption spectra thusexplaining the presence of the dark lines in the solar spectra.Ground State AtomWith that brief history of the development of the atomic absorption procedure and Varian atomic absorption instruments, we will now examine the atomic theory that explains how an atomic absorption signal is generated.In order to understand the atomic absorption process one must first understand the structure of the atom and its orbitals. The atom consists of the central core, or nucleus, made up of positively charged protons and neutral neutrons. Surrounding the nucleus in precisely defined energy orbitals are the electrons. All neutral atoms have an equal number of protons in the nucleus. This means that each element has a unique number of electrons and protons, The outermost electrons are known as the valence electrons and atomic spectroscopy involves energy changes in these valence electrons.Beer - Lambert LawThe relationship that converts the intensity of the light beam to concentration is called the Beer - Lambert Law or simply Beer' s law. Beer' s Law states that the absorbance, A, is equal to the molar absorptivity or extinction coefficient, a. times the path length over which the measurement is made. b, times the concentration of the analyte, c. For a given set of conditions, the molar absorptivity, a, is a constant. The path length of the determination, b, is also a constant.Therefore, the absorbance is equal to a constant times the concentration.A = abc = Kc, whereA = absorbancea = absorptivity constant, b = sample thicknesspath length, c = concentrationK = a constantIf this expression is plotted, a curve of absorbance versus concentration is drawn, Beer's Law predicts that a straight line will result. In practice we find that deviation from the linear calibration is observed at higher concentrations.Normal AbsorbanceThe important thing to remember in the use of Beer' s Law is that A refers to absorbance, not absorption. Absorbance is defined by the equation:A = log (lo/1), whereA = absorbancelo = the initial intensityI = the intensity after absorptionCalibrationThe concentration of the unknown is determined by comparing the samples with a seriesof standards. AA is always a comparative technique where the determination is performed usingfreshly prepared matrix matched standards.Flame Emission and Atomic Absorption SpectroscopyThe following are the 3 main types of Flame Emission and Atomic Absorption Spectroscopy:a) Atomic Emission (with thermal excitation), AESb) Atomic Absorption, (with optical photon unit) AASc) Atomic Florescence, AFSAll of the following methods use the same or similar steps:1. Atomization: Breakdown of the molecule into its atomic components in the gas phase.(Aerosol> Desolvation>Vaporization> Atomization)2. Excitation: Thermal excitation for AES and Optical excitation for AAS and/or AFS3. Measurement: Absorption (AAS) Emission (AFS) & (AES)A powerful technique of measurement is the ICP-AES which stands for Inductively Coupled Plasma-Atomic Emission Spectroscopy. In terms of simplicity Atomic Emission Spectroscopy (AES) is the most complex because of the atomization part which is a function of temperature. Furthermore, in terms of cost AES is the most expensive and in terms of efficiency and precision AES is also the most efficient and precise. In terms of sensitivity, AES is the least sensitive.Simplicity: AAS>AFS>AESCost: AES>AAS>AFSSensitivity: AFS>AAS>AFS.It should be noted that in AES, one would like the excited state of the elements to be populated by the electrons.ATOMIC EMISSIVE SPECTROMETRY WITH PLASMAS.Atomic emissive spectrometry (AES) can be performed where the flame is replaced with either aplasma or electrodes. A plasma is an electrical conducting gaseous mixture containing a significantconcentration of cations and electrons. The concentration of the two are such that the net charge approaches zero. Argon plasmas are used most often for nonflame AES. The high temperatures that are achieved in argon plasmas cause more efficient excitation of atoms and ions than isachieved with flames. As a result, the intensities of the emitted lines are greater and more spectrallines are observed. Three types of high-temperature plasmas are encountered and these are: 1) the inductively coupled plasma (ICP), (2) the direct current plasma (DCP), and (the microwave induced plasma (MIP). The most important of these plasmas is the inductively coupled plasma (ICP).The Inductively Coupled Plasma Source.The figure below is a shematic of a typical inductively coupled plasma source called a torch. It consists of three concentric quartz tubes through which streams of argon gas flow. Depending upon the torch design, the total rate of argon consumption is 5 to 20 l/min. Surrounding the top