Atomic Absorption Spectroscopy (AAS) PDF

ATOMIC ABSORPTION SPECTROSCOPY (AAS) Nurul Farahana Kamaludin (PhD) Environmental Health & Industrial Safety Programme Faculty of Health Sciences Universiti Kebangsaan Malaysia ATOMIC ABSORPTION SPECTROSCOPY (AAS) ATOMIC ABSORPTION SPECTROSCOPY  INS...

ATOMIC ABSORPTION SPECTROSCOPY (AAS) Nurul Farahana Kamaludin (PhD) Environmental Health & Industrial Safety Programme Faculty of Health Sciences Universiti Kebangsaan Malaysia ATOMIC ABSORPTION SPECTROSCOPY (AAS) ATOMIC ABSORPTION SPECTROSCOPY  INSTRUMENTATION ATOMIZER  In order for the sample to be analyzed, it must first be atomized.  This is an extremely important step in AAS because it determines the sensitivity of the reading.  The most effective atomizers create a large number of homogenous free atoms. There are many types of atomizers, but only two are commonly used: flame and electrothermal atomizers.  INSTRUMENTATION i) FLAME ATOMIZERS  are widely used for a multitude of reasons including their simplicity, low cost, and long length of time that they have been utilized.  Accept an aerosol from a nebulizer into a flame that has enough energy to both volatilize and atomize the A schematic diagram of a flame sample. atomizer showing the oxidizer inlet (1) and fuel inlet (2).  When this happens, the sample is dried, vaporized, atomized, and ionized.  INSTRUMENTATION ii) ELECTROTHERMAL ATOMIZER  Were developed before flame atomizers.  Employ graphite tubes that increase temperature in a stepwise manner.  Electrothermal atomization first dries the sample and evaporates much of the solvent and impurities, then atomizes the sample, and then rises it to an extremely high temperature to clean the graphite tube.  Some requirements: ability to maintain a constant temperature during atomization, have rapid atomization, hold a large volume of solution, and emit minimal radiation. Schematic diagram of an electrothermal atomizer showing the:  external gas flow inlet (1),  the external gas flow outlet (2),  the internal gas flow outlet (3),  the internal gas flow inlet (4),  and the light beam (5).  INSTRUMENTATION RADIATION SOURCE  The radiation source irradiates the atomized sample. The sample absorbs some of the radiation, and the rest passes through the spectrometer to a detector.  Radiation sources can be separated into two broad categories: line sources and continuum sources.  Line sources: Excite the analyte and thus emit its own line spectrum. Hollow cathode lamps (HCL) and electrodeless discharge lamps (EDL) are the most commonly used examples of line sources.  Continuum sources: Have radiation that spreads out over a wider range of wavelengths. These sources are typically only used for background correction. Deuterium lamps and halogen lamps are often used for this purpose.  INSTRUMENTATION SPECTROMETER  Spectrometers are used to separate the different wavelengths of light before they pass to the detector.  Can be either single-beam or double-beam.  Single-beam spectrometers only require radiation that passes directly through the atomized sample, while double-beam spectrometers require two beams of light; one that passes directly through the sample, and one that does not pass through the sample at all.  SINGLE BEAM AAS 5 MAIN COMPONENTS: Light source – eg: most commonly used is HCL. Chopper - to modulate the intensity of a light beam. Atomizer - to generate a flame to produce atoms of the same elements that are present in the sample. Monochromator - disperses the incident light beam and permits the selected wavelength to reach the detector. Detector - produces a signal proportional to the amount of light received by it.  AAS quantitatively measures the concentrations of elements present in a liquid sample (aqueous or organic solution), or may even be solid provided it can be dissolved successfully.  The elements in the gas phase absorb light at very specific wavelengths.  This will gives the technique excellent specificity and detection limits.  AAS can measure down to parts per billion of a gram (µg dm–3) in a sample.  PRINCIPLES OF AAS The liquid is drawn in to a flame where it is ionized in the gas phase. Light of a specific wavelength appropriate to the element being analyzed is shone through the flame, the absorption is proportional to the concentration of the element. Quantification is achieved by preparing standards of the element. WHAT IS AAS USED FOR?   To identify the presence and concentration of substances by analyzing the spectrum produced when a substance is vaporized and absorbs certain frequencies of light.  For detecting the concentrations of metal ions in solutions.  APPLICATIONS Can be divided into the broad categories of:  Biological analysis  Environmental and marine analysis  Geological analysis  BIOLOGICAL ANALYSIS Human Tissue Sample  To determine the amount of various levels of metals and other electrolytes, within tissue samples (eg: blood, bone marrow, urine, hair, and nails).  Sample preparation is dependent upon the sample.  Many elements (eg: As, Hg & Pb) are toxic in certain concentrations in the body, and AAS can analyze what concentrations they are present in.  Eg: measurement of the electrolytes Na and K in plasma - indicative of various diseases when outside of the normal range.  BIOLOGICAL ANALYSIS Food Samples  Provides analysis of vegetables, animal products, and animal feeds. - these kinds of analyses are some of the oldest application of AAS.  An important consideration that needs to be taken into account in food analysis is sampling.  The sample should be an accurate representation of what is being analyzed. Because of this, it must be homogenous, and many it is often needed that several samples are run.  Food samples are most often run in order to determine mineral and trace element amounts so that consumers know if they are consuming an adequate amount.  Samples are also analyzed to determine heavy metals which can be detrimental to consumers.  ENVIRONMENTAL & MARINE ANALYSIS  Typically refers to water analysis of various types (from drinking water to waste water to sea water).  Unlike biological samples, the preparation of water samples is governed more by laws than by the sample itself.  The analytes that can be measured also vary greatly and can often include Pb, Cu, Ni, and Hg.  GEOLOGICAL ANALYSIS  Geological analysis encompasses both mineral reserves and environmental research.  When prospecting mineral reserves, the method of AAS used needs to be cheap, fast, and versatile because the majority of prospects end up being of no economic use.  Eg: analysis of lake and river sediment for Pb and Cd.  MORE EXAMPLES ON THE APPLICATIONS  Monitoring of trace metals in industrial effluent streams.  Trace elements in product / raw materials along with ICP-MS.  Analysis of additives and purity in steels and other metal alloys.  Analysis of low level contaminants.  SAMPLE PREPARATION  Sample preparation is extremely varied because of the range of samples that can be analyzed.  Certain considerations should be made including the laboratory environment, the vessel holding the sample, storage of the sample, and pretreatment of the sample.  Sample preparation begins with having a clean environment to work in. AAS is often used to measure trace elements, in which case contamination can lead to severe error.  Possible equipment includes laminar flow hoods, clean rooms, and closed, clean vessels for transportation of the sample.  It also needs to be conserved in terms of pH, constituents, and any other properties that could alter the contents.  When trace elements are stored, the material of the vessel walls can adsorb some of the analyte leading to poor results.  To correct for this, perfluoroalkoxy polymers (PFA), silica, glassy carbon, and other materials with inert surfaces are often used as the storage material.  Acidifying the solution with hydrochloric or nitric acid can also help prevent ions from adhering to the walls of the vessel by competing for the space.  The vessels should also contain a minimal surface area in order to minimize possible adsorption sites.  Pretreatment of the sample is dependent upon the nature of the sample. Sample pretreatment methods for AAS CALIBRATION TECHNIQUES Standard calibration technique Bracketing technique Analyte addition technique  CALIBRATION TECHNIQUES The standard calibration technique and bracketing technique both require that the standards have a similar matrix to that of the sample and that is not possible when the matrix is unknown. Analyte addition technique can be used if the composition of the sample is unknown.  STANDARD CALIBRATION TECHNIQUE  The simplest and the most commonly used.  The concentration of the sample is found by comparing its absorbance or integrated absorbance to a curve of the concentration of the standards versus the absorbances or integrated absorbances of the standards.  STANDARD CALIBRATION TECHNIQUE  In order for this method to be applied the following conditions must be met:  Both the standards and the sample must have the same behavior when atomized. If they do not, the matrix of the standards should be altered to match that of the sample.  The error in measuring the absorbance must be smaller than that of the preparation of the standards.  The samples must be homogeneous. The curve is typically linear and involves at least five points from five standards that are at equidistant concentrations from each other. An example of a calibration curve made for the standard calibration technique.  BRACKETING TECHNIQUE  A variation of the standard calibration technique.  Only 2 standards are necessary with concentrations c1 and c2. They bracket the approximate value of the sample concentration very closely. Where c x and A x : the concentration and adsorbance of the unknown A 1 and A 2 : the adsorbance for c 1 and c 2  BRACKETING TECHNIQUE  Very useful when the concentration of the analyte in the sample is outside of the linear portion of the calibration curve because the bracket is so small that the portion of the curve being used can be portrayed as linear.  This method can be used accurately for nonlinear curves, however the further the curve is from linear the greater the error will be.  In order to reduce this error, the standards should bracket the sample very closely.  ANALYTE ADDITION TECHNIQUE  The analyte addition technique is often used when the concomitants in the sample are expected to create many interferences and the composition of the sample is unknown.  To compensate for this, this technique uses an aliquot of the sample itself as the matrix.  The aliquots are then spiked with various amounts of the analyte.  This technique must be used only within the linear range of the absorbances. MEASUREMENT INTERFERENCE  Interference Caused by contaminants within the sample that absorb at the same wavelength as the analyte.  Corrections Variety of methods can be made including background correction, addition of chemical additives, or addition of analyte. Examples of interference in AAS  THANK YOU Email: [email protected] Ebon, L., Fisher, A. & Hill, S.J. 1998. An Introduction to Analytical Atomic Spectrometry, Ed. Evans, E.H. Wiley, New York. Robinson, J.W. 1996. Atomic Spectroscopy. 2 nd Ed. Marcel Dekker, Inc., New York. Welz, B. & Sperling, M. 1999. Atomic Absorption Spectrometry. 3 rd Ed. Wiley-VCH, New York.