Introduction AtomicAbsorption Spectroscopy (AAS) Atomic absorption spectroscopy iscommonly used in calculating the concentration of aparticular element present in a sample. It isbased on the principle which requires standards withthe element concentrationis already known to establish the relationship between the element concentration and theabsorbance of the light and therefore relies on the Beer-Lambert Law which states that absorbance is directly proportional to the concentration of the element in the sample. Beer-Lambert LawDifferent atoms of elements absorb different wavelengths of light. The radiation may either be absorbed or transmitted depending on thewavelength of the radiation when a beam of electromagnetic radiation is passed througha sample. Theabsorption of radiation of specific wavelength would increase the energy of themolecule. These electrons can be excited to higher energylevel by absorbing a specific quantity of energy.
The energy gained by the moleculeis directly proportional to the wavelength of radiation. This amount of energy isspecific for a particular element. In general, each wavelengthcorresponds to only one element. Electrons are excited to higher state The unknown concentrationis usually determined by comparing the amount of light absorbed by thesample to the amount of light absorbed using a series of standards ofknown concentration.
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This involves the use ofa calibration graph. Example of obtaining an unknown concentration from the calibration graph For example with calcium, a light source containing calcium emits light with the wavelengths to be absorbed by any calcium atoms from the sample and is passed through avaporized sample. In AAS, thesample which contains calcium ions is atomized (convertedinto atoms in ground statevapour form) by heating using a flame. Some of the radiation is absorbed by the calcium atoms, while others are transmitted. The greater the number of atoms there is in the form of vapour, the more light will be absorbed.
The the number of calcium atoms in vapour form is proportional to amount of light absorbed. A calibration curve isconstructed by using some samples of known calcium concentration under the same conditions as theunknown. The absorbance of the unknown sample is determined by comparing with the standard by using a calibration curve. Thisenables the concentration of the calcium in the unknown sample to be calculated. Calibration graph for Calcium Light source: Hollow cathodelamp are the most common radiation source in AAS. It contains a tungstenanode and a hollow cylindrical cathode made of the element to bedetermined. These are sealed in a glass tube filled with an inert gas (neon orargon) at a pressure of between 1Nm-2 and 5Nm-2. The ionisation of somegas atoms occurs by applying a potential difference of about 300-400V betweenthe anode and cathode and eject metal atoms from the cathode in aprocess called sputtering.
Some sputtered atoms are in excited states and emitradiation characteristics of the metal as they fall back to the ground state. Theshape of the cathode concentrates the radiation into a beam which passesthrough a quartz window, and the shape of the lamp is such that most of thesputtered atoms are redeposited on the cathode. A typical atomic absorptioninstrument holds several lamps each for a different element. The lamps are housed ina rotating turret so that the correct lamp can be quickly selected.Nebulizer: Suck up liquidsamples at controlled rate. Create a fine aerosol spray for introduction intoflame.
Mix the aerosol, fuel and oxidant thoroughly for introduction intoflame.Atomizer: Elements to beanalysed needs to be in atomic state. Atomization is separation of particles intoindividual molecules and breaking molecules into atoms. This is done by exposing theanalyte to high temperatures in a flame or graphite furnace.Flame Atomizer: Theoldest and most commonly used atomizers in AAS are flames. To createflame, we need to mix an oxidant gas and a fuel gas.
In most of the cases, air-acetyleneflame or nitrous oxide-acetylene flame is used. A flexiblecapillary tube connects the solution to the nebulizer. At the tip of thecapillary, the solution is ‘nebulised’ which is broken into small drops. The largerdrops fall out and drain off while smaller ones vaporise in the flame. Only1% of the sample is nebulised.GraphiteTube Atomizer: 25?l of sample is placed through the sample hole and onto the platformfrom an automated micropipette and sample changer. The tube is heated electrically by passing acurrent through it in a pre- programmedseries of steps. The details will vary with the sample buttypically they might be 30-40 second at150? to evaporate the solvent,30 seconds at 600? to drive off any volatile organic materialand char the sample to ash, and with a very fast heating rate to2000-2500? for 5-10 seconds to vaporise and atomise elements.
Finally heating the tube to a still higher temperature cleansit ready for the next sample. During this heating cycle, the graphitetube is flushed with argon gas to prevent the tube burning away.In electrothermal atomisation, almost 100% of the sample is atomised.
This makes the technique much more sensitive than flame AAS.Monochromator: This is avery important part in an AA spectrometer. It is used to separate out all ofthe thousands of lines. A monochromator is used to select the specificwavelength of light which is absorbed by the sample, and to exclude otherwavelengths. The selection of the specific light allows the determinationof the selected element in the presence of others. The light selected by the monochromator is directedonto a detector that is typically a photomultipliertube. This produces an electrical signal proportional to the lightintensity.
Double beam spectrometers:Modern spectrometers incorporate a beam splitter so that one part of the beam passes through the sample cell and the other is the reference. The intensity of the light source may not stay constant during an analysis. If only a single beam is used to pass through the atom cell, a blank reading containing no analyte would have to be taken first, setting the absorbance at zero. If the intensity of the source changes by the time the sample is put in place, the measurementwill be inaccurate. In the double beam instrument, there is a constant monitoring between the reference beam and the light source.
To ensure that the spectrum does not suffer from loss of sensitivity, the beam splitter is designed so that as high a proportion as possible of the energy of the lamp beam passes through the sample.Detector: The lightselected by the monochromator is directed onto a detector that is typically a photomultiplier tube,whose function is to convert the light signal into an electrical signalproportional to the light intensity. The processing of electrical signal is fulfilled by asignal amplifier.
The signal could be displayed for readout, or further fed into adata station for printout by the requested format. Background absorption: Itis possible that other atoms or molecules apart from those of the elementbeing determined will absorb or scatter some radiation from thelight source. These species could include unvaporised solvent droplets,or compounds of the matrix (chemical species, such as anions, that tend to accompany themetals being analysed) that are notremoved completely. This means that there is a background as wellas that of the sample. One way of measuring and correcting this backgroundabsorption is to use two light source, one of which is the hollowcathode lamp appropriate to the element being measured.
Thesecond light source is a deuterium lamp. The deuterium lamp producesbroad band radiation, not specific spectral lines as with a hollow cathode lamp.By alternating the measurements of the two lightsources generally at 50-100Hz, the total absorption is measured withthe specific light from the hollow cathode lamp and the backgroundabsorption is measured with the light from the deuteriumlamp. Subtracting the background from the total absorption givesthe absorption arising from only analyte atoms.Calibration Curve: Acalibration curve is used to determine the unknown concentration of an element ina solution. The instrument is calibrated using several solutions of knownconcentrations. The absorbance of each known solution is measuredand then a calibration curve of concentration vs absorbance is plotted.
The sample solutionis fed into the instrument, and the absorbance of theelement in this solution is measured. The unknown concentration of theelement is then calculated from the calibration curve. Specific UsesAgriculture – analysingsoil and plants for minerals necessary for growthTrace metals areessential for plant growth. Atomic spectroscopy facilitates precise soilanalysis to ensure that metals are not at levels that could unduly affect the foodsource (livestock and/or crops). Plants may be sampled to monitor nutrientuptake efficiency and also to check for toxic metal accumulation for healthreasons. Industrial and Chemical –analysing raw chemicals as well as fine chemicalsFrom the analysis of rawmaterials and components to finished product testing and quality control,industrial and chemical manufacturers require accurate analytical techniques toensure the safety and performance of their products. Many raw materials areexamined and AAS is widely used to check that the major elements are presentand that toxic impurities are lower than specified – eg. in concrete, wherecalcium is a major constituent, the lead level should be low because it istoxic.
Environmental Study –determination of heavy metals in water, soil, and airIn the environment welive in, understanding heavy-metal contamination is critical. The accuratemeasurement of concentrations of these metals is imperative to maintain cleanair, water and soil for a safer world. AAS is used to monitor our environment-eg. finding out the levels of various elements in rivers, seawater, drinkingwater, air, petrol and drinks such as wine, beer and fruit drinks. Food Industry – qualityassurance and testing for contaminationAccurate analysis of foodfor nutritional content, contamination or authenticity – the exact geographicsource of the product – is critical for regulatory and quality assurance. Forensic Science –substance identificationAAS functions indetermination of trace elements, mode of poisoning, and ammunitionmanufacturers, elemental profiles of biological samples, trace elements inartificial fibres, hair analysis for heavy metal poisons and discrimination ofobjects/elements. Mining – testing theconcentration of valuable substances in potential mining areasAtomic spectroscopyoffers a fast, accurate solution for broad geological surveys as well as aninvaluable means of testing potential mining areas before incurring the high costsassociated with digging. By using AAS, the amount of metals such as gold inrocks can be determined to see whether it is worth mining the rocks to extractthe gold Nuclear Energy –monitoring potentially hazardous elements in water and waste outputOperating under constantscrutiny, the nuclear field is required to monitor and measure the levels of avariety of elements to an exacting degree.
Atomic spectroscopy is commonly usedto determine trace elements in everything from process water to low-levelwaste. Petrochemical – analysingproducts for metals and other substances that can have adverse effects such asoil and gasFrom petroleum refiningto a broad spectrum of applications using lubricants and oils, many industriesrequire the determination of metals – particularly analytes that can lead to degradationand contamination – to ensure conformity as well as monitor and controlprocesses. Pharmaceutical – manyapplications from quality control to detecting impurities in drugsDrug research, developmentand production is dependent on elemental analysis, starting with the testing ofindividual ingredients and continuing through production to final quality control,as impurities can affect drug efficacy and metabolism.
Most PharmaceuticalCompanies these days develop drugs which are targeted at specific cells in thebody. These drugs must be tested for correct activity but more importantly forthe absence of any adverse side reactions. In some pharmaceutical manufacturingprocesses, minute quantities of a catalyst used in the process (usually ametal) are sometimes present in the final product. By using AAS, the amount ofcatalyst present can be determined.
ADVANTAGESAccuracy.Atomic absorption spectroscope is a great way of producingaccurate results, with a rate of 0.5-5%, the result can be even better rate ifappropriate standards are used. Sensitivity. AAS is a sensitive methodof detection, it can measure down to parts per billion of a gram (µg dm–3 ) ina sample. As such, it has many uses in different areas of chemistry.
For example, in medicine, it can be usedto detect trace toxin levels of atmosphere or medication. Similarly, inpharmaceuticals, manufacturing processes, minute quantities of a catalyst usedin the process (usually a metal) are sometimes present in the final product, byusing AAS the amount of catalyst present can be determined. In industry, AAS isused to check that the major element are present and the toxic impurities arelower than specified. Cost AAS uses less argon than other methods, thusits running costs are often lower compare to other methods. Accessibility.
Since the process relying upon radiation and lightabsorption, it can reach previously inaccessible places. For example, minerscan now use AAS to determine if a rock contains enough elements of gold orother precious metals to be worthwhile mining. DISADVANTAGESLack of VersatilitySample must be in solution or at least volatile. Thisis because the substances have to be vaporised before it can be analysed.Liquids lend themselves to this much more than solids. Besides, the techniquethat allow for solid-substance testing can not be used on non-metals. Thistechnique has also not proved very successful for the estimation of elementslike V, Si, Mo, Ti and A1 because these elements give oxides in the flame. Low precisionOther chemicals that are found in the sample or in thesurrounding atmosphere can have an interfering and distorting effect on theresults of the study.
WeaknessIt is a destructiveanalysis method and therefore scarce sample should be analysed by anon-destructive method such as raman. Due to the sample preparation is timeconsuming, it cannot give information as detailed as other techniques such as NuclearMagnetic Resonance (NMR). It is also qualitative rather than quantitative andthere are lots of compounds which are not infrared (IR) active and thereforecannot be detected.