atomic absoption


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BASIC PRINCIPLE,INSRUMENTATION,LIGHT SOURCE,BACK GROUND CORRECTION &APPLICATION OF ATOMIC ABSOPTION SPECTROSCOPY (AAS) Presented by: Vikram patel M.pharm –Pharmaceutics (sem-1) Roll no.03 Guided by: Mr. Nishit patel M.pharm (Quality Assurance) Assistant professor DHARMAJ DEGREE PHARMACY COLLEGE, DHARMAJ.

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BASIC PRINCIPLE When the solution of metallic salt spray onto a flame, the fine droplets are formed. Due to the thermal energy of the flame , the solvent in the droplets evaporate, leaving a fine residue, which are converted to neutral atoms. These neutral atoms absorb radiation of specific wavelength, emmited by hollow cathode lamp(HCL). Hollow cathode lamp is filled with the vapour of element, which gives specific wavelength of radiation. For the determination of every elements, hollow cathode lamp is selected, which contains vapour of the element to be analyzed. Although this appears to be the demerit of AAS, specificity can be achieved only by the use of HCL.

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The intensity of light absorbed by the neutral atoms is directly proportional to the concentration of element and obeys Beer’s law over a wide concentration range.The intensity of radiation absorbed by neutral atoms is measured using photometric detectors(Photo tube or Photo multiplier tube).

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Mathematically , the total amount of light absorbed may be given by expression as follows: At v the total amount of light absorbed= π e 2 Nf/mc Where e is charge on the electron of mass m , c is the speed of the light , N is the total number of atom that can be absorb at frequency v in the light path and f is the oscillator strength or ability for each atom to absorb at frequency, v. As π , e,m , and c are constant. Above equation can be simplified to following expression : Total amount of Light absorbed = constant * N* f

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From the above expression it follows that (1)Firstly , there is no term involving the wavelength (or frequency ) of absorption other than indication of actual absorption wavelength. (2)Secondly there is no term involving temperature. In AAS, the temperature of the flame is not critical, since the thermal energy of flame is used just to atomize the sample solution to fine droplets, to form a fine residue and later to neutral atoms. The excitation of neutral atoms is brought about only by radiation from hollow cathode lamp and not by the thermal energy of the flame .


INSTRUMENTATION The three flame spectrophotometric procedures are required. Nebuliser -burner: which produces gaseous metal atoms by using suitable combustion flame involving the fuel gas-oxidant mixture.But with so called non-flame cells,the burner is not required. Spectrophotometer: This include a suitable optical train, a photosensitive detector & appropriate suitable device for the output from detector.

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Resonance line source: For both atomic absorption spectroscopy and atomic fluorescence spectroscopy , a resonance line source is required for each elements determined this line source are usually modulated . A Schematic diagram showing disposition of this essential components for different technique is given in figure.

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The relationship between the ground state and excited state population is given by the BOLTZMANN EQUATION N 1 /N 0 =(g 1 /g 0 ) e -ΔE/Kt Where, N 1 =number of atoms in excited state N 0 =number of atoms in ground state G 1 /G 0 =ratio of statistical weights for ground & excited states ΔE=energy of excitation= hv K=the Boltzmann constant T=temperature in kelvins

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The ratio N 1 /N 0 depends on the excitation energy ΔE and the temperature T. An increase in temperature & decrease in ΔE will each result in higher value for the ratio N 1 /N 0 . Calculation shows that only small fraction of atoms are excited, even under most favorable conditions,i.e. when the temperature is high & the excitation energy is low. This is illustrated by data in following Table

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There are mainly two types of sources: Continuum source : deuterium lamp, xenon arc lamp, mercury lamp, tungston lamp Line source : Hollow cathode lamp, Electrode discharge lamp.

Components of atomic absorption spectrophotometer :

Components of atomic absorption spectrophotometer Hollow cathode lamp (HCL): The lamp or source of light in AAS is a hollow cathode lamp. The cathode is made up of specific element or alloys of elements . When current is applied between anode and cathode,metal atoms emerges from the hollow cup and collides with filler gas,which normally argon or neon. Due to these collisions,number of metal atoms are excitedand emit their characteristic radiation.This characteristic radiation is absorbed by neutral atoms of the same element in ground state,which occur in the flame, when sample solution is sprayed.

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It is not possible to use a source of light with monochromator because this arrangements gives a radiation with a band width 1nm, where as the hollow cathode lamp gives a band width 0.001 to 0.01 nm, which is highly desirable to achieve specificity.

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The neon gas filled in the hollow cathode lamp performs three functions: (1)It is the main source of current –carrying capacity in the hollow cathode. (2)It dislodges atoms from the surface of cathode. (3)It primarily responsible for excitation of groud state metal atoms.

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The pressure maintained in the HCL 1to 1.5torr.If higher pressures are maintained, the discharges tends to be unstable and if lower pressures are maintained, the vaporization of the hollow cathode metal increase and the operating temperature also increase. The separate line produced by the hollow cathode lamp are so narrow that they are completely absorb by atom. By this method one can easily detect and measure the atomic absorption.

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Each hollow cathode lamp emits the spectrum of the metal which is use in cathode, for example , copper cathode emits the copper spectrum, zinc oxide emits the zinc spectrum and so on . At the same time, the narrow spectral line emitted by copper cathode are only absorb by the copper atom present in the sample to be analysed by atomic absorption spectroscopy. Similarly zinc atom will absorb spectral line emitted by zinc cathode. For this reason different hollow cathode has to be used for each element to be analysed by atomic absorption spectroscopy.

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The demerit in AAS is that , for determination of every element,separate HCL has to be used. This can be overcome by using multi-element lamps. Examples : Two element lamps like Na/K, Ca/Mg, Cu/Zn and three elements lamps like Ca/Mg/Zn are available.

Multielement HCL:

Multielement HCL

Mountable HCL:

Mountable HCL

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(b) Electrodeless discharge lamp It Is difficult to make stable hollow cathodes from certain elements, particularly those that are volatile such as arsenic,germanium or selenium. An alternative light source has been developed in the electrode discharge lamp. It consist of an evacuated tube in which the metal of interest is placed. The tube is filled with argon at low pressure and sealed off.

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The sealed tube is then placed in a microwave discharge cavity. Under this condition the argon becomes a plasma and cause excitation of the metal sealed inside the tube. The emission from the metal is that of its spectrum including the resonance line. The intensity of these lamp is very high, and they have been quite stable in recent years.

Electrodeless discharge lamp:

Electrodeless discharge lamp

Background corrector :

Background corrector (A) Deuterium arc background correction. This system uses two lamps, a high-intensity deuterium arc lamp producing an emission continuum over a wide wavelength range and the hollow cathode lamp of the element to be determined. The deuterium arc continuum travels the same double-beam path as does the light from the resonance source (see Fig.). The background absorption affects both the sample and reference beams and so when the ratio of the intensities of the two beams is taken, the background effects are eliminated.

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The deuterium arc method is widely used in many instruments and is often satisfactory for background correction. In addition, due to the low output of a deuterium lamp in the visible region of the spectrum, its use is limited to wavelengths of less than 340 nm.

Deuterium lamp:

Deuterium lamp

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(b) Zeeman background correction. (B) Zeeman background correction In a strong magnetic field, the electronic energy levels of atoms may be Split, resulting in the formation of several absorption lines for each electronic transition (the Zeeman effect). The simplest form of splitting pattern is shown in Fig. In the presence of the magnetic field three components are observed, the π component having the same energy as the transition in the absence of the magnetic field, and the σ components observed at lower and higher energies, typically 0.01 nm distant from the π c omponent.

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The π peak is plane- polarised parallel to the direction of the magnetic field, whilst the σ peaks are polarised perpendicular to the direction of the magnetic field (see Fig.) In practice, the emission line is split into three peaks by the magnetic field. The polariser is then used to isolate the central line which measures the Absorption A π which also includes absorption of radiation by the analyte. The polariser is then rotated and the absorption of the background A σ is measured. The analyte absorption is given by A π - A σ

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( c ) The Smith- Hieftje system. This method is based on the principle of self-absorption. If a hollow cathode lamp is operated at low current the normal emission line is obtained. At high lamp currents the emission band is broadened with a minimum appearing in the emission profile that corresponds exactly to the wavelength of the absorption peak. Hence, at low current the total absorbance due to the analyte and the background is measured, whilst at a high lamp current essentially only the background absorbance is obtained. Thus, the hollow cathode lamp is run alternatively at low and high current (see Fig.).

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Thus, the analyte absorption is given by the difference between the absorbance measured at low lamp current and the absorbance at high lamp current. Both the Zeeman and the Smith- Hieftje systems have the advantages that only one light source is used and that the background is measured very close to the sample absorption.

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The Zeeman background correction is, however, normally limited to use in furnace AAS and can suffer from a lack of sensitivity. The Smith- Hieftje system, although less expensive than the Zeeman method, suffers the disadvantage that the lifetime of hollow cathode lamps may be shortened, particularly those incorporating the more volatile elements.


INTERFERENCE: The technique of AAS is especially free from cationic interference. This is because of sharp resonance lines from HCL. However, there are seven types of interference are as follows:

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(a) Spectral interference This type of interference may be caused by overlapping of any radiation with that of characteristic radiation of the test element to be estimated. The interfering radiation mat be an emission line of another element , radical or molecule , unresolved band spectra or general background radiation from flame , solvent etc. In this case , the lines will be read together in proportion to the decrease of overlap if the resolving power and spectral band pass of the monochromator permit the undesired radiation to reach the photo receptor .

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An interesting example of serious overlapping is the manganese triplet (4031, 4033,4035Å) the gallium line (4033 Å ) and potassium doublet (4033, 4035 Å) This type of overlapping can be overcome by selecting other spectral line. If the spectral interference is due to sample matrices or flame components, this can be overcome working with AC amplifiers turned to the frequency at which the source is chopped or modulated.

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The band spectra are due to rotational & vibrational characteristics of undissociated molecules or complexes ions remaining in the flame. A flame background background is also presents.

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(b) Chemical, ionization and bulk (matrix)interference If compounds or complex ions of element being determined incompletely dissociate into their atoms, low results will occur. Generally the more concentrated solution the greater will be the deviation from the correct value.This incomplete dissociation is a chemical interferance and might be removed by the use of higher flame temperature. In situations where a hotter flame can not be utilized, chemicals means are suggested. For eg . Aluminium and magnesium form a thermally stable mixed oxide.

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Because of their stability, low results for magnesium are obtained in the presence of aluminium . The interference may be removed by the addition of lanthanum to the aluminum, lanthanum-aluminum oxide is formed and magnesium oxide is released. Magnesium is then determined after this latter compound is reduced and vaporized in the flame.

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Thermally stable metal phosphates and sulfates may be converted to thermally unstable metal salts of ethylene diamine tetra acetic acid, the other respective inorganic acid. Chemical interference which cause high results may also arise. For instance, the absorption of zirconium atoms is enhanced by nitrogen-containing compounds.

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In fact this enhancement is proportional to the concentration of nitrogen-containing compounds and has been used as procedure for the determination of ammonia over a limited range. The zirconium nitrogen compounds are more efficiently atomized than the other available zirconium oxides.

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( c) Ionization interference: Ionization interference arises in atomic absorption if flame temperature is to high.When this occurs the number of vapourised atoms become ionized by the flame. The resulting ions absorbed at the different wavelength than the vapourised atoms the new wavelength will not be selected by the monochromator ,and the low values results. The interference is usually minimized by the addition of more easily ionizable elements. For eg . Ionisation interference of calcium may be corrected by the addition of large quantities of Na or K salts to the solution.

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Since each of these elements possesses a lower ionization potential than does calcium their electrons will be easily removed calcium will remain as the vaporized metals. The process is represented by the following reaction : My Δ M° +Y° And the interfering reaction is M° Δ M + + e – Where My is molecular compound , M° & Y°are the vaporized elements, and M + is the metal ion.

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(d)Matrix or bulk interference : A change in the viscosity of solution caused by either change in solvent or by change in concentration may result in matrix or bulk interference. The addition an bulk solvent result in decrease viscosity and increase in absorbance. On the other hand, an in increase the concentration result in increase viscosity, a slower flow through burner, and corresponding decrease absorbance.

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(e) Solvent Interferences: In general, metals in aqueous solutions gives lower absorbance than same concentration of such elements in an organic solvent.

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(f) Dissociation metal compounds: When metals like La, Al aspired into the flame metal atoms are not obtained but stable refractory oxides are obtained. So for such elements nitrous oxide- acetylene flames used which dissociate this metal oxides . (g) Role of solvents: In general metals in aqueous solution yield the lower absorbance than concentration of such metals when present in organic solvent.

Application of atomic absorption spectroscopy :

Application of atomic absorption spectroscopy AAS is used for the determination of all metal and metalloid elements. Qualitative Analysis : The radiation source used in AAS is an HCL or an EDL, and a different lamp is needed for each element to be determined. Because it is essentially a single-element technique, AAS is not well suited for qualitative analysis of unknowns. For a sample of unknown composition, multielement techniques such as, inductively coupled plasma-optical emission spectrometry, and other atomic emission techniques are much more useful and efficient.

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AAS is an excellent quantitative method. 1 . Quantitative Analytical Range The relationship between absorbance and concentration of the analyte being determined in AAS follows Beer’s Law over some concentration range. There is an optimum linear analytical range for each element at each of its absorption lines. The ultimate limiting factor controlling the detection limit is the noise level of the instrument being used.

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The linear working range for AAS is small; generally only one to two orders of magnitude at a given wavelength. The calibration curve deviates from linearity, exhibiting a flattening of the slope at high absorbance values.

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2. Calibration Calibration of AAS methods can be performed by the use of an external calibration curve. External calibration curves are prepared from solutions of known concentrations of the sample element. High-purity metals dissolved in high-purity acids are used to make the stock standard solution . For AAS, stock standard concentrations are either 1000 or 10,000 ppm as the element. Working standards are diluted from the stock standard. For example, if we wanted to make a calibration curve for copper determination in the range of 2–20 ppm, we would make a stock standard solution of 1000 ppm Cu by dissolving Cu metal in nitric acid.

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In this example, the calibration blank would be prepared from deionized water and the same high purity nitric acid used to make the standard solutions. These standard solutions would then be aspirated into the flame and the absorbance of each standard would be measured. The absorbance would be plotted vs. concentration. A calibration curve like that in Fig.

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To obtain reliable quantitative data the following should be the same for the samples and standards: 1. The same solvent (e.g., water, 5% nitric acid, alcohol); same matrix modifier if used. 2. The same predominant anion (e.g., sulfate, chloride) at the same concentration. 3. The same type of flame (air–acetylene or nitrous oxide–acetylene) or the same graphite tube/platform. 4. Stable pressure in the flame gases during the analysis. 5. Absorbance measured at the same position in the flame/furnace. 6. Background correction carried out on each sample, blank, and standard using the same correction technique.

Difference between AAS & AES:

Difference between AAS & AES


Reference Instumental method of chemical analysis by G.R.Chatwal, Himalaya publishing house, fifth edition, page no: 2.340-2.366 Vogel ‘s Text book of quantitative chemical analysis by J.Mendham, sixth edition, page no: 613-631 Text book of pharmaceutical analysis by Dr. Ravishankar, third edition, page no: 27-1 to 27-8

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