atomic absorption spectroscopy and mass spectroscopy

basic principle and working model of atomic absorption spectroscopy and mass spectroscopy

1. ATOMIC ABSORPTION AND MASS SPECTROSOPY G.Vanitha

2. ATOMIC ABSORPTION SPECTROSOPY

3. INTRODUTION Atomic Absorption Spectroscopy is a very common technique for detecting metals and metalloids in samples. It is very reliable and simple to use. It can analyze over 62 elements. It also measures the concentration of metals in the sample.

4. HISTORY The first atomic absorption spectroscopy was built by CSIRO scientist Alan Walsh in 1954. The first commercial atomic absorption spectroscopy was introduced in 1959.

5. Elements detectable by atomic absorption are highlighted in pink in this periodic table.

6. PRINCIPLE The technique uses basically the principle that free atoms (gas) generated in an atomizer can absorb radiation at specific frequency. Atomic Absorption spectroscopy quantifies the absorption of ground state atoms in the gaseous state. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. The analyte concentration is determined from the amount of absorption. Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration.

7. INSTRUMENTATION

8. LIGHT SOURCE Hollow Cathode Lamp are the most common radiation source in AAS . It contains a tungsten anode and a hollow cylindrical cathode. These are sealed in a glass tube filled with an inert gas (neon or argon ) . Each element has its own unique lamp which must be used for that analysis .

9. Hollow Cathode Lamp for Aluminum (Al)

10. NEBULIZER Suck up liquid samples at controlled rate. Create a fine aerosol spray for introduction into flame. Mix the aerosol and fuel and oxidant thoroughly for introduction into flame.

11. ATOMIZER Elements to be analysed needs to in atomic state. Atomization is separation of particles into individual molecules and breaking molecules into atoms. This is done by exposing the analyte to high temperature in a flame or graphite furnace.

12. Flame Atomization Nebulizer suck up liquid samples at controlled rate . Create a fine aerosol spray for introduction into flame . Mix the aerosol and oxidant thoroughly for introduction into flame . An aerosol is a colloid of fine solid particles or liquid droplets, in air or another gas.

13. Flame Atomization nebulization volatilization d e solvation dissociation

14. Disadvantages of Flame Atomization Only 5-15% of the nebulized sample reaches the flame . A minimum sample volume of 0.5-1.0 ml is needed to give a reliable reading . Samples which are viscous require dilution with a solvent.

15. Electro Thermal Atomization Uses a graphite coated furnace to vaporize the sample . samples are deposited in a small graphite  coated tube which can then heated to vaporize and atomize the analyte . The graphite tubes are heated using a high current power supply.

16. Advantages Small sample size Very little or no sample preparation is needed Sensitivity is enhanced Direct analysis of solid samples Analyte may be lost at the ashing stage The sample may not be completely atomized Analytical range is relatively low Disadvantages

17. MONOCHROMATOR This is very important part in an AAS. It is used to separate out all of the thousands of lines. A monochromator is used to select the specific wavelength of light which is absorbed by the sample, and to exclude other wavelengths. The selection of the specific light allows the determination of the selected element in the presence of others.

18. DETECTOR The light selected by the monochromator is directed onto a detector that is typically a photomultiplier tube, whose function is convert the light signal into an electrical signal proportional to the intensity. The processing of electrical signal is fulfilled by a signal amplifier. The signal could be displayed for readout, or further fed into a data station for printout by the requested format

19. Calibration Curve A calibration curve is used to determine the unknown concentration of an element in a solution. The instrument is calibrated using several solutions of known concentrations. The absorbance of each known solution is measured and then a calibration curve of concentration vs absorbance is plotted. The sample solution is fed into the instrument, and the absorbance of the element in this solution is measured. The unknown concentration of the element is then calculated from the calibration curve

20. Applications Determination of even small amounts of metals (lead, mercury, calcium, magnesium, etc.) Environmental studies: drinking water, ocean water, soil. Food industry. Pharmaceutical industry. Presence of metals as an impurity or in alloys could be done easily Level of metals could be detected in tissue samples like Aluminum in blood and Copper in brain tissues

21. MASS SPECTROSOPY

22. Introduction Mass Spectroscopic method is one of the most popular molecular analysis methods today. Mass Spectroscopy is an analytical spectroscopic tool primarily concerned with the separation of molecular (and atomic) species according to their mass. It is a microanalytical technique requiring only a few nanomoles of the sample to obtain characteristic information pertaining to the structure and molecular weight of analyte . It is not concerned with non- destructive interaction between molecules and electromagnetic radiation.

23. Principle Mass spectroscopy is the most accurate method for determining the molecular mass of the compound and its elemental composition . In this technique, molecules are bombarded with a beam of energetic electrons . The molecules are ionised and broken up into many fragments, some of which are positive ions.

24. Mass spectra is used in two general ways : To prove the identity of two compounds . To establish the structure of a new a compound. The mass spectrum of a compound helps to establish the structure of a new compound in several different ways: It can give the exact molecular mass . It can give a molecular formula or it can reveal the presence of certain structural units in a molecule .

25. TYPICAL DIAGRAM OF MS

26. METHODOLOGY Gaseous or liquid substances that vaporize under vacuum are admitted to a mass spectroscopy. The gas is diluted by being partially pumped down to a low pressure (molecular flow range) in a vacuum chamber and ionized through electron bombardment. The ions thus generated are introduced to a mass filter and separated on the basis of their charge to mass ratio.

27. IONISATION The atom is ionised by knocking one or more electrons off to give a positive ion. (Mass spectrometers always work with positive ions). The particles in the sample (atoms or molecules) are bombarded with a stream of electrons to knock one or more electrons out of the sample particles to make positive ions .

28. ACCELERATION The ions are accelerated so that they all have the same kinetic energy .

29. The positive ions are repelled away from the positive ionization chamber and pass through three slits with voltage in the decreasing order . The middle slit carries some intermediate voltage and the final at ‘0’ volts . All the ions are accelerated into a finely focused beam.

30. DEFLECTION The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected. The amount of deflection also depends on the number of positive charges on the ion -The more the ion is charged, the more it gets deflected.

31. Different ions are deflected by the magnetic field by different amounts. The amount of deflection depends on : The mass of the ion: Lighter ions are deflected more than heavier ones . The charge on the ion : Ions with 2 (or more) positive charges are deflected more than ones with only 1 positive charge.

32. DETECTION The beam of ions passing through the machine is detected electrically. When an ion hits the metal box, its charge is neutralized by an electron jumping from the metal on to the ion .

33. That leaves a space among the electrons in the metal, and the electrons in the wire shuffle along to fill it . A flow of electrons in the wire is detected as an electric current which can be amplified and recorded. The more ions arriving, the greater the current .

34. APPLICATIONS Pharmaceutical analysis Bioavailability studies Drug metabolism studies, pharmacokinetics Characterization of potential drugs Drug degradation product analysis Screening of drug candidates Identifying drug targets Biomolecule characterization Proteins and peptides Oligonucleotides

35. Environmental analysis Pesticides on foods Soil and groundwater contamination Forensic analysis/clinical

36. Reference Mass Spectroscopy Amruta S. Sambarekar Mass Spectroscopy - An Overview Dr. M. Vairamani, IPFT http://www.uga.edu/~ sisbl/aaspec.html http:// www.clu-in.org/char/technologies/graphite.cfm B. Welz, M. Sperling, Atomic Absorption Spectrometry, Wiley-VCH, Weinheim, Germany, ISBN 3-527-28571-7. Skoog, Douglas (2007). Principles of Instrumental Analysis (6th ed.). Canada: Thomson Brooks/Cole. ISBN 0-495-01201-7 .

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