FUNDAMENTAL OF ATOMIC ABSORPTION SPECTROSCOPY Wedad H. Al- Dahhan.

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2 FUNDAMENTAL OF ATOMIC ABSORPTION SPECTROSCOPY Wedad H. Al- Dahhan

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5 Atomic absorption spectroscopy  Atomic absorption spectroscopy (AAS) is a spectro analytical procedure for the quantitative determination of chemical elements employing the absorption of optical radiation (light) by free atoms in the gaseous state.  In analytical chemistry the technique is used for determining the concentration of a particular element (the analyte) in a sample to be analyzed.  AAS can be used to determine over 70 different elements in solution or directly in solid samples

6 Elements detectable by atomic absorption

7 Flame Emission Atomic Absorption Atomic Fluorescence ICP-MS Year 1950196019701980 1990 Atomic Spectra Development

8 Absorption and fluorescence by atoms in a flame

9 Concentration Coverage

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11 Atomic Absorption E2E2 E1E1 e-e- h e-e- h Nucleus

12 E2E2 E1E1 e-e- h E 2 = Excited state E 1 = Ground state h = Planck’s constant = Spectral frequency C = Speed of light Atomic Absorption E = E 2 - E 1 = h  = c /  hc/ (E 2 - E 1 )

13 Sodium Lines 2.2 eV 589 nm Ground state eV 2 4 6 3.6 eV 330.3 nm

14 Lambert-Beer’s Law I = I o e -klc T = (I/I o ) x 100% A = log (I o /I) = klc where T = transmittance, A = absorbance, k = molar absorptivity, c = concentration of atomic vapour

15 1)Radiation source (hollow cathode lamp) 2)Atomizer (Sample conversion to free atoms ) 3)Optical system (single/double beam) 4)Monochromator 5)Detector (photomultiplier tube) 6)Signal processor and output display AAS Instrumentation

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18 (1) Light Source

19 The light source is usually a hollow cathode lamp of the element that is being measured. It contains a tungsten anode and a hollow cylindrical cathode made of the element to be determined. 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.

20 Hollow Cathode Lamp (HCL)

21 Hollow Cathode Lamp M M M M M Ar Cathode Ar Anode Ar + e - Ar + + 2e - Ar + + M (s) M (g) M (g) M* (g) M* (g) M (s) Light e -, Ar +

22 (2) Atomizer Creation of Free Atomic Vapor!

23 The process of converting the analyte into free atoms to emit or absorb light energy. Nebulization Aerosol Free Atomic Vapor Furnace heating Flame Atomization Solution

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25 2 Types Of Atomizer 1)Flame 2)Flameless a)graphite furnace AAS (GFA) b)hydride vapor generator (HVG) c) Cold Vapour Technique

26 Atomizer IoIo I Atomic Vapor l In flame, l = length of burner head In GFA, l = length of graphite tube

27 Flames are formed by combustion of an oxidant and a fuel mixture, i.e. air, nitrous oxide, Ar, H 2, acetylene. Flame Atomization Flame Atomization

28 Flame Atomization Process

29 Flame Atomization Steps  Nebulization.  Desolvation of droplets.  Vaporization of solids.  Dissociation of molecular species.  Ionization of analyte atoms.

30 Nebulization  The sample introduction system disperses the sample solution onto the impact bead  This causes the sample solution to change into fine spray or mist which can be carried by gases upwards to the flame.

31 Laminar Flow Burner

32 Desolvation Of Droplets  Organic solvents evaporate more rapidly than water.  The desolvation leaves a dry aerosol of the molten or solid particles.  This often begins in the nebulization step.

33 Vaporization of Solids The solids or molten particles remaining after desolvation must be vaporized to obtain free atoms [ MX (gas) M (gas) + X (gas) ]

34 Vaporization Efficiency  Depends on bond dissociation energies of compounds.  Flame conditions - high temperatures and a reducing environment tend to increase the volatilization efficiency and reduce the formation of refractory oxides.  Aerosol size - the vaporization increases as the size of droplets introduced into the chamber decreases.  Incomplete vaporization - results in non-linearity in calibration curve and continuous background emission by molecular species.

35 Flame Structure The burning velocity is a fundamental parameter of the gas mixture and it is important in determining the flame shape and stability.

36 Temperature profiles for a natural gas - air flame

37 Flame selection -These flames vary in temperature, reducibility and transmission characteristics. -Selected according to the element being analyzed, and properties of the sample. Argon-hydrogen: Max. temp. 1,577 °C Air- hydrogen : Max. temp. 2,045 °C Air- acetylene : Max. temp. 2,300 °C Nitrous oxide-acetylene : Max. temp. 2,955 °C (For elements are hard to combine with oxygen (Al, Si, V, Ti, etc.))

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39 Atomic Vapour Population Burner height and fuel composition do not have much affect on atomic mist distribution. Burner height and fuel composition affect the atomic mist distribution.

40 Atomic Vapour Population

41 -element rapidly oxidizes - largest [atom] near burner -element poorly oxidizes - largest [atom] away from burner most sensitive part of flame for AAS varies with analyte

42 Graphite Furnace AAS (GFA)

43 GFA  Also known as electro thermal.  The graphite furnace tube is continually bathed in an inert gas (i.e. Ar) to prevent the furnace from oxidation.  Inert gas reduces the oxide formation and increases the atomization efficiency.

44 GFA Atomization Process

45 GFA Atomization Steps  Drying or desolvation step  Ashing step  Atomization step  Cleaning step (optional)

46  Like the desolvation step in flame AAS, the solvent is removed.  Generally the heating temperature is set at 60-150 o C for water-based samples and 50-100 o C for organic-based samples.  The chosen temperature should ideally evaporate the solvent as rapidly as possible without spattering. (1) Drying Step

47  During this stage, organic matter in the sample is ashed or converted into water, CO 2 and volatile inorganic compounds.  Ideally, the temperature should be high enough to remove all volatile components without loss of the analyte. (2) Ashing Step

48  The analyte is vaporized and atomized to produce atomic vapor at around 2000- 3000 o C.  At the end of the atomization stage, the atomic vapor is rapidly diffused out of the observation zone. (3) Atomization Step

49  To evaporate remaining metal and salt which remains in the graphite tube.  Carried out at 3000 o C but lower temperature desirable.  Cleaning temperature is normally atomization temperature plus 200 o C. (4) Cleaning Step

50 Drying step Ashing step Atomization step Temperature ( o C) Time (s) Outer gas (Ar) Inner gas (Ar) Inner gas (O 2 ) 0 20355054 Cleaning step (option) GFA Heating Steps

51 - Drying stage (100°C ) - Ashing stage (400-1000°C ) - Atomizing stage (1400-3000°C )

52 Graphite Tubes 1)High density graphite tube 2) Pyrolytic graphite tube 3) Platform type graphite tube

53 High density graphite tube Pyrolytic graphite tube Graphite Tubes Sample Injection Port

54 Platform Type Graphite Tube Sample Injection Port

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56 High Sensitivity GFA

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58 Flame/GFA Auto- Switching Single-Body Dual Atomizer

59  Easy switch over between GFA and flame atomizer.  High Precision x- and y-axis motorized movement.  Available in model AA-6701 and AA-6800. Dual Atomizer

60 Manual Switching  Available in AA-6300.  To change from flame to furnace mode, just remove the burner head, place the furnace unit, and fix it with the screw. No tools are required Remove the burner head. Fit the furnace. Remove the furnace. Fit the burner head.

61 ASC-6000 ASC-6000/ASC-6100 ASC-6100

62 Auto-Sampling Unit

63 Mixing Mode

64 Cu Calibration Curve. Preparation of 40, 80, 120, 160 ppb with automatic dilution of 200 ppb Cu. Auto-Dilution - GFA

65 HVG  Hydride Vapour Generator.  For volatile elements such as arsenic (As), selenium (Se), antimony (Sb), tin (Sn), bismuth (Bi), tellurium (Te).  Conversion of elements to metal hydrides by sodium borohydride under acidic condition.  Detection limit improved to ppb level.  Suitable for environmental analysis. As calibration curve

66 HVG - Reaction 3BH 4 - + 3H + + 4H 3 AsO 3 4AsH 3 + 3H 2 O + 3H 3 BO 3 As, Bi, Sb, Se, Sn, Te Gaseous Hydride Atomization Flame Electrical heating cell HVG-1

67 HVG-1 - Flow Line ElementsConcentration (ppb) As5~20 Sb5~20 Te5~20 Bi5~20 Se10~40 Hg20~80 Sn30~90

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69 Typical Detection Limits (ppb)

70 Cold Vapor Technique

71 Flame vs GFA

72 (3) Optical System ATOMIZER OPTICS

73 AAS Optical System

74 Single-Beam Optical Systems AA-6800/6650 AA-6701 AA-6300 GFA

75 Double-Beam Optical Systems AA-6200

76 Double-Beam Optical System

77 (4) Monochromator

78 Monochromator Consists of: 1)Dispersive element (diffraction grating) 2) Image transfer system (entrance slit, mirror or lenses, and exit slit)

79 Dispersive Grating  Consists of a plane or concave plate that is ruled with closely spaced grooves.  Different wavelengths are obtained when the plate is rotated at different angles.

80 Monochromator

81 (5) Detector Photomultiplier Tube

82 (6) Signal Processing Unit

83 INTERFERENCE IN AAS ANALYSIS & SOLUTIONS

84 Interference In AAS Analysis (1) Physical Interference (2)Chemical Interference (3)Spectrophotometer Interference a)molecular absorption b)light scattering c)spectral interference

85 (1) Physical Interference  Causes Interference from fluid characteristics of the sample e.g. viscosity, surface tension, etc. Flame AASGraphite Furnace AAS Flame Dispensing tube

86 Physical Interference  Solutions Flame AAS - carry out large dilution (10 to 50 times) or small dilution with acetone or butanol. GFA - use pyrolytic/platform graphite tube.

87 (2) Chemical Interference  Causes Generation of various compounds from chemical reactions between components in the sample, e.g.  in flame AAS, phosphate interference with regards to Ca, Mg, etc. (alkaline earth metals).  in GFA, target elements scattering at ashing step caused by chlorine compounds, etc.

88 Chemical Interference  Solutions Removal of obstructing materials by ion exchange and solvent extraction. Target element extraction. Use the hotter N 2 O/C 2 H 2 flame. Addition of interference suppressant e.g. Sr and La or EDTA (Ethylenediaminetetraacetic acid) for alkaline earth elements. Use matrix modifier in GFA.

89 Chemical Interference  Solutions  Ionization Buffers  Provide an excess of electrons, to increase the free atom population of elements with low ionization potential  E.g. Cesium chloride, Cesium nitrate, Lithium chloride, Lithium nitrate, Potassium chloride, Potassium nitrate

90 Chemical Interference  Solutions  Use Matrix Modifier in GFA  Reduce the volatility of the analyte.  Increase the atomization efficiency of the analyte by changing its chemical composition.  Permits the use of higher charring temperatures to volatile interfering substances and improve sensitivity.  Increase the volatility of the matrix.  Magnesium nitrate, Palladium nitrate, Calcium nitrate, Ammonium phosphate, Ammonium nitrate, Nickel nitrate

91 Chemical Interference  Matrix Modifier effect in GFA. NaCl + NH 4 NO 3 NH 4 Cl + NaNO 3 Volatile Elements, e.g. + H 3 PO 4 Cd (PO 4 ) Cd, Pb Decomposed at 400 o C Less volatile

92 Chemical Interference

93 (3) Spectrophotometric Interference  Causes a)molecular absorption b) light scattering c) spectral interference

94 (a) Molecular Absorption  Caused by un dissociated molecules in the sample path, the absorption bands from molecules are usually broad in UV region. Caused by particles in the sample path, and also produces a broad-band effect. (b) Light Scattering

95 (c) Spectral Interference  Caused by overlapping of the atomic absorption of an analyte and other free atoms in sample (two spectral with close absorption wavelengths).

96 Spectral Interference

97 Spectrophotometric Interference  Solution Removal of obstructing materials by solvent extraction. Background correction by instrument.

98 SAMPLEPREPARATION

99 Sample Preparation

100 Sample Pretreatment  To clean up samples.  Removal of interfering materials.  Separation of the element.  To decompose the organic substances by dry ashing, wet ashing methods, etc.  Method depends on nature of element, sample, potential interference, and analysis method.

101 Sample Pretreatment  Precautions for pretreatment: Dissolve all the elements into the same solution evenly - check with certified reference material. Ensure that elements are not lost (i.e. due to vaporization or sedimentation) in the solution - check with recovery test. Contamination from purified water, reagent (e.g. acid), container, environment - check with blank operation. Ensure that the solution to be analyzed is stable for a long time (i.e., no hydrolysis, sedimentation, or adsorption). Consider the effect (interference) of the reagent (e.g. acid, salt concentration) on the analysis values.

102 Sample Pretreatment  Dilution method: The sample is diluted using purified water, dilute acids, and organic solvents. Effective only for homogenous/uniform samples. E.g. food products (e.g. dairy products), pharmaceutical, wastewater, plating solution, lubricants, biological samples (e.g. blood, urine, etc).

103 Sample Pretreatment  Treat inorganic samples in mineral acids (HNO 3, HCl, H 3 PO 4, H 2 SO 4, HF, HClO 4 ) normally with heating.  Convert organic samples by oxidative treatment to CO 2 and H 2 O with: Dry ashing Wet ashing (digestion) High pressure decomposition e.g. microwave digestion.

104 Dry Ashing  The sample is heated (400 to 550ºC) and combusted in an electrical furnace.  Decomposes in a short time (a few hours).  E.g. food products, plastics, organic powders, etc. Sample CO 2

105 Dry Ashing Drying Oxidation, Ashing Acid dissolution

106 Wet Ashing  The sample & acid are heated at low temperatures (up to 300ºC) - suitable for volatile elements.  E.g. iron & steel, non-ferrous metals, living organisms, food products, plastics etc.  Boiling by using HCl or HNO 3 - for extremely small amount of organic substances and suspensions.  Decomposing by using HCl or HNO 3 - for samples contain -OH, oxide, sulfide phosphate.  Decomposing by HNO 3 and HClO 4 - organic substances hard to be oxidized.

107 Wet Ashing Clean, readily oxidized sample HNO 3 - H 2 SO 4 HNO 3 -HCl HNO 3 - HClO 4 or HNO 3 - HClO 4 -HF Difficult-to-oxidized organic samples Samples containing -OH, oxide, sulfide, phosphate HNO 3

108 Wet Ashing – H 2 SO 4 & H 2 O 2 Sample Oxidation Remove H 2 O 2 Digestion & Oxidation H 2 SO 4 H2O2H2O2 Boiling

109 Wet Ashing - H 2 SO 4 & H 2 O 2

110 Wet Ashing  Kjeldahl flask.  Compared to normal wet decomposition, there is little volatilization or external contamination.  But not suitable to process multiple samples.  For organic samples such as plastics e.g. based on EN1122.  When analyzing As, Se, Hg, etc. Cooling tube Nitric acid Sample + sulfuric acid

111 Wet Ashing - Advantages  Minimize the loss of volatile elements.  Oxidation at less drastic temperature.

112 Wet Ashing - Disadvantages  Organic matter takes a long time to decompose (from a few hours to several days).  High possibility of contamination by oxidizing reagents - must watch out for contamination from the acid or the operating environment, such as the container and atmosphere..  Unavoidable loss of elements like As, Hg and Se. Use catalyst such as Mo(VI) or (V).

113 Thank you!