Introduction to Atomic Spectroscopy Lecture 10 1 Introduction

Introduction to Atomic Spectroscopy Lecture 10 1

Introduction to Atomic Spectroscopy 2

Technique – Flame Test 3

An Introduction to Optical Atomic Spectroscopy The prerequisite for performing atomic spectroscopy is the: Atomization Method 1 - Compounds are first converted to gaseous molecules. 2 - Gas molecules then converted to gaseous atoms. 3 - Gaseous atoms absorb energy from a beam of radiation or simply heat. v. Measuring the absorption: (needs external light source). v. Measuring the emission: resulted from excited atoms (don’t need external light source). Both Abs. and Em. Is related to concentration of analyte. 4

Atomic Energy Level Diagrams v Only valence electrons are responsible for atomic spectra observed in a process of absorption or emission of radiation in the UV-Vis region. v Valence electrons in their ground states are assumed to have an energy equal to zero e. V. v As an electron is excited to a higher energy level, it will absorb energy exactly equal to the energy difference between the two states. 5

Portion of the sodium energy level diagram where sodium got one electron in the 3 s orbital: (3 S 1 Val. El. ) 6

Energy Level Diagram Sodium 5 Energy, electron volts 4 3 2 1 0 7 s 6 s 5 s 6160 4 s 6 p 5 p 4 p 4 p 5 d 4 d 5682 3 d 5688 8183 6154 11404 1138 2 2852. 8 2853 3 s 8195 3 p 3 p 3302 3303 5890 5896 Values: in Ao

8 more intense than others (most prop. And dense, (resonance lines (preferred for measurements)

4000 4400 4800 5200 5600 Wavelength A 5682. 7 5688. 2 Ao Relative intensity 5890 and 5896 Ao Flame Emission Spectrum of Sodium 6000 6400

- The dark lines represent most probable transitions and in an atomic spectrum they would appear more intense than others. - Two transitions, of very comparable energies (589. 0 and 589. 6 nm), from the 3 s ground state to 3 p excited state do take place. - This suggests splitting of the p orbital into two levels that slightly differ in energy. Explanation: Splitting may be presented as a result of electron spin where the electron spin is either in the direction of the orbital motion or opposed to it. 10

Both spin and orbital motion create magnetic fields that may lead to: Lower E line: interact in an attractive manner, if motion is in opposite direction, lower energy. Higher E line: interact in a repulsive manner when both spin and orbital motion are in the same direction. The same occurs for both d and f orbitals but the energy difference is so small to be observed. A Mg+ ion would show very similar atomic spectrum as Na since both have one electron in the 3 s orbital. 11

For atoms of large numbers of electrons: Atomic spectra become too complicated and difficult to interpret. This is mainly due to presence of a large numbers of closely spaced energy levels. Selection Rule: v Transition from ground state to excited state is not arbitrary and unlimited. v Transitions follow certain selection rules that make a specific transition allowed or forbidden. 12

Atomic Emission and Absorption Spectra At room temperature: All atoms are in the ground state. Excitation of electrons: From ground state atoms requires an input of sufficient energy to transfer the electron to one of the excited state through an allowed transition. Excited electrons: Spend a short time in the excited state (shorter than a ms). Relaxation an excited electron: Emit a photon and return to the ground state. 13

Preferred transition (most probable): • Each type of atoms would have certain preferred or most probable transitions (sodium has the 589. 0 and the 589. 6 nm). • Relaxation would result in very intense lines for these preferred transitions where these lines are called resonance lines. • Absorption of energy is most probable for the resonance lines of each element. • Thus intense absorption lines for sodium will be observed at 589. 0 and 589. 6 nm. 14

Atomic Fluorescence Spectra When gaseous atoms at high temperatures are irradiated with a monochromatic beam of radiation of enough energy to cause electronic excitation: • Emission takes place in all directions. • The emitted radiation from the first excited electronic level, collected at 90 o to the incident beam, is called resonance fluorescence. • Photons of the same wavelength as the incident beam are emitted in resonance fluorescence. • This topic will not be further explained in this text as the merits of the technique are not very clear compared to instrumental complexity involved 15

Fluorescence of Thallium Atom 6 s 27 s 2 S 1/2 = 535. 0 nm = 377. 6 nm 6 s 26 p 2 P 3/2 6 s 26 p 2 P 1/2 Energy level diagram of thallium

Atomic Line Width • Should have infinitesimally small (or zero) line width since transition between two quantum states requires an exact amount of energy. • However, careful examination of atomic lines reveals that they have finite width. For example, try to look at the situation where we expand the x-axis (wavelength axis) of the following line: 17

expand x-axis 18

The effective line width in terms of wavelength units : Is equal to D 1/2 Defined as the width of the line, in wavelength units, measured at one half maximum signal (P). The question which needs a definite answer is what causes the atomic line to become broad? 19

Reasons for Atomic Line Broadening There are four reasons for broadening observed in atomic lines. These include: 1. The Uncertainty Principle t. E h For un. of E = zero t must be • Nature places limits on the precision by which two interrelated physical quantities can be measured. • Uncertainty present for calculation of the energy required for a transition when the lifetime of the excited state is short. Error in E leads to error in 20

DE>const/Dt DE = hc/D Const’/D > Dt Therefore, atomic lines should have some broadening due to uncertainty in the lifetime of the excited state. The broadening resulting from the uncertainty principle is referred to as natural line width and is unavoidable. 21

The uncertainty effect The uncertainty principle Dn. Dt >1 • Dt = 2*10 -8 D =10 -5 nm •

2. Doppler Broadening Atoms moving fast either toward or away the detector (transducer): Fast moving atom toward a transducer: More wave crests and thus higher frequency will be measured. Fast moving atom away from a transducer: Lest wave crests and thus lower frequency will be measured. The wavelength of radiation emitted by the two types are different. The same occurs for sound waves 23

Assume your ear is the transducer, when a car blows its horn toward your each successive wave crest is emitted from a closer distance to your ear since the car is moving towards you. Thus a high frequency will be detected. On the other hand, when the car passes you and blows its horn, each wave crest is emitted at a distance successively far away from you and your ear will definitely sense a lower frequency. 24

higher frequency detected Lower frequency detected 25

The line width (D ) due to Doppler broadening can be calculated from the relation: D / o = v/c Where: o : is the wavelength at maximum power and is equal to ( 1 + 2)/2, (resulted from an atom moving perpendicular to the transducer will keep its original frequency and will not add to line broadening by the Doppler effect) v : is the velocity of the moving atom. c : is the speed of light. . 26

The Doppler Effect

Moving atoms towards source: v. In the case of absorption lines, you may visualize the line broadening due to Doppler effect since fast atoms moving towards the source will experience more wave crests and thus will absorb higher frequencies. v. On the other hand, an atom moving away from the source will experience less wave crests and will thus absorb a lower frequency. v. The maximum Doppler shifts are observed for atoms of highest velocities moving in either direction toward or away from a transducer (emission) or a source (absorption). 28

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3. Pressure Broadening (Collisional broadening) At high pressure atoms Line broadening caused by collisions of emitting or absorbing atoms with other atoms, ions, or other species in the gaseous matrix. v. These collisions result in small changes in ground state energy levels. v. Thus the energy required for transition to excited states will be different and dependent on the ground state energy level distribution. 30

Used as source for Fluorescence Spectroscopy 200 to 1100 nm 31

Pressure Broadening result in important line broadening. This phenomenon is most astonishing for xenon where a xenon arc lamp at a high pressure produces a continuum from 200 to 1100 nm instead of a line spectrum for atomic xenon. A high pressure mercury lamp also produces a continuum output. Both Doppler and pressure contribution to line broadening in atomic spectroscopy are far more important than broadening due to uncertainty principle. 32

4. Magnetic Effects Gaseous atoms in presence of a magnetic field: Splitting of the degenerate energy levels. The complicated magnetic fields exerted by electrons in the matrix atoms and other species will affect the energy levels of analyte atoms. The simplest situation: Is one where an energy level will be split into three levels 1)one of the same quantum energy. 2) Second one of higher quantum energy, 3)The third assumes a lower quantum energy state. A continuum of magnetic fields exists due to complex matrix components, and movement of species, thus exist. 33 Electronic transitions from the thus split levels will result in line broadening.

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The Effect of Temperature on Atomic Spectra Atomic spectroscopic methods require the conversion of atoms to the gaseous state. This requires the use of high temperatures (in the range from 2000 -6000 o. C). Thee high temperature can be provided through: 1) A flame. 2) Electrical heating. 3) An arc or a plasma source. It is essential that the temperature should be : v Of enough value to convert atoms of the different elements to gaseous atoms and, v In some cases, provide energy required for excitation. v Remain constant throughout the analysis especially in atomic emission spectroscopy. 35

Quantitative assessment: The effect of temperature on the number of atoms in the excited state can be derived from Boltzmann equation: Where: Nj : is the number of atoms in excited state No : is the number of atoms in the ground state, Pj and Po : are constants determined by the number of states having equal energy at each quantum level Ej: is the energy difference between excited and ground states. K : is the Boltzmann constant. T: is the absolute temperature. 36

Boltzmann distribution 37 Atom Wavelength Cs 852. 1 nm Nj /N 0 at 3000 K 7. 24 10 -3 Na 589. 0 nm 5. 88 10 -4 Ca 422. 7 nm 3. 69 10 -5 Zn 213. 9 nm 5. 58 10 -10

Application of Boltzmann equation: Let us consider the situation of sodium atoms in the 3 s state (Po = 2) when excited to the 3 p excited state (Pj = 6) at two different temperatures 2500 and 2510 K. Now let us apply the equation to calculate the relative number of atoms in the ground and excited states: Usually we use the average of the emission lines from the 3 p to 3 s where we have two lines at 589. 0 and 589. 6 nm which is: 38

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Therefore: At higher temperatures: The number of atoms in the excited state increases. Let us calculate the percent increase in the number of atoms in the excited state as a result of this increase in temperature of only 10 o. C: 40

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Introduction to Atomic Spectroscopy Lecture 11 42

Effect of Temperature on Atomic Absorption and Emission Which technique would be affected more as a result of fluctuations in temperature? The answer to this important question is rather simple. Atomic emission: It will be severely affected by fluctuations in temperature since signal is dependent on the number of atoms in the excited state. This number is significantly affected by fluctuations in temperature as seen from the example above. Atomic absorption: The signal depends on the number of atoms in ground state that will absorb energy. 43

very high as related to the number of excited atoms: Nj/No = 1. 72 x 10 -4 or 172 excited atoms for each 106 atoms in ground state. This suggests a very high population of the ground state even at high temperatures. Therefore: Atomic absorption will not be affected to any significant extent by fluctuations in temperature, if compared to atomic emission spectroscopy. 44

Some indirect effects of temperature on atomic absorption spectroscopy. 1. Better sensitivities are obtained at higher temperatures since higher temperatures can increase the number of vaporized atoms at any time. 2. Higher temperatures will increase the velocities of gaseous atoms, thus causing line broadening as a result of the Doppler and collisional effects. 3. High temperatures increase the number of ionized analyte and thus decrease the number of atoms available for absorption (decrease of signal). 45

Band Continuum Spectra Associated with Atomic Spectra When the atomization temperature is insufficient to cause atomization of all species in the sample matrix: • Existent molecular entities, at the temperature of the analysis, impose very important problems on the results of atomic absorption and emission spectroscopy. • The background band spectrum should be removed for reasonable determination of analytes. • Otherwise, the sensitivity of the instrument 46 will be significantly decreased.

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As the signal for the blank is considered zero and thus the instrument is made to read zero, when the analyte is to be determined, it got to have an absorbance greater than the highest point on the continuum and the instrument will assume that the absorbance related to analyte is just the value exceeding the background blank value. This will severely limit the sensitivity of the technique. ﻳﺠﺐ ﺍﻟﺘﺨﻠﺺ ﻣﻦ ﺍﻟﻤﻨﺤﻨﻰ ﺑﻄﺮﻳﻘﺔ ﺳﻠﻴﻤﺔ ﺳﺘﺪﺭﺱ ﻻﺣﻘﺎ ﺑﺘﻘﻨﻴﺔ Back ground by correction 48

Putting this conclusion in other words we may say that if the analyte signal is less than the background blank, the instrument will read it as zero. Therefore, it is very important to correct for the background or simply eliminate it through use of very high temperatures that will practically atomize all species in the matrix. We will come to background correction methods in the next chapter. 49

Atomization Methods It is essential, as we have seen from previous discussion, that all sample components (including analytes, additives, etc. ) should be atomized. The atoms in the gaseous state absorb or emit radiation and can thus be determined. Many atomization methods are available which will be detailed in the next two chapters. 50

Generally, flame atomization methods can be summarized by: 51

Sample Introduction Methods The method of choice for a specific sample will mainly depend on: whether the sample is in solution or solid form. The method for sample introduction in atomic spectroscopy affects the precision, accuracy and detection limit of the analytical procedure. 52

Introduction of Solution Samples 1. Pneumatic ﻫﻮﺍﺋﻲ Nebulizers : Samples in solution are usually easily introduced into the atomizer by a simple nebulization, aspiration, process. Nebulization: Converts the solution into an aerosol of very fine droplets using a jet of compressed gas. The flow of gas carries the aerosol droplets to the atomization chamber or region. Several versions of nebulizers are available and few are shown in the figure below: 53

Predominates in most instruments 54 Concentric Tube Nebulizer

Concentric Tube Pneumatic Nebulizer Capillary tube High velocity gas flow High-velocity gas breaks the liquid up into fine droplets. Suction Sample High pressure gas flow Gas inlet

Also, predominates in most instruments 56 Cross Flow Nebulizer

Cross-Flow Pneumatic Nebulizer ` High pressure gas flow Sample

Babington Nebulizer 58

Babington Pneumatic Nebulizer Sample High pressure gas flow Solution film

Fritted Disc Nebulizer Porous base 60

Fritted Disk Pneumatic Nebulizer Sample High pressure gas flow

2. Ultrasonic Nebulizers • In this case samples are pumped onto the surface of a piezoelectric crystal (that vibrates in the k. Hz to MHz range. • Such vibrations convert samples into homogeneous aerosols that can be driven into atomizers. • Ultrasonic nebulization is preferred over pneumatic nebulization: Because: finer droplets and more homogeneous aerosols are usually achieved. However, most instruments use pneumatic nebulization. 62

Vibrated plate gives aerosol droplets 63

3. Electrothermal Vaporization ﻛﻬﺮﺑﻲ ﺗﺴﺨﻴﻦ An accurately measured quantity of sample (few m. L) is introduced into an electrically heated cylindrical chamber through which an inert gas flows. Constituent: Usually, the cylinder is made of pyrolytic carbon but tungsten cylinders are now available. ﺍﻟﻌﺎﻟﻴﺔ ﺍﻟﺤﺎﺭﺓ ﺗﺘﺤﻤﻞ ﻣﻮﺍﺩ The signal produced by instruments which use electrothermal vaporization (ETV) is a discrete signal for each sample injection (not continuous). Electrothemal vaporizers are called discrete atomizers to differentiate them from nebulizers which are called continuous atomizers 64

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4. Hydride Generation Techniques Samples that contain arsenic, antimony, tin, selenium, bismuth, and lead ( )ﺳﻬﻠﺔ can be vaporized by converting them to volatile hydrides by addition of sodium borohydride. Volatile hydrides are then swept into the atomizer by a stream of an inert gas. 66

Sodium borohydride + acid on sample 67

For production of gas 68

Introduction to Atomic Spectroscopy Atomic Absorption Spectroscopy Lecture 12 69

Introduction of Solid Samples A variety of techniques were used to introduce solid samples into atomizers. These include: 1. Direct Sample Insertion: Samples are first powdered and placed in a boat-like holder (from graphite or tantalum) which is placed in a flame or an electrothermal atomizer. 70

2) If the sample is conductive and is of a shape that can be directly used as an electrode: v (like a piece of metal or coin), that would be the choice for sample introduction in arc and spark techniques. v Otherwise, powdered solid samples are mixed with fine graphite and made into a paste. § Upon drying, this solid composite can be used as an electrode. § The discharge caused by arcs and sparks interacts with the surface of the solid sample creating a plume ( )ﺳﺤﺐ of very fine particulates and atoms that are swept into the atomizer by a flow of an inert gas. § This process of sample introduction is called ablation

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3. Laser Ablation ﺑﻌﺜﺮﺓ v Sufficient energy from a focused intense laser will interact with the surface of samples (in a similar manner like arcs and sparks) resulting in ablation. v The formed plume of vapor and fine particulates are swept into the atomizer by the flow of an inert gas. v Laser ablation is becoming increasingly used since it is applicable to conductive and nonconductive samples 73

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Introduction of Solid Samples • Direct sample insertion • Electrothermal vaporization • Arc and spark ablation • Laser ablation

4. The Glow Discharge Technique A low pressure envelope (1 to 10 torr argon) with two electrodes with the conductive solid sample is the cathode, as in the figure below. v The technique is used for sample introduction and atomization as well. v. The electrodes are kept at a 250 to 1000 V DC. v. This high potential is sufficient to cause ionization of argon which will be accelerated to the cathode where the sample is introduced. v. Collision of the fast moving energetic argon ions with the sample (cathode) causes atomization by a process called 77 sputtering.

Sputtering 78

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Atomic Absorption Spectroscopy We will cover two main techniques of atomic absorption spectroscopy (AAS), depending of the type of atomizer. Two atomization techniques are usually used in AAS: 82

1. Flame Atomization Flames are regarded as continuous atomizers: Because samples are continuously introduced and a constant or continuous signal is obtained. Samples in solution form are nebulized by one of the described nebulization techniques discussed previously. The most common nebulization technique is the pneumatic nebulization. Nebulized solutions are carried into a flame where atomization takes place. 83

Several processes occur during atomization including: a. Nebulization: Sample aspirated and nebulized samples are sprayed into a flame as a spray of very fine droplets. b. Desolvation: Droplets will lose their solvent content due to very high flame temperatures and will thus be converted into a solid aerosol. c. Volatilization: The solid aerosol is volatilized to form gaseous molecules 84

d. Dissociation: Gaseous molecules will then be atomized and neutral atoms are obtained which can be excited by absorption of enough energy. Note: If energy is not enough for atomization, gaseous molecules will not be atomized and we may see molecular absorption or emission. e. Excitation: Atoms in the gaseous state can absorb energy and are excited. Note: If energy is too much, we may observe ionization. 85

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The different processes occurring in flames are complicated and are not closely controlled and predicted. Therefore, it can be fairly stated that the atomization process in flames may be one of the important parameters limiting the precision of the method. It is therefore justified that we have a closer look at flames and their characteristics and the different variables contributing to their performance. 88

Types of Flames can be classified into several types depending on fuel/oxidant used. For example, the following table summarizes the features of most familiar flames. Therefore, it can be clearly seen that significant variations in flame temperatures can be obtained by changing the composition of fuel and oxidant. 89

Max. Burning velocity 90

On the other hand, flames are only stable at certain flow rates and thus the flow rate of the gas is very important. (the flow rate is defined the volume of gas in second). At low flow rates: ( i. e. less than the maximum burning velocity) the flame propagates into the burner body causing flashback and, in some cases, an explosion. As the flow rate is increased: The flame starts to rise above the burner body. Best flames are obtained when the flow rate of the gas is equal to the maximum burning velocity. At this equity ratio the flame is most stable. At higher ratios: flames will reach a point where they 91 will no longer form and blow off the burner.

Flame Structure Three well characterized regions can be identified in a conventional flame. 1) The lower region: Which close to the burner tip, with blue luminescence. This region is called the primary combustion zone. v Characterized by: The existence of some non atomized species and presence of fuel species (C 2 and CH, etc. ) that emit in the blue region of the electromagnetic spectrum. 2)The second is interzonal region: It is well defined region which is the just above the primary combustion zone. v. Characterized by: 1)is rich in free atoms and is the region of choice for performing atomic spectroscopy. 2) It also contains the regions of highest temperatures. 92

3) The third region is the outer region : which is called the secondary combustion region. Characterized by: Reformation of molecules as the temperature at the edges is much lower than the core. 93

Schematically representation of flame: 94

Flame Absorption Profiles We have seen that there are different temperature profiles in a flame and temperature changes as the distance from the burner tip is change 95