1 Introduction to Atomic Spectroscopy Lecture 10

2 Introduction to Atomic Spectroscopy

3 Technique – Flame Test

4 An Introduction to Optical Atomic Spectroscopy In optical atomic spectrometry, compounds are first converted to gaseous molecules followed by conversion to gaseous atoms. This process is called atomization and is a prerequisite for performing atomic spectroscopy. Gaseous atoms then absorb energy from a beam of radiation or simply heat. Absorbance can be measured or emission from excited atoms is measured and is related to concentration of analyte.

5 Atomic Energy Level Diagrams As a start, we should be aware that only valence electrons are responsible for atomic spectra observed in a process of absorption or emission of radiation in the UV-Vis region. Valence electrons in their ground states are assumed to have an energy equal to zero eV. As an electron is excited to a higher energy level, it will absorb energy exactly equal to the energy difference between the two states. Let us look at a portion of the sodium energy level diagram where sodium got one electron in the 3s orbital:

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8 The dark lines represent most probable transitions and in an atomic spectrum they would appear more intense than others. It should also be indicated that two transitions, of very comparable energies (589.0 and nm), from the 3s ground state to 3p excited state do take place. This suggests splitting of the p orbital into two levels that slightly differ in energy. Explanation of this 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.

9 Both spin and orbital motion create magnetic fields that may interact in an attractive manner (if motion is in opposite direction, lower energy), or in a repulsive manner when both spin and orbital motion are in the same direction (higher energy). 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 3s orbital.

10 In cases where atoms of large numbers of electrons are studied, atomic spectra become too complicated and difficult to interpret. This is mainly due to presence of a large numbers of closely spaced energy levels It should also be indicated that transition from ground state to excited state is not arbitrary and unlimited. Transitions follow certain selection rules that make a specific transition allowed or forbidden.

11 Atomic Emission and Absorption Spectra At room temperature, essentially all atoms are in the ground state. Excitation of electrons in 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 will only spend a short time in the excited state (shorter than a ms) where upon relaxation an excited electron will emit a photon and return to the ground state.

12 Each type of atoms would have certain preferred or most probable transitions (sodium has the and the 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 and nm.

13 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 90o 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

14 Atomic Line Width It is taken for granted that an atomic line 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:

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16 The effective line width in terms of wavelength units is equal to  1/2 and is 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?

17 Reasons for Atomic Line Broadening There are four reasons for broadening observed in atomic lines. These include: 1. The Uncertainty Principle We have seen earlier that Heisenberg uncertainty principle suggests that nature places limits on the precision by which two interrelated physical quantities can be measured. It is not easy, will have some uncertainty, to calculate the energy required for a transition when the lifetime of the excited state is short.

18 The ground state lifetime is long but the lifetime of the excited state is very short which suggests that there is an uncertainty in the calculation of the transition time. We have seen earlier that when we are to estimate the energy of a transition and thus the wavelength (line width), it is required that the two states where a transition takes place should have infinite lifetimes for the uncertainty in energy (or wavelength) to be zero:

19  E>const/  t  E = hc/  Const’/  >  t 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.

20 2. Doppler Broadening The wavelength of radiation emitted by a fast moving atom toward a transducer will be different from that emitted by a fast atom moving away from a transducer. More wave crests and thus higher frequency will be measured for atoms moving towards the transducer. The same occurs for sound waves

21 Assume your ear is the transducer, when a car blows its horn toward your ear 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.

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23 The line width (  ) due to Doppler broadening can be calculated from the relation:  o = v/c Where o is the wavelength at maximum power and is equal to ( )/2, v is the velocity of the moving atom and c is the speed of light. It is noteworthy to indicate that an atom moving perpendicular to the transducer will always have a o, i.e. will keep its original frequency and will not add to line broadening by the Doppler effect.

24 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. On the other hand, an atom moving away from the source will experience less wave crests and will thus absorb a lower frequency. 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).

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26 3. Pressure Broadening Line broadening caused by collisions of emitting or absorbing atoms with other atoms, ions, or other species in the gaseous matrix is called pressure or collisional broadening. These collisions result in small changes in ground state energy levels and thus the energy required for transition to excited states will be different and dependent on the ground state energy level distribution.

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28 This will definitely 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.

29 4. Magnetic Effects Splitting of the degenerate energy levels does take place for gaseous atoms in presence of a magnetic field. 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, one of the same quantum energy and one of higher quantum energy, while 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. Electronic transitions from the thus split levels will result in line broadening

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31 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 o C). Thee high temperature can be provided through a flame, electrical heating, an arc or a plasma source. It is essential that the temperature be of enough value to convert atoms of the different elements to gaseous atoms and, in some cases, provide energy required for excitation. The temperature of a source should remain constant throughout the analysis especially in atomic emission spectroscopy.

32 Quantitative assessment of the effect of temperature on the number of atoms in the excited state can be derived from Boltzmann equation: Where N j is the number of atoms in excited state, N o is the number of atoms in the ground state, P j and P o are constants determined by the number of states having equal energy at each quantum level, E j is the energy difference between excited and ground states, K is the Boltzmann constant, and T is the absolute temperature.

33 Boltzmann distribution WavelengthAtom N j /N 0 at 3000 K nmNa 5.88  nmCa 3.69  nmZn 5.58  nmCs 7.24  10 -3

34 To understand the application of this equation let us consider the situation of sodium atoms in the 3s state (P o = 2) when excited to the 3p excited state (P j = 6) at two different temperatures 2500 and 2510K. 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 3p to 3s where we have two lines at and nm which is:

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36 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:

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38 Introduction to Atomic Spectroscopy Lecture 11

39 Effect of Temperature on Atomic Absorption and Emission The question here is which technique would be affected more as a result of fluctuations in temperature? The answer to this important question is rather simple. Atomic emission is the technique that 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. However, in the case of atomic absorption, the signal depends on the number of atoms in ground state that will absorb energy.

40 very high as related to the number of excited atoms: N j /N o = 1.72x10 -4 or 172 excited atoms for each 10 6 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.

41 However, there are some indirect effects of temperature on atomic absorption spectroscopy. These effects can be summarized as: 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.

42 Band and Continuum Spectra Associated with Atomic Spectra When the atomization temperature is insufficient to cause atomization of all species in the sample matrix, the 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 will be significantly decreased.

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44 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.

45 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.

46 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 ionization methods are available which will be detailed in the next two chapters. Generally, atomization methods can be summarized below:

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48 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.

49 Introduction of Solution Samples 1. Pneumatic 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:

50 Concentric Tube Nebulizer

51 Cross Flow Nebulizer

52 Babington Nebulizer

53 Fritted Disc Nebulizer

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

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56 3. Electrothermal Vaporization An accurately measured quantity of sample (few  L) is introduced into an electrically heated cylindrical chamber through which an inert gas flows. Usually, the cylinder is made of pyrolytic carbon but tungsten cylinders are now available. The signal produced by instruments which use electrothermal vaporization (ETV) is a discrete signal for each sample injection. Electrothemal vaporizers are called discrete atomizers to differentiate them from nebulizers which are called continuous atomizers

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58 4. Hydride Generation Techniques Samples that contain arsenic, antimony, tin, selenium, bismuth, and lead 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.

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61 Introduction to Atomic Spectroscopy Atomic Absorption Spectroscopy Lecture 12

62 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. 2. If the sample is conductive and is of a shape that can be directly used as an electrode (like a piece of metal or coin), that would be the choice for sample introduction in arc and spark techniques. 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 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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64 3. Laser Ablation 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. The formed plume of vapor and fine particulates are swept into the atomizer by the flow of an inert gas. Laser ablation is becoming increasingly used since it is applicable to conductive and nonconductive samples

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67 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. The technique is used for sample introduction and atomization as well. The electrodes are kept at a 250 to 1000 V DC. This high potential is sufficient to cause ionization of argon which will be accelerated to the cathode where the sample is introduced. Collision of the fast moving energetic argon ions with the sample (cathode) causes atomization by a process called sputtering.

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