CHAPTER 8 Introduction to Optical Atomic Spectrometry

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1 CHAPTER 8 Introduction to Optical Atomic Spectrometry From: Principles of Instrumental Analysis, 6 th Edition, Holler, Skoog and Crouch. CMY 383: Dr Tim Laurens Introduction. Three major types of spectrometric methods for identification of the elements in a sample and to determine their concentrations. 1. Optical spectrometry elements converted into gaseous atoms/elementary ions by a process called atomization UV/VIS absorption/emission, or fluorescence is then recorded 2. Mass spectrometry Samples also atomized but and converted into positive ions and measured according to their mass-to-charge ratio. 3. X-Ray spectroscopy Atomization not required because X-Ray spectra of most elements independent of their chemical composition in a sample, i.e. how the atoms is binded into the molecule. 1

2 8A: Optical atomic spectra 8A-1 Energy Level diagram Note: The scale for the energy level diagram of sodium extends to about 5.14 ev The 3s electron requires 5.14 ev to produce a sodium ion ionization energy = 5.14eV Horizontal lines = energy levels of atomic orbitals 2

3 The p orbitals are split into two levels that differ slightly in energy (5896nm and 5890nm) Reason: An electron spins on an axis and the direction of spin may be in either in o the same direction as its orbital motion or o the opposite direction of its orbital motion Both the electron spin as well as the orbital motion create a magnetic field as a result of the rotation of the charge on the electron. The two magnetic fields interact in an o Attractive sense if these two motions are in opposite direction o Repel one another when the motions are in the same direction Energy of the electron increased The result of this phenomenon is that the energy of the electron with opposing spin & orbital motion is slightly smaller as compared to the electron with parallel spin. Similar differences in the d and f orbitals exist, however the differences are so small that its goes undetectable only a single energy level for the d orbitals The splitting of higher p, d, f orbitals are characteristic of all species containing a single outer-shell electron. See the energy level diagram for Mg + in figure 8-1b o Much the same general appearance as that of atomic/uncharged sodium. o The same applies to Al 2+ and the rest of the alkali-metal ions o However, their energy levels (3s 3p) will differ due to the difference in nuclear charges. (twice as much in Mg + compared to Na) 3

4 Compare the one outer electron energy level diagram with that of a two outer electron shell energy level diagram. (Fig 8-2) For Mg 2+ and Mg Triplet state Singlet state The two 1s electrons of Mg have different energy states (ways of spinning) o The excited singlet state electron spins around its own axis are opposed paired spin o The excited triplet state electron spins around its own axis are parallel non-paired spin lower in energy than the corresponding singlet state 4

5 Although the energy level diagram for Na and Mg are relatively straight forward that of corresponding transition elements are much more complex to interpret... Li 106 lines; Na 170 lines; K 124 lines; Rb 294 lines.cr 792; Fe 2340 Atomic Emission Spectra 5

6 Atomic Absorption Spectra Atomic Fluorescence Spectra 8A-2: Atomic Line Widths 1. Narrow atomic line widths will result in good selectivity, i.e will reduce interference due to overlapping lines 2. Atomic Absorption/emission lines are generally found to e made up of a symmetric distribution of wavelengths that centres on a mean wavelength λ 0 6

3. From an energy level diagram, the impression can be created that the atomic lines are infinitely narrow due to the discreteness of the energy levels, however several phenomena can cause line

7 3. From an energy level diagram, the impression can be created that the atomic lines are infinitely narrow due to the discreteness of the energy levels, however several phenomena can cause line broadening: The uncertainty effect The Doppler effect Pressure effects Electric and magnetic field effects (Later) The Uncertainty effect Spectral lines have a finite width due to uncertainties in the transition times between energy states An atomic line will approach zero or (become very narrow) only if the transition times are infinite. The Doppler Broadening effect Pressure effects due to collisions 7

8 8.A-3 Effect of Temperature on Atomic Spectra Temperature has a profound effect on the ratio of the no atoms in excited state over that in the ground state (N j / N o ) N N j o g g j o e E j kt k = Boltzman constant =1.38 X J/K T = Absolute temperature (K) E j = Energy difference between excited and ground state. Example 8-2 : Illustrates that a 10K increase in temperature results in a 4% increase in the no of excited atoms. Atomization temp requires careful control Temperature fluctuations do exert the following indirect influence on atomic absorption and emission/fluorescence measurements in several ways: 1. Increase in T improves the efficiency of atomization more atoms in vapour phase 2. Line broadening more collisions pressure broadening 3. More Doppler bradening atoms travel at a greater rate 4. Influence the degree of ionization decreases the no of non-ionized analyte on which the analysis is actually based. 8

9 Because of the above mentioned reasons the flame temperature requires careful control in the case of atomic absorption (AAS) and emission (AES)/fluorescence (AFS) measurements The large ratio of unexcited to excited atoms in atomization media also leads to the following interesting comparison of the three atomic methods: Atomic absorption and fluorescence measure ment are made on a much larger population of atoms, these two procedures are expected to be more sensitive than the emission procedure. This apparent advantage is offset in the case of Absorption methods (A=logP o /P) where P and P o are nearly equal (poor absorbers/ dilute solutions). This will case a relative larger error inmeasurement. AAS and AES can therefore be regarded as complimentary in sensitivity depending on the group of elements to be analysed. Based on the ative atom population AFS methods should be the most sensitive of the three at least in principle 8A-4 Band and Continuum Spectra Associated with Atomic Spectra 9

8B: ATOMIZATION METHODS To obtain atomic optical (AAS, AES, AFS) and atomic mass spectra (MS) constituents of the sample needs to be converted to gaseous atoms/ions.

10 8B: ATOMIZATION METHODS To obtain atomic optical (AAS, AES, AFS) and atomic mass spectra (MS) constituents of the sample needs to be converted to gaseous atoms/ions. The precision and accuracy of the atomization and sample introduction method. 8C SAMPLE INTRODUCTION METHODS Sample introduction also referred to as the Achilles heel of atomic spectroscopy since it has a profound effect on precision, accuracy and detection limits. A reproducible and representative portion of the sample needs to be transferred into an atomizer. 10

11 8C-1 Introduction of Solution Samples Atomization devices/methods can be grouped in two categories: 1. Continuous atomizers Plasma, Flame : sample introduced in a steady manner 2. Discreet atomizers Electrothermal atomizer: Sample introduced in a discontinuous manner, eg with a syringe or an auto sampler. 11

spray/aerosol Nebulizers 1. Pneumatic nebuliser p 224 2.

12 Direct nebulisation most often used by which a constant supply of sample is introduced in the form of a spray/aerosol Nebulizers 1. Pneumatic nebuliser p Ultrasonic nebuliser p Electrothermal Vaporizers p225/226 12

13 4. Hydride generation techniques BH 4 - (aq) + 3H + (aq) + 4H 3 AsO 3(aq) 3H 3 BO 3(aq) +4AsH 3(g) +3H 2 O (l) 8C-2 Introduction of Solid Samples Direct sample Insertion Electrothermal Vaporizers Arc and Spark Ablation Laser Ablation The glow discharge technique 13

Ch. 8 Introduction to Optical Atomic Spectroscopy

Ch. 8 Introduction to Optical Atomic Spectroscopy 8.1 3 major types of Spectrometry elemental Optical Spectrometry Ch 9, 10 Mass Spectrometry Ch 11 X-ray Spectrometry Ch 12 In this chapter theories on