Ch. 8 Introduction to Optical Atomic Spectroscopy

Transcription

1 Ch. 8 Introduction to Optical Atomic Spectroscopy major types of Spectrometry elemental Optical Spectrometry Ch 9, 10 Mass Spectrometry Ch 11 X-ray Spectrometry Ch 12 In this chapter theories on sources properties of optical atomic spectra atomizing techniques sample introducing methods 8A. Optical Atomic Spectra 8A-1. Energy Level Diagram 8.2 Splitting of p,d,f orbitals Spin & orbital motion create magnetic field due to rot of charge carried by electrons Same direction rep. Opposite -attractive

2 8A-1. Energy Level Diagram 8.3 Atomic Emission Atomic Absorption 3P excited state 3P heat h 3S 5890, 5896 o A ground state 5890, 5896 h o A energy absorbing 3S Na Na Heat source: flame plasma arc spark Atomization needed Atomic Fluorescence 8.4 h 1 Atoms or Ions Mg h 2852A o 3P h 2852A o h 2 3S Na 4P Rapid transition Heat loss 3P h A o h A o 3S

3 8A-2. Atomic Line Widths 8.5 Line Widths: Narrower, Better for absorption spectroscopy emission Definition: 1/2 line widths at half the max. signal Sources of broadening 1. Uncertainty effect in transition time 2. Doppler effect 3. Pressure effects due to collisions 4. Electric & magnetic field effect 8A-2. Atomic Line Widths 8.6 Broadening from Uncertainty effect From Heisenberg s Uncertainty Principle t 1 Minimum time for a measurement t Life time of transition state - finite Spectral lines have finite widths 1 Pressure Broadening Caused from collisions among atoms or ions. Collision alters ground state E. level changes range of abs or emission

4 8A-2. Atomic Line Widths 8.7 Broadening from Doppler effect Doppler shift 8A-3. Effect of Temp. on Atomic Spectra 8.8 Temperature affects number of excited (N j ) & unexpected atomic particles N N j 0 P P j 0 exp E j kt k: Boltzman cons. 1.38x10-23 J/K E j : E difference bet. Ex.-gr.state P j, P 0 : statistical factor 3S 2 quantum number 3P 6 T: 10K % increase in # of ex. Na atoms excited at 2500K % Na excited Emission Spec. : based on this limited number Absorption Spec : based on 99.98% unexcited

5 8B. Atomization Methods 8.9 For spectroscopy, sample should be converted in gaseous atom or ionized atom then determined by emission absorption fluorescence mass spectrometry Atomization : sample conversion to atomic vapor 8C. Sample Introduction Methods (Achilles Heal) 8.10 Critical for Accuracy, Precision, Detection limit With high Efficiency With no interference mostly from solution status low reproducibility for solids

6 Continuous sample introduction into a plasma or flame C-1. Solution samples 8.12 Nebulization methods : finely divided droplets (aerosols) by jet or compressed gas Pneumatic Nebulizer a) Concentric tube: Bernoullie effect b) Cross-flow a) Fritted disk: finer than a, b b) Babington type less clogging of high salt samples

7 8C-1. Solution samples 8.13 Ultrasonic Nebulizers piezoelectric crystal, vibrating at 20kHz ~ Mhz Electrothermal vaporizers (ETV) Ar Conductor (carbon rod) Apply currents to evaporate samples Hydride Generation For volatile metals to enhance D.L. (10~100) As, Sb, Sn, Se, Bi, Pb Good for small vol. liquid or solid sample 3BH H + + 4H 3 AsO 3 3H 3 BO 3 + 4AsH 3 + 3H 2 O 8C-2. Solid samples 8.14 : directly to flame or plasma as a form of powder, metals, particulates adv: reduce sample dissolution process dis-adv: calibration sample conditioning precision accuracy main difference from solution introduction discrete signal vs. continuous

8 8C-2. Solid samples 8.15 Types A. Direct insertion: powder on atomizing probe use electric arc or spark B. Electrothermal vaporization C. Arc & Spark ablation Ablation: plume (particulate & vaporized sample) at surface, transferred to atomizer by inert gas flow D. Laser ablation work for both conducting and non-cond. Solid inorganic & organic powder & metallic E. Glow Discharge Technique 8C-2. Solid samples 8.16 DC 250~1000V causes Ar into Ar + + e- By electric field, Ar + hits cathode (sample) SPUTTERING : Eject neutral sample atoms 100g/min