Atomic Absorption & Emission Spectroscopy Chapter 5

Sub Topics Introduction to Atomic Spectroscopy Atomic Absorption Spectroscopy AAS Atomic Emission Spectroscopy AES Interferences and Comparisons

Atomic Absorption and Emission Spectroscopy What is it used for? Study of trace metals in the environment Examples: Contamination of water Food stuffs Petrol products Quality control Qualitative and Quantitative method for >70 elements with detection levels in ppm/ppb range Fast and convenient with high selectivity

Three Types Done in the Gas Phase Why is this significant? No vibrational or rotational energy levels, only electronic transitions Therefore spectra made of quantised spectral lines Spectral ranges of interactions? 180 – 800 nm (UV-Vis) Where is electromagnetic radiation absorbed / emitted in each spectroscopy?

Spectrum Related to Electronic Configuration(s)

Basic Instrumentation Requirements: Atoms / Ions in gas phase AES: No Radiation Source AAS: As Shown AFS: Radiation Source at 90’ to Detector

Atomisation Sample needs to be in gas phase Atomisation is therefore crucial Several ways: AES Inductively Coupled Plasma (ICP) AAS Flames Electrothermal Atomizers Two Types of Atomisation Continuous Sample Continuously Introduced Discrete Sample introduced by syringe/autosampler

Sub Topics Introduction to Atomic Spectroscopy Atomic Absorption Spectroscopy AAS Atomic Emission Spectroscopy AAS Interferences and Comparisons

Process of Atomisation Nebulization Production of mist / spray Commonly used for solutions. Solids: Electric Spark or Laser Sample Spray Dry-Aerosol Free Atoms / Molecules / Ions Nebulization Desolvation Volatilization

Example of Atomisation Process Flames Geometry of a Flame Different Fuels Different fuels give different temperatures Normal Flame Alkali/Alkaline Metals 1700-2400’C O2 or N2O Heavy Metals 2500-3100’C

Sample Introduction and Atomisation Pneumatic Nebulizer Continuous Achieved using Venturi Effect Jet effect to form mist / spray

Interferences Any effect that changes signal from proper signal Blank Interferences Spectral Analyte (multiplicative) Interferences Physical (matrix) Chemical Ionisation

Spectral Interferences Independent of Analyte concentration From elements other than Analyte E.g. Na (285.28 nm) overlaps Mg (285.21)nm Can subtract blank Na spectrum Results in Line Broadening Spectral lines have finite widths, but determined by spectrometry properties Not needed to know different types

What if too concentrated?

Sub Topics Introduction to Atomic Spectroscopy Atomic Absorption Spectroscopy AAS Atomic Emission Spectroscopy AES Interferences and comparisons

Inductively Coupled Plasma Plasma Atomisers Plasma is a conducting gaseous mixture containing cations and anions. Argon typically used for AAS Once Argon ions formed, can absorb enough power from external source to reach high temperatures so further ionisation can occur (T > 10,000 K) Power Sources DC Electric Powerful Radio Frequency (RF) Generators Powerful Microwave Frequency (MW) Generators (ICP) RF / ICP provide best advantage, sensitivity and freedom from interference DC Plasma cheaper and simpler

Inductively Coupled Plasma Three concentric quartz tubes End of torch is a RF induction coil which induces magnetic field Gases (Ar or N2) flow in outer tubes to cool torch. Stabilizes plasma. Torch is seeded with electrons using spark. H accelerates to high energy to ionise gases. White hot fireball formed (8000 – 10000 K)

Inductively Coupled Plasma Sample (from nebulizer, + Ar gas) is injected into it via central tube. Advantages Higher Excitation Temperature than any flame Lower detection limits Wide range of metals and non-metals Disadvantages Many Lines Initial and Running Costs Large! Other Points Long, Linear Calibration Curves (4-5 orders magnitude) Useful for both Atomic Spectroscopy and Mass Spectrometry

Comparison ICP vs DC ICP Advantages Chemically inert environment Unlike a flame (violent, highly reactive) Temperature cross-section constant Thin optical path Reduces self-absorption Calibration linear over large orders of concentration Disadvantages Does not like organic solvents Clogging due to carbon build up DC and other Plasma Sources Advantages Less Argon Needed Can cope with organic solvents Less expensive Less sensitive Good Reproducibility Disadvantages Useful for atoms, not ions Good alignment of optics Electrodes need to be replaced

Recap so far Elemental Analysis Need Light Source ICP Most common source Need Light Source Flame most common source

Some more equations Intensity is donated by 𝐼= 𝐼 0 𝑒 − 𝑘 𝑣 𝑙𝑐 Atomic absorption transmission is given by 𝑇= 𝐼 𝐼 0 = 𝑒 − 𝑘 𝑣 𝑙𝑐 Therefore, with some higgery-pokery Absorbance is given by 𝑨=ε𝒄𝒍

Absorbance 𝐴=− log 𝑇 =ε𝑐𝑙 Characteristic Concentration The concentration in mg/L which gives 1% absorbance for a certain element 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝐶ℎ𝑎𝑟𝑎𝑐𝑡𝑒𝑟𝑖𝑠𝑡𝑖𝑐 =0.0044 𝑥 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛𝑜𝑓𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑎𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 Why? 1% absorbance = 99% transmission Log(1/0.99) = 0.0044

Requirements for Source Atomic Absorption Spectroscopy AAS Narrow resonance line profile from lamp Stable and reproducible output Enough intensity for good S/N ratio Durability Easy Operation Hollow Cathode Lamp Good Choice!

Atomization AAS Flame Air or N2O combined with acetylene Eletrothemal Atomizer Graphite furnace Background Corrections may be required Chemical Interference Continuum Source Corrections Pulsed Hollow Cathode Lamp Corrections Zeeman Effect

Electrothermal Atomizers: Graphite Furnace AAS Drawbacks of AAS/AES/AFS in General Large volumes Poor efficiency nebulization Aqueous solutions required Electrothermal Atomizer: Graphite Furnace (AAS/AES only) Heated with inert purge gas All sample is atomized in confined space! Small volume in furnace with a syringe Programmed heating

GF-AAS Procedure Introduce sample Add modifier Drying Pyrolysis/Ashing 5-50 microlitres or solid Add modifier Drying 110’C Pyrolysis/Ashing 300-1100’C Evaporation of matrix? Atomisation and measurement time resolved 2000-3000’C Clean out Cool down Whole process takes milliseconds!

GF-AAS Interferences Corrections are needed Matrix Modifier Dual Lamp setup T selective programing Zeeman Correction

Zeeman Effect Correction Strong Magnetic Field splits degenerate spectral lines into several lines according to their Mj values, which have different polarization characteristics Example, Magnesium

Zeeman Background Correction

Sub Topics Introduction to Atomic Spectroscopy Atomic Absorption Spectroscopy AAS Atomic Emission Spectroscopy AES Interferences and comparisons

Interferences Ionization Loss of atoms as ions – give completely different spectra Can be supressed by adding ionization suppressor (e.g. Cs) to sample and reference Chemical Key problem for AAS Formation of chemical species which prevent/promote dissociation, giving rise to depressed/enhanced signals Requires releasing/protective agents

Calibration Very Important Encompass range of expected values for samples Blank Triplicate to determine detection limit Standard addition method required for high matrix samples

Comparison What is less prone to spectral interferences? AAS AES Temperature affects signal more in? AES / AFS Because ground state population is largest

Comparison Rank the following in Limit of Detection? AES, ICP-AES, AAS, GF-AAS, AFS AES>AFS>AAS>ICPAES>GF-AAS Rank the following in dynamic working range? ICP-AES>AFS>AES>AAS>GF-AAS

Other Comparisons AAS Ratio method removes systematic errors but sensitivity affected by difference in two large quantities ICP-AES and AFS can be multi element, less so than ASS AAS most cost effective, ICP-AES most versatile AFS good for vapour phase elements Cost of instrumentation ICP-AES>GF-AAS>AAS>AFS>AES