INSTRUMENTAL ANALYSIS CHEM 4811 CHAPTER 7 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

CHAPTER 7 ATOMIC EMISSION SPECTROSCOPY

ATOMIC EMISSION - Technique is also known as OPTICAL EMISSION SPECTROSCOPY (OES) - The study of radiation emitted by excited atoms and monatomic ions - Relaxation of atoms in the excited state results in emission of light - Produces line spectra in the UV-VIS and the vacuum UV regions

- Used for qualitative identification of elements present in the sample - Also for quantitative analysis from ppm levels to percent - Multielement technique - Can be used to determine metals, metalloids, and some nonmetals simultaneously Emission wavelength and energy are related by ΔE = hc/λ ATOMIC EMISSION

- Does not require light source - Excited atoms in the flame emit light that reaches the detector (luminescence) Techniques Based on Excitation Source - Flame Photometry (flame OES) - Furnace (Electrical Excitation) - Inductively Coupled Plasma (ICP) ATOMIC EMISSION

FLAME ATOMIC EMISSION SPECTROSCOPY - Known as Flame OES - Also called flame photometry - Solutions containing metals (or some nonmetals) are introduced into a flame - Very useful for elements in groups 1A and 2A

INSTRUMENTATION OF FLAME OES - No external lamp is needed - Flame serves as both the atomization source and the excitation source Main Components - Burner assembly - Flame - Wavelength selection device - Detector

INSTRUMENTATION OF FLAME OES Burner Assembly - The most commonly used is the Lundegarth or the premix burner - Is the heart of the emission spectrometer - Nebulizer introduces sample aerosol into the base of the flame - Free atoms are formed and excited in flame - Excited free atoms emit radiant energy - Only about 5% of the aspirated sample reach the flame

INSTRUMENTATION OF FLAME OES General Process in Flame - Liquid samples enter nebulizer - Sample droplets of liquid enter flame - Fine solid particles form - Particles decompose to free atoms - Excited atoms form - Excited atoms relax and emit radiation - Oxidation of atoms occur

INSTRUMENTATION OF FLAME OES Nebulizers commonly used - Pneumatic and - Cross-flow

INSTRUMENTATION OF FLAME OES Wavelength Selection Device Two wavelength selectors used - Monochromators and - Filters

INSTRUMENTATION OF FLAME OES Wavelength Selection Device Monochromators - Diffraction grating is used as the dispersion element Filters - Good for detection of alkali metals due to simple spectrum - Material is transparent over a narrow spectral range - Desired radiation passes through filter and others are absorbed - One element is determined at a time (single channel)

INSTRUMENTATION OF FLAME OES Wavelength Selection Device Multichannel Flame Photometers - Two or more filters are used simultaneously - Each filter transmits its designated radiation - Detector is PMT - Permits the use of internal standard calibration

INSTRUMENTATION OF FLAME OES Detectors - PMT - Solid-state detectors (CCD, CID) - PDA

INSTRUMENTATION OF FLAME OES Flame Excitation Source - Two gases (fuel and oxidant) are used - Oxidant: air or nitrous oxide - Fuel: acetylene (commonly used), propane, butane, natural gas - Increase in flame temperature increases emission intensity of most elements (exception: Na, K, Li) - Maxwell-Boltzmann equation applies (see chapter 6)

INSTRUMENTATION OF FLAME OES - Each element emits different characteristic wavelength of light - Emission lines are characterized by wavelength and intensity Emission intensity depends on - Analyte element concentration in sample - Rate of formation of excited atoms in flame - Rate of introduction of sample into flame - Flame composition - Flame temperature

INSTRUMENTATION OF FLAME OES S = kN S = intensity k = proportionality constant N = number of atoms in the excited state - Increasing temperature increases N - Atomic emission spectrometry is very sensitive to temperature - Temperature must be carefully controlled for quantitative analysis

INSTRUMENTATION OF FLAME OES - Elements with emission lines at shorter wavelengths give weak emission intensity at low temperature - High-temperature nitrous oxide-acetylene flame is used for such elements - High-energy electrical or plasma excitation sources may also be used - Ratio of fuel to oxidant also affects emission intensity - The highest temperature is achieved when stoichiometric mixture is used

Two Classes - Spectral interference and - Nonspectral interference INTERFERENCE

Spectral Interference Two types Background Radiation - Broad band emission by excited molecules and radicals in flame Overlapping emission lines - Emission by different elements of the same wavelength as the analyte element INTERFERENCE

Nonspectral Interference Chemical Interference - Occurs if anions that combine strongly with analyte element are present in sample Excitation Interference - Result of collisions between unexcited atoms of an element with excited atoms of a different element in sample Ionization Interference - Occurs when atoms ionize in flame and cannot emit atomic λs INTERFERENCE

- For measurement of alkali metals in clinical samples such as serum and urine - Excellent method for qualitative determination of multiple elements in sample - Characteristic emission lines of analyte are compared with literature (appendix 7.1) - Also used for quantitative analysis (application of Beer’s Law) - Deviation from linearity is generally observed at high concentrations APPLICATIONS OF FLAME OES

- More free atoms are liberated in organic solvents than in aqueous solutions - Implies emission intensity is relatively higher in nonaqueous solutions - Atomization is exothermic and rapid in organic solvents - Atomization is endothermic and relatively slow in aqueous solutions - External calibrations and standard addition methods are used APPLICATIONS OF FLAME OES

- Excitation and emission with the aid of electrical discharge, glow discharge, or plasma excitation source - Higher energy excitation sources than the flame source - All metals, metalloids, and some nonmetals can be detected at low concentrations - Electrical and glow discharge sources are used for solids only - Plasma source is used for liquids and solids - Electrical source can be used for gases in a sealed quartz tube ATOMIC OPTICAL EMISSION SPECTROSCOPY

Two Types of Line Spectra - Atomic emission spectra from neutral atoms (designated with I in tables of emission lines) - Emission lines from ions (ion lines) (lines from singly charged ions are designated II) (lines from doubly charged ions are designated III) ATOMIC OPTICAL EMISSION SPECTROSCOPY

- Produces electrical discharge between two electrodes (the sample electrode and the counter electrode) - A piece of metal analyte is the sample electrode - Counter electrode is an inert electrode (tungsten or graphite) Examples of Sources - DC arc - AC arc - AC spark FURNACE (ELECTRICAL) EXCITATION

- DC is primarily used for qualitative analysis of solids - Spark source makes use of a switch, a capacitor, an inductor, and a resistor - Temperature is higher in spark than in DC arc - More complex spectra in spark than in DC but more reproducible - Spark is better for quantitative analysis - Spark is used for precision and arc is used for sensitivity FURNACE (ELECTRICAL) EXCITATION

Solid Sample Holders - High purity carbon electrodes - Well drilled in one end to hold powdered solid samples - Powdered sample may be mixed with alumina or silica (improves precision) - Metallic samples in the form of rod or wire may be used directly as one of the electrodes FURNACE (ELECTRICAL) EXCITATION

Liquid Sample Holders - Liquid samples are analyzed directly using rotating disk electrodes - This method is used for the determination of metals in lubricating and fuel oils FURNACE (ELECTRICAL) EXCITATION

- Spectrometers are multichannel Three Types Spectrographs - Uses photographic film or plate to detect and record emitted radiation Polychromators - Multichannel with PMTs as detectors Array-Based Systems (Electronic Spectrographs) - Radiation intensity is measured by PMT or array detectors FURNACE (ELECTRICAL) EXCITATION

Detectors - 2D array detectors are used - Consists of multiple arrays of detectors so that different wavelengths fall on each individual detector - Charge transfer devices (CTD) are used (silicon based) Examples of CTD - Charge-injection device (CID) - Charge coupled device (CCD) FURNACE (ELECTRICAL) EXCITATION

- Limited to analysis of solids of trace elements Matrix Effect - Emission intensity of trace elements is greatly affected by the matrix Spectral Interference - Two types - Background interference and line overlap - Background is due to thermal radiation, molecular emission, and polyatomic species INTERFERENCE IN ARC & SPARK

- Qualitative identification of elements - Also used for quantitative analysis APPLICATIONS OF ARC & SPARK

Plasma - State of matter that contains electrons, ions, neutral species, and radicals - Highly energetic ionized gas - Electrically conductive - Affected by a magnetic field PLASMA EMISSION SPECTROSCOPY

Excitation Sources - Inductively Coupled Plasma (ICP) – operates at radiofrequency - Direct Current Plasma (DCP) - Helium Microwave Induced Plasma (MIP) - Temperature in plasma excitation source is between 6500 K and K PLASMA EMISSION SPECTROSCOPY

Dispersion Devices Sequential spectrometer systems - One wavelength is measured at a time Simultaneous systems - Contains either a polychromator or an Echelle spectrometer - Measures multiple wavelengths at the same time Combination systems - Consists of both polychromators and monochromators PLASMA EMISSION SPECTROSCOPY

Detectors - PMTs - CIDs - Segmented Array CCD (SCD) PLASMA EMISSION SPECTROSCOPY

Nebulizers - Concentric - Cross-flow - Babington: the V-groove is used for ICP - Microconcentric or direct insertion nebulizer (DIN) - Untrasonic nebulizer (USN) PLASMA EMISSION SPECTROSCOPY

Spectral Interference - Much more common in plasma than in flame Nonspectral Interference Chemical Interference - Rare in plasma emission due to efficiency of atomization Ionization Interference - Results in suppression and enhancement of signals from easily ionized elements (EIE: alkali metals) INTERFERENCE IN PLASMA EMISSION

- For analysis of environmental samples, geological samples, clinical samples - For characterization of metal alloys, glasses, ceramics, polymers, oils - For food and nutrition - Forensics APPLICATIONS OF PLASMA EMISSION

ICP-MS HPLC-ICP GC-MIP HYPHENATED METHODS

Glow Discharge (GD) - Reduced-pressure gas discharge generated between two electrodes - Tube is filled with inert gas (Ar) Excitation Source - DC GD - RF GD GLOW DISCHRGE EMISSION SPECTROSCOPY

- Involves emission of a photon from a gas phase atom that has been excited by the absorption of a photon - Different from the excitation by thermal or electrical means Interferences - Chemical interference - Spectral interference ATOMIC FLUORESCENCE SPECTROMETRY (AFS)

Source Monochromator (λ selector) Signal Processor Detector Atomizer Instrumentation - Fluorescence signal is measured at an angle of 90 o with respect to the excitation source - This minimizes scattered radiation ATOMIC FLUORESCENCE SPECTROMETRY (AFS)