INSTRUMENTAL ANALYSIS CHEM 4811

CHAPTER 7

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment 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 byΔE = 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 = intensityk = 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 10000 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)

SignalProcessor

Detector Atomizer

Instrumentation- Fluorescence signal is measured at an angle of 90o with

respect to the excitation source

- This minimizes scattered radiation

ATOMIC FLUORESCENCE SPECTROMETRY (AFS)