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
- 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)

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

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

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

Nebulizers commonly used

- Pneumatic

and

- Cross-flow

Wavelength Selection Device

Two wavelength selectors used

- Monochromators

and

 - Filters

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)

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

Detectors

- PMT

- Solid-state detectors (CCD, CID)

- PDA

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)

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

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

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

INTERFERENCE

Two Classes

- Spectral interference

and

- Nonspectral 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

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

APPLICATIONS OF FLAME OES

- 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

- 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