INTRODUCTION
ICP/OES is one of the most powerful and popular analytical
tools for the determination of trace elements in a sample types. The technique
is based upon the spontaneous emission of photons from atoms and ions that have
been excited in a RF discharge. Liquid and gas samples may be injected directly
into the instrument, while solid samples require extraction or acid digestion
so that the analytes will be present in a solution. The sample solution is
converted to an aerosol and directed into the central channel of the plasma. At
its core the inductively coupled plasma (ICP) sustains a temperature of
approximately 10 000 K, so the aerosol is quickly vaporized. Analyte elements
are liberated as free atoms in the gaseous state. Further collisional
excitation within the plasma imparts additional energy to the atoms, promoting
them to excited states. Sufficient energy is often available to convert the
atoms to ions and subsequently promote the ions to excited states. Both the
atomic and ionic excited state species may then relax to the ground state via
the emission of a photon. These photons have characteristic energies that are
determined by the quantized energy level structure for the atoms or ions. Thus
the wavelength of the photons can be used to identify the elements from which
they originated. The total number of photons is directly proportional to the
concentration of the originating element in the sample.The instrumentation
associated with an ICP/OES system is relatively simple. A portion of the
photons emitted by the ICP is collected with a lens or a concave mirror. This
focusing optic forms an image of the ICP on the entrance aperture of a
wavelength selection device such as a monochromator. The particular wavelength
exiting the monochromator is converted to an electrical signal by a photo
detector. The signal is amplified and processed by the detector electronics,
then displayed and stored by a computer.
ICP OES OPERATION
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The ICP is generated as follows. RF power, typically 700–1500 W, is
applied to the load coil and an alternating current oscillates inside the coil
at a rate corresponding to the frequency of the RF generator. For most ICP/OES instruments,
the RF generator has a frequency of either 27 or 40 MHz. The oscillation of the
current at this high frequency causes the same high-frequency oscillation
of electric and magnetic fields to be
set up inside the top of the torch. With argon gas flowing through the torch, a
spark from a Tesla coil is used to produce ‘‘seed’’ electrons and ions in the
argon gas inside the load coil region. These ions and electrons are then
accelerated by the magnetic field, and collide with other argon atoms, causing
further ionization in a chain reaction manner. This process continues until a
very intense, brilliant white, tear drop shaped, high-temperature plasma is
formed. Adding energy to the plasma viaRF-induced collision is known as
inductive coupling, and thus the plasma is called an ICP. The ICP is sustained
within the torch as long as sufficient RF energy is applied in a cruder sense, the coupling of RF power to
the plasma can be visualized as positively charged Ar ions in the plasma gas
attempting to follow the negatively charged electrons flowing in the load coil
as the flow changes direction 27 million times per second. Figure shows the
temperature gradient within the ICP with respect to height above the load coil.
It also gives the nomenclature for the different zones of the plasma .The
induction region at the base of the plasma is ‘‘doughnut-shaped’’ as described above,
and it is the region where the inductive energy transfer occurs. This is also
the region of highest temperature and it is characterized by a bright continuum
emission. From the IR upward towards to the tail plume, the temperature decreases. An aerosol, or very
fine mist of liquid droplets, is generated from a liquid sample by the use of a
nebulizer. The aerosol is carried into the center of the plasma by the argon
gas flow through the induction region. Upon entering the plasma, the droplets
undergo three processes. The first step is desolvation, or the removal of the
solvent from the droplets, resulting in microscopic solid particulates, or a
dry aerosol. The second step is vaporization, or the decomposition of the
particles into gaseous state molecules. The third step is atomization, or the
breaking of the gaseous molecules into atoms. These steps occur predominantly
in the preheating zone . Finally, excitation and ionization of the atoms occur,
followed by the emission of radiation from these excited species. These
excitation and ionization processes occur predominantly in the initial
radiation zone, and the normal analytical zone from which analytical emission
is usually collected.
ICP OES INSTRUMENTATION
In inductively coupled plasma-optical emission spectrometry, the sample
is usually transported into the instrument as a stream of liquid sample. Inside
the instrument,the liquid is converted into an aerosol through a process known
as nebulization. The sample
aerosol is then transported to the plasma where it is desolvated, vaporized,
atomized, and excited and/or ionized by the plasma. The excited atoms and ions
emit their characteristic radiation which is collected by a device that sorts
the radiation by wavelength. The radiation is detected and turned into
electronic signals that are converted into concentration information for the
analyst. A representation of the layout of a typical ICP-OES instrument is
shown in Figure.
![]() |
Major components and ICP-OES instrument |
SAMPLE INTRODUCTION
A sample introduction system is used to transport a sample into
the central channel of the ICP as either a gas, vapor, aerosol of fine droplets, or solid
particles. The general requirements for an ideal sample introduction system include amenity to samples in all
phases (solid, liquid, or gas), tolerance to complex matrices, the ability to
analyse very small amount of samples (<1mL or <50 mg),
excellent stability and reproducibility, high transport efficiency, simplicity, and low
cost. A wide variety of sample introduction methods have been developed, such
as nebulization, hydride generation (HG), electro thermal vaporization (ETV),
and laser ablation.
- NEBULIZERS
1. CONCENTRIC NEBULIZER
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2. CROSS-FLOW NEBULIZER
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3. BABINGTON NEBULIZER
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SPRAY CHAMBER
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The material from which a spray chamber is constructed can be an
important characteristic of a spray chamber. Spray chambers made from
corrosion-resistant materials allow the analyst to introduce samples containing
hydrofluoric acid which could damage glass spray chambers.
TORCH
TORCH
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RADIO FREQUENCY GENERATOR
The radio frequency (RF) generator is the device that provides the power
for the generation and sustainment of the plasma discharge. This power,
typically ranging from about 700 to 1500 watts, is transferred to the plasma
gas through a load coil surrounding the top of the torch. The load coil, which
acts as an antenna to transfer the RF power to the plasma, is usually made from
copper tubing and is cooled by water or gas during operation.Most RF generators
used for ICP-OES operate at a frequency between 27 and 56 MHz, most ICP
generators were operated at 27.12 MHz. However, an increasing number of
instruments now operate at 40.68 MHz because of improvements in coupling
efficiency and reductions in background emission intensity realized at this
frequency. Frequencies greater than 40 MHz also have been used but have not
been as successful commercially. There are two general types of RF generators
used in ICP instruments. Crystal-controlled
generators use a piezoelectric quartz crystal to produce an RF
oscillating signal that is amplified by the generator before it is applied at
the load coil. The proper electrical parameters, such as output impedance,
needed to keep the generator operating efficiently are controlled by a matching
network that utilizes manual or automatic (servo mechanical) components. The
speed and accuracy of this matching network are critical to the operation of
this type of generator.
DETECTION OF EMISSION
DETECTION OF EMISSION
Most of the analytically useful emission lines
for ICP-OES are in the 190 - 450 nm region; thus, spectrometers used for
ICP-OES are usually optimized for operation in this wavelength region. However,
there are also some important ICP emission lines between 160 and 190 nm and
above 450 nm. Unfortunately, electromagnetic radiation in the 160 - 190 nm
wavelength region is readily absorbed by oxygen molecules, and instruments must
be specially designed to remove the air from the spectrometer to observe
emission in this wavelength region. Removing oxygen from the spectrometer is
done either by purging the
spectrometer with a gas, usually nitrogen or argon, that doesn’t absorb the
emission, or by removing the air from the spectrometer with a vacuum system.
Recently, nitrogen filled optics maintained at atmospheric pressure and
incorporating a catalyst for scrubbing the recycled nitrogen have been
introduced.
1. PMT DETECTOR
Once the proper emission line has been isolated by the spectrometer, the
detector and its associated electronics are used to measure the intensity of
the emission line. By far the most widely used detector for ICP-OES is the
photomultiplier tube or PMT. To know more about the PMT read the topic
Photomultiplier Tube.
2. ARRAY CCD DETECTOR
Charge transfer devices include a broad range of solid-state
silicon-based array detectors. They
include the charge injection device (CID) and the charge-coupled device (CCD).
The CCD has found extensive use in nonspectroscopic devices such as video
cameras, bar code scanners, and photocopiers. With the CTDs, photons falling on
a silicon substrate produce electron–hole pairs.The positive electron holes
migrate freely through the p type silicon semiconductor material, while the
electrons are collected and stored temporarily by an array of metal oxide
semiconductor (MOS) capacitors The CCD differs from the CID mainly in the
readout scheme. The CCD is read out in a sequential charge shifting manner
towards the output amplifier. The CID on the other hand may be read out in a
non-destructive manner by shifting charge between adjacent electrodes, and then
shifting it back again. The CID thus benefits from quick random access, even
during long integration periods. Spectroscopic
applications of CTD’s has been hampered by the physical mismatch between
the relatively small surface area of the detector and the large sometimes two
dimensional focal plane associated with polychromators. This mismatch may be
overcome, however, and one commercial
ICP spectrometer employs a CID detector having more than 250 000 pixels
positioned upon an echelle focal plane. Alternative approaches have been
successful with the CCD detector. In one case, a group of several CCD arrays
are arranged around a Circular Optical System (CIROS) based upon a Rowland
circle design. Rather than monitoring discrete wavelengths as is the case with
the multiple PMT Rowland circle systems, the CIROS system provides total
wavelength coverage from 120 to 800 nm, with resolution on the order of 0.009
nm
References
1. Inductively Coupled Plasma/Optical Emission Spectrometry by Xiandeng Hou and Bradley T. Jones
2. Concepts, Instrumentation and Techniques in Inductively Coupled Plasma Optical Emission Spectrometry by Charles B. Boss and Kenneth J. Fredeen (Second edition)
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