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ATOMIC EMISSION SPECTROMETRY
Contents
Principles and Instrumentation
Interferences and Background Correction
Flame Photometry
Inductively Coupled Plasma
Microwave-Induced Plasma
Principles and
Instrumentation
R M Twyman, University of York, York, UK
& 2005, Elsevier Ltd. All Rights Reserved.
Introduction
Atomic (or optical) emission spectrometry (AES, OES)
is an important technique for the multielement analysis
of a wide range of materials. Many elements have been
discovered using emission spectrometry and it is the
most commonly used procedure for the measurement
of trace elements in rocks, water, soil, manufactured
goods, and biological specimens. The technique is used
to monitor the levels of different chemicals and trace
elements in the environment and to determine the compositions of solids, liquids, and gases. In geoanalysis,
emission spectrometry has been instrumental in the
exploration of economic mineral deposits. In metallurgy and in the semiconductor industry, emission
spectrometry is of prominent importance in the production control of both raw materials and finished
products. Finally, emission spectrometry allows the elements present in the sun and stars to be identified,
helping us to understand better the nature of the
universe. These are only a few examples of scientific and
technical disciplines in which the technique of emission
spectrometry has made a significant contribution.
Theory and Signal Generation
Atomic Spectra
AES involves the measurement of electromagnetic
radiation emitted from atoms. Both qualitative and
quantitative data can be obtained from this type of
analysis. In the former case, the identity of different
elements reflects the spectral wavelengths that are
produced, while in the latter case, the intensity of the
emitted radiation is related to the concentration of
each element. Atomic spectra are derived from the
transition of electrons from one discrete electron orbital in an atom to another. These spectra can be
understood in terms of the Bohr atomic model.
ATOMIC EMISSION SPECTROMETRY / Principles and Instrumentation 191
In the Bohr model, the atom is depicted as a nucleus surrounded by discrete electron orbits, each
associated with energy of the order hn. Every atom
has a certain number of electron orbitals, and each
electron orbital has a particular energy level. When
all the electrons are present in the orbitals, the atoms
are in the most stable form (the ground state). When
energy (either thermal, resulting from collision, or
radiational, resulting from the absorption of electromagnetic radiation) is applied to an atom and is
sufficient to lift an electron from a shell with energy
Ei to one with Ej, the atom is said to be in the excited
state. The state of excitation is unstable and decays
rapidly. The residence time of the unstable excited
state is very short, in the order of 10 8 s. When
electrons return to the stable ground state, energy is
emitted and that energy is equal to the difference in
the energies between the ground and excited states.
The energy is released in the form of electromagnetic
radiation and defines the wavelength of the transition. The relationship between the energy and
wavelength is described by the Planck equation:
Ej Ei ¼ hn ¼ hc=l
where Ej Ei is the energy difference between the two
levels (and Ej4Ei); h is Planck’s constant, 6.624 10 34 J s 1; n is the frequency of the radiation; c is
the velocity of light in a vacuum, 2.9979 108 m s 1;
and l is the wavelength of the radiation in meters.
If enough energy is absorbed by the atom, electrons
may escape completely, leaving the atom in the ionized
state. The energy required for ionization is called the
ionization potential. Ions also possess ground and excited states, through which they can absorb and emit
energy by the same processes described for an atom.
Figure 1 illustrates the electron shell configuration
in terms of energy levels. Horizontal lines represent
energy levels and vertical lines depict permissible transitions between them. The arrows show the direction
of energy input or output (ascending arrows show the
absorption of energy while descending arrows show
energy radiation). When an electron in a quantum
level j is captured by an ionized atom, energy is liberated according to the following equation:
hn ¼ hnj þ mn2 =2 ¼ Ej þ mn2 =2
The wavelength for the emitted radiation due to a
transition Ej Ei is
l ¼ hc=E
where E is the energy difference and l is the wavelength of the emitted radiation.
Spectra of neutral excited atoms are denoted as I,
and correspond to those deexciting to the ground state
(resonance lines) or close to the ground state (nearresonance lines). They are observed in low-energy
sources such as flames. Spectra of singly ionized atoms
are denoted as II, and they are observed in highenergy sources such as electrical sparks, inductively
coupled plasmas (ICPs), and glow discharges. Every
element has a characteristic emission spectrum, which
is the basis of spectrochemical analysis.
Molecular Spectra
Molecular spectra consist of numerous densely
grouped lines. These are called band spectra because
they appear as luminous bands. The fine structure can
only be observed with high-resolution instruments.
Molecular spectra of excited molecules are related to
the energy states of a molecule rotating around the
principal axes of inertia. Band spectra in the nearinfrared are produced by energy transitions related to
oscillatory vibrations of individual molecules.
Continuum
Excitation
Emission
Ion excited state
4
e
Energy
Ion ground state
Excited
states
a
b
h
f g
c d
This is radiation distributed continuously over the
wavelength range and can be attributed to recombination processes and other background factors. The intensity of the background increases with temperature.
3
2
1
Ground State
Figure 1 Electron shell configurations in terms of energy levels.
Arrows depict permissible transitions by absorption and excitation
(ascending) or radiation and photon emission (descending), a and
b represent excitation, c is ionization, d is ionization plus excitation,
e is ion emission, and f, g, and h are atom emissions. (Reproduced
with permission from Boss CB and Freeden KJ (1989) Concepts,
Instrumentation and Techniques. Inductively Coupled Plasma
Atomic Emission Spectrometry, Perkin Elmer Corp.)
Instrument Design – Overview
A spectrometer consists of three main parts: (1) an
emission source, which produces the spectrum; (2) an
optical system, which scatters the spectrum; and (3) a
device to measure the emitted lines. The two major
types of instrument for the analysis of emission spectra are sequential and simultaneous spectrometers,
although there are many variants of each in terms of
mechanical and optical characteristics. The spectral
192 ATOMIC EMISSION SPECTROMETRY / Principles and Instrumentation
wavelength range of interest is B160–900 nm, but
not all instruments are capable of covering this
range, and the resolution may vary with wavelength.
Oxygen and water vapor absorb short-wave UV
emissions (o190 nm) and this obscures the emission
lines of some common and important elements (e.g.,
hydrogen, carbon, oxygen, nitrogen, chlorine, phosphorus, and sulfur). Therefore, oxygen and water
vapor must be eliminated from the instrument, either
by evacuation or by flushing with nitrogen or argon.
Nitrogen flushing does not interfere with the analysis
of samples containing nitrogen compounds since
molecular nitrogen (N2) does not obscure the emission lines from atomic nitrogen in the sample.
Instrument Design – Emission
Sources
In spectrochemical analysis, atomization and excitation can be achieved using various different emission
sources. The spectra derived from low-energy sources such as flames are simpler than those from electrical discharges, although the temperature of flames
and furnaces (2000–4000 K) is inadequate to excite
many of the elements. Nevertheless, flame emission
spectrometry is widely used for the determination of
the alkali elements (lithium, sodium, and potassium),
whose excitation states are low enough to be populated at flame temperatures. Higher-energy sources
produce higher temperatures and therefore more
emission lines. In electrical discharges, arcs and
sparks are created by applying currents and potentials across conducting electrodes, and a large quantity of the sample surface is evaporated in this
process. Better quantitative analysis is achieved using
plasma sources: ICP, direct current plasma (DCP),
and microwave induced plasma (MIP), which generally achieve temperatures of 7000–8000 K. Glow
discharge sources, which use high-energy argon atoms and ions to excite atoms ejected from the analyte
surface, are often used for the analysis of metals.
The degree of excitation by a thermal source can
be described by the Boltzmann distribution equation.
If N1 is the number of atoms in the excited state and
N0 is the number in the ground state, then the excited
fraction is given by
Flame Sources
In flame emission spectrometry, the sample solution
is sprayed or aspirated into a flame as a fine mist or
aerosol. The sample is vaporized in the flame, and
atomized by a combination of heat and the action of
a reducing gas. The atoms are excited into higher
electronic states by the heat, and as they revert to the
ground state they emit photons, which are measured
by the detector. The layout of a flame photometer is
shown in Figure 2.
Discharge Sources (Arcs and Sparks)
The first discharge sources produced direct current
(DC) arcs by electrically heating the sample in
an electrode cup and vaporizing the analytes into a
low-voltage, high-current discharge. The temperature of the arc plasma varies from 4000 to 5000 K.
The limits of detection are good and the entire sample can be consumed. However, due to variations
in the volatilization process, the accuracy is usually
poor. Reproducibility can be improved by using
internal references and optimizing the conditions of
vaporization (graphite electrode designs, addition of
modifiers). The spectral range of the source is limited
due to the presence of cyanogen bands with heads at
421.6, 388.3, and 359.0 nm when graphite is used.
Argon can be injected into the arc to minimize these
effects. In the so-called cathode region, intensity is
relatively higher (10–50 times) than in the central
and anode regions of the arc.
Spark sources produce lower average temperature
than arcs, but the local temperature can be as high
as 40 000 K. Like arc sources, sparks produce atomic lines, but also more pronounced lines for ions,
which are known as spark lines. The emission source
consists of a sparking stand and a spark generator.
A spark forms between the cathode and the sample (which acts as the anode). The adjustable gap
Mirror
Lens Slit Filter Photodetector
Aerosol
enters flame
N1 =N0 ¼ ðg1 =g0 Þ expðE=kTÞ
where E is the energy difference between the ground
and excited states, T is the absolute temperature (K),
k is the Boltzmann constant (1.38 10 23 J K 1),
and g1 and g0 are quantum statistical weighting
factors.
Fuel
Air from
compressor
Sample
Drain
Figure 2 Schematic of a flame emission spectrophotometer.
ATOMIC EMISSION SPECTROMETRY / Principles and Instrumentation 193
between the cathode and the sample is sometimes
termed the entrode, and is filled with argon. The
generator initially produces a brief low-energy discharge that ionizes the argon and creates the plasma.
A second, high-energy discharge then causes the
sample to vaporize at the sparking point, exciting the
atoms and generating the emission spectrum. A large
number of sparks is generated, each lasting only a
few microseconds. Due to the lower sample consumption, the limits of detections are poorer than in
the arc. The high-voltage alternating current (AC)
arc is intermediate in analytical performance between the DC arc and the high-voltage spark. These
sources are used mainly in the metallurgical industry.
water-cooled discharge tube inserted in a reentranttype cavity that does not require tuning. It is generally
considered that a laminar flow torch is more suitable
than tangential flow for GC detection. In its usual
form, the MIP operates with a low helium flow rate
(1 l min 1). MIPs are very suitable for the determination of halides and other nonmetals. A microwave
plasma torch (MPT) has been developed consisting of
concentric copper tubes. The carrier gas and aerosol
enter the inner tube while the outer tube serves as the
microwave cavity. The plasma is formed at the top of
the MPT and extends out like a flame, but with a
central channel for the introduction of the aerosol. The
MPT is superior to the conventional MIP since it allows the introduction of wet aerosols at lower power.
Direct Current Plasmas
The DCP evolved from DC arcs and can be classified
into two types: discharges confined within a chamber
(wall-stabilized) and unconfined plasmas. Various
designs and configurations of the electrode exist for
injecting the sample aerosol-carried gas into the
plume, one of which is shown in Figure 3. Magnetic
fields can be used to enhance the coupling of the
sample into the plasma, as is useful for the analysis of
very complex materials. It is characterized by ease of
operation, robustness, and optimization for a great
variety of complex matrices. Analyte signals are
observed in the tail flame or close to the region where
the sample is injected into the discharge and where
the density of excited species is greatest. The discharge has been shown to be suitable for the analysis
of slurries and solutions containing very high salt
concentrations.
Microwave-Induced Plasmas
MIPs have been used widely in gas chromatography
(GC) detection. Low-power MIPs (50–150 W) do not
accept liquid aerosols efficiently. One way of overcoming this disadvantage is to desolvate the sample
solution or to employ electrothermal vaporization.
A commercial system is available consisting of a
Cathode
Inductively Coupled Plasmas
ICP sources have brought about a revolution in multielement analysis. ICPs are generated from radiofrequency (RF) magnetic fields induced by a water- or
air-cooled copper coil looped around a quartz tube.
The RF magnetic field oscillates at 27.12 or
40.68 MHz, at incident powers ranging from 0.5 to
2.5 kW. Higher powers are usually applied when
organic solvents are aspirated. Argon gas flows
through a torch, which consists of three concentric
tubes usually constructed from fused silica. The plasma is initiated by seeding the argon stream with
electrons provided from a Tesla coil. The electrons,
detached from the argon atoms, collide with further
argon atoms and populate the coil region with positive and negative charges. Because of the magnetic
field, the particles flow in a closed annular path. Due
to the conductance of the gases in the coil region, the
charged particles are heated by inductive coupling to
a temperature equaling the ionization temperature of
the support gases B– 7000–8000 K in the case of
argon. A chain reaction of collisional ionization occurs, resulting in the formation of the ICP. In practice, the plasma impedance is monitored along with
the tube grid current, grid voltage, plate current, and
voltage. These data are fed back to a loop to control
the plasma power. The configuration of an ICP-AES
system is shown in Figure 4.
Glow Discharge Sources
Anode
Anode
Sample
Figure 3 A schematic design of a direct current plasma source.
Glow discharge is based on a phenomenon called
sputtering, where atoms ejected from the surface of
the analyte by high-energy argon atoms and ions achieve the excited state in the resulting plasma. A
copper tube filled with argon is juxtaposed with the
sample and a potential difference applied across the
gap, with either a direct current or a RF alternating
current. Electrons jump from the negatively charged
194 ATOMIC EMISSION SPECTROMETRY / Principles and Instrumentation
sample toward the positively charged copper electrode and collide with argon atoms, creating positively charged argon ions that are attracted toward
the sample surface. En route, they collide with other
Tailflame
Fireball
Induction coil
Spectrometer
Radio frequency
generator
Coolant gas
Torch
Auxillary gas
Sample
capillary
Coating gas
Nebulizer
Spray chamber
Peristaltic
pump
Injector
gas
Instrument Design – Sample
Introduction
To drain
Sample
solution
argon atoms and ions, and then strike the analyte
surface with sufficient energy to displace electrons
and atoms from the sample (sputtering). These analyte atoms also collide with the electrons and highenergy argon atoms/ions, causing them to be excited
to higher energy states. As they deexcite, they emit
photons resulting in a ‘glow’ extending 2–3 mm from
the sample (Figure 5).
Glow discharge sources are based on three principal designs. The Grimm source, which consists of a
copper cathode block in direct contact with the metal
sample, is used with DC voltage. The Renault source
is based on the Grimm source, but utilizes a ceramic
cathode block that allows the use of RF voltages. The
most recent development is the Marcus source, which
also operates in RF mode. It has a ceramic cathode
block and a very short anode tube to facilitate rapid
plasma expansion. Although DC and RF plasmas are
similar, RF plasmas are more stable and show a
greater sputtering depth. The most important difference is that RF glow discharges can be used to analyze
both conducting and nonconducting analytes, while
DC glow discharges are restricted to conductors.
Figure 4 Schematic of an inductively coupled plasma source
for atomic emission spectrometry.
Some of the emission sources discussed above are
designed for use with solid samples, which can be
Window
Anode
D
Argon
Electrons
Cathode
block
Ions
O-ring
d
Sample
Figure 5 Principle of glow discharge atomic emission spectrometry. D ¼ diameter of the anode and d ¼ distance from anode to
sample.
ATOMIC EMISSION SPECTROMETRY / Principles and Instrumentation 195
attached to or used as electrodes. For plasma sources,
solid samples can be converted into slurries if the
particle size is small, but an alternative is to use spark
ablation or laser ablation to explode the sample,
generating a small amount of vapor that is carried
into the plasma in a gas stream. Gaseous samples can
be analyzed directly, but liquid samples are first
converted into aerosols, so that they can desolvate
and atomize completely. The most common sample
delivery system consists of a peristaltic pump and
capillary tube to deliver a constant flow of analyte
liquid into a nebulizer. The nebulizer turns the analyte liquid into fine droplets, which are carried by gas
into the plasma. Larger droplets (45 mm) are captured in the spray chamber, and are drained from the
instrument.
Analysis of Solids in Plasma Sources
Spark ablation In this method, conducting samples
are vaporized with an electrical discharge, while
nonconducting samples are first modified by mixing
with copper or carbon powder. The dry aerosol is
carried by an argon stream into the plasma. The
system is calibrated with samples of similar physical
and chemical composition. Slight differences can be
compensated through the use of internal standards.
Large particles can be eliminated using traps, as long
as internal references are available.
Laser ablation This method is used as a microchemical sampling procedure for localized determinations. A pulsed neodymium–yttrium aluminum
garnet (Nd–YAG) laser is used to ablate material
from solid samples. Repetitive laser pulses and sample translation can be used to improve the precision
and accuracy of the analysis. Refractory materials
and geological samples can be analyzed for trace and
major elements. Powdered samples can be pelleted
under high pressure for bulk analysis.
Nebulizers
Many different types of nebulizer are available, some
of which are suitable for general analytical purposes
while others have more specialized uses. Pneumatic
nebulizers are general-purpose devices in which the
aerosol is formed by the shattering effect of a highvelocity jet of gas. The most commonly used pneumatic nebulizer is the Meinhard glass concentric nebulizer, in which the sample is introduced along a
narrow capillary tube located within a larger glass or
quartz tube. The outer tube contains argon gas,
which flows to a 10–20 mm gap surrounding the
sample capillary. The aerosol is formed by the
venturi effect produced by the gas stream as it is
forced through the annulus, which fragments the
liquid into fine droplets. The Meinhard nebulizer is
sensitive to clogging, so particulates and solutions
with high salt concentrations should be avoided.
Cross-flow nebulizers consist of a capillary tube
that directs a stream of argon gas at 901 to the sample delivery tube, creating an aerosol due to its
shearing effect over the sample tube. Again, these
nebulizers are designed for general-purpose use, although they are less prone to blockage where the
sample has a high salt concentration. Parallel flow
nebulizers, such as the Burgener nebulizer, consist of
parallel sample and gas capillaries with adjacent exits, so that the liquid sample is drawn into the gas
stream. These are designed specifically to deal with
inert samples with a high concentration of dissolved
solids. In the Babington nebulizer, the liquid sample
flows over a smooth surface containing a small orifice, through which argon flows at a high velocity,
shearing the liquid into tiny droplets. As above, this
device is not susceptible to clogging and can handle
viscous solutions and suspensions. Another nebulizer
designed for these difficult samples is the V-groove
nebulizer, a special type of Babington nebulizer in
which the liquid flows down a vertical V-shaped
groove toward the gas orifice, from which argon
flows at a pressure of 210–1050 kPa. A further Babington-type nebulizer, the Frit nebulizer, produces
very fine aerosols but has very long washout times.
Ultrasonic nebulizers offer enhanced sensitivity in
detection limits by using a vibrating piezoelectric
transducer to set up standing waves in the liquid to
produce uniformly sized droplets. The efficiency of
nebulization is so high that the solvent loading of the
aerosol needs to be reduced by thermal desolvation,
otherwise cooling of the plasma takes place. Direct
injection nebulizers (DINs) are effective devices for
introducing liquids directly into the plasma when
flows are slower than 0.1 ml min 1. By avoiding the
use of a spray chamber, DINs allow 100% transport
into the plasma. However, solvent loading in the
plasma can exceed the optimum level and can cause a
reduction in the plasma temperature, resulting in
lower intensities.
Instrument Design – Optical Systems
Once the sample has been introduced into the emission source, atomized, and excited, the emitted photons are diffracted by an optical system consisting of
slits, mirrors, and gratings, which focus the spectral
lines onto a detector. This section discusses the types
of gratings and spectrophotometer designs that can
be used in AES.
196 ATOMIC EMISSION SPECTROMETRY / Principles and Instrumentation
Gratings
Gratings are reflective surfaces containing parallel,
equally spaced lines. Their resolving power is proportional to the number of lines, which in turn depends on the line spacing. For radiation arriving with
an angle of incidence a, the angle b at which it will be
diffracted by a grating of N lines per millimeter depends on the wavelength l of the radiation, and is
defined by the equation
sin a þ sin b ¼ kNl
In this equation, k is the order of diffraction, meaning
that constructive interference occurs at wavelength l
(first order), l/2 (second order), l/3 (third order), and
so on. The spectrum produced by a grating thus consists of several superimposed orders, with the first and
second orders carrying most of the energy. The blaze
angle of a diffraction grating is the mirroring angle of
each line. The blaze angle is not arbitrarily selected;
radiation can be concentrated into the first order, rather than in multiple orders and the performance of the
grating for a particular spectral range can be optimized
by varying the blaze angle. To cover the entire analytical spectral range, two gratings may be employed, or
the spectra can be observed in several orders.
Ruled gratings have been replaced by holographic
gratings produced from the interference patterns
generated by two synchronous lasers. The interference pattern is etched into a photosensitive film, and
replicas of these gratings can be manufactured readily. The grating pattern created in this way is greatly
superior to those produced mechanically, being much
more accurate, having higher linearity, and being free
from imperfections and distortions that can give rise
to ghosts and stray light. However, the brightness
achieved by the conventional holographic gratings is
not as high as that of ruled gratings because of the
sinusoidal groove profile.
The echelle grating is designed to operate in multiple orders to produce high-resolution spectra. In
contrast to normal gratings, echelles are ruled at 30–
300 grooves per millimeter by ion bombardment,
and are frequently in the range of 30–120 orders.
The grating consists of saw-like grooves varying in
height and 6–13 mm in depth. The echelle grating is
designed to separate one order of diffraction from
another. This is achieved by inserting a quartz prism,
which acts as an order sorter, in front of the grating.
Successive spectra are diffracted to different degrees,
so that the various orders appear as a two-dimensional array of wavelengths. These gratings are installed in polychromators, sequential systems, and in
charge-injected and charge-transfer detector instruments (see below).
The resolving power of a grating is a measure of its
ability to separate to the baseline two adjacent spectral wavelengths l1 and l2. The resolution R is expressed as
R ¼ l=ðl1 l2 Þ
Monochromators
A monochromator measures a single wavelength, but
can be scanned through a wide wavelength range.
Polychromatic light passes through an entrance slit
and is dispersed by diffraction gratings. These instruments are used in sequential mode, element determinations being performed one after the other.
Two designs are usually employed in commercial instrumentation: Czerny–Turner and echelle.
Czerny–Turner monochromators Two mirrors are
used to reflect and focus the polychromatic and diffracted beams. Wavelengths are selected by using a
computer to rotate the grating in various ways. This
design is shown in Figure 6. As the grating rotates, a
different wavelength is focused onto the exit slit.
Echelle monochromators These are high-resolution
instruments that readily achieve resolutions of 5 pm
in contrast to the 10–20 pm that is normal for
conventional sequential instruments. Because the
spectra are recorded one above the other, such instruments can be very compact.
Simultaneous Spectrometers
There are several types of simultaneous instruments,
which carry out elemental determinations in parallel
rather than in series. They differ in optical design and
the type of detector that is used.
Polychromators These instruments can measure
numerous spectral lines simultaneously, and several
approaches can be used to achieve this. In the classical Paschen–Runge design, light is directed onto a
diffraction grating that diffracts the polychromatic
radiation into its individual wavelengths. Exit slits
are located at predetermined positions on a Rowland
circle to focus the diffracted wavelengths onto a specific photomultiplier tube (PMT). In this design, the
number of elements that can be determined simultaneously is limited by the geometric configuration of the exit optic area of the instrument. This is
a disadvantage compared with the state-of-theart charge-coupled and charge-injection devices discussed below. Geometric alignment is also controlled
by the space requirements of the PMTs, and as a
ATOMIC EMISSION SPECTROMETRY / Principles and Instrumentation 197
Source
Variable entrance slit
ICP
Focusing
mirrors
Direct
drive
Grating
PMT
Variable exit slit
Figure 6 Czerny–Turner monochromator. ICP ¼ inductively coupled plasma and PMT ¼ photomultiplier tube. Reproduced from
www.thespectroscopynet.com
result a series of mirrors is employed to direct the
spectral radiation to the measuring surface of the
detector. For the determination of the spectral lines
in the low part of the UV spectrum (o190 nm), as is
necessary in the detection of aluminum and phosphorus, the spectrometer must be contained in a vacuum
or purged of oxygen using nitrogen or argon gas.
Polychromators have several advantages, including
high sample throughput, lower running costs, and
the ability to measure more than 20 elements, in
duplicate, with background correction in less than
5 min using only 5 ml of solution. However, disadvantages include the high costs associated with
the individual electronic systems required for each
spectral line measurement, and the inflexibility of the
instrument due the static optical system.
Instrument Design – Detection
Photomultiplier Tubes
Photons emerging from the exit slits of the spectrophotometer are detected by one of two types of
device, a PMT or a solid-state component. Photomultipliers are often used as detectors in AES. Incident
photons emerging from the exit slit fall on the photocathode, liberating electrons, and the current is amplified by a set of dynodes. The final anode current is
proportional to the incident photon signal received by
the photocathode. The measurement dynamic range is
very broad, i.e., 1015, and sensitivity is high. These
detectors allow the detection of low intensities emitted by trace elements, as well as strong signals from
major elements. They have very fast response times,
typically 1–2 ns for a 10–90% change in signal. The
main inconvenience of photomultipliers is their cost.
There are several types of photomultipliers, which
differ in the nature of the entrance window, either
crystal or fluoride, and in the nature of the sensitive
layer on the photocathode. Some are only sensitive in
the far-UV while others are more sensitive in the visible. The type of photomultiplier to be used is selected according to the wavelength of the line to be
detected. A fatigue lamp (a small incandescent light
source) is often used with photomultipliers to keep
the temperature of the tube and its associated electronics constant. The fatigue lamp is switched on
when the emission source is off and vice versa.
Charge-injection devices (CIDs) CIDs can be used
in combination with an echelle spectrometer to produce a flexible detection system for multielement
analysis using a direct current arc, AC spark, or inductively coupled or direct current argon plasmas.
The CID consists of a two-dimensional array of detector elements and when it is coupled to a polychromatic dispersive system, simultaneous multielement
analysis is provided over the spectral range 170–
800 nm. In addition to the multielement capability
and large dynamic range (eight orders of magnitude),
the system allows random access integration, where
each detector element can be nondestructively processed until an appropriate signal-to-noise ratio is attained. Background can be read simultaneously, and
alternate spectral lines can be selected for the analysis of spectrally complex materials. The detection
limits are similar to those obtained by conventional
198 ATOMIC EMISSION SPECTROMETRY / Interferences and Background Correction
detection systems. The system is ideally suited to
semiquantitative screening analysis.
Charge coupled devices (CCDs) and echelle polychromators CCDs and echelle polychromators are
now available, and in such systems the high-energy
echelle spectrometer utilizes two detector focal
planes and two cross-dispersers. At 200 nm, the resolution is 0.007 nm but this degrades substantially in
the visible region. The cross-disperser for the UV
region (167–375 nm) is a grating (374 lines per millimeter) with a Schmidt correction incorporated into
its surface. Aberration is also corrected for a 400 mm
radius focal plane. The cross-disperser for the visible
region is a fused 601 quartz prism. A segmented array
CCD consists of 224 addressable subarrays with over
6000 pixels on a 13 18 mm silicon substrate. One
of the drawbacks of the CCD is the inability to read
nondestructively. When the charge is read on a subarray, it is destroyed and cannot be monitored to
adjust the integration time.
Photodiode arrays (PDAs) A spectrally segmented
PDA spectrophotometer has also been developed.
Radiation from the plasma is predispersed and transmitted through an optical mask prior to dispersion
by an echelle grating onto a PDA. Limits of detection
are similar to those obtained by conventional detection systems, except at low wavelengths where
degradation has been observed.
See also: Atomic Emission Spectrometry: Interferences and Background Correction; Flame Photometry; Inductively Coupled Plasma; Microwave-Induced Plasma.
Further Reading
Beauchemin D, Le Blanc JCY, Peters GR, and Craig JM
(1992) Plasma emission spectrometry. Analytical Chemistry Reviews 64: 442R–467R.
Bings NH, Bogaerts A, and Broekaert JAC (2002) Atomic
spectroscopy. Analytical Chemistry 74: 2691–2711.
Bogaerts A and Gijbels R (1998) Fundamental aspects and
applications of glow discharge spectrometric techniques.
Spectrochimica Acta B 53: 1–42.
Cave MR, Butler O, Chenery SRN, et al. (2001) Atomic
spectrometry update. Environmental Analysis. Journal of
Analytical Atomic Spectrometry 16: 194–235.
Evans EH, Day JA, Price WJ, et al. (2002) Atomic spectrometry update. Advances in atomic emission, absorption and fluorescence spectrometry, and related techniques. Journal of Analytical Atomic Spectrometry 17:
622–651.
Evans EH, Day JA, Price WJ, et al. (2003) Atomic spectrometry update. Advances in atomic emission, absorption and fluorescence spectrometry, and related
techniques. Journal of Analytical Atomic Spectrometry
18: 808–833.
Evans EH, Fisher A, and Hill S (1998) An Introduction to
Analytical Atomic Spectrometry. New York: Wiley.
Fisher A, Hinds MW, Nelms SN, Penny DM, and
Goodall P (2002) Atomic spectrometry update. Industrial analysis: Metals, chemicals and advanced materials.
Journal of Analytical Atomic Spectrometry 17:
1624–1649.
Hill SJ (1999) Inductively Coupled Plasma Spectrometry
and its Applications. Boca Raton: CRC Press.
Marcus RK and Broekaert JAC (2003) Glow Discharge
Plasmas in Analytical Spectroscopy. New York: Wiley.
Montaser A and Golightly DW (1992) Inductively Coupled
Plasmas in Analytical Atomic Spectrometry. Weinheim:
VCH.
Payling R (1998) Glow discharge optical emission spectrometry. Spectroscopy 133: 36–48.
Payling R and Larkins P (2000) Optical Emission Lines of
the Elements. New York: Wiley.
Taylor A, Branch S, Halls D, Patriarca M, and White M
(2003) Atomic spectrometry update. Clinical and biological materials, foods and beverages. Journal of Analytical Atomic Spectrometry 18: 385–427.
Thomsen VBE (1996) Modern Spectrochemical Analysis of
Metals: An Introduction for Users of Arc/Spark Instrumentation. Ohio: ASM International.
Interferences and Background Correction
R M Twyman, University of York, York, UK
& 2005, Elsevier Ltd. All Rights Reserved.
Introduction
When a spectral line produced by one element is very
close to a line produced by another element, interference between the two lines may take place. If this
happens, the measured intensity is a combination of
the intensities of the two emission lines and does not
give a true indication of the abundance of either element. Spectral line interference in atomic emission
spectrometry (AES) can severely affect the accuracy
of trace and minor element determinations. Therefore, measurement of the analytical line of interest
must take into account the possibility of interference
from adjacent spectral lines and the interference