Atomic Emission
Spectroscopy
1
Atomic Emission Spectroscopy
Atomic emission spectroscopy (AES), in
contrast to AAS, uses the very high
temperatures of atomization sources to
excite atoms, thus excluding the need for
lamp sources. Emission sources, which are
routinely used in AES, include plasma, arcs
and sparks, as well as flames. We will study
the different types of emission sources, their
operational principles, features, and
operational characteristics. Finally,
instrumental designs and applications of
emission methods will be discussed.
2
Plasma Sources
The term “plasma” is defined as a
homogeneous mixture of gaseous
atoms, ions and electrons at very high
temperatures. Two types of plasma
atomic emission sources are
frequently used:
1. Inductively coupled plasma
2. Direct current plasma
3
4
Inductively Coupled Plasma
(ICP)
A typical ICP consists of three concentric
quartz tubes through which streams of
argon gas flow at a rate in the range from 520 L/min. The outer tube is about 2.5 cm in
diameter and the top of this tube is
surrounded by a radiofrequency powered
induction coil producing a power of about 2
kW at a frequency in the range from 27-41
MHz. This coil produces a strong magnetic
field as well.
5
Ionization of flowing argon is achieved by a
spark where ionized argon interacts with the
strong magnetic field and is thus forced to
move within the vicinity of the induction coil
at a very high speed. A very high temperature
is obtained as a result of the very high
resistance experienced by circulating argon
(ohmic heating). The top of the quartz tube
will experience very high temperatures and
should, therefore, be isolated and cooled.
6
This can be accomplished by passing
argon tangentially around the walls of
the tube. A schematic of an ICP (usually
called a torch plasma) is shown below:
7
8
9
10
The torch is formed as a result of the
argon emission at the very high
temperature of the plasma. The
temperature gradients in the ICP torch
can be pictured in the following
graphics:
11
12
The viewing region used in elemental
analysis is usually about 6000 oC,
which is about 1.5-2.5 cm above the top
of the tube. It should also be indicated
that argon consumption is relatively
high which makes the running cost of
the ICP torch high as well. Argon is a
unique inert gas for plasma torches
since it has few emission lines. This
decreases possibility of interferences
with other analyte lines.
13
Sample Introduction
There are several methods for sample
introduction; the most widely used is, of
course, the nebulization of an analyte
solution into the plasma. However, other
methods, as described earlier, are fine where
vapors of analyte molecules or atom from
electrothermal or ablation devices can be
driven into the torch for complete
atomization and excitation. For your
convenience, sample introduction methods
are summarized here again:
14
Samples in Solution
1. Pneumatic Nebulizers
Samples in solution are usually easily
introduced into the atomizer by a simple
nebulization, aspiration, process.
Nebulization converts the solution into an
aerosol of very fine droplets using a jet of
compressed gas. The flow of gas carries the
aerosol droplets to the atomization chamber
or region.
15
Ultrasonic Nebulizers
In this case samples are pumped onto the
surface of a piezoelectric crystal that
vibrates in the kHz to MHz range. Such
vibrations convert samples into
homogeneous aerosols that can be driven
into atomizers. Ultrasonic nebulization is
preferred over pneumatic nebulization since
finer droplets and more homogeneous
aerosols are usually achieved. However,
most instruments use pneumatic
nebulization for convenience.
16
Electrothermal Vaporization
An accurately measured quantity of
sample (few mL) is introduced into an
electrically heated cylindrical chamber
through which an inert gas flows.
Usually, the cylinder is made of
pyrolytic carbon but tungsten cylinders
are now available. The vapors of
molecules and atoms are swept into the
plasma source for complete
atomization and excitation.
17
Hydride Generation Techniques
Samples that contain arsenic, antimony,
tin, selenium, bismuth, and lead can be
vaporized by converting them to
volatile hydrides by addition of sodium
borohydride. Volatile hydrides are then
swept into the plasma by a stream of an
inert gas.
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19
Introduction of Solid Samples
A variety of techniques were used to introduce solid
samples into atomizers. These include:
1. Conductive Samples
If the sample is conductive and is of a shape that can
be directly used as an electrode (like a piece of metal
or coin), that would be the choice for sample
introduction in arc and spark techniques. Otherwise,
powdered solid samples are mixed with fine graphite
and made into a paste. Upon drying, this solid
composite can be used as an electrode. The
discharge caused by arcs and sparks interacts with
the surface of the solid sample creating a plume of
very fine particulates and atoms that are swept into
the plasma by argon flow.
20
Laser Ablation
Sufficient energy from a focused intense laser
will interact with the surface of samples (in a
similar manner like arcs and sparks)
resulting in ablation. The vapors of
molecules and atoms are swept into the
plasma source for complete atomization and
excitation. Laser ablation is becoming
increasingly used since it is applicable to
conductive and nonconductive samples.
21
The Glow Discharge Technique
The technique is used for sample introduction and
atomization as well. The electrodes are kept at a 250
to 1000 V DC. This high potential is sufficient to
cause ionization of argon, which will be accelerated
to the cathode where the sample is introduced.
Collision of the fast moving energetic argon ions
with the sample (cathode) causes atomization by a
process called sputtering. Samples should thus be
conductive to use the technique of glow discharge.
The vapors of molecules and atoms are swept into
the plasma source for complete atomization and
excitation by flowing argon. However, nonconductive
samples were reported to be atomized by this
technique where they were mixed with a conductor
material like graphite or powdered copper.
22
Plasma Appearance and Spectra
A plasma torch looks very much like a flame
but with a very intense nontransparent
brilliant white color at the core (less than 1
cm above the top). In the region from 1-3 cm
above the top of the tube, the plasma
becomes transparent. The temperatures
used are at least two to three orders of
magnitude higher than that achieved by
flames which may suggest efficient
atomization and fewer chemical
interferences.
23
Ionization in plasma may be thought to
be a problem due to the very high
temperatures, but fortunately the large
electron flux from the ionization of
argon will suppress ionization of all
species.
24
The Direct Current Plasma (DCP)
The DCP is composed of three electrodes
arranged in an inverted Y configuration. A
tungsten cathode resides at the top arm of
the inverted Y while the lower two arms are
occupied by two graphite anodes. Argon
flows from the two anode blocks and
plasma is obtained by momentarily
bringing the cathode in contact with the
anodes. Argon ionizes and a high current
passes through the cathode and anodes.
25
It is this current which ionizes more argon and
sustains the current indefinitely. Samples are
aspirated into the vicinity of the electrodes
(at the center of the inverted Y) where the
temperature is about 5000 oC. DCP sources
usually have fewer lines than ICP sources,
require less argon/hour, and have lower
sensitivities than ICP sources. In addition,
the graphite electrodes tend to decay with
continuous use and should thus be
frequently exchanged. A schematic of a DCP
source is shown below:
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27
28
A DCP has the advantage of less argon
consumption, simpler instrumental
requirements, and less spectral line
interference. However, ICP sources are
more convenient to work with, free
from frequent consumables (like the
anodes in DCP’s which need to be
frequently changed), and are more
sensitive than DCP sources.
29
Advantages of Plasma Sources
1. No oxide formation as a result of two
factors including
• Very high temperature
• Inert environment inside the plasma (no
oxygen)
2. Minimum chemical interferences
3. Minimum spectral interferences except for
higher possibility of spectral line
interference due to exceedingly large
number of emission lines (because of high
temperature)
30
4. Uniform temperature which results in
precise determinations
5. No self-absorption is observed which
extends the linear dynamic range to
higher concentrations
6. No need for a separate lamp for each
element
7. Easily adaptable to multichannel
analysis
31
Plasma Emission Instruments
Three classes of plasma emission instruments
can be presented including:
1. Sequential instruments
In this class of instruments a single channel
detector is used where the signal for each
element is read using the specific
wavelength for each element sequentially.
Two types of sequential instruments are
available:
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a. Linear sequential scan instruments where
the wavelength is linearly changed with
time. Therefore, the grating is driven by a
single speed during an analysis of interest
b. Slew scan instruments where the
monochromator is preset to provide
specific wavelengths; moving very fast in
between wavelengths while moving slowly
at the specific wavelengths. Therefore, a
two-speed motor driving the grating is thus
used.
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Radial vs. Axial Viewing
Radial – traditional side view, better for concentrated samples.
Axial – direct view into plasma, lower sensitivity, shifts detection range lower.
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Detector
35
Sequential vs. multichannel
• Sequential instrument
– PMT moved behind aperture plate,
– or grating + prism moved to focus new l on exit slit
– Pre-configured exit slits to detect up to 20 lines, slew scan
• characteristics
– Cheaper
– Slower
• Multichannel instrument
– Polychromators (not monochromator) - multiple PMT's
– Array-based system
• charge-injection device/charge coupled device
• characteristics
– Expensive ( > $80,000)
– Faster
36
Slew scan spectrometer
• Two slewscan
gratings
• Two PMTs
for VIS and
UV
• Most use
holographic
grating
37
2. Multichannel Instruments
This class of instruments is also referred
to as simultaneous instruments in
which all signals are reported at the
same time using two types of
configurations:
38
a. Polychromators
Multiple detectors, usually
photomultiplier tubes are used. Beams
of radiation emerging from the grating
are guided to exit slits (each
representing the wavelength of a
specific element) are focused at several
PMTs for detection. Detection, thus,
takes place simultaneously
39
Detectors
Grating
40
b. Array-based systems
This multichannel type instrument uses a
multichannel detector like a charge injection
device or a charge-coupled device. Diffracted
beams from a grating pass through a prism
where further resolution of diffracted beams
takes place by a prism. The prism will
disperse the orders of each diffracted beam.
The multichannel detector can also be a
linear photodiode array as in the figure
below:
41
Diode Array Detector
Grating
42
CCD or CID Detector
Grating
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44
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3. Fourier transform instruments
(FT)
Instruments in which the signal is coded will need a
decoding mechanism in order to see the signal. FT is
a very common technique for decoding time domain
spectra. In such instruments, the detector records
the change of signal with time, which is practically
not useful. However, Fourier transformation of the
time domain signal yield a frequency domain
spectrum, which is the usual signal, obtained by
conventional methods. Instruments that rely on
decoding a coded signal is also said to have a
multiplex design.
46
Applications of Plasma Sources
1. Since plasma sources result in a very large
number of emission lines, these sources can
be used for both qualitative and quantitative
analysis.
2. The signal obtained from plasma sources is
stable, has a low noise and background, as
well as freedom from interferences.
3. Requires sample preparation similar to AAS
47
4. Plasma sources are usually best suited for
operation in the ultraviolet region, therefore,
elements having emission lines below 180
nm (like B, P, S, N, and C) can be only
analyzed under vacuum since air
components absorb under 180 nm. Also,
alkali metals are difficult to analyze since
their best lines under plasma conditions
occur in the near infrared.
5. An analytical emission line can easily be
located but will depend on the other
elements present since spectral line
interferences are encountered in plasma
sources due to the very high temperatures
used.
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6. Linear calibration plots are usually obtained but
departure from linearity is observed at high
concentrations; due to self absorption as well as
other instrumental reasons. An internal standard is
often used in emission methods to correct for
fluctuations in temperature as well as other factors.
The calibration plot in this case is a plot between the
concentration of analyte and the ratio of the analyte
to internal standard signal. The internal standard is a
substance that is added in a constant amount to all
samples, blanks, and standards; therefore it must be
absent from initial sample matrix. The internal
standard should have very close characteristics
(both chemically and physically) to analyte.
49
Elements by ICP-AES
50
Different elements have different emission intensities.
Alkalis (Na, K, Rb, Cs) are weakly emitting. Alkaline Earths
(Be, Mg, Ca, Sr, Ba ) are strongly emitting.
Concepts, Instrumentation, and Techniques in Inductively Coupled Plasma Optical Emission
Spectrometry, Boss and Freeden, Perkin Elmer
51
ICP/OES INTERFERENCES
• Spectral interferences:
– caused by background emission from continuous or recombination
phenomena,
– stray light from the line emission of high concentration elements,
– overlap of a spectral line from another element,
– or unresolved overlap of molecular band spectra.
• Corrections
– Background emission and stray light compensated for by subtracting
background emission determined by measurements adjacent to the
analyte wavelength peak.
– Correction factors can be applied if interference is well characterized
– Inter-element corrections will vary for the same emission line among
instruments because of differences in resolution, as determined by
the grating, the entrance and exit slit widths, and by the order of
dispersion.
52
Physical interferences of ICP
• cause
– effects associated with the sample nebulization and transport
processes.
– Changes in viscosity and surface tension can cause significant
inaccuracies,
• especially in samples containing high dissolved solids
• or high acid concentrations.
– Salt buildup at the tip of the nebulizer, affecting aerosol flow
rate and nebulization.
• Reduction
– by diluting the sample
– or by using a peristaltic pump,
– by using an internal standard
– or by using a high solids nebulizer.
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Interferences of ICP
Chemical interferences:
include molecular compound formation,
ionization effects, and solute vaporization
effects.
Normally, these effects are not significant with
the ICP technique.
Chemical interferences are highly dependent
on matrix type and the specific analyte
element.
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Memory interferences:
When analytes in a previous sample contribute to
the signals measured in a new sample.
Memory effects can result
from sample deposition on the uptake tubing to
the nebulizer
from the build up of sample material in the
plasma torch and spray chamber.
The site where these effects occur is dependent on
the element and can be minimized
by flushing the system with a rinse blank between
samples.
High salt concentrations can cause analyte signal
suppressions and confuse interference tests.
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INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY
(ICP-MS)
- Very sensitive and good for trace analysis
- Plasma produces analyte ions
- Ions are directed to a mass spectrometer
- Ions are separated on the basis of their mass-to-charge ratio
- A very sensitive detector measures ions
- Very low detection limits
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SUMMARY
Inductively Coupled Plasma Emission
- High cost
- No lamp required
- Low background signals
- Low interference
- Moderate sensitivity
Inductively Coupled Plasma-Mass Spectrometry
- Very high cost
- No lamp required
- Least background signals
- Least interference
- Very high sensitivity
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Techniques for elemental analysis
Detection Limits
Productivity
LDR
Precision
Spectral interference
Chemical interference
Ionization
Mass efffects
Isotopes
Dissolved solids
No. of elements
Sample usage
Semi-quantitative
Isotope analysis
routine operation
Method development
Running costs
Capital costs
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ICP-MS
ICP-AES
FAAS
GFAAS
Excellent
Excellent
10 5
1-3 %
Few
Moderate
Minimal
High on low
Yes
0.1-0.4 %
~75
low
yes
yes
Skill required
skill required
high
very high
Good
Very good
10 6 /10 10 HDD
0.3-2 %
Common
Few
Minimal
none
none
up to 30 %
~73
medium
yes
no
easy
skill required
high
high
Good
Excellent
Good
Low
10 3
10 2
0.1-1 %
1-5 %
Almost none Very few
Many
Many
Some
Minimal
none
none
none
none
0.5-3 %
up to 30 %
~68
~50
high
very low
no
no
no
no
easy
skill required
easy
skill required
low
medium
low
medium
Emission Spectroscopy Based
on Arcs and Sparks
Samples are excited in the gap between a
pair of electrodes connected to a high
potential power supply (200 VDC or
2200-4400 VAC). The high potential
applied forces a discharge between the
two electrodes to occur where current
passes between the two separated
electrodes (temperature rises due to
very high resistance).
59
The very high temperature (4000-5000 oC)
realized in the vicinity between the two
electrodes provide enough energy for
atomization and excitation of the samples in
this region or when the sample is, or a part
of, one of the electrodes.
Arc and spark methods are mainly used as
qualitative techniques and can also be used
as semiquantitative techniques.
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10B. Arc and Spark AES
• Arc and Spark Excitation Sources:
–Limited to semi-quantitative/qualitative
analysis (arc flicker)
–Usually performed on solids
–Largely displaced by plasma-AES
• Electric current flowing between two C
electrodes
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Sample Handling and
Preparation
If the sample is conductive and is of a shape that can
be directly used as an electrode (like a piece of metal
or coin), that would be the choice for sample
introduction in arc and spark techniques. Otherwise,
powdered solid samples are mixed with fine graphite
and made into a paste. Upon drying, this solid
composite can be used as an electrode. The
discharge caused by arcs and sparks interacts with
the surface of the solid sample creating a plume of
very fine particulates and atoms that are excited and
emission is collected. The figure below shows some
common shapes of graphite electrodes used in arc
and spark sources.
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Carbon electrodes
Sample pressed into
electrode or mixed with Cu
powder and pressed Briquetting (pelleting)
Cyanogen bands (CN) 350420 nm occur with C
electrodes in air -He, Ar
atmosphere
Arc/spark unstable
each line measured >20 s
needs multichannel
detection
63
Instruments for Arcs and Sparks
In most cases, emission from atoms in
an arc or spark is directed to a
monochromator with a long focal
length and the diffracted beams are
allowed to hit a photographic film. This
typical instrument is called a
spectrograph since it uses a
photographic film as the detector.
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spectrograph
Beginning 1930s
photographic film
Cheap
Long integration times
Difficult to develop/analyze
Non-linearity of line "darkness“
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Potential Source
Graphite Electrodes
Photographic Film
66
67
The blackness of the lines on the photographic
film is an indication of the intensity of the
atomic line and thus the concentration of the
analyte. The location of emission lines as
compared to standard lines on a film serves
to identify the wavelengths of emission lines
of analyte and thus its identity. The use of
spectrographs is not very convenient since a
lot of time and precautions must be spent on
processing and calibrating the photographic
film.
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Qualitative analysis is accomplished by
comparison of the wavelengths of
some emission lines to standards while
the line blackness serves as the tool for
semiquantitative analysis.
Polychromators are also available as
multichannel arc and spark
instruments. However, these have fixed
slits at certain wavelengths in order to
do certain elements and thus they are
not versatile.
69
Potential Source
Detectors
Grating
70
Recently, arc and spark instruments
based on charge injection and charge
coupled devices became available.
These have extraordinarily high
efficiency and performance in terms of
easier calibration, short analysis time,
as well as superior quantitative results.
71
CCD or CID Detector
Potential Source
Grating
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Characteristics of Arc Sources
1. Typical temperatures between 4000-5000 oC are high
enough to cause atomization and excitation of
sample and electrode materials.
2. Usually, cyanogens compounds are formed due to
reaction of graphite electrodes with atmospheric
nitrogen. Emission bands from cyanogens
compounds occur in the region from 350-420 nm.
Unfortunately, several elements have their most
sensitive lines in this same region which limits the
technique. However, use of controlled atmosphere
around the arc (using CO2, Helium, or argon) very
much decreases the effect of cyanogens emission.
73
3. The emission signal should be integrated over a
minute or so since volatilization and excitation of
atoms of different species differ widely. While
some species give maximum signal, others may
still be in the molecular state.
4. Arc sources are very good for qualitative analysis of
elements while only semiquantitative analysis is
possible. It is mandatory to compare the emission
spectrum of a sample with the emission spectrum
of a standard. In some cases, a few milligrams of a
standard is added to the sample in order to locate
the emission lines of the standard and thus
identify the emission wavelengths of the different
elements in the sample. A comparator
densitometer can be used to exactly locate the
wavelengths of the standard and the sample
components.
74
Standard
Sample
The lines from the standard are projected on the lines
of the combined sample/standard emission spectra in
order to identify sample components. Only few lines
are shown in the figure.
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76
77
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Why use Carbon in Atomic
Spectroscopy?
We have previously seen the use of graphite in
electrothermal AAS as well as arc and spark
AES, even though molecular spectra are real
problems in both techniques due to
cyanogens compounds absorption and
emission. The reasons after graphite
common use in atomic spectroscopy can be
summarized below:
79
1.
2.
3.
4.
5.
6.
80
It is conductive.
It can be obtained in a very pure state.
Easily available and cheap.
Thermally stable and inert.
Carbon has few emission lines.
Easily shaped.
Spark Sources
Most of the instruments in this category are arc
based instruments. Spark based instruments
are of the same idea except for a spark
source substituting an arc source. The spark
source is constructed as in the figure below
where an AC potential in the order of 10-50
KV is discharged through a capacitor which
is charged and discharged through the
graphite electrodes about 120 times/s;
resulting in a discharge current of about
1000 A.
81
This very high current will suffer a great deal of
resistance, which increase the temperature to
an estimated 40000 oC. Therefore, ionic
spectra are more pronounced.
Potential Source
Transformer
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