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Atomic Emission Spectroscopy - DOC

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Atomic Emission Spectroscopy

- this technique has many advantages over absorption methods:

1. lower susceptibility to chemical interferences
→ this is a direct result of their higher temperatures
2. good emission spectra result for most elements under a single set of excitation conditions

 emission spectra for dozens of elements can be recorded simultaneously
→ this is important for multi-element analysis of very small samples

→ flames are less good (than plasma etc) as emission sources because the optimum excitation conditions
vary widely from element to element
→ high temperatures are required for some, low temperature for others and the best region of the flame to
take a reading from varies

→ plasma sources also permit determination of low concentration elements that tend to form refractory
compounds
i.e. compounds that are highly resistant to thermal decomposition e.g. oxides of B, P, W, Zr, U, and Nb
→ plasma sources permit the analysis of the non-metals e.g. Cl, Br, I and S

→ plasma methods usually have concentration ranges of several orders of magnitude compared to 2-3
decades for absorption methods

- emission spectra from plasmas, arc and spark sources are often highly complex and are frequently
made up of hundreds or even thousands of lines
→ this is a good thing for qualitative information
→ but increase the probability of spectral interferences in quantitative analysis
 require higher resolution and more expensive optical emission equipment than is needed for AA
methods

→ but unlikely that emission methods will ever completely displace absorption methods
→ absorption procedures require less operator 1skill to get good results

Flame Emission Spectroscopy (FES)

 there are certain features common to both  in particular, the use of a flame to atomize the sample
 in AAS, the primary purpose of the atomizer is to produce the highest possible concentration of neutral,
ground-state atoms, capable of absorbing characteristic radiation from an external source

 the atomizer has an additional responsibility in FES  to excite the analyte atoms so they may emit
their characteristic radiation upon relaxing to a lower electronic energy state
 the thermal excitation occurs as a result of high energy collisions between the analyte atoms and
particles formed during combustion of the flame gas

- flame emission spectra consist of fairly simple series of narrow lines whose s are characteristic of the
emitting element
 the  (or energy) of the most intense line usually corresponds to the energy of the transition between
      st
the 1 excited electronic state and the ground state

Instrumentation

- the schematic diagram of a simple flame emission spectrometer looks very much like that of a AAS
without the lamp and its power supply
since the flame serves as both atomizer and “exciter”, no external radiation source is required
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
 the monochromator removes background radiation from the flame, whose  is different from that being
measured
 unfortunately, the flame is likely to produce some background radiation whose  is the same as that
being measured (ie passed by the monochromator)
 modulation, as used in AAS to discriminate against this radiation cannot be used in flame emission
because both analyte and background emission originate in the flame

 this is the major limitation of the flame emission technique
 in cases where the background radiation is known to be of a broad-band type, and the desired
emission line is fairly well isolated from other emission lines, a correction may be possible by making 2
measurements at slightly different s

Methodology

- many of the problems associated with sample preparation and atomization for AAS are encountered also
in flame emission methods
 but that doesn’t mean that the optimum conditions and procedures used are always the same for both
methods

e.g. the spectral emission of the flame itself is of much greater consequence in flame emission than in
AAS because background correction is more difficult (no modulation)

 the various flame conditions selected depend partly on the spectral characteristics of the flame gases
and some organic solvents may have to be avoided due to their high spectral emission during the burning
process

- a successful flame emission method depends not only on the ability of the flame to atomize the analyte
but also to excite the atoms
 T is a critical factor in determining the extent of both processes
 flame parameters that might affect the T must be controlled much more carefully in flame emission than
in AAS

 the high percentage of ground-state atoms in flames can cause problems:

self-absorption  the outer portion of a flame is cooler that the centre and  contains a higher
concentration of unexcited atoms

 some of the photons emitted from the centre of the flame are absorbed as they attempt to pass through
this region, causing a decrease in the signal
 self-absorption becomes more severe as the analyte concentration increases and ultimately causes the
signal-concentration relationship to become non-linear

Applications

- in theory, the scope of FES is somewhat broader than that of AAS because there is no need for a line
source characteristic of the element being determined
 however, the atomic emission lines of most non-metals fall in the UV spectral range below 200 nm,
where special, expensive equipment is required for their measurement

 presently, the most important applications of flame emission spectrometry are for the determination of
the alkali and alkaline earth metals  especially in biological fluids and tissues
 it has also found use in the determination of lanthanides (rare earths)
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 with the exception of the alkali and alkaline earth metals, detection limits for FES are not as low as
those obtained for AAS

 unlike AAS, FES has the inherent capability for simultaneous, multi-element determinations
 however, one of the limitations lies in the fact that the optimum atomization and excitation conditions
vary considerably from element to element

Plasma and Electrical Discharge Emission Spec

- these techniques are similar in principle to FES but use much more energetic atomization-excitation
processes
 the energy available in these atomizers is sufficient to excite atoms to many upper levels
 in addition, a substantial number of ions are formed, which also become excited

 as a result, the emission spectra are complex, containing dozens of lines char of each element
 this complexity can be an advantage in identifying which elements are present in a sample

 these high-energy atomizers also are more efficient than flames at breaking down the stable oxides
formed by refractory elements such as Bo, P, niobium, Zr and W

 the optimum conditions for excitation with plasmas and electrical discharges do not vary much from
element to element
 good spectra can be obtained for most element at a single set of atomization-excitation conditions
 the techniques are conducive to simultaneous, multi-element determinations

Emission Spectra with Plasma Sources

- a plasma is an electrically conducting gaseous mixture containing a significant concentration of cations
and electrons
→ Ar is the most common plasma used for analytical purposes  Ar ions and electrons are the
conducting species

→ Ar ions, once formed absorb sufficient power from an external source to maintain the temperature at a
level where ionization sustains the plasma indefinitely
→ temperatures can be as high as 10,000 K

There are 3 types of plasmas

1. ICP
2. DCP
3. microwave (MIP)

→ MIP is not widely used for general elemental analysis

The ICP Torch

→ the resistance of the ions and electrons to this flow of charge caused ohmic heating of the plasma
→ the temperature of the plasma formed in this way is high enough to require thermal isolation of the
outer quartz cylinder

→ to do this argon is forced to flow tangentially around the walls of the tube
→ this cools the inside walls of the centre tube and centres the plasma radially

→ the argon flow rate through the typical torch is great enough to produce a significant operating cost
(thousands per year)
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- the plasma source is a relative recent innovation that is rapidly replacing the older electrical discharge
sources
 a plasma consists of a gas or mixture of gases in which a significant fraction of the atoms are ionized
 as an electrical conductor, it can be heated inductively by coupling it with an oscillating magnetic filed
 the sample is nebulized in much the same way as in a laminar flow burner and the sample aerosol and
argon nebulizing gas are fed into the torch through a small quartz tube above which is placed a high-
power radio-frequency induction coil

→ the argon gas flows through three concentric quartz tubes

→ typical Ar consumption is 5-10 L/min (depending on design)

→ a water-cooled induction coil that is powered by a radio-frequency generator which radiates 0.5 – 2 kW
of power surrounds the top of the tube (the largest is ~ 2.5 cm wide)
 a spark from a Tesla coil initiate ionization in the flowing gas, and the resulting ions and electrons
rapidly acquire enough energy from the oscillating field of the induction coil to sustain a high degree of
ionization
 a second stream of Ar gas flows up and around the sample tube helping to aspirate the sample into the
plasma and to thermally isolate the outer wall from the hot plasma
 the geometry of the torch causes the ions and electrons to move in close annular paths

 the plasma consists of a brilliant white core (shaded area) and a flame-like tail
 the volume just about the core can reach a T of 7000 K and is remarkably free from background
radiation, making it an ideal location from which to measure the analyte emission
 sample fragments reaching the observation point have spent about 2 ms in the plasma at Ts from 6000
to 8000 K
 this is 2 – 3 x longer and hotter than in a typical combustion flame
: atomization is much more complete and the concentration of emitting atoms is larger

 in addition, the atomization process takes place in a chemically inert atmosphere (Ar) which largely
prevents the formation of stable oxides

Sample Introduction

→ samples are introduced flowing argon (~ 1 L/min) through the central quartz tube
→ samples can be an aerosol, a thermally generated vapour or a fine powder

→ the glass nebulizer is the most common way of sample intro
→ the sample is aspirated to the tip
→ the high velocity gas breaks up the liquid into fine droplets of various sizes which are then carried to the
plasma

→ there are many other types of nebulizers for high efficiency nebulisation, for samples with high solids
content and for production of ultrafine mists

→ can also introduce liquid and solid samples via electrothermal vaporization
i.e. the sample is vaporized in a furnace similar to GFAA (electrothermal atomic absorption)
→ but the furnace is used only for sample introduction
→ the atomization occurs in the plasma
→ this allows the microsampling (~ 5L) and low absolute detection limits (~ 1ng) of electrothermal
furnaces while maintaining the wide linear working range, acceptable sample-to-sample precision (~5-
10%), freedom from interferences and the multielement capabilities of ICP
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→ with ablation devices for solids, the plume of vapour and particulate matter produced by interaction of
the sample with an electric arc or spark or with a laser beam are transported by a flow of argon into the
torch where further atomization and excitation occur

Plasma Spectra

→ typically has a very intense brilliant white nontransparent core topped by a flamelike tail
→ the core, which extends a few mm above the tube produces the atomic spectrum of argon
superimposed on a continuous spectrum
→ in the region 10-30 mm above the core, the continuum fades and the plasma is optically transparent

→ spectral observations are generally made at a height of 12-20 mm above the induction coil where the
temperature is 6000-6500 K
→ in this region the background radiation is remarkably free of argon lines and is well suited to analysis
                                                                                                   +    +
→ many of the most sensitive analytic lines in this region of the plasma are from ions such as Ca , Cd ,
Cr+, and Mn+

Atomization and ionization

→ sample atoms reside in the plasma for ~ 2 ms before they reach the observation point
→ during the residence time they are subjected to temperatures of 5500-8000 K
→ the time and temperatures are ~ 2-3 times greater than those found in the hottest combustion flames
(acetylene NO) used in flame spec
 atomization is more complete and there are fewer chemical interferences

→ ionization interference effects are small
→ probably because the large electron concentration from ionization of the argon maintains a fairly
constant electron concentration in the plasma

→ several other advantages are associated with the plasma source:

1. atomization occurs in a chemically inert environment which tends to enhance the lifetime of the analyte
by preventing oxide formation
2. in contrast to arcs, sparks, and flames, the temperature cross section of the plasma is relatively uniform
and  self-absorption and self reversal effects do not occur as frequently
 calibration curves are usually linear over several orders of magnitude concentration
3. the plasma produces significant ionization, which makes it an excellent source for ICP-MS

DCP Source

→ plasma jet source → consists of three electrodes
→ argon flows from the two anode blocks toward the cathode

→ the plasma jet is formed by bringing the cathode into momentary contact with the anodes
→ ionization of the argon occurs and a current develops that generate additional ions to sustain the
current indefinitely
→ temperature at arc core is > 8000 K and the viewing region ~ 5000 K

→ spectra from DCP tend to have fewer lines than those produced by the ICP and the lines formed are
largely from atoms, rather than ions
→ sensitivities rage from an order of magnitude less than those form ICP to about the same as ICP
→ reproducibility of the two systems are similar

→ significantly less argon is required and auxiliary power supply is simpler and less expensive
→ can handle organic solutions and aqueous solutions with high solid content better than the ICP
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→ however, sample volatilization is often incomplete due to the short residence times in the high
temperature region
→ also, the optic have to be carefully aligned and electrodes have to be constantly replaced whereas ICP
requires relatively little maintenance

Plasma Emission Instruments

- the real power of plasma and electrical discharge spec lies in their ability to provide multi-element
determinations on a single sample

→ the ideal spec is not available because some of the desirable properties are mutually exclusive

→ most commercial instruments cover the entire UV-visible spectrum from 170-800 nm
→ some are equipped for vacuum operation → this extends the UV range to 150-160 nm
→ important because elements such as P, S and C have emission lines in this range

→ there are 3 types of instruments:

1. sequential
2. simultaneous multichannel
3. Fourier transform (not widely used in emission spec)

→ sequential instruments are usually programmed to move from the line for one element to that of a
second, pausing long enough (a few seconds) at each to measure line intensities with a reasonable
signal-to-noise ratio

→ multichannel instruments are designed to measure simultaneously, (or nearly) the intensity of emission
lies for a large number of elements (up to 50-60)

→ when several elements are determined sequential instrument required significantly greater time for
samples to be introduced than is required with the other two types
 these instruments , although simple, are costly in terms of sample consumption and time

→ sequential instruments no longer used

Applications

- these are high performance instruments
 excellent stability coupled with low background and insensitivity to sample matrix makes it the source of
choice in a lot of applications

- produce spectra with many characteristic emission lines
→ this makes them useful for both qualitative and quantitative analysis

→ the ICP and DCP yield significantly better analytical data than other emission sources
→ due to high stability, low noise, low background and freedom from interferences when operating under
appropriate experimental conditions
→ the quality of the ICP source is somewhat better than the DCP source in terms of detection limits
→ however, DCP is cheaper

Sample Prep

→ ICP emission spectroscopy is used primarily for qualitative and quantitative analysis of samples that
are dissolved or suspended in aqueous or organic liquids

→ techniques for preparation are similar to those for flame absorption methods
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→ but with plasma emission, it is possible to analyze solid samples directly

→ procedures include incorporating electrothermal vaporization, laser and spark ablation and glow-
discharge vaporization
→ suspensions of solids in solutions can also be handled with a Babington nebulizer (see 8-11d)

Elements Determined

→ in principle, all metallic elements can be determined
→ a vacuum spec is required for P, N, S and C because the emission lines for these elements lie at
wavelengths greater than 180 nm where components of the atmosphere absorb radiation

→ alkali metal determination is limited due to:
1. the operating conditions that can be used to accommodate most other elements are unsuited for the
alkali’s
2. the most prominent lines of Rb, Li, K and Cs are located at near-IR wavelengths which lead to detection
problems with many plasma specs that are designed primarily for UV

 plasma emission spectroscopy is usually limited to ~60 elements

Line Selection

→ most elements have several prominent lines that can be used for identification and determination
purposes
→ wavelength data with appropriate intensity information for greater than 70 elements have been
determined
→ selection depends on a consideration of what elements other than the analyte might be present in the
sample and whether there is any likelihood that lines of these elements will overlap analyte lines

Calibration Curves

→ usually plot electrical signal (which is proportional to the line intensity) vs. analyte concentration
→ may need log-log plots if a high concentration range is used
→ often plots are linear

→ a large cause of non-linearity (when large concentration ranges are covered) is self-absorption i.e. the
output signal is decreased due to absorption by ground state atoms in the medium
→ only evident at high concentrations

→ non-linearity can also be due to erroneous background correction from ionization and from nonlinear
responses of the detector systems

e.g. Nb and Tl

Interferences

→ chemical interactions and matrix effects are significantly lower with plasma sources that with other
atomizers
→ at low analyte concentration, however, the background emission due to recombination of Ar ions with
electrons is large enough to require corrections
i.e. take background readings on either side of the line of interest

→ because ICP spectra are so rich in lines, spectral interferences are always possible
→ to accommodate for this must know all components of the sample and use reference works
→ there is software available for this
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Detection Limits

→ in general, comparable to or better than other atomic spectral methods
→ but ICP-MS improves the detection limit by 2-5 orders of magnitude

Emission Spec Based on Arc and Spark Sources

→ these are based on excitation of emission spectra of elements with electric arcs or high-voltage sparks
→ allows qualitative and quantitative determination of metallic elements in a variety of sample types,
including metals, alloys, soils, minerals and rocks
→ still used, especially in the metals industry but is gradually being replaced by plasma

→ excitation occurs between a pair of electrodes → this provides the necessary energy to atomize the
sample and produced atoms or ions in electronic excited states

→ mostly limited to solids because the liquids are more easily handled by plasma methods
→ for metals, one or both electrodes can be formed from the analyte
→ for non-metals, the sample is often supported on an electrode whose emission spectrum will not
interfere with the analysis
→ carbon is ideal material because it is a good conductor, has good heat resistance and is easily shaped

→ in another common method, the finely ground sample is mixed with a relatively large amount of
powdered graphite, Cu or other conduction mixture which is compressed at high pressure into an
electrode

Arc Source Emission Spec

→ usually formed with a pair of graphite or metal electrodes spaced a few mm apart
→ the arc is initially ignited by a low current spark that causes momentary formation of ions for electrical
conduction in the gap

→ arc sources are particularly useful for qualitative and semi-quantitative analysis of non-metallic samples
e.g. soils, plant material, rocks and minerals
→ typically 2-50 mg of sample in the form of a powder, small chips grindings or filings are often mixed with
a weighed amount of graphite, is packed into cavity of a graphite electrode
→ usually the sample containing the electrode is the anode and a second graphite counter-electrode is
the cathode

→ arc excitation may be used for either qualitative or quantitative analysis
→ but generally poorer than spark and much poorer than plasma or flame
→ also, emission intensities of samples are highly sample dependent
 matrix matching of standards and samples is usually required

→ internal standard can be used to partly offset this problem

Application of Spark Source Spec

→ quantitative analysis requires precise control of many variables in sample prep and excitation
→ requires a set of carefully prepped standards for calibration
→ standards should approximate as closely as possible the components and physical properties of the
sample
→ generally spark sources analyses are based on ratios of intensity of the analyte line to that of an
internal standard line (usually a major constituent of the sample)
→ currently the primary use of spark source emission spec is for the analysis of metals and other
conducting materials
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Miscellaneous Sources for Optical Emission Spec

1. Flame Emission Spec

→ most single element determinations
→ absorption are better, plasma sources way better
→ but is still used for alkali metals and Ca

2. Glow Discharge Sources

→ used for metals, alloys and other solids

Laser Based Atomic Emission Spec

1. Laser microprobe Sources

→ when a powerful laser pulse is focused on a 5-50 m spot on a sample surface, a small amount of the
solid is vaporized regardless of whether it is a conductor
→ the resulting plume is made up of atoms, ions and molecules
→ in the microprobe the contents of the plume are excited by a spark between a pair of small electrodes
located immediately above the surface of the sample
→ the resulting radiation is then focused on a suitable monochromator and detector system
→ it is then possible to determinate trace element composition of single blood cells, and tiny inclusions in
alloys

→ laser can be scanned across a surface to obtain a spatially resolved representation of surface
composition

Laser Induced Breakdown Specs

→ if the pulsed laser beam focused on a sample is powerful enough (~ 1 GW/cm 2) not only can solid
samples be ablated but the plume can become superheated and converted into a highly luminous plasma
→ near the end of the typical 10 ns laser pulse, the plasma cools and the excited atoms and ions emit
radiation

→ by monitoring the plasma at the appropriate time, atoms and ions lines of the elements can be
observed

→ time-gated detection is usually used with LIBS to avoid the intense spectral continuum emitted early
during the plasma formation and growth stage and to allow detection of the mission lines later during
plasma decay
→ LIBS has found application in metals, semiconductors, ceramics, polymers and pharmaceuticals

				
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