Atomic emission spectroscopy

1. Atomic Emission Spectroscopy
MAHENDRA G S
M.Pharm

2. Introduction
• Technique is also known as OPTICAL EMISSION
SPECTROSCOPY (OES)
• The study of radiation emitted by excited atoms and
monatomic ions
• Relaxation of atoms in the excited state results in emission
of light
• Produces line spectra in the UV-VIS and the vacuum UV
regions

3. Introduction
• Used for qualitative identification of elements present in the
sample
• Also for quantitative analysis from ppm levels to percent
• Multielement technique
• Can be used to determine metals, metalloids, and some
nonmetals simultaneously
• Emission wavelength and energy are related by ΔE = hc/λ

4. Introduction
• Does not require light source
• Excited atoms in the flame emit light that reaches the
detector (luminescence)
Techniques Based on Excitation Source
• Flame Photometry (flame OES)
• Furnace (Electrical Excitation)
• Inductively Coupled Plasma (ICP)

5. Principle
Atomic emission spectroscopy is also an analytical
technique that is used to measure the concentrations
of elements in samples.
It uses quantitative measurement of the emission from
excited atoms to determine analyte concentration.

6. The analyte atoms are promoted to a higher energy
level by the sufficient energy that is provided by
the high temperature of the atomization sources .
The excited atoms decay back to lower levels by
emitting light. Emissions are passed through
monochromators or filters prior to detection by
photomultiplier tubes.

7. The instrumentation of atomic emission
spectroscopy is the same as that of atomic
absorption, but without the presence of a radiation
source .
In atomic Emission the sample is atomized and the
analyte atoms are excited to higher energy levels.

8. Schematic Diagram of an Atomic Emission
spectrometer

9. Sources
• Flame – still used for metal atoms
• Alternating current arc (AC arc)
• Direct current (DC arc)
• Alternating current spark (AC spark)
• Direct current Plasma
• Microwave Induced Plasma
• Inductively Coupled Plasma – the most
important technique

10. Flame
• It is used for those molecules which do not require very high
temperatures for excitation into atoms during quantitative
analysis.
• Sample in solution form and sprayed into the flame of a burner.
Fuel Oxidant Flame temp (O
C)
H2 O2 2800
H2 Air 2100
Acetylene O2 3000
Acetylene Air 2200

11. Alternating current spark (AC spark)
• High voltage transfer 10 – 50 KV across two electrodes gives a
spark.
• The use of condenser increases the current.
• Brilliant spark is obtained at 0.005 microfarad capacity to the
capacitor.
Advantages:
• Reproducible
• Less material is consumed
• High concentration solution can be used
• Heating effect is less which is useful for analysis of low
melting materials.
• No interference

12. AC Arc
• It employs potential difference of 1000 volts or more.
• The arc is drawn at a distance of 0.5 – 3 mm.
Advantages:
• Best source for qualitative analysis
• Stable
• Reproducible

13. DC Arc
• The DC arc is generated with a potential gradient of 50 – 300
volts.
• The current used is about 1 to 300 amps and emission is due to
neutral atoms.
• The temperature in the arc stream ranges from 2273 – 5273 K.
• Used for identification and determination of elements present
in very small concentrations.

14. Three types of high-temperature
plasma
Plasma
• an electrically conducting gaseous mixture containing
significant concentrations of cations and electrons.
• The inductively coupled plasma (ICP)
• The direct current plasma (DCP)
• The microwave induced plasma (MIP)
The most important of these plasmas is the inductively coupled
plasma (ICP).

15. Sources
Advantages of plasma
• Simultaneous multi-element Analysis – saves sample amount
• Some non-metal determination (Cl, Br, I, and S)
• Concentration range of several decades (105
– 106
)
Disadvantages of plasma
• very complex Spectra - hundreds to thousands of lines
• High resolution and expensive optical components
• Expensive instruments, highly trained personnel required

16. The Direct Current Plasma Technique
• The direct current plasma is created by
the electronic release of the two
electrodes.
• The samples are placed on an electrode.
• In the technique solid samples are
placed near the discharge to encourage
the emission of the sample by the
converted gas atoms.

17. Inductively Coupled Plasma (ICP)
• Plasma generated in a device called a Torch
• Torch up to 1" diameter
• Argon cools outer tube, defines plasma
shape
• Rapid tangential flow of argon cools outer
quartz and centers plasma
• Rate of Argon Consumption 5 - 20 L/Min
• Radio frequency (RF) generator 27 or 41
MHz up to 2 kW
• Telsa coil produces initiation spark
• Ions and electrons interact with magnetic field
and begin to flow in a circular motion.
• Resistance to movement (collisions of electrons
and cations with ambient gas) leads to ohmic
heating.
• Sample introduction is analogous to atomic
absorption.

18. A typical inductively coupled plasma source called a
torch

19. Inductively-coupled plasma atomic emission
spectrometer

20. 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 electrodes

21. Sample holder
•The function of sample holder is to introduce the
sample into the electrical discharge.
•Two types, for
• Solid samples
• Liquid samples

22. Monochromators
•Prism
•Grating

23. Detectors
•Photomultiplier tubes
•Photographic plate

24. Detectors
• For Quantitative analysis photographic plate is used on
which all the emission lines from the samples are recorded.
• This photograph of the emission spectrum helps in the
measurement of wavelength of radiation lines.
• From these lines emitting elements can be identified.
• The instrument with photographic recording is called
spectrograph and the one using photoelectric device as
spectrometer.

25. Advantages of Photographic plates
over Photomultiplier tubes
• Large number of spectral lines can be recorded.
• Photographic plates provides a permanent record of the
spectrum.
• Emission spectrum can be integrated by a photographic
emulsion for long time.
• Photographic emulsions are preferred because of their high
sensitivity in UV and visible region.

26. Disadvantages of Photographic plates
over Photomultiplier tubes
• Photographic plates do not show quick response to spectral
lines and their interpretation is not easy.
• In Photographic detection, controlled photographic
development is required. This requires much time and
increases the risk of error.

27. WAVELENGTH SELECTION AND
DETECTION FOR AES
•Arc and spark instruments normally contain non
scanning monochromators.
•Either a series of slits is cut in the focal plane of the
monochromator and a photomultiplier tube is placed
behind each slit that corresponds to the wavelength
of a line that is to be measured, or one or more
photographic plates or pieces of film are placed on
the focal of the monochromator.

28. QUALITATIVE ANALYSIS WITH ARC AND
SPARK AES
•Qualitative analysis is performed by comparing the
wavelengths of the intense lines from the sample
with those for known elements.
•It is generally agreed that at least three intense lines
of a sample must be matched within a known
element in order to conclude that the sample
contains the element

29. QUANTITATIVE ANALYSIS WITH
ARC AND SPARK AES
• Regardless of the type of detection used for the assay, the
precision of the results can be improved by matrix-matching
the standards with the sample.
• Use of the internal-standard method also improves precision.
• Usually a working curve is prepared by plotting the ratio or
logarithm of the ratio of intensity of the standard's line to the
internal standard's line as a function of the logarithm of the
concentration of the standard.
• The corresponding ratio for the analyte is obtained and the
concentration determined from the working curve.

30. AES WITH ELECTRICAL
DISCHARGES
• An electrical discharge between two electrodes can be used to
atomize or ionize a sample and to excite the resulting atoms or
ions.
• The sample can be contained in or coated on one or both of the
electrodes or the electrode(s) can be made from the analyte.
• The second electrode which does not contain the analyte is the
counter electrode.
• Electrical discharges can be used to assay nearly all metals and
metalloids.
• Approximately 72 elements can be determined using electrical
discharges.

31. AES WITH ELECTRICAL
DISCHARGES
• For analyses of solutions and gases the use of plasma is
generally preferred although electrical discharge can be used.
• Solid samples are usually assayed with the aid of electrical
discharges.
• Typically it is possible to assay about 30 elements in a single
sample in less than half an hour using electrical discharges.
• To record the spectrum of a sample normally requires less
than a minute.

32. ELECTRODES FOR AES
• The electrodes that are used for the various forms of AES are
usually constructed from graphite.
• Graphite is a good choice for an electrode material because it
is conductive and does not spectrally interfere with the assay
of most metals and metalloids.
• In special cases metallic electrodes (often copper) or
electrodes that are fabricated from the analyte are used.
• Regardless of the type of electrodes that are used, a portion of
each of the electrodes is consumed during the electrical
discharge.
• The electrode material should be chosen so as not to
spectrally interference during the analysis.

33. Sample introduction
• Nebulizer
• Electrothermal vaporizer

34. Nebulizer
•convert solution to fine spray or
aerosol
•Ultrasonic nebulizer
• uses ultrasound waves to "boil" solution
flowing across disc
•Pneumatic nebulizer
• uses high pressure gas to entrain solution

35. Electro-thermal vaporizer ETV
• electric current rapidly heats
crucible containing sample
• sample carried to atomizer
by gas (Ar, He)
• only for introduction, not
atomization

36. Plasma structure
•Brilliant white core
• Ar continuum and lines
•Flame-like tail
• up to 2 cm
•Transparent region
• where measurements are made (no
continuum)

37. Plasma characteristics
• Hotter than flame (10,000 K) -
more complete atomization/
excitation
• Atomized in "inert" atmosphere
• Ionization interference small due
to high density of electrons
• Sample atoms reside in plasma
for ~2 msec and
• Plasma chemically inert, little
oxide formation
• Temperature profile quite stable
and uniform.

38. Direct Current plasma (DC)
• First reported in 1920s
• DC current (10-15 A) flows
between anodes and cathode
• Plasma core at 10,000 K, viewing
region at ~5,000 K
• Simpler, less Ar than ICP - less
expensive
• Less sensitive than ICP
• Should replace the carbon anodes in
several hours

39. Atomic Emission Spectrometer
• May be >1,000 visible lines (<1 Å) on continuum
• Need
• higher resolution (<0.1 Å)
• higher throughput
• low stray light
• wide dynamic range (>1,000,000)
• precise and accurate wavelength calibration/intensities
• stability
• computer controlled

40. AES instrument types
Three instrument types:
• Sequential (scanning and slew-scanning)
• Multichannel - Measure intensities of a large number of elements (50-60)
simultaneously
• Fourier transform FT-AES

41. Desirable properties of an AE
spectrometer

42. 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

43. Sequential monochromator
Slew-scan spectrometers
• even with many lines, much spectrum contains no
information
• rapidly scanned (slewed) across blank regions (between
atomic emission lines)
• From 165 nm to 800 nm in 20 msec
• slowly scanned across lines
• 0.01 to 0.001 nm increment
• computer control/pre-selected lines to scan

44. Slew scan spectrometer
•Two slew-scan
gratings
•Two PMTs for
Visible & UV
•Holographic
grating

45. Scanning echelle spectrometer
• PMT is moved to monitor signal from slotted aperture.
• About 300 photo-etched slits
• 1 second for moving one slit
• Can be used as multi channel spectrometer
• Mostly with DC plasma source

46. Multichannel photoelectric
spectrometer
•multichannel PMT instruments
• for rapid determinations (<20 lines) but not versatile
• For routine analysis of solids
•metals, alloys, ores, rocks, soils
• portable instruments
•Multichannel charge transfer devices
• Recently on the market
• Orignally developed for plasma sources

47. Multichannel polychromator AES
• Rowland circle
• Quantitative determination of more than 20 elements within 5
minutes

48. Applications of AES
• Qualitative analysis is done using AES in the same manner in
which it is done using FES. The spectrum of the analyte is
obtained and compared with the atomic and ionic spectra of
possible elements in the analyte. Generally an element is
considered to be in the analyte if at least three intense lines
can be matched with those from the spectrum of a known
element.
• Quantitative analysis with a plasma can be done using either
an atomic or an ionic line. Ionic lines are chosen for most
analyses because they are usually more intense at the
temperatures of plasmas than are the atomic lines.

49. Applications of AES
• In practice ~60 elements detectable
• 10 ppb range most metals
• In determining the impurities of Ni, Mn, Cu, Al etc., in iron
and steel in metallurgical processes. The percentage
determined is 0.001% in iron to 30 in steel.
• Lubricating oils can be analysed for Ni, Fe, Mn etc.,
• Solid samples and animal tissues have been analysed for
several elements including K, Na, Ca, Zn, Ni, etc.,
• To detect 40 elements in plants and soils, thus metal
deficiency in plants and soils can be diagnosed.

50. Detection power of ICP-AES

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.

53. 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.

54. 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.

55. Typical Calibration ICP curves

56. Calibration curves of ICP-AES

57. Advantages of AES
• Highly specific
• Sensitive (low concentration 0.0001%)
• Metalloids (arsenic, silicon, selenium) have been identified by
this technique
• Samples in solid or liquid state and rarely gas samples can be
used
• Techniques requires minimum sample preparation
• No preliminary treatment of sample is required
• Spectra can be taken simultaneously for more than 2 elements and
no separation is required. (1 – 10 mg)
• Rapid results, if automated, times required is 30 – 60 seconds

58. Disadvantages of AES
•Equipment is costly and experience is required for
handling and interpretation of spectra.
•Recording is done on a photographic plate which takes
sometime to develop, print and interpret the results.
•The spectrograph is essentially a comparator. For
quantitative analysis, standards of similar composition
are required.
•Radiation intensities are not always reproducible
•Method limited to the analysis of elements

59. Comparison Between Atomic Absorption
and Emission Spectroscopy
Absorption
- Measure trace metal
concentrations in
complex matrices .
- Atomic absorption
depends upon the
number of ground state
atoms .
Emission
- Measure trace metal
concentrations in
complex matrices .
- Atomic emission depends
upon the number of
excited atoms .

60. - It measures the
radiation absorbed by
the ground state atoms.
- Presence of a light
source ( HCL ) .
- The temperature in
the atomizer is adjusted to
atomize the analyte
atoms in the ground
state only.
- It measures the
radiation emitted by
the excited atoms .
- Absence of the light
source .
- The temperature in the
atomizer is big enough to
atomize the analyte
atoms and excite them to
a higher energy level.