Dong-Sun Lee / cat - lab / SWU
2010
Chapter
Fall version
28
Atomic spectroscopy
Atomic spectroscopy
Atomic spectroscopy deals with the absorption, emission, or fluorescence by
atom or elementary ions. Two regions of the spectrum yield atomic
information- the UV-visible and the X-ray.
As atoms have no rotational or vibrational energy, transitions occur only
between electronic levels and bandwidths in atomic spectra are very narrow.
Atomic spectroscopic methods normally are classified according to the type of
spectral process involved and the method of atomization used.
Absorption, emission, fluorescence
Schematic representation of absorption, emission, and fluorescence.
Origin of three sodium emission lines
(resonance line)
(a)
Partial absorption spectrum for sodium
vapor.
(b)
Electronic transitions responsible for
the absorption lines in (a)
Atomic energy level diagram.
Atomic absorption spectrometry
Atomic absorption is the process that occurs when a ground state atom absorbs
energy in the form of electromagnetic radiation at a specific wavelength and is
elevated to an excited state. The atomic absorption spectrum of an element
consists of a series of resonance lines, all originating with the ground electronic
state and terminating in various excited states. Usually the transition between the
ground state and the first excited state is the line with the strongest absorptivity,
and it is the line usually used.
Transition between the ground state and excited state occur only when the incident
radiation from a source is exactly equal to the frequency of a specific transition.
Part of the energy of the incident radiation Po is absorbed. The transmitted
radiation P is given by
P = Poe(kb)
where k is the absorption coefficient of the analyte element and b is the horizontal
path length of the radiation through the flame. Atomic absorption is determined by
the difference in radiant power of the resonance line in the presence and absence
of analyte atoms in the flame. The width of the line emitted by the light source
must be narrower than the width of the absorption line of the analyte in the flame.
The concentration value in the AAS is based on the Lambert-Beer law:
with :
E - extinction,
T - light transmission expressed as a percentage (transmission),
ID - intensity of the transmitted signal,
I0 - intensity of the original signal,
e - extinction coefficient (proportionality factor),
c - concentration
l - layer thickness (= distance traversed by the light ray through the
flame/atomic cloud).
Atomic emission spectrometry with flame
A solution if the sample is sprayed into a flame possessing the thermal energy
required to excite the element to a level at which it will radiate its characteristic
line emission spectrum. For an atom or molecule in the ground electronic state to
be excited to a high electronic energy level, it must absorb energy from the flame
via thermal collisions with the constituents of the partially burned flame gases.
Upon their return to a lower or ground state, the excited atoms and molecules
emit radiation characteristic of the sample components.
Band spectra arise from electronic transitions involving molecules. For each
electronic transition there will be a whole suite of vibrational levels involved.
This causes the emitted radiation to be spread over a portion of the spectrum.
Band emissions attributed to triatomic hydroxides(CaOH) at 554 nm and
monoxides (AlO, strongest band at 484 nm) are frequently observed and
occasionally employed in FES. The boron oxide system gives very sensitive
bands at 518 and 546 nm.
A photodetector measures the radiant power of the selected radiation that is
correlated with the concentration of analyte in the sample and in standards.
Basic components of an atomic spectrophotometer
Thermo Elemental SOLAAR 969 AA spectrometers
http://www.thermoelemental.com/instruments/aa/solaar_969_overview.asp
http://neon.zal.tu-cottbus.de/zal/prakt/aas.htm
Sources
Hollow cathode lamp
These lamps consist of a cylindrical metallic cathode(the same element as that being
analyzed) and tungsten anode sealed in a glass tube containing neon or argon at a
pressure of about 1 to 5 torr. When high voltage is applied between the anode and
cathode, the filler gas is ionized and positive ions are accelerated toward the cathode.
They strike the cathode with enough energy to “sputter” metal atoms from the cathode
surface into the gas phase. The free atoms are excited by collisions with high-energy
electrons and then emit photons to return to the ground state. This radiation has the
same frequency as that absorbed by analyte atoms in the flame or furnace.
A hollow cathode lamp.
Atomic absorption bandwidths are so narrow, generally in the range 0.002 to 0.005 nm. The
narrowest band of wavelengths that can be isolated from a continuum with best
monochromator is about 0.5 nm.
At a proper conditions, the bandwidth of emitted radiation with hollow cathode lamp is even
narrower than the atomic absorption bandwidth.
Comparison of atomic absorption and
monochromator spectral bandwidths.
Relative line widths for copper
emission and absorption.
Line broadening
The linewidth of the source must be narrower than the linewidth of the atomic vapor for
Beer’s law to be obeyed.
Doppler broadening :
The wavelength of radiation emitted or absorbed by a fast moving atom decreases if the
motion is toward a detector and increases if the atom is receding from the detector. The
linewidth, ,due to the Doppler effect, is given approximately by
  (7×10–7)(T/M)–1/2
where is the frequency(Hz) of the peak, T is temperature(K), M is the mass of the atom.
Pressure broadening :
Pressure, or collisional , broadening arises from collisions of emitting or absorbing
species with other atoms or ions in the heated medium.
Cause of Doppler broadening.
(a) When an atom moves toward a photon detector and emits radiation, the detector sees
wave crests more often and detects radiation of higher frequency. (b) When an atom
moves away from a photon detector and emits radiation, the detector sees crests less
frequently and detects radiation of lower frequency. The result in an energetic medium is
a statistical distribution of frequencies and thus a broadening of spectral lines.
Atomic absorption of a narrow emission line from
a source. The source lines in (a) are very narrow.
One line is isolated by a monochromator. The line
is absorbed by the broader absorption line of the
analyte in the flame (b) resulting in attenuation ( c)
of the source radiation. Since most of the source
radiation occurs at the peak of the absorption line,
Beer’s law is obeyed.
The mass spectrum of a standard rock sample obtained by laser ablation / ICP-MS.
Atomizers
1) Flame atomization : the laminar flow burner
Laminar flow burner.
Processes occurring during atomization .
Solution of analyte
Nebulization
Spray(small droplets)
Desolvation
Solid/Gas aerosol
Volatilization
Gaseous
Excited
h
molecules
molecules
molecular
Dissociation(reversible)
Atoms
Excited atoms
h atomic
Ionization(reversible)
Atomic
ions
Excited ions
h atomic
A) Schematic diagram of a premix burner.
B) End view of flame.
Processes leading to atoms.
Continuous sample introduction methods.
Region of a flame
Properties of flames.
Temperature oC
Maximum burning velocity(cm s–1)
Fuel
Oxidant
Natural gas
Air
1700~1900
39~ 43
Natural gas
Oxygen
2700~2800
370 ~ 390
Hydrogen
Air
2000~2100
300 ~ 440
Hydrogen
Oxygen
2550~2700
900 ~1400
Acetylene
Air
2100~2400
158 ~266
Acetylene
Oxygen
3050~3150
1100 ~ 2480
Acetylene
Nitrous oxide
2600~2800
285
Comparison of detection limits for various elements by flame absorption and flame
emission methods.
Flame emission
Sensitivity
Flame absorption
more sensitive
about the same
more sensitive
Al, Ba, Ca, Eu,
Cr, Cu, Dy, Er,
Ag, As, Au, B,
Ga, Ho, In, K,
Gd, Ge, Mn, Mo,
Be, Bi, Cd, Co,
La, Li, Lu, Na,
Nb, Pd, Rh, Sc,
Fe, Hg, Ir, Mg,
Nd, Pr, Rb, Re,
Ta, Ti, V, Y,
Ru, Sm, Sr, Tb
Zr
Tl, Tm, W, Yb
E. E. Pickett and S. R. Koirtyohann, Anal. Chem., 1969, 41(14), 42A.
Ni, Pb, Pt, Sb,
Se, Si, Sn, Te,
Zn
Inductively coupled plasmas
By definition, a plasma is an electrical conducting gaseous mixture
containing a significant concentration of cations and electrons. (The
concentrations of the two are such that the net charge approaches zero.) In
the argon plasma employed for emission analyses, argon ions and electrons
are the principal conducting species, although cations from the sample will
also be present in lesser amounts. Argon ions, once formed in a plasma, are
capable of absorbing sufficient power from an external source to maintain the
temperature at a level at which further ionization sustains the plasma
indefinitely; temperatures as great as 10,000 K are encountered. Three power
sources have been employed in argon plasma spectroscopy. One is a dc
electrical source capable of maintaining a current of several amperes between
electrodes immersed in a stream of argon. The second and third utilize
powerful radio-frequency and microwave-frequency fields through which the
argon flows. Of the three, the radio-frequency, or inductively coupled plasma
(ICP), source appears to offer the greatest advantage in terms of sensitivity
and freedom from interference. On the other hand, the dc plasma source
(DCP) has the virtue of simplicity and lower cost. Both will be described
here. The microwave induced plasma source (MIP) is not widely used for
analysis (primarily because it is not available from instrument manufacturers).
Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES,
or ICP).
ICP-AES, often referred to simply as ICP, is a multi-element analysis
technique that uses an inductively coupled plasma source to dissociate the
sample into its constituent atoms or ions, exciting them to a level where they
emit light of a characteristic wavelength. A detector measures the intensity of
the emitted light, and calculates the concentration of that particular element in
the sample.
When undergoing ICP analysis, the sample experiences temperatures as high
as 10,000oK, where even the most refractory elements are atomized with high
efficiency. As a result, detection limits for these elements can be orders of
magnitude lower with ICP than with FAAS techniques, typically at the 1-10
parts-per-billion level.
http://www.thermoelemental.com/instruments/icpoes/fundamental_icp.asp
Principles of ICP
An ICP source consists of a flowing stream of argon gas ionized by an
applied radio frequency field typically oscillating at 27.1 MHz. This field is
inductively coupled to the ionized gas by water-cooled coil surrounding a
quartz torch that supports and confines the plasma. A sample aerosol is
generated in an appropriate nebulizer and spray chamber and is carried into
the plasma through an injector tube located within the torch. The sample
aerosol is injected directly into the ICP, subjecting the constituent atoms to
temperature of about 6000 to 8000oK. Because this results in almost
complete dissociation of molecules, significant reduction in chemical
interferences is achieved. The high temperature of the plasma excites atomic
emission efficiently. Ionization of a high percentage of atoms produces ionic
emission spectra. The ICP provides an optically thin source that is not
subject to self absorption except at very high concentrations. Thus linear
dynamic ranges of four to six orders of magnitude are observed for many
elements. The efficient excitation provided by the ICP results in low
detection limits for many elements.
Advantages of an ICP source
1. The analytes are confined to a narrow region.
2. The plasma provides simultaneous excitation of many elements.
3. The analyst is not limited to analytical lines involving ground state transitions but can
select from first or even second ionization state lines. For the elements Ba, Be, Mg, Sr, Ti,
and V, the ion lines provide the best detection limits.
4. The high temperature of the plasma ensures the complete breakdown of chemical
compounds (even refractory compounds ) and impedes the formation of other interfering
compounds, thus virtually eliminating matrix effects.
5. The ICP torch provides a chemically inert atmosphere and an optically thin emission
source.
6. Excitation and emission zones are spatially separated : this results in a low background.
The optical window used for analysis lies just above the apex of the primary plasma and
just under the base of the flame-like afterglow.
7. Low background, combined with a high S/N ratio of analyte emission, results in low
detection limits, typically in the parts-per-billion range.
Inductively coupled plasma optical emission spectrometry (ICP-OES) is a
major technique for elemental analysis. The sample to be analysed, if solid, is
normally first dissolved and then mixed with water before being fed into the
plasma.
Atoms in the plasma emit light (photons) with characteristic wavelengths for
each element. This light is recorded by one or more optical spectrometers and
when calibrated against standards the technique provides a quantitative
analysis of the original sample.
ICP. http://www.thespectroscopynet.com/techniques/ICP.HTM
Instrumental Lay-out
An ICP-OES instrument consists of a sample delivery system, an IC plasma to
generate the signal, one or more optical spectrometers to measure the signal,
and a computer for controlling the analysis.
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 device which produces the IC plasma is commonly referred to as the
ICP torch. It consists of two to four Argon flows depending on the
manufacturer:
Nebuliser gas (inner Argon flow), at about 1 L/min, carries the analyte
aerosol
Sheath gas (JY patent), for producing a laminar flow to improve low
excitation energy elements eg group I & II elements
Auxiliary gas (if present), lifts the plasma above the injector tube, used when
measuring organics
Plasma gas, at about 12-16 L/min, sets the plasma conditions, eg excitation
temperature
The argon and analyte flow into a toroidal radio frequency (RF) field,
usually at 40.68 MHz. The plasma is ignited by a Tesla spark.
Inductively coupled plasma source
Schematic diagram showing the different
regions in the IC Plasma
Temperature profile of an ICP.
Schematic diagram of an ICP torch.
Demountable Torch
The ICP torch must be kept clean and open. Periodically, then, the torch is
dismounted for cleaning.
ICP torch and components of an ICP torch
Author: Geoff Tyler, Jobin-Yvon Horiba, France , First published on the web: 15 May
2000. http://www.thespectroscopynet.com/techniques/ICP.HTM
Vaporization, Atomization and Excitation
Aerosol vapour is transported to the plasma
Vapour desolvates
Atomization occurs within the plasma
Atoms get excited to atomic and ionic states
Rich spectra produced because of presence of both atomic and ionic lines
Because of their different excitation energies, different emission lines will
have maximum intensities at different vertical positions in the plasma
The droplets coming from the nebuliser can vary
greatly in size, from less than 1 µm to more than
10 µm. Since droplets going into the IC plasma
should be kept below 5 µm in size, it is necessary
to remove the large droplets. This is done in a
spray chamber.
The liquid spray from the nebuliser enters the
spray chamber. By sheer size, the larger droplets
fall to the bottom of the chamber and exit through
the drain. The finer droplets in the vapour are
transported to the plasma.
Various types of spray chambers commonly used
are:
Scott
Cyclonic
Inert
Cooled
Low Volume
Spray chamber
Electrothermal atomization : the graphite furnace
The electrically heated furnace offers greater sensitivity than that afforded by flames and
requires a smaller volume of sample. The main part of the atomizer is a small graphite tube
about 5 cm in length and 1 cm in diameter. The maximum recommended temperature for a
graphite furnace is 2550oC for not more than 7 sec.
A graphite furnace atomizer.
Flame Atomic Absorption Spectrometry (FAAS)
In flame atomic absorption spectrometry, either an air/acetylene or a nitrous
oxide/acetylene flame is used to evaporate the solution and dissociate the sample into its
component atoms. When light from a hollow cathode lamp (selected based on the element
to be analyzed) passes through the cloud of atoms, the atoms of interest absorb the light
from the lamp. This is measured by a detector, and used to calculate the concentration of
that element in the original sample.
The use of a flame limits the excitation temperature reached by a sample to a maximum of
approximately 2600oC (with the N2O/acetylene flame). For many elements this is not a
problem. Compounds of the alkali metals, for example, and many of the heavy metals such
as lead or cadmium and transition metals like manganese or nickel are all atomized with
good efficiency with either flame type, with typical FAAS detection limits in the sub-ppm
range.
However, there are a number of refractory elements like V, Zr, Mo and B which do not
perform well with a flame source. This is because the maximum temperature reached, even
with the N2O/acetylene flame, is insufficient to break down compounds of these elements.
As a result, flame AAS sensitivity for these elements is not as good as other elemental
analysis techniques.
http://www.thermoelemental.com/instruments/aa/fundamental_aa.asp
Atomic absorption spectrophotometer
Single beam design
Double beam design
Background correction
Background signal arises from absorption, emission, or scatter by everything in the
sample besides analyte ((the matrix), as well as absorption, emission, scatter by the
flame, the furnace, or the plasma.
Background correction methods :
Beam chopping
D2 lamp ---------- The difference between absorbance measured with the hollow
cathode and absorbance measured with the D2 lamp is the
absorbance due to analyte.
Zeeman ---------- When an atomic vapor is exposured to a strong magnetic field (0.1
to 1 tesla), a splitting of electronic energy levels of the atoms
takes place, which leads to formation of several absorption lines
for each electronic transition.
Smith-Hieftje --- based on the self-reversal, or self-absorption, behavior of radiation
emitted from hollow cathode lamps when they are operated at high
currents.
A beam chopper for subtracting the signal due to flame background emission.
Resulting square-wave signal.
Interference
Types of interference
1) Spectral : unwanted signals overlapping analyte signal
2) Chemical : chemical reactions decreasing the concentration of analyte atoms
3) Ionization : ionization of analyte atoms decreases the concentration of neutral atoms.
Methodology :
Establishing a relationship between absorbance and concentration
Standard curve method
Standard addition method
Internal standard method
Standard calibration curve for copper.
Internal standard calibration curve.
Graphite Furnace Atomic Absorption Spectrometry (GFAAS)
This technique is essentially the same as flame AA, except the flame is replaced by a
small, electrically heated graphite tube, or cuvette, which is heated to a temperature up
to 3000oC to generate the cloud of atoms. The higher atom density and longer residence
time in the tube improve furnace AAS detection limits by a factor of up to 1000x
compared to flame AAS, down to the sub-ppb range. However, because of the
temperature limitation and the use of graphite cuvettes, refractory element performance
is still somewhat limited.
Features of the electrothermal (nonflame) atomizers
1. Only small amounts (10–8 to 10–11 g absolute) of analyet are required.
2. Solid can be analyzed directly, often without any pretreatment.
3. Small amounts of liquid samples, 5 to 100l, are needed.
4. Background noise is very low.
5. Sensitivity is increased because the production of free analyte is more efficient than
with a flame.
Detection limits
Flame, furnace, and ICP detection limits.
Detection limit of ICP-MS
ICP emission calibration curves for several metals.
The toxicological effect of metallic mercury: Tea party from Lewis Carroll’s Alice in
Wonderland.
Biological concentration of mercury in the environment.
Applications
Trace elements in a wide variety of aqueous matrices: drinking water, river,
lake and ground water, waste water and effluent, and seawater.
Trace elements in solids after digestion: sediment, soil, sludge, road dust, air
particulate matter, plant tissue and grain, rocks and minerals, etc.
Trace elements in samples of body fluids, including blood, plasma, and urine.
Isotope ratio measurement
Sample Requirements and Preparation
Minimum liquid sample volume required varies from a few l to a few mL,
depending on the number of elements to be determined and the concentration
levels required (i.e. if sample can be diluted). Ideally 10 ml.
Solid samples must be dissolved before they can be analyzed. (Note: when the
laser ablation accessory will be available, solid samples can be analyzed
directly).
Digestion facilities for solids are available.
Minimum solid sample size for dissolution varies from 0.1 mg to 20 mg,
depending on the sample matrix, the homogeneity of the sample, the digestion
method and the concentration level of the elements interested.
The content of the total dissolved solid (TDS) in the final solution to be
analyzed should be less than 1%. However, the high sensitivity of the HR ICPMS allows high dilutions, and hence the ability to analyze samples with high
TDS.
http://www.mri.psu.edu/mcl/icpms.htm
Summary
AAS
AES
Graphite furnace
Hollow cathode lamp
Atomizer
Nebuliser
ICP
Plasma
ICP-AES
ICP-MS
Q n A
Thanks
Home page
http://mail.swu.ac.kr/~dslee
Electronic mail
dslee@mail.swu.ac.kr