ATOMIC
SPECTROSCOPY
Atomic
spectroscopy deals with the line spectra from atoms. It can be of both types
either atomic emission or absorption spectra. In atomic emission, the atoms are
raised to excited state which will emit a specific line spectra when returns to
ground state while in case of absorption, the atoms in gaseous state absorbs a
specific wavelength of light. The line spectra generated from atoms depends on
the electron transitions in atoms. It is strictly limited by availability of
orbitals within an atom. The orbitals could not be overfilled. The transitions in
an atom tend do occur between closer energy levels. So, the most obvious wavelength
(l) are associated
with minimal energy changes. Each element has their own characteristic emission
and absorption lines which are generated during electronic transitions. Hence,
line spectra can be helpful to deduce electronic structure of an atom.
PRINCIPLE:
For
sample processing, atoms are volatilized (ashing) in a flame or electrothermally
in an oven so that, the elements will readily emit or absorb monochromatic
radiation at the appropriate wavelength (l).
The absorption or emission energy is directly proportional to number of atoms
however standard conditions are difficulties to achieve because of flame
instability and variation in temperature within the system. Different atoms
absorbs different wavelength so the line spectra should be selected according
to the atoms to be analyzed.
During
analysis some interference are encountered. The presence of alkali metals such
as sodium, potassium etc. may enhance emission in the sample while the presence
of certain elements like phosphate, silicate, aluminate suppress the emission.
They require very high temperature to excite and emit line spectra. Therefore,
repeated analysis with refined interfering substances and standards are
required in atomic spectroscopy. In this technique each sample is processed
with triplicate assays to overcome flame instability problems. Moreover the
samples are stored in polythene bottles as metal ions are both absorbed and
released by glass.
Absorption
or emission energy µ Number
of atoms
The atomic spectroscopy can be further classified to three
different types on the basis the mechanism of line spectra generated from the
atoms.
- Atomic Emission Spectroscopy
- A. Simple Flame Emission Spectroscopy
- B. Plasma Emission Spectroscopy
- Atomic Absorption Spectroscopy
- Atomic Fluorescence Spectroscopy
1. ATOMIC EMISSION SPECTROSCOPY
In atomic emission, a sample is subjected to a high energy,
thermal environment in order to produce excited state atoms, capable of
emitting light. The energy source can be an electrical arc, a flame, or more
recently, plasma (a gas becomes plasma when it is heated until it loses all
electrons leaving highly electrified collection of nuclei and free electrons.
The emission spectrum of an element exposed to such an energy source consists
of a collection of the allowable emission wavelengths, commonly called emission
lines, because of the discrete nature of the emitted wavelengths. This emission
spectrum can be used as a unique characteristic for qualitative identification
of the element. Atomic emission using electrical arcs has been widely used in
qualitative analysis. Emission techniques can also be used to determine how
much of an element is present in a sample. For a "quantitative"
analysis, the intensity of light emitted at the wavelength of the element to be
determined is measured. The emission intensity at this wavelength will be
greater as the number of atoms of the analyte element increases. The technique
of flame photometry is an application of atomic emission for quantitative
analysis.
A. SIMPLE FLAME EMISSION SPECTROSCOPY:
It is based on the
emission phenomenon when atoms are excited into the high temperature flaming.
It is simple and conventional technique for analysis of alkali and alkaline
earth metals.
WORKING
PRINCIPLE:
Simple Flame photometry relies upon emission phenomenon. The
compounds of the alkali and alkaline earth metals when dissociated in a flame,
some of the atoms produced are further excited to a higher energy level. When
these atoms return to the ground state they emit radiation which lies mainly in
the visible region of the spectrum. Each element emits radiation at a wavelength
specific for that element. The table below gives details of the measurable
atomic flame emissions of the alkali and alkaline earth metals in terms of the
emission wavelength and the color produced. The emitted radiation is then
measured.
Element
Emission Wavelength (nm) Flame Color
Sodium (Na) 589
Yellow
Potassium (K) 766
Violet
Barium (Ba) 554
Lime Green
Calcium (Ca) 622*
Orange
Lithium (Li) 670
Red
*Note:
Calcium is measured by using the calcium hydroxide band emission at 622 nm
as the Calcium main atomic emission occurs at 423nm.
INSTRUMENTATION:
A Simple Flame Emission Spectroscopy has following
different working parts.
- Sample delivery
- Nebulization
- Atomization
- Monochromatic system (Filter-interference type)
- Detector and data recording
Figure
1:
Optical diagram of a Simple Flame Emission Spectroscopy
1.
Sample Delivery:
The
device that introduces the sample into the flame plays a major role in
determining the accuracy of the analysis. The most popular sampling method is
nebulization of a liquid sample to provide a steady flow of aerosol into a
flame. Flame FES requires that the analyte be dissolved in a solution in order
to undergo nebulization. An introduction system for liquid samples consists of
three components:
1. A nebulizer that
breaks up the liquid into small' droplets,
2. An aerosol
modifier that removes large droplets from the stream, allowing only droplets
smaller than a certain size to pass, and
3. The flame or
atomizer that converts the analyte into free atoms.
Different
states of samples are processed in different ways.
Solid
samples: The
solid samples must be first dissolved into suitable solution and then analyzed.
The examples of solid samples include metals, alloys, soil, animal tissue,
plant material, fertilizers, ores, cement, bone ash etc.
Liquid
samples: The
liquid samples are directly analyzed. However if sample is concentrated, it is diluted
and if it is very dilute solution, then it is concentrated by evaporation
before analysis. E.g. Blood, urine, electroplating solutions, petroleum
products, wines, polluted water etc.
Gas
samples: In
conventional samples, metal components are changed to gaseous form. In hydride
analysis: some metal converted to a gaseous hydride form and analyzed. E.g.
Arsenic, selenium, uranium etc.
2.
Nebulization:
Pneumatic
nebulization is the technique used in most atomic spectroscopy determinations.
The sample solution is introduced through an orifice into a high- velocity gas
jet, usually the oxidant gas. The sample stream may intersect the gas stream in
either a parallel or perpendicular manner. Liquid is drawn through the sample
capillary by the pressure differential generated by the high-velocity gas
stream passing over the sample orifice. The liquid stream begins to oscillate,
producing filaments. Finally, these filaments collapse to form a cloud of
droplets in the aerosol modifier or spray chamber. In the spray chamber the
larger droplets are removed from the sample stream by mixer paddles or broken
up into smaller droplets by impact beads or wall surfaces. The final aerosol,
now a fine mist, is combined with the oxidizer/fuel mixture and carried into
the burner. Droplets larger than about 20 µm are trapped in the spray chamber
and flow to waste. The distribution of drop sizes is a function of the solvent
as well as the components of the sampling system. In AAS only a small
percentage (usually 2% or 3%) of the nebulized analyte solution reaches the
burner.
3.
Atomization:
The
atomization step must convert the analyte within the aerosol into free analyte
atoms in the ground state for FES analysis. Very small sample volumes (5-100 ml) or solid samples can be
handled by flameless electrothermal methods.
Flame
Atomizers: The
sequence of events involved in converting a metallic element, M, from a
dissolved salt, MX, in the sample solution to free M atoms in the flame is
carried out in flame atomizers. After the aerosol droplets containing metals
enter the flame, the solvent is evaporated, leaving small particles of dry
solid sample. Next, solid sample is converted to vapor form. Finally, a portion
of the MX molecules are dissociated to give neutral free atoms. The efficiency
with which the flame produces neutral analyte atoms is of equal importance in
all the flame techniques.
4.
Monochromatic system
Simple
color filters (interference type): a means of isolating light of the wavelength
to be measured from that of extraneous emissions.
5.
Photo-detector:
A
photo-detector similar to spectrophotometer is used as means of measuring the
intensity of radiation emitted by the flame.
APPLICATIONS:
- — Most applications of FES have been the determination of trace metals, especially in liquid samples.
- — It should be remembered that FES offers a simple, inexpensive, and sensitive method for detecting common metals, including the alkali and alkaline earths, as well as several transition metals such as Fe, Mn, Cu, and Zn.
- — FES has been extended to include a number of nonmetals: H, B, C, N, P, As, O, S, Se, Te, halogens, and noble gases.
- — FES detectors for P and S are commercially available for use in gas chromatography.
- — FES has found wide application in agricultural and environmental analysis, industrial analyses of ferrous metals and alloys as well as glasses and ceramic materials, and clinical analyses of body fluids.
- — FES can be easily automated to handle a large number of samples. Array detectors interfaced to a microcomputer system permit simultaneous analyses of several elements in a single sample.
- Widely used in clinical laboratories for determination of metals in body fluids, Acid diagnosis as well as valuable in monitoring of therapeutic regimes. Elements such as Na, K, Ca, Mg, Cd, Zn etc can be directly measured. However elements such as Cu, Pb, Fe, Hg etc require prior extraction for analysis.
B.
PLASMA EMISSION SPECTROSCOPY
Atoms
can be excited using the high energy levels associated with Inductively Coupled
Plasma (ICP) instead of a flame such a method of excitation is for a more
effective and permits the analysis of elements beyond the scope of simple flame
emission techniques, such as the refractory elements of boron, phosphorous and
tungsten. The high temperature eliminates many of interference effects and the instrumentation
is designed with series of photo multiplier tubes set for the different
emission wavelength (l) of spectroscopy
elements, permitting multi element analysis of samples. The method is very
sensitive and specific.
The
ICP discharge is caused by the effect of a radio frequency field on argon gas
flowing through a quartz tube. The high power frequency causes a changing
magnetic field in the gas and this in turn result in a heating effect. Temperatures
of 9000-10000°C can be
produced in this way.
Inductively
Coupled Plasma: ICP
is a type plasma sources in which the energy is supplied by electrical current
which are produced by electromagnetic induction that is by time varying
magnetic field.
When
a time varying electric current is passed through the coil, it creates a time
varying magnetic field around it which in turn induces azimuthal (an angular
measurement in spherical coordinates) electric currents in the rarefield gas
leading to break down and formation of a plasma. Argon is commonly used
rarefied gas. The temperature of the plasma ranges between 6000-10000°C. ICP discharges high electron
density of 1015 cm-1. It is relatively free of
contamination because electrodes are completely outside the reaction chamber.
Plasma:
It
is a gas in which a certain proportion of its particles are ionized. The
presence of a non negligible number of charge carriers makes the plasma
electrically conductive so that it responds strongly to electromagnetic field. Plasma
therefore is quite unlike those of solids, liquids or gases-considered to be a
distinct states of matter. Like gaseous state, plasma doesn't have a definite
shape and volume unless enclosed in a container. Unlike gas, in the influence
of a magnetic field it may form structures such as filaments, beams and double
layers. Some examples of plasma are fire, lightening and the corona of the sun.
2. ATOMIC ABSORPTION SPECTROSCOPY
If light of just the right wavelength impinges on a free,
ground state atom, the atom may absorb the light as it enters an excited state
in a process known as atomic absorption. Atomic absorption measures the amount
of light at the resonant wavelength which is absorbed as it passes through a
cloud of atoms. As the number of atoms in the light path increases, the amount
of light absorbed increases in a predictable way. By measuring the amount of
light absorbed, a quantitative determination of the amount of analyte element
present can be made. The use of special light sources and careful selection of
wavelength allow the specific quantitative determination of individual elements
in the presence of others. The atom cloud required for atomic absorption
measurements is produced by supplying enough thermal energy to the sample to
dissociate the chemical compounds into free atoms. Aspirating a solution of the
sample into a flame aligned in the light beam serves this purpose. Under the
proper flame conditions, most of the atoms will remain in the ground state form
and are capable of absorbing light at the analytical wavelength from a source
lamp. The ease and speed at which precise and accurate determinations can be
made with this technique have made atomic absorption one of the most popular
methods for the determination of metals.
INSTRUMENTATION:
An Atomic Absorption Spectroscopy has following different
working parts.
- Source of Radiation
- Modulator / Chopper
- Atomization / Nebulization
- Monochromatic system (Filter-interference type)
- Detector and data recording
Figure 2: Optical diagram of an
atomic absorption spectroscopy
1. Source of Radiation:
The absorption bands due to atoms are very narrow and the
use of white light as the incident radiation would swamp even the best
monochromating system with unabsorbed radiation on either side of the
absorption band. It is fundamental, therefore to the technique of atomic
absorption spectroscopy that the incident radiation is of the correct
wavelength, bandwidth and intensity. This radiation is produced by a lamp in
which the cathode is coated with atoms of the element under investigation and
which emits radiation of precisely the same wavelength as that which will be absorbed
by non-excited atoms of the same element in the flame.
A hollow cathode lamp consists of two electrodes sealed in
a glass envelop filled with an inert gas, usually argon or neon. The end window
of the lamp must be of an appropriate material in order to transmit the emitted
radiation and is either quartz or silica. The cathode of the lamp is usually
cup shaped is either made of the element whose spectrum is required or coated
with the element, and the application of a potential of 300-500V is usually
required to cause excitation of the atoms and discharge of the appropriate
radiation. Lamps are available with cathodes which contain two or more elements
with emission lines that are easily distinguishable. These are such
multi-elements lamps that tend to give less satisfactory performance than
single element lamps. For certain elements, electrodeless discharge lamps have
been designed in which the excitation of atoms is achieved by radio frequencies
that induce resonance effects and the energy liberated causes vaporization and
excitation of the element.
2. Moduator:
The flame, as well as containing the unexcited atoms of the
element will also emit radiation due to the thermal excitation of a small
proportion of atoms, and it is essential that the detector is capable of
distinguishing between the identical radiation that is transmitted by the flame
and that emitted from the flame. This is achieved by introducing a
characteristic signal or modulation into the incident radiation by means of a
rotating segmented mirror (Chopper) or an electrically induced pulse. This
pulsed beam is detected as an alternating signal, which is superimposed on the
relatively constant signal generated by the emission from the flame. The differences
between the two signals are automatically measured by the instrument and only
the light emitted from the flame is recorded.
3. Atomization / Nebulization:
The design of the burner head and the method of atomization
of the sample both influence the sensitivity which can be achieved. The burner
head is designed to give a long narrow flame so that as many atoms as possible
are presented in the light path. It needs to keep spotlessly clean to
minimize background emission and the
position in the light path has to be adjusted to give maximum sensitivity, the
precise position depending not only on the gases being burnt but also on their
flow rate. The proportion of fuel to oxidant alters the characteristic of the
flame, a high proportion of fuel resulting in a flame with reducing properties
while as excess of oxidant gives an oxidizing flame. For each element analyzed
the optimum proportion must be determined.
The basic design of an atomizer is the same as that for
flame emission spectroscopy. The method of producing an aerosol involves
spraying the sample in air or oxidant gas. The larger drops precipitates ion
the baffles of the expansion chamber and flow to waste. The fuel gas is introduced
and the components are mixed before passing to the burner.
4. Monochromatic systems:
High quality monochromatic systems are necessary to isolate
the required emission line of the element from those emissions lines due to the
gases that are also present in the lamp. Owing to the very narrow bandwidth of
atomic emission lines, it is not adequate simply to select the required
wavelength using the monochromator.
5. Detector and data recording:
The sample is introduced into the cuvette using a micropipette.
The first step in the analysis is the heating of the sample (Ashing) to remove
the solvents and to destroy the matrix. When ashing is complete, the
temperature of the cuvette is rapidly raised to the atomizing temperature,
which is usually about 1000°C higher than the ashing temperature. This causes
vaporization of the sample and a resulting transient increase (2-5 seconds) in
the absorbance. The detector in atomic absorption spectrophotometer then
detects and records this maximum absorbance value.
Differences
between Atomic absorption and Flame Emission spectroscopy
— Excited atoms quickly emit a photon of light in Emission
spectroscopy.
— Signal is due to difference between the intensity of source
without and with sample in optical path.
— Emission intensity depends upon no. of exciting atoms, influenced
by temperature variation.
— In atomic absorption spectroscopy, relation between
absorbance and concentration is linear.
— Applications are more wide than
that of simple flame emission spectroscopy.
Advantages
of Atomic absorption spectroscopy:
— Specific technique
— Higher signal in atomic
absorption
— Detection limits are almost
similar
— More sensitive by flame emission:
Al, Ba, K, Ca, Eu, Ho, In, La
— Equally sensitive: Cr, Cu, Mo, Pd,
Rh, Ni, V
— More sensitive by absorption: Ag, As, Au, B, Bi, Cd, Co, Fe
3. ATOMIC FLUORESCENCE SPECTROSCOPY
The third field of atomic spectroscopy is atomic
fluorescence. This technique incorporates aspects of both atomic absorption and
atomic emission. Like atomic absorption, ground state atoms created in a flame
are excited by focusing a beam of light into the atomic vapor. Instead of
looking at the amount of light absorbed in the process, however, the emission
resulting from the decay of the atoms excited by the source light is measured.
The intensity of this "fluorescence" increases with increasing atom
concentration, providing the basis for quantitative determination. The source
lamp for atomic fluorescence is mounted at an angle to the rest of the optical
system, so that the light detector sees only the fluorescence in the flame and
not the light from the lamp itself. It is advantageous to maximize lamp
intensity since sensitivity is directly related to the number of excited atoms
which in turn is a function of the intensity of the exciting radiation.
