FLAME EMISSION SPECTROSCOPY

ATOMIC EMISSION SPECTROSCOPY

FLAME PHOTOMETRY

INTRODUCTION –
Atomic emission spectroscopy (AES) is a method of chemical analysis
that uses the intensity of light emitted from flame, plasma, arc, or spark
at a particular wavelength to determine the quantity of an element in a
sample. The wavelength of the atomic spectral line gives the identity of
the element while the intensity of the emitted light is proportional to the
number of atoms of the element. A frequent application of emission
spectrometry is measurement of alkali metals in pharmaceuticals.
PRINCIPLE -
A sample of a material (analyte) is brought into the flame as either a gas,
sprayed solution, or directly inserted into the flame by use of a small
loop of wire, usually platinum. The heat from the flame evaporates the
solvent and breaks chemical bonds to create free atoms. The thermal
energy also excites the atoms to higher electronic states that
subsequently emit light when they return to the ground electronic state.
Each element emits light at a characteristic wavelength, which is
dispersed by a grating or prism and detected in the spectrometer. Thus in
flame emission spectrometry, atoms in the form of atomic vapor are
created in a high temperature flame. A portion of the atomic vapor is
thermally or collisionally excited to higher electronic energy level. These
excited atoms return to ground state while emitting photons to create
sharpline emission spectra. The intensity of emission is directly
proportional to concentration of the analyte being aspirated. Hence a
calibration curve of emission intensity versus concentration can be
plotted to find unknown concentration of given sample.
INSTRUMENTATION
The emission spectrometer consists of following components –
1) Nebulizer
2) Burner
3) Atomizer
4) Filter or monochromator,
5) Detector
6) Readout devices

Dia : Schematic representation of a Flame Photometer

1) Nebuliser is a means of transporting a homogeneous solution into the
flame at a steady rate and converting it into a fine spray. 2) Burners are
used to generate the flames. In burners fuel and oxidants are burnt to give
flames of desired temperature.
3) Atomizers - Flame atomizers are widely used in emission
spectroscopy.
4) Filter or monochromator – Generally a grating or a prism
monochromator is used in emission spectroscopy. Its role is to disperse
the radiation coming from the flame.
5) Detector - The dispersed radiation from the monochromator goes to
the detector which measures intensity of radiation falling on it.
Photomultiplier tubes are widely used detectors.
6) Readout devices/amplifiers – The output from the detector is
suitably amplified and displayed on the readout device.

ATOMIZERS
1) PLASMA SOURCES –
INDUCTIVELY COUPLED PLASMA SOURCE – ICP Inductively
coupled plasma is used to produce excited atoms and ions that emit
electromagnetic radiation at wavelengths characteristic of a particular
element.
Inductively coupled plasma atomic emission spectroscopy (ICP-AES)
uses inductively coupled plasma to generate excited atoms. A plasma is
an electrically conducting gaseous mixture containing a significant
concentration of cations and electrons. Argon plasma is frequently used
for emission spectroscopy.

Dia :Schematic presentation of Inductively Coupled Plasma (ICP)

The ICP torch consists of a quartz torch. Radio frequency (RF)
generator surrounds part of this quartz torch. When the torch is turned
on, an intense electromagnetic field is created within the coil due to RF
generator. The argon gas flowing through the torch gets ignited and
initiate the ionization process. Thus the plasma is ignited.
The argon gas is ionized in the intense electromagnetic field and flows
towards the magnetic field of the RF coil. A stable, high temperature
plasma of about 7000 K is then generated as the result of the collisions
created between the neutral argon atoms and the charged particles.
A peristaltic pump delivers the sample into a nebulizer where it is
changed into mist and introduced directly inside the plasma flame. The
sample immediately collides with the electrons and charged ions in the
plasma and is itself broken down into charged ions. The various
molecules break up into their respective atoms which then lose electrons
giving off radiation at the characteristic wavelengths of the elements
involved.
Advantages of Inductively coupled plasma source :
1) excellent limit of detection
2) linear dynamic range
3) multi-element capability
4) low chemical interference
5) stable and reproducible signal.

Disadvantages :
1) spectral interferences (many emission lines)
2) cost

DIRECT CURRENT PLASMA SOURCE –

It is a power source employed in argon plasma spectroscopy. This dc
electrical source is capable of maintaining a current of several amperes
between electrodes immersed in a stream of argon. A direct-current
plasma (DCP) is created by an electrical discharge between two
electrodes. A plasma support gas is necessary and Argon is common.
Samples can be deposited on one of the electrodes. This sputtering
process is often referred to as glow-discharge excitation.

FLAME ATOMIZATION

In AES flames are widely used atomizers. Atomization is a process of

converting analyte molecules in sample solution to atomic state.
Nebulized samples are sprayed into a flame as fine droplets. Droplets
lose the solvent due to very high flame temperatures in a process called
desolvation and will be converted to solid aerosol which is volatilized to
form gaseous molecules. Gaseous molecules will then be atomized and
neutral atoms are obtained which can be excited by absorption of enough
energy.

2) FLAME ATOMIZERS
TYPES OF FLAMES
Flames can be classified into several types depending on fuel/oxidant
used. Significant variations in flame temperatures can be obtained by
changing the composition of fuel and oxidant. The temperatures
provided by the burning of natural or manufactured gas in air are so low
that only the alkali and alkaline-earth metals, with very low excitation
energies, produce useful spectra.
Acetylene/air mixtures give a somewhat higher temperature. Oxygen
or nitrous oxide must be employed as the oxidant in order to excite the
spectra of many metals; with the common fuels, temperatures of 2500 to
3100°C are obtained. The hottest practical flame results from the
combustion of cyanogen in oxygen.
STRUCTURE OF FLAME
Three well characterized regions can be identified in a conventional
flame.

Dia : Regions of flame

1) Primary combustion zone - A lower region, close to the burner tip,
with blue luminescence. This region is characterized by existence of
some non atomized species and presence of fuel species (C2 and
CH, etc.) that emit in the blue region of electromagnetic spectrum.
2) Interzonal region – This is second well defined region just above the
primary combustion zone. The interzonal region is rich in free atoms
and is the region of choice for performing atomic spectroscopy. It also
contains the regions of highest temperatures.
3) Secondary combustion region - The third region in the flame is the
outer region which is characterized by reformation of molecules as the
temperature at the edges is much lower than the core.

DIA :TEMPERATURE PROFILES OF FLAME –

The maximum temperature is located somewhat above the inner cone.
Clearly, it is important-particularly for emission methods-to focus the
same part of the flame on the entrance slit for all calibrations and
analytical measurements.
The flame temperature also determines the fraction of species that
exists in excited states, and thus influences emission intensities. The
Boltzmann equation permits calculation of this fraction of excited state.

FLAME ATOMIZERS FOR ATOMIC SPECTROSCOPY The most

common atomization device for atomic spectroscopy consists of a
nebulizer and a burner. The nebulizer produces a fine spray or aerosol
from the liquid sample, which is then fed into the flame. Two types of
widely used burners in AES are
1) Total consumption burner (turbulent flow)
2) Premixed (laminar flow) burner
3)
1) TOTAL CONSUMPTION BURNER (TURBULENT FLOW)

In a turbulent flow burner, the nebulizer and burner are combined into a
single unit. The sample is drawn up the capillary and nebulized by
venturi action caused by the flow of gases around the capillary tip.
Typical sample flow rates are 1 to 3 ml/min.
Advantages of Turbulent flow burners
 ✔ relatively large sample reaches the flame
✔ no possibility of flashback and explosion exists.
Disadvantages of total consumption or turbulent burners
✔ Relatively short flame path length
✔ Problems with clogging of the tip
✔ Noisy both from the electronic and auditory standpoint.

2) LAMINAR FLOW BURNER –

In Laminar Flow Burner, sample is nebulized by the flow of oxidant
past a capillary tip. The resulting aerosol is then mixed with fuel and
flows past a series of baffles that remove all but the finest droplets. As a
result of the baffles, the majority of the Sample collects in the bottom
of the mixing chamber where it is drained to a waste container. The
aerosol, oxidant, and fuel are then burnt in a slotted burner that provides
a flame which is usually 5 or 10 cm in length. Advantages of Laminar
flow burners
✔ relatively quiet flame
✔ significantly longer path length
✔ enhanced sensitivity and reproducibility
✔ less clogging
Disadvantages of laminar flow burners :
✔ Lower rate of sample introduction
✔ Possibility of selective evaporation of mixed solvents in the mixing
chamber which may lead to analytical uncertainties.
✔ Mixing chamber contains a potentially explosive mixture which
can be ignited by a flashback. This burner may be equipped with
pressure relief vents for this reason.

SAMPLE PREPARATION -
Flame and plasma sources are best suited for samples in the form of
diluted and filtered solution. Although a solid sample can be analyzed
by directly inserting it into the flame or plasma, they usually are first
brought into solution by digestion or extraction.
Chemical form of analyte often makes no difference since it will finally
be dissociated into atoms. Thus several elements can be determined from
various biological fluids like blood,plasma etc. Usually dilution with
water is required to avoid clogging of the burner.
In preparation of standards matrix matching is necessary. A simulated
solvent matrix must be used for quantitative analysis by AES. During
sample preparation chemical interferences can often be overcome by
simple addition of reagents. For example, sodium, potassium are added
to calcium standard to overcome ionization interference.
Organic samples can be solubilized on wet basis. These methods
guarantee the elimination of organic material and the quantitative
recovery of the analyte in the solution.
In ICP-AES, samples are carried into the torch by argon flowing through
the central quartz tube. Samples are introduced into the argon flow by
nebulizers.
Another method of introducing liquid and solid samples into a plasma is
by electrothermal vaporization. Here, the sample is vaporized in a
furnace.
CATIONIC, ANIONIC AND PHYSICAL INTERFERENCES IN
FLAME PHOTOMETRY –

1) CATIONIC INTERFERENCE –
Filter instruments give adequate resolution of the principal lines of
sodium (589 nm), potassium (767nm), lithium (671 nm) to allow assay
of each element free of interference from other.
When Na and Ca both are present in the sample, interference may occur
because λ of emission lines too close. Emission at 589 nm from high
concentration of sodium may interfere in the assay of calcium, when
measured at 626 nm, its principal wavelength of emission. But sodium
does not interfere when measured at less intense line of calcium i.e. 423
nm.

2) ANIONIC INTERFERENCE –
Anions do not emit light but some polyvalent ions decrease emission of
certain cations by forming less volatile salts in the flame e.g. solution of
calcium containing phosphate or sulphate emits less intensely than one
containing chloride. This is due to formation of less volatile calcium
phosphate or calcium sulphate which give fewer free excited calcium
atoms than an equimolar solution of calcium chloride.
The interference is avoided by adding excess of releasing agent e.g.
lanthanum chloride which releases calcium by competitive
complexation of lanthanum with phosphate or EDTA which complexes
with calcium and removes interference.

3) PHYSICAL INTERFERENCE –
For accurate results sample and standard solutions should be aspirated
into the flame at the same rate and should give identical dropsize in
nebulizer chamber. Substance that alter physical properties of sample
may affect flow rate and dropsize e.g. substances like sugar increase
viscosity of sample, reduce flow rate and dropsize .
Alcohols in high concentration increase emission because lower surface
tension reduce dropsize and increases thermal excitation. These effects
can be minimized by preparing standard solution containing same
concentration of interferent.
DIFFERENCE BETWEEN AAS AND AES
Atomic Absorption Spectroscopy is an atomic technique which
measures intensity of light absorbed by ground state atoms. Light source
is employed to cause excitation of atoms in AAS. Flame Emission
photometry (FP) is an atomic technique which measures the wavelength
and intensity of light emitted by atoms in a flame resulting from the drop
from the excited state (formed due to absorption of energy from the
flame) to lower states. No light source is required since the energy
imparted to the atoms comes from the flame. Thus, flame photometry is
different from atomic absorption spectroscopy (AAS).

APPLICATIONS OF AES -
1) Pharmaceuticals: In some pharmaceutical manufacturing processes,
minute quantities of a catalyst that remain in the final drug product
Determination of alkali and alkaline earth metals in pharmaceutical and
biological samples.
2) Clinical analysis: Analyzing metals in biological fluids and tissues
such as whole blood, plasma, urine, saliva, brain tissue, liver, muscle
tissue, semen.
3) Water analysis: Analyzing water for its metal content. •