Flame Photometry
Objectives
By the end of this session, students will be able to:
• Explain the principle of flame photometry
and instrumentation
• Outline the factors affecting the
intensity of flame emission
Atomic
Spectroscopy
To understand the relationship of these techniques to each
other, It is important to understand the atom itself and the atomic process
involved in each technique.
Practically, the ratio of the excited to ground state atoms
is extremely small. Therefore, The absorption spectrum is usually only
associated with transitions from the ground state to higher energy states
N1: No. of excited atoms N°: No. of ground state
atoms ΔE: excitation energy
K: Boltzmann constant
T: Temperature in kelvin
The process of excitation and decay to ground state is involved in the two
techniques of atomic spectroscopy.
We measure the energy absorbed or emitted and use it for
quantification process
Atomic
Spectroscopy Methods
- Atomic
Emission Spectroscopy (Flame Photometry) - Atomic
Absorption Spectrometry (AAS)
Atomic
Emission Spectroscopy (AES) (Flame Photometry)
Principle: Flame photometry is based upon those
particles that are electronically excited in the medium.
Flame: is the
source of excitation energy. (Low energy source).
Uses: Flame
photometry is used mainly for the determination of alkali metals and easily
excited elements (Na, K, Li, Ca, etc.) particularly in biological fluids and
tissues
Functions
of Flame
1. To convert the constituents of liquid sample into the
vapor state.
2. To decompose the constituents into atoms or simple
molecules:
M+ + e– (from flame)
-> M + hn
3. To electronically excite a fraction of the resulting
atomic or molecular species
M ->
M*
The flame is composed
of: a fuel gas and oxidant gas
|
Oxidant – Fuel |
Max. Temp. (oC) |
|
Air- propane |
1725 |
|
Air- acetylene |
2400 |
|
Oxygen- acetylene |
3100 |
|
Nitrous oxide-acetylene |
3000 |
|
Air-hydrogen |
2000 |
|
Oxygen-hydrogen |
2700 |
|
Air + argon -hydrogen |
1577 |
Factors
affecting intensity of flame emission:
1- The concentration of the analyte in solution
2- The rate at which excited atoms are formed in the flame.
3- The rate at which the sample is introduced into the
flame.
4- Temperature of the flame.
5- Composition of the flame.
6- The ratio of fuel to oxidant in the flame.
7- Solvent used to dissolve the sample.
The flame temperature
is the most important factor. Increase in flame temperature causes an increase
in emission intensity. This is controlled by composition of the flame.
High temperature flames should not be used for elements that
ionized easily e.g. Na, K, Li or Ce. However, high temperature flames are
generally favored for transition elements and alkaline earth metals.
Effect of the solvent
used to dissolve the sample; if the solvent is water the process is slow
and if it is organic solvent the process is fast and emission intensity is
increased.
It is therefore very important that calibration curves be
prepared using the same solvent.
The stochiometric ratio of fuel to oxidant in the flame must
be used, in which both fuel to oxidant are totally consumed.
The
nebulizer-burner system
To convert the test sample into gaseous atoms
Nebulizer produces
an aerosol of the test solution
Burner system –
the mixing of fuel and oxidant for flame
Types of burner
system
1. Pre-mix or laminar flow burner
2. Total consumption burner
1. Pre-mix
or laminar flow burner
Advantages
1. Homogenous flame
2. Suitable for AAS and AES as the pathway could be
increased
Disadvantages
Suffers from explosion hazards
2. Total
consumption burner
3 concentric tubes, the sample, fuel and oxidant only mix at
the tip of burner
Used mainly for FES (short path)
Advantages
1. Simple to manufacture
2. Allows a total representative sample to reach the flame
3. Free from explosion hazards
Disadvantages
1. Aspiration rate varies with different solvents
2. Suitable only for AES
Non Flame
Atomizers
For example: Heated
Granite Furnace
Sample evaporation→ time and temp. Controlled drying and
ashing
Advantages
1. Small samples are analysed
2. 1000-fold more sensitive than flame
3. Oven is adaptable for determination of solid samples
Disadvantages
1. Low accuracy
2. Low precision
3. More ionic interferences due to very high temp.
Monochromators
Detectors Analytical technique
As in UV photomultipliers
Choice of the
wavelength: of max. Sensitivity
and min. spectral interferences
Sample preparation:
It is very important to obtain the sample in a form of
solution, where the spectral and chemical interferences are absent
Demineralized dist. Water and very pure reagents are to be
used because of the high sensitivity of the technique
Because of the instability of the very dil. Solution, it is
advisable to dilute the soln just before use.
Several elements can be determined in blood, urine,
cerebrospinal fluid and other biological fluids by direct aspiration of the
sample after dilution with water.
Chemical
interferences: can often be
overcome by simple dilution with a suitable reagent solution e.g. serum is
diluted by EDTA solution for the determination of calcium in order to prevent
interference from phosphate.
Standard curves
Deviations from linearity may occur
Qualitative
analysis
Flame photometry is useful mostly for the detection of
elements in group I and II of the periodic table. The presence of certain
elements can be detected by the use of a filter or monochromator.
Quantitative
analysis
To perform quantitative analysis, the sample is introduced
into the flame and the intensity of radiation is measured. The concentration of
the emitting substance is then calculated from a calibration curve or using
standard addition method.
Application
of flame photometry in pharmaceutical analysis
1. Metals are major
constituents of several pharmaceuticals such as dialysis solutions, lithium
carbonate tablets, antacids and multivitamin – mineral tablets.
2. The elements Na,
K, Li, Mg, Ca, Al and Zn are among the most common elements subjected to
pharmaceutical analysis using flame emission technique.
3. Sodium and potassium levels in biological
fluids are difficult to analyze by titrimetric or colorimetric techniques.
Their analysis is very important for control of infusion and dialysis solutions
which must be carefully monitored to maintain proper electrolyte balance.
Advantages:
1. Flame emission is
the simplest and least expensive technique.
2. The analysis may be carried out without prior separation
as other components such as dextrose, do not interfere.
Summary
• Flame
photometry is an example of atomic and emission spectroscopy
• Nebulizing
burner systems are the unique components found in flame photometers.
• Premix or laminar flow type and complete
combustion type nebulizer – burner systems are the two types of burner systems
used in flame photometers
• Flame
photometry is used determine the elements of I and II group of periodic table
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