Transcript Atomic Spectroscopy

Atomic Absorption
Spectroscopy
Lecture 13
1
Flame Absorbance profiles
The temperature of a flame depends on the position from
its tip.
 So, it is necessary to concentrate our work on one
spot in a flame and preferably adjust the height of the
flame to get best signal.
 In fact, not all elements require a specific height above
burner tip but rather each element has its own
requirements which largely reflect some of its
properties.
 For example:
For Silver (Ag) analysis:
Using higher distances from the tip so that higher
temperatures are achieved to analyze for silver.
This
is possible since silver will not be easily oxidized.
2
3
For Chromium analysis:
Occurs at lower heights (fuel rich flames) since at
higher heights oxygen from atmosphere will force
chromium to convert to the oxide which will not be
atomized at flame temperatures.
For magnesium analysis:
Increasing the height above tip will increase the signal
due to increased atomization at higher temperatures.
However, at higher distances the oxide starts to form
leading to a decrease in signal.
4
Flame Atomizers (Continuous Atomizers)
There are several types of flame atomizers available.
1) The turbulent flow burner:
The simplest type that is very similar to conventional
Bunsen burner.
Disadvantages:
 This type of burner suffers from fluctuations in
temperature since there is no good mechanism for
homogeneous mixing of fuel and oxidant.
 The drop size of nebulized sample is also
inhomogeneous which adds to fluctuations in signal.
 The path length of radiation through the flame is
small which suggests a lower sensitivity of the
technique.
 Turbulent flow burners are also susceptible to
flashback.
5
2) Laminar flow burner (also called premix burner) :
The
drawbacks of the turbulant burnaer are
overcome using this most widely used burner.
Advantages:
 Quite flames obtained.
 Long path length are obtained.
 Flashback is avoided.
 Very homogeneous mixing between fuel, oxidant,
and droplets take place.
 Larger droplets are excluded and directed to a
waste container.
6
A schematic representation of the burner
7
8
9
10
11
Fuel and Oxidant Regulators
The adjustment of the fuel to oxidant ratio and
flow rate is undoubtedly very crucial.
1) Stoichiometric ratios are usually required.
2) Optimization is necessary in order to get
highest signal.
In the determination of metals that form stable
oxides:
A flame with excess fuel is preferred in order to
decrease oxide formation.
12
Performance Characteristics of
Flame Atomizers
Reproducibility of flame methods are usually superior to
other atomization techniques.
However, the residence time of an atom in a
flame is in the order of 10-4 s which is very
short.
This is reflected in a lower sensitivity of flame
methods as compared to other methods.
Also, conventional flames with reasonable
burning velocities can produce relatively low
temperatures which make them susceptible to
interference from molecular species.
13
2. Electrothermal Atomization
Advantages:
These have better sensitivities than flame
methods.
Explained by:
 A longer atom residence time is achieved (can
be more than 1 s).
 Also, atomization of the whole sample in a
very short time.
Procedure:
As the name implies, a few mL of the sample are
injected into the atomization chamber (a cylinder
of graphite coated with a film of pyrolytic
carbon).
14
15
Electrothermal Atomization processes:
a. Evaporation:
The solvent associated with the sample is evaporated
in a low temperature (~120 oC) slow process (seconds).
b. Ashing:
Sample is ashed to burn organics associated with the
sample at moderate temperatures (~600 oC, seconds).
c. Atomization:
The current is rapidly increased after ashing so that a
temperature in the range from 2000-3000 oC is obtained
in less than1 second.
16
Electrothermal Atomizers (Discrete
Atomizers)
The cylindrical graphite tube:
 The heart of the atomizer.
 Contain efficient heating elements and electronics.
 Cylindrical graphite tube opened from both ends
and has a central hole for sample introduction.
What type of Graphite:
It was found that porous graphite results in
poor reproducibility since some of the
analyzed materials will diffuse through porous
graphite and will thus lead to a history effect.
17
18
Pyrolytic graphite :‫النوع المفضل‬
The cylindrical graphite is made from a special
type of nonporous high quality graphite.
Description:
 The length of the cylinder is 2-5 cm.
 The diameter is less than 1 cm.
 When the tube is fixed in place electrical
contacts are achieved which are water cooled.
 Two inert gas streams (argon) flow at the
external surface and through the internal space
of the tube to prevent oxidation and clean the
tube after each measurement.
 Usually, samples are analyzed in triplicates
where three consecutive reproducible signals
19are required for each sample.
20
Atomic Absorption
Spectroscopy
Lecture 14
21
Performance Characteristics of
Electrothermal Atomizers
Advantages:
 Electrothermal atomization is the technique of
choice in case of small sample size.
 Higher sensitivities than flames are ordinarily
obtained.
Disadvantages:
 The analysis time is in the few minutes range.
 The relative precision is in the range of 5-10% as
compared to 1% in flame methods.
 In addition, the linear dynamic range is usually small
(~ two orders of magnitude) which requires extra
sample manipulation.
 It may be also mentioned that better experienced
22 personnel can achieve the merits of the technique.
Atomic Absorption Instrumentation
Atomic absorption instruments consist of:
1) A source of radiation.
2) A monochromator.
3) A flame or electrothermal atomizer in which
sample is introduced.
4) and a transducer.
23
1) Radiation Sources
1) Radiation in the UV-Vis region is required.
2) We can not use broad band sources.
Because:
 The best monochromators can not provide a
bandwidth that is narrower than the atomic
absorption line.
 If the bandwidth of the incident radiation is wider
than the line width, measurement will fail as
absorption will be only a tiny fraction of a large
signal which is difficult to measure and will result in
very low sensitivities (Figure a).
 Therefore, line sources with bandwidths narrower
than that of the absorption lines must be used.
24
X
25

 This can be achieved by using a lamp
producing the emission line of the element
of interest where analyte atoms can absorb
that line.
 Conditions are established to get a narrower
emission line (from lamp) than the
absorption line.
 This can in fact be achieved by getting an
emission line of interest at the following
conditions:
26
Conditions for getting an emission line of interest:
1.Low temperatures:
To decrease Doppler broadening (which is easily
achievable since the temperature of the source is
always much less than the temperature in flames).
2. Lower pressures:
This will decrease pressure broadening and will
thus produce a very narrow emission line.
27
Difficulty:
 Separate lamp for each element is neede for a
which is troublesome and inconvenient.
Overcome:
 However, recent developments lead to
introduction of multielement lamps.
Requirement:
 In this case, the lines from all elements
should not interfere and must be easily
resolved by the monochromator so that, at a
specific time, a single line of one element is
leaving the exit slit .
28
Types of Line sources:
1)Hollow Cathode Lamp (HCL)
 This is the most common source in atomic absorption
spectroscopy.
 It is formed from a tungsten anode and a cylindrical
cathode the interior surface of which is coated by the
metal of interest.
 The two electrodes are usually sealed in a glass tube
with a quartz window and filled with argon at low
pressure (1-5 torr).
 Ionization of the argon is forced by application of about
300 V DC where positively charged Ar+ heads rapidly
towards the negatively charged cathode causing
sputtering.
 A portion of sputtered atoms is excited and thus emit
photons as atoms relax to ground state.
 The cylindrical shape of the cathode serves to
concentrate the beam in a limited region and enhances
redeposition of sputtered atoms at the hollow surface.
29
30
31
Properties:
 High potentials usually result in high
currents which, in turn, produce more
intense radiation.
 However, Doppler broadening increases as
a result.
 In addition, the higher currents will produce
high proportion of unexcited atoms that will
absorb some of the emission beam which is
referred to as self absorption (a lower
intensity at the center of the line is observed
in this case).
32
1) Electrodeless Discharge Lamps (EDL)
 Is a sealed quartz tube containing a few torr of an
inert gas and a small quantity of the metal of interest.
 Excitation of the metal is achieved by a
radiofrequency or a microwave powered coil through
ionization of argon, due to high energetic
radiofrequency.
 Ionized argon will hit the metal causing excitation of
the atoms of the metal of interest.
 The output power of the EDL lamp is higher than the
HCL lamp.

 However, compared to HCL lamps, EDL lamps are
33 rarely used.
34
35
Emission in Flames
There can be significant amounts of emission
produced in flames: due to
- Presence
of
flame
constituents
(molecular
combustible products).
- Sometimes impurities in the burner head.
This emitted radiation must be removed for successful
sensitive determinations by AAS.
 What happened If it not removed??
1) In case of absorption measurments:
A negative error will always be observed.
We can visualize this effect by considering the
schematic below:
36
37
The detector will see the overall signal:
Which is the power of the transmitted beam (P) in addition
to the power of the emitted radiation from flame (Pe).
Therefore if we are measuring absorbance, this will result
in a negative error as the detector will measure what it
appears as:
A high transmittance signal (actually it is P + Pe).
38
Explanation:
Absorbance is defined as:
A = log (Po/P)
However, in absence of a sample the detector will
measure S1 , where:
S1 = Po + Pe
In presence of a sample, the detector will measure S2,
where:
S2 = P + Pe
Therefore A = log (Po + Pe)/(P + Pe)
At high absorbances, Pe may become much larger than P
and the absorbance will be a constant since both Po and
Pe are constants:
A = log (Po + Pe)/(Pe)
39
2) In case of emission measurements:
Here will always be a positive error since emission
from flame is an additive value to the actual sample
emission.
It is therefore obvious that we should get rid of this
interference from emission in flames.
40
Correction method for background of flame emission
Source Modulation
For excluding the emission signal from flames
can easily.
1) Can be done by an addition of a chopper to
the instrumental design.
What is the chopper:
The chopper is a motor driven device that has
open and solid (mirrors in some cases)
alternating regions as in the schematic:
41
42
The function of the chopper:
Is to chop the light leaving the source so that when
the incident beam hits the chopper at the solid
surface, the beam will be blocked and detector will
only read the emitted signal from the flame.
As the chopper rotates and the beam emerges to
the detector, the detector signal will be the sum of
the transmitted signal plus that emitted from the
flame.
The signal processor will be able to subtract the
first signal from the second one, thus excluding
the signal from emission in flames.
43
Beam chopper for
subtracting flame
background emission
Lamp and flame (a)
emission reach detector
Only flame (b)
emission reaches
detector
(c) Resulting signal
This can be represented by the following
equations:
Signal 1 (Blocked Beam) = Pe
Signal 2 (Transmitted Beam) = P + Pe
Overall Difference Signal = (P + Pe) - Pe = P
(Corrected Signal)
This correction method for background
emission in flames is called source
modulation.
45
The schematic of the AAS instrument with source
modulation correction can be represented by the
following schematic:
46
It should be recognized that addition of extra
components to an instrument will decrease the
signal to noise ratio and addition of a moving
component is usually regarded as a disadvantage
due to higher need for maintenance.
2) Another procedure:
Which can overcome the emission from flames is
to use a modulated power supply that will give
fluctuating intensities at some frequency (say for
example pulsed radiation at a specific frequency).
47
The emission from flames is a continuous signal.
 But that from the source is modulated.
Now if we use a high pass RC filter, only the fluctuating
signal will be measured as signal while the DC signal
will be considered zero as it can not pass through the
electronic filter.
The high pass RC filter:
Is a device which uses a resistor and a capacitor the
impedance of which is inversely proportional to the
frequency of the modulated signal.
Therefore, only high frequencies will have low
impedance and can pass through the capacitor while
signals of low frequencies will suffer very high
resistance and will not be able to go through the
capacitor.
48
AAS Instruments
Instruments in AAS can be regarded as 1) Single
or 2) double beam instruments.
1)
Single
Beam
spectrophotometers.
Atomic
Absorption
A single beam instruments is the same as the
one described above (source modulation
section) or generally:
49
The term “spectrophotometer” implies that:
1- The instrument uses a dispersive monochromator
(containing a prism or a grating).
2) Also, the detector is a photomultiplier tube in most
cases.
50
51
52
2) Double Beam Atomic Absorption
Spectrophotometers
In this type of instruments:
- The incident beam is split into two beams of equal
intensity by a chopper with the solid surface being a
mirror.
- One of the beams will traverse the sample in the
atomizer while the other is considered as a
reference.
- Detector signals will be consecutive readings of
both the reference and sample beams.
- The ratio of the reference to the sample beams is
recorded to give the final signal.
53
A schematic representation of a double beam
instrument is shown below:
54
It should be emphasized here that in the absence
of sample:
 Pr is not equal to P since the reference beam
traverses through air while the other beam
traverses through the flame.
 In flames, particulates and molecular species
scatter and absorb a portion of incident radiation,
which results in a lower intensity of the beam.
 To act as a real double beam, The AA
spectrophotometer reference beam should pass
through a reference flame.
55
But even if we do that:
There are no guarantees that both beams will
be of equal intensities,,,
Because it is almost impossible to obtain
exactly equivalent flames.
It is therefore important to understand that:
The excellent features of a double beam
configuration are not achievable in AAS
instrumentation.
56
Atomic Absorption
Spectroscopy
Lecture 16
57
Interferences in Atomic Absorption
Spectroscopy
There are two major classes of interferences in AAS:
1)The first class: Spectral interferences.
Related to spectral properties of components other than
atomized analyte.
2) The other class: Chemical interferences.
Related to the chemical processes occurring in flames
and electrothermal atomizers and their effects on signal.
*These are usually more important than spectral
interferences.
58
First: Spectral Interferences
1. Spectral line Interference
Usually, interferences due to overlapping lines
is rare since atomic lines are very narrow.
In cases of line interference, it can be simply
overcome by:
Choosing to perform the analysis using
another line that has no interference with other
lines.
Therefore, line interference is seldom a
problem in atomic spectroscopy.
59
2. Scattering
Caused by particulates from combustion products and
sample materials which lead to scatter radiation that
will result in positive analytical error.
The error from scattering can be corrected for:
By making a blank measurement.
Scattering phenomenon is most important when
concentrated solutions containing elements that form
refractory oxides (like Ti, Zr, and W) are present in
sample matrix.
60
Types of particles that make scattering:
1) Metal oxide particles with diameters larger than the
incident wavelength (a real problem).
2) Samples containing organic materials or organic
solvents which can form carbonaceous (especially
in cases of incomplete combustion) particles.
61
3. Broad Band Absorption
Broad band spectrum limit the sensitivity of the
technique.
Resulted from:
Molecular species from combustion products or
sample matrix that formed in flames or electrothermal
atomizers.
Spectral interferences by matrix products are not
widely encountered in flame methods.
Largely overcome by:
1) Adjusting various experimental conditions like
fuel/oxidant ratio or temperature.
62
2) Using radiation buffer :
Much higher concentration of interferent is used than
that initially present in sample material, in both sample
and standards.
The contribution from sample matrix will thus be
insignificant.
Spectral interferences due to matrix are severe in
electrothermal methods and must thus be corrected
for.
63
Background Correction Methods
a.
The Two Line Correction Method:
In this method, a reference line from the source
(from an impurity in cathode or any emission
line) is selected where this line should have the
following properties:
1.
Very close to analyte line
2.
Not absorbed by analyte
If such a line exists, since the reference line is
not absorbed by the analyte, its intensity
should remain constant throughout analysis.
64
65
However, if its intensity decreases:
This will be an indication of absorbance or
scattering by matrix species.
The decrease in signal of the reference line is
used to correct for the analyte line intensity (by
subtraction of the absorbance of the reference
from that of the analyte).
This method is very simple but unfortunately it
is not always possible to locate a suitable
reference line.
66
b. The Continuum Source Method
Is the most common method although, for
reasons to be discussed shortly, it has major
drawbacks and fails a lot.
In this technique:
 Radiation from a deuterium lamp and a HCL
lamp alternately pass through the graphite
tube analyzer.
 It is essential to keep the slit width of the
monochromator sufficiently wide in order to
pass a wide bandwidth of the deuterium
lamp radiation.
67
In this case, the absorbance by analyte
atoms is negligible and absorbance can
be attributed to molecular species in
matrix.
The absorbance of the beam from the
deuterium lamp is then subtracted from
the analyte beam (HCL) and thus a
background correction is obtained.
68
69
70
Problems Associated with Background Correction Using
D2 Lamp
1. The very hot medium inside the graphite
tube is inhomogeneous and thus signal is
dependent on the exact path a beam would
follow inside the tube.
Therefore, exact alignment of the D2 and HCL
lamps should be made.
2. The radiant power of the D2 lamp in the
visible is insignificant which precludes the
use of the technique for analysis of analytes
in the visible region.
3. Addition of an extra lamp and chopper will
decrease the signal to noise ratio.
71
c. Background Correction Based on Zeeman
Effect
Theory: Zeeman has observed that:
When gaseous atoms (but not molecules) are placed
in a strong magnetic field (~ 1 tesla), splitting of
electronic energy levels takes place.
The simplest splitting:
One energy level results in three energy levels:
- One at a higher energy, (- s satellite)
- Second at a lower energy (+ s satellite lines)
- Third remains at the same energy as the level in
absence of the magnetic field (central p line).
Furthermore, the p line has twice the absorbance of a s
line and absorbs polarized light parallel ║to direction of
the magnetic field while the two s lines absorb light
72
perpendicular
┴ to magnetic field.
┴
║
┴
73
 Light from a HCL lamp will pass through a
rotating polarizer that passes polarized light
parallel to external magnetic field at one cycle
and passes light perpendicular to field in the
other cycle.
 The idea of background correction using this
method :
Is to allow light to traverse the sample in the
graphite furnace atomizer and record the
signal for both polarizer cycles using the
wavelength at the p line.
75
76
a. First cycle:
Light parallel to field; the p line of the analyte
absorbs in addition to absorbance by matrix
(molecular matrix absorb both polarized light
parallel or perpendicular to field)
Signal a = Ap + Amatrix
b. Second cycle:
Light perpendicular to field; the p line of
analyte will not absorb light perpendicular to
field and s lines will also not affect absorbance
at the p line wavelength.
Only matrix will absorb.
77
Signal b = AMatrix
The overall signal is the difference of the two
signals = Ap
Therefore, excellent background correction is
achieved using the Zeeman effect.
This background correction method results in
good correction and is usually one of the best
methods available.
78
Zeeman effect correction
• B - Rotating polarizer
• E – absorption at // polarization
• Very sensitive correction technique
Atomic Absorption
Spectroscopy
Lecture 17
80
Second: Chemical Interferences
These are interferences resulting from chemical
processes occurring in flames and electrothermal
atomizers and affect the absorption signal.
Chemical processes occurring in flames:
 The burnt gases as behaving like a solvent.
 This is necessary since our knowledge of
gaseous state reaction equilibria is rather
limited.
81
Chemical interferences include three major processes:
1. Formation of Compounds of Low Volatility
1) Anionic species: (-ve error)
Forming compounds of low volatility are the most
important.
Decrease the sensitivity.
For example: the absorption signal of calcium will be
decreased as higher concentrations of sulfate or
phosphate are introduced.
2) Cations species: (-ve error)
Forming combined products with the analyte will also
decrease the signal obtained for the analyte.
For example: aluminum forms a heat stable compound
with magnesium.
82
Elimination of Low Volatility Compounds
1. Addition of a releasing agent: cations that can
replace the analyte (preferentially react with the
anion).
In this case the analyte is released from the compound
of low volatility and replaced by the releasing agent.
Examples of RA:
Lanthanum or strontium are good releasing agents in
the determination of calcium in presence of phosphate
or sulfate.
Also, lanthanum or strontium are good releasing agents
in the determination of magnesium in presence of
aluminum since both can replace magnesium.
83
2. Addition of a protective agent: organic
ligands that form stable volatile species
with analytes are called protective agents.
Example:
EDTA and 8-hydroxyquinoline which will form
complexes with calcium even in presence of
sulfate and phosphate or aluminum.
3. Use of higher temperature: is the simplest
procedure to try if it is possible
84
2. Dissociation Equilibria
Dissociation reactions occur in flames:
where the outcome of the process is
desired to produce the atoms of analyte.
For
example:
metal
oxides
and
hydroxides will dissociate in flames to
produce the atoms as in the equations
MO = M + O
M(OH)2 = M + 2 OH
85
Remember that: we are not working in solution
to dissociate the compounds into ionic
species.
In fact: not much is known about equilibrium
reactions in flames.
Alkaline earth oxides and hydroxides are
relatively stable and will definitely show
characteristic broad band spectra (more
intense than line spectra), except at very high
temperatures.
Alkali metals oxides and hydroxides: Show
opposite behavior for which are instable even
at lower flame temperatures and thus produce
line spectra.
86
An equilibrium can be established for the
dissociation of compounds containing atoms
other than oxygen, like NaCl where:
NaCl = Na + Cl
Now, if the signal from a solution of NaCl was
studied in presence of variable amounts of Cl
(from HCl, as an example):
The signal will be observed to decrease as the
concentration of Cl is increased; a behavior
predicted by the Le Chatelier principle in
solutions.
87
The same phenomenon is observed when a
metal oxide is analyzed using a fuel rich flame
(low O2 ) or a lean flame (high O2).
Signal will be increased in fuel rich flames
since the dissociation of metal oxides is easier
due to less oxygen while the opposite takes
place in lean flames (oxygen rich).
88
A good example on dissociation equilibria can
be presented for the analysis of vanadium in
presence of aluminum and titanium:
Fuel rich flames:
Result in higher absorbance signal for
vanadium since the little oxygen present in
flames will be mainly captured by Al and Ti,
thus more V atoms are available.
However, in lean flames:
Excess oxygen is present and thus vanadium
will form the oxide and addition of extra Ti and
Al will not affect the signal.
89
3. Ionization Equilibria
Ionization in fuel/air flames is very limited due
to relatively low temperatures.
However, in fuel/nitrous oxide or fuel/oxygen
mixtures, ionization is significant.
Therefore, at higher temperatures an important
portion of atoms can be converted to ions:
M = M+ + e
K = [M+][e]/[M]
90
Ionization in flames:
Lead to the decrease in absorption signal for
alkali metals at very high temperatures.
After signal increase by increasing Temp. it
starts to decrease as temperature is further
increased; a consequence of ionization.
Therefore:
Usually lower flame temperatures are used for
determination of alkali metals.
Ionization suppressor:
A material that is added to samples in order to
produce large number of electrons, which
91
results
in higher sensitivities.
92
Practical Details in AAS
Sample Preparation:
- The need for introduction of samples in the
solution form.
- This necessitates the dissolution of the
sample (requires very good experience).
- Care taken in order not to lose any portion of
the analyte.
- Make sure that the reagents, acids, etc. used
in the dissolution and pretreatment of the
sample are free from analyte impurities.
93
For direct introduction of sample to an
electrothermal
atomizer
without
pretreatment (like serum samples),
definitely, electrothermal atomizers will
have an obvious advantage over flame
methods which require nebulization.
94
Organic Solvents
1. Increased nebulization rate due to lower
surface tension of organic solvents which
produces smaller droplets as well as faster
evaporation of solvents in flames will result
in better sensitivities.
2. Immiscible organic solvents containing
organic ligands are used to extract metal
ions of interest and thus concentrate them
in a small volume (thus increasing
sensitivity)
and
excluding
possible
interferences due to matrix components.
95
Calibration Curves
The absorbance of a solution is directly
proportional to its concentration.
Deviation from linearity:
Due to the large number of variables in AAS.
Using standard procedure to construct a
relation
between
the
absorbance
and
concentration for a series of solutions of
different concentrations.
The thus constructed graph is called a
calibration curve.
96
The unknown analyte absorbance is found and
the concentration is calculated or located on
the curve.
Neither interpolation nor extrapolation is
permitted to the calibration curve.
A sample can be diluted or the calibration
curve may be extended but always the analyte
absorbance should be within the standard
absorbance range recorded.
Usually, the concentration axis has the ppm or
ppb units.
97
Standard Addition method
Used to overcome interferences.
In addition, the use of this method provides
better correlations between standards and
sample results due to constant nebulization
rates.
 The method involves addition of the same
sample volume to a set of tubes or
containers.
98
Variable volumes of a standard are
added to the tube set followed by
completion to a specific volume.
Now, all tubes contain the same
amount of sample but different
concentrations of analyte.
A plot is then made for the volume of
standard and absorbance.
This plot will have an intercept (b) with
the y axis and a slope equals m.
99
100
The concentration of the analyte can be
determined by the relation:
Cx = bCs/mVx
Where, Cx and Vx are concentration and
volume of analyte and Cs is the
concentration of standard.
One can only use two points to get the
analyte concentration using the relation:
Cx = AxCsVs/(At –Ax)Vx
101
Detection Limits
Usually,
atomic
absorption
based
on
electrothermal
atomization
has
better
sensitivities and detection limits than methods
based on flames.
In general:
Flame methods:
have detection limits in the range from 1-20 ppm.
Electrothermal methods:
have detection limits in the range from 1-20 ppb.
102
This range can significantly change for
specific elements where not all elements
have the same detection limits.
For example, detection limits fro mercury
and magnesium using electrothermal
atomization are 100 and 0.02 ppb.
While the detection limits for the same
elements using flame methods are 500
and 0.1 ppm, respectively.
103
Accuracy
 Flame methods are superior to electrothermal
methods in terms of accuracy.
 The relative error in flame method can be less
than 1% while that for electrothermal method
occurs in the range from 5-10%.
 Also, electrothermal methods are more
susceptible to molecular interferences from the
matrix components.
 Therefore, unless a good background correction
method is used, large errors can be encountered
in electrothermal methods depending on the
nature of sample analyzed.
104
Flame Photometry
It is a flame emission technique.
We introduce it here because we will not be
back to flame methods in later chapters.
The basics of the technique are extremely
simple:
- Sample is nebulized into a flame.
- Atomization occurs due to high flame
temperatures.
- Also excitation of easily excitable atoms can
occur.
105
- Emission of excited atoms is proportional to
concentration of analyte.
- Flame emission is good for such atoms that
do not require high temperatures for
atomization and excitation, like Na, K, Li, Ca,
and Mg.
- The instrument is very simple and excludes
the need for a source lamp.
- The filter is exchangeable in order to
determine the analyte of interest and, in most
cases, a photomultiplier tube is used as the
detector.
106
107