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We were discussing about the oxide formation in flames, and we have discussed that spinal
types of the type MeO metal oxide, Me 2 O 3 that is metal dioxide and of pervoskites
MeOMeO 2 type; they form very stable lattices.
The examples include the metal such as titanium, zirconium, hafnium, molybdenum, etcetera and
in all these cases the signal depression occurs if the salts are of the type of sulfides which
gives maximum signal depression compared to chlorides, and that is much more than nitrates.
So, the formation of carbides also follows a similar series that is 4th group elements
are; they give you still smaller signal depression compared to the 6th group, 5th group and that
is much more than 4th group; that is valancy four. If the metal oxide is more volatile
than the metal or the carbide, usually signal enhancement occurs.
So, we will discuss now about vapor phase interferences; the equilibrium and incomplete
conversion of the analyte into spectroscopically active form that is atoms resulting in the
automation of the signal may be considered as vapor phase interference. First of all,
we have to remember that the metal atoms formed they need to get into vapor phase system also.
So, the equilibrium and incomplete conversion are the only factors that can change the concentration
of the atoms in the vapor phase. So, such alterations usually occur in the primary reactions
only. So, diatomic and triatomic compounds such
as sodium hydroxide, barium hydroxide, barium oxide, cyanides, oxides; they such compounds
are readily formed in the flame and they can alter the degree of ionization markedly.
So, this is another type of vapor phase interference. Usually, disassociation processes take place
between the chemical species and the flame gases. Variation in the concentration of the
halides, free air radicals, oxygen, and OH radicals, cyanogens radials, hydrogen it is
all these gases are there present in the flame of the atomic absorption. So, there presence
in the flame gases can influence the disassociation equilibria. Usually, we expect this kind of
reactions; that is metal oxide reacting with carbon monoxide giving you metal plus CO 2;
metal oxide reacting with carbon to give you metal and carbon monoxide plus carbon, and
carbon monoxide can also react with oxygen to form CO 2. Cyanogens can pick up an electron
from the flame to form cyanide ion; and CHO radical can change over to CH plus NO. These
are ions if they form, they can readily suppress the ionization of the analyte.
So that means, the signal will become smaller; that is it gets attenuated and then, we have
other types of interferences that is special distribution of the interferences. These interferences
are caused by the changes in the flow rate of or flow pattern of the sample in the flame.
The quantities of combustion products can change the mass flow rate or flow pattern,
which in turn are influence by the size and rate of volatilization of the particles. Examples
of this type include signal enhancement of aluminum, barium, calcium, lithium, transom
etcetera.
So, the... In general, several types of interferences can occur in flame AAS due to several factors.
Therefore, even in the absence of specificity, still signal enhancement or attenuation can
occur. This is what we want to understand in atomic absorption. That is even though
it is an element specific technique, the signal at attenuation can occur due to various reasons
that we had discussed so far. We will discuss again the actual chemical reactions that take
place with specific metal ions in the later stages.
Now, the back ground... Let us discuss about the back ground correction of flame AAS. So,
the whenever there is signal attenuation the back ground correction also becomes very important
because, will have to keep on adjusting the back ground depending upon the level of attenuation.
Therefore, most of the spectral interference if they are there and all other types of interferences
result in the attenuation on the AAS signal to some extent. In general, attenuation varies
from negligible to several percent depending upon the matrix. So, if you are having very
pure samples, standards etcetera where the matrix elements are much less; that is other
types of elements which in which you are not interested as an analyte are much less then,
the attenuation becomes much less. So, the signal is more commonly known as back ground
absorption when there is no absorption no matrix element.
There is suppose, you use aqueous solutions only containing only one analyte or two analyte
then, there is whatever signal you get without the sample is known as back ground absorption,
which can be easily estimated by aspiring a closely matching reference solution or blank,
we call it as blank. And, it becomes fairly important to run a blank in almost every determination.
So, the absorbance of reference or the blank must be subtracted from that of the calibration
curve as well as the samples. So, alternately what you can do, radiation from the deuterium
lamp can be measured at the resonant wavelength only and to determine the back ground absorption
because, the radiation from the continuum is almost represented by the deuterium lamp
which is used for back ground this thing. So, a schematic representation of a deuterium
lamb back ground corrector can be measured at the reference wavelength to determine the
back ground absorption. This is the figure of the back ground correction unit in atomic
absorption.
So, I want you to see this figure. Here, what I have is an analyte hallow cathode lamb;
that is the source and here, I have a deuterium lamb signals come from each site and then,
there is a rotating chopper; both beams are combined and then passed through the flame
and again they will be separated and then they lead to the monochromator. This is the
schematic arrangement.
And in this figure, the exit slit of the monochromator separates the resonance line from the emission
spectrum of the hallow cathode lamp equivalent to band pass width of about 0.2 to 0.7 nano
meter. This is this number 0.2 to 0.7 represents these actual slit width in the atomic absorption
instruments which you can set manually or it you can change it automatically. The intensity
I PS of the primary source is equalized to the intensity I CS; CS means continuum source.
So, the primary source that is hallow cathode lamp intensity I PS is equalized to I CS before
the determination so that I PS to I CS ratio is unity. This is very important for us to
standardize the measurements. So, for normal measurements what we do is, we measure the
usually less than 1 percent absorption if the back ground concentration back ground
absorbent is less than 1 percent, we can neglect that. That is I CS that is continuum source.
If the absorption of the reference relation is less than 1 percent you can neglect. At
higher absorbance, the signal from the hollow cathode lamp is attenuated proportionally
to the concentration of the analyte in this case. So, in effect I CS that is continuum
source of the radiation serves as the reference beam.
So, this is how the signal works; for example, here I have a primary source. This is a slit
and then here, it is continuum source same slit and then I PS and I CS are equalized
in height. And then, we have atomic absorption signal I PS is greater than I CS. So, the
emission absorption occurs in this signal making a small gap here in the third figure.
And then, the I PS and I CS are equalized again and then the atomic absorption occurs
that is I PS is less than I CS. So, we have a atomic absorption signal; that is this is
how the mechanism of back ground correction works.
So, back ground correction; it will work provided the continuum sources are less than 0.5 absorbance.
If it is very high then, this technique will not work. Further suppose, it is very high
then what happens; the noise also increases by a factor of two or three.
So against this, we generally go for measurements with low back ground; that is the solutions
should be as less as possible in terms of complexity; that means, it should not you
should not determining the samples in very high salt water content such as body fluids
or urine or sea water or several other alloys etcetera. So, another way of back ground correction
is Zeeman effect back ground correction. So, let us discus a little more about the Zeeman
Effect back ground correction. So, the basic theory is very simple. When an atomic vapor
is placed in a magnetic field of the order of 10 kilo gauss, the energy levels split
or terms split which manifest a spectral line splitting. In the simplest case, the spectral
line split into three components. One is sigma component and other is pi component. So, pi
component occurs at the exactly the same frequency as the original signal, and sigma components
are separated slightly to the left and right of the pi component. The components are of
the order of about the separation of the sigma components are of the order of about 0.01
nano meter. So, if the energy level split, the energy
also is split automatically and the ratio of sigma plus and sigma minus 2 pi corresponds
to approximately 25 to 50 is to 25; that is pi component is maximum with 50 percent and
the one which shifts to the left is a about 25 percent in intensity and which shifts to
the right is also a approximately 25 percent intensity; that means, if the separation occurs
only between sigma plus and sigma minus and pi component, you get three peaks in the presence
of magnetic field and this is known as Zeeman effect.
Now simultaneously, when you place a magnet in on the atomic cloud, the terms get split
into three components. But simultaneously, the radiation also is polarized; that means,
the direction changes of the direction of the radiation also changes. So, the extend
of polarization direction change of direction depends upon the direction of the magnetic
field. So, higher the magnetic field more is the rotation. Some elements split into
only three components just like I was telling you and this is known as normal Zeeman splitting.
Elements such as barium, calcium, beryllium, calcium, magnesium, mercury, lead, etcetera;
they exhibit normal Zeeman effect splitting. Some elements split into more components and
but odd numbers. Some elements split into more even numbers of components. These are
sort of enameling because the normal is only three components. So, when there are more
components, the effect is known as anomalous Zeeman splitting. This kind of splitting can
be used in atomic absorption measurement for back ground correction.
How? It is very simple. Here, I am showing you the normal splitting; that is the central
component here is the pi component and here are two sigma components. One is sigma minus
another is sigma plus. So, the elements corresponding to this are barium, beryllium, calcium, cadmium,
mercury, etcetera, magnesium, etcetera; here are anomalous examples of anomalous Zeeman
effect. Here, we can see that there are more number of pi components, more number of sigma
components also. Here, you can see again pi component is missing,
but there are equal number of sigma components. The elements corresponding showing this type
of pattern or splitting are aluminum, arsenic, beryllium and then bismuth, antimony etcetera
and all in all most of the elements what I have listed here, there are about 35 elements
which will show you different kinds of the splitting Zeeman splitting and this effect
we can use it for back ground correction.
So, if the magnetic field is applied at right angles to the radiation beam; that is the
radiation beam is going like this; I am going to put the magnetic field like this; one at
the top; one at the bottom. So, the radiation is going in between these two magnetic fields.
So, that is a pi component is in such case; pi component is polarized perpendicular to
the applied field; that means, it is rotated by 90 degrees. So this configuration is known
as transverse Zeeman effect. So, when the magnetic field is parallel to the radiation
beam then, it is called as longitudinal Zeeman effect. In this configuration, pi component
is totally missing and only two sigma components you will be seeing. And, both of them would
be having approximately 50 percent of the intensity because, the pi component is missing.
So, the total of sigma plus and sigma minus would be 50 percent. Now, this is a schematic
representation of the Zeeman effect.
This figure tells you that the atom cloud how it can affect the signal. Now, the in
the center you will see there is an atom cloud here; there is no magnetic field. When there
is no magnetic field, I get a signal at mu 0 which is approximately this side. Now, if
I apply the field perpendicular to the direction of the beam; here is the direction of the
beam and then what happens; I have a transverse Zeeman effect. The this is pi component; this
is sigma minus; and this is sigma plus. So in longitudinal, how I am going to apply the
field; put the magnet on the top and bottom and this is the magnetic field direction.
So in this case, pi component is rotated out of the out of the beam and you will see only
sigma plus and sigma minus peaks. So, this effect can be... Suppose, you put magnet like
this and measure the absorbance; when the magnet is on, you will be measuring only the
whatever signal you get will be the background because, the metal atoms signals sigma plus
and sigma minus are already shifted away. So, what you see would be the pi component
is missing because, it is already rotated ninety degrees. So, what you would be measuring
is, whatever signal you get is only the sigma component.
So, when the magnetic field is applied to the radiation source. Now, in the previous
figure I have shown you that this is the atom cloud on which the magnetic field has been
applied. Now, what you want to do is suppose, we apply to the source that is hallow cathode
lamp itself then, it is known as direct Zeeman effect. The magnetic field also can be applied
to atomic could as we have seen earlier, this is known as inverse Zeeman effect.
So in the inverse Zeeman effect, the energy levels of the absorbing atoms are split and
the absorbance values also change. Since sigma components are rotated out of the radiation
line and only pi component absorbs which can be measured. For the direct Zeeman effect,
absorption can be measured at both sigma and pi component wavelength also.
So, a number of possibilities exists in which we can organize the Zeeman effect application
of the magnetic field to the either to the atomic could or to the source of radiation
that is hallow cathode lamp. So, if a magnetic field is applied permanent magnet or by a
direct current, rotating polarizer must be applied to measure the total absorbent because,
when you apply a permanent magnetic field it is already rotated. So, you have to re-rotate
it back to measure the absorbance. So, by apply... Suppose, you apply alternating current
then, alternating magnetic field is generated which splits the energy levels only the field
is on. So, you will have a continuous on off on off magnetic field.
So, if you look at all this possibilities, there are eight possible configuration of
application of Zeeman effect back ground; for example, here you can see that the...
We have at the... suppose, I apply to the radiation source that is on the hallow cathode
lamp, I can orient the magnetic field parallel; that is longitudinal or perpendicular to the
radiation source then, I can apply constant field or alternating field. So, rotating if
I apply constant field, I need a rotating polarizer; if I apply a alternating field
then, there is no polarizer required. Similarly, we have a... Suppose, I do with transverse
magnetic field then again, I have a constant and alternating, but I need a rotating polarizer
and fixed polarizer for the for measuring the signal.
So suppose, I apply the field magnetic field at the atomizer that is atomic could then
again, I have parallel and perpendicular positions. I can use constant magnet or alternating magnetic
field then off course, this is not applicable because to a flame you cannot apply constant
magnetic field, but to the alternating field you can apply. So, no polarizer required.
So, perpendicular transverse arrangement gives me a rotating polarizer and fixed polarizer.
So in all these cases, it is possible to measure. Now, in atomic absorption instruments inverse
Zeeman effect; that is magnet at the atomizer are preferable compared to a source. So, what
do to you would like to do is, around the flame we can put a magnet without touching
the magnet, but keeping the field on at the same time. In that case, a rotating require
is required if a constant magnet is used. Therefore, alternating magnetic field is used
because, it does not require rotating polarizer also.
So, the addition of a rotating polarizer puts further restrictions of the servo mechanism
to rotate the field, non rotate, d rotate the etcetera. In this case, configuration
absorbance is measured with field off; that is when the field is off your measuring total
absorbance that is normal AAS. And with field is on when the field is on, the sigma components
are shifted, pi component is rotated. So, there is nothing at the at the measurement
wavelength, and you will be measuring only the back ground. So, this is a true double
beam technique since both beams originate from the same source. Usually, measurements
are made at the same frequency therefore, they follow the same optical path, and they
fall on the same detector. Only difference is, once we are measuring the total absorbance,
once you are measuring the total back ground. So, the sensitivity remains unaltered and
back ground gets corrected. This is the beauty of Zeeman atomic absorption and several commercial
instruments are available with this type of arrangement.
And, another one is new way of measuring the absorbance; that is back ground correction
is known as smith Hieftje back ground correction technique, and this method is based on the
self absorption behavior of the radiation emitted by the hallow cathode lamps when there
are operated at high currents. Now, I have referred to this self absorption earlier in
my introductory remarks, and I have also mentioned that several of this street lights what you
see sodium. In sodium, vapor lamps in the streets they switch on and off as if somebody
is switching with a button you know on off. But actually, it is not so because, the at
high currents the atomic could increases of atomic cloud of the sodium increases and it
absorbs all the radiation emitted by the sodium vapor lamp. So, this principal is used in
the back ground correction. What happens; application of high current
produces large concentration of the unexited atoms in the hallow cathode lamp itself. So,
the these atoms are capable of absorbing the radiator; where ever you produce either in
the flame or in the lamp if there are un exited atoms there would be absorbing the radiation.
So suppose, you increase the concentration of the unexcited atoms in the hallow cathode
lamp itself then, the hallow cathode lamp itself will absorb all the radiation and no
radiation will pass through the optic flame and atomic could and reach the detector, this
is the principal. Now what happens; these atoms are capable of absorbing radiation produced
from the exited atomic spaces, and high currents also broaden the emission lines of the exited
species; net effect is to produce a line that has a minimum at its center; that is the resonance
line. So at the resonance line, when complete absorption takes place, there is no absorbance
and only back ground gets measured. So, what we had to do is to operate the hallow
cathode lamp alternately with high current, low current, high current and low current
like that. If you are able to do and measure then, the atomic absorption usually is can
be measured with good back ground correction. This is known as smith Hieftje correction.
I will show you a figure shortly corresponding to this. To obtain the correct absorbance,
the lamp must be programmed to run alternately at normal current and high current in quick
succession at the rate of every 5 milli seconds; 5 milli seconds you take one measurements
and then apply high current no absorbance only back ground gets measured; again apply
a low current. Like that, if you keep on doing every 5 milli seconds then, you can collect
the all the back ground measurements for about 1 second or something like that add all of
them then, add the signal without high current, that will give you back ground plus the atomic
absorption signal. So, if you subtract the two what you will be getting is the normal
atomic absorption signal which is corrected for the background, and during these first
part back ground and atomic absorption is measured and during the second part only absorption
peak use at the minimum and only the back ground is measured. So, the data acquisition
system must be there to subtract the two signals to give a this thing.
So, if you take a look at this figure, this is a what I had discussed so far; it becomes
fairly clear that is, when at low current, this is the atomic absorption signal. At high
current, the signal gets submerged and only the what you will be measuring is only the
difference that is there is no atomic absorption at all
and this is the mechanism of an atomic absorption. So, you can either use even the... Nowadays,
instruments are available which will give you the an instrument which is automatically
fitted with Smith Hieftje correction method; that is to supply electric current to the
hallow cathode lamp alternately and such instruments are available in the market.
One can definitely go and choose between all the three all the that is deuterium back ground
correction, Zeeman effect back ground correction or Smith Hieftje back ground correction all
the systems are available, and one has to evaluate critically, what are the requirements
of an analytical method and then go for the most suitable back ground correction system.
I will be listing out the effects, the advantages and disadvantages of such back ground correction
in the coming slides. So, what I want to do now is, I want to discuss with you the chemical
reactions in atomic absorptions spectrometry. So, in the light of back ground correction
how we can evaluate the chemical reactions and the types of interference also. Earlier,
we had discussed solute vaporization etcetera and vapor phase and interference etcetera.
But, in theory atomic absorptions spectrometry is an element specific technique; that means,
there should not be any doubt regarding the presence of an element if you get an AAS signal.
At the same time, it is element specific; that means, no other elements will interference.
Now, because of the chemical reactions in atomic absorptions flame, we will have interferences.
So, any analytical instrumental technique; however, sensitive, simple and rapid it is
not free from all interference. This point, I want to stress again and again that no instrumental
technique is ever interference free. So as analytical scientist, we must be aware of
the origin and source of such interferences to provide accurate analytical results and
precise results also. So, atomic absorption spectrometer is one such analytical instrument
and it has got inherent interferences from the sample introduction stages to the detector.
Sometimes, even contamination during the preparation of the sample itself may occur and then originally,
it was thought that AAS is basically free from all interferences as we measure only
very narrow resonance lines from the hallow cathode lamp, but it is actually not so. So
the interferences in chemical terms and physical terms,
we can classify like this; that is first is physical interferences, chemical interferences,
ionization interference then, we have spectral interference we have discussed a little bit
earlier and then non specific interference. So, the physical interference normally occurs
at the sample introduction stage and the remaining chemical ionization etcetera, they the first
one occurs at the sample introduction stage and all the other four occur during their
stay in the flame.
So, with this will not discuss physical interference because, we have already discussed the different
types of interference that is due to viscosity and then transport and all these things. But,
what I want to stress at this stage is that the transport and other aerosol effects etcetera,
the all the effect all the instruments equally. So, the signal gets attenuated, but you cannot
really call it an interference, but attenuation all the same. So, they constitute interference
not specific to particular element this point will have to remember.
Now, the physical processes that occurred during nebulisation; they are large influence
on the sensitivity and selectivity of the film methods. The nebulisation efficiency
depends upon the nature of the nebulising gas and the samples solvent. The variation
in the viscosity, surface tension, density and temperature of the sample all these things
interference in the nebulisation process and therefore, they affect the sensitivity. This
interference can be controlled to some extent by the preparation of standard solutions used
to construct the calibration curve under similar physical conditions of the solvent of the
sample. So, this interference can be eliminated by diluting the sample solutions also or by
the method of standard addition. These things we will discuss later about the analytical
techniques of analytical methodology how we go about doing this standard addition and
other techniques. Now, the presence of high concentration of
dissolved salts; for examples, it can reduce an analytical signal. It also leads to formation
and incrust station of the nebulizer and the burner head. You know burner head is a basically
a small metallic piece with several salts and if the salts are there, they can block
the gas passage. So, if it blocks the gas passage, you will not be getting the signal
at all. So in general, it can be said that physical interferences can result if the sample
and the standard solution vary in bulk composition. The normal sample solution up take in atomic
absorption is about 6 to 7 milli liters per minute, and the nebulisation efficiency is
of the order of about 10 percent maximum. Nowadays, it is about 50 13 to 15 percent
not more than; that means, only 15 percent of the 6 to 7 ml of the sample gets into the
flame as aerosol. So, any change in these normal values causes physical interference
and this can lead to spurious signals also. So, I do not want to say more about physical
interference because, all these things are common to all elements. Therefore, good maintenance
of atomic absorption should take care of such interference. Now, let us talk about chemical
interference. We have already explained that the physical interference occur from sample
introduction stage before nebulisation until it reaches the burner. Now, once the burner
the sample reaches the burner then all types of chemical reactions can occurs. This we
also we have discussed earlier with the flame component reactions. You remember, that I
want to go back to the slide what I had shown you today only if you could look at this.
These are some of the reactions we had discussed earlier metal oxide etcetera, but these are
all general terms; that is M represents only a metal and CO, C 2, oxygen, electrons these
are all these things represents the flame components.
So now, we will discuss a little more about the chemical interference in specific because,
we feel that it is possible to know the reaction correction much more thoroughly if you understand
the processes. So, the schematic diagram again I had shown you this figure earlier that,
metal has to get evaporated; it must form a solution; and it must form an aerosol then,
it forms a solid aerosol then, it comes to gaseous form and then, a metal atoms and then,
metal atoms can form oxides and then, as they can get exited they can form react with carbide
carbon to form carbides etcetera and are this reaction, this we have seen in earlier and
discussed also.
So, the sample solution first enters the nebulisation where it gets fragmented etcetera. The bigger
size droplets fall into the collection system and they get lose to drains and smaller 10
percent goes to the atmosphere goes into the spar chamber and then enters this thing enters
the burner, and the... It is the free metals undergo a variety of reactions like oxygen,
hydrogen, carbon, etcetera and the to get optimum sensitivity, we need the metal atom
concentrations. So, chemical interferences are the most common interferences encountered
in the atomic absorption spectroscopy. If the sample enters thermally forms a thermally
stable compound then, the dissociation of the metal into atoms does not occur. This
is a very simple very, straight forward interpretation of the interference in the atomic absorption.
So, if the signal gets attenuated, we have a chemical interference. So, chemical interference
can either enhance a signal or attenuate the signal also. Now, let us take an example of
calcium chloride. Now, the... In this exit slide,
I am going to show you the calcium as an example what happens to the solution. Suppose, I start
with calcium in solution, I can represent it as calcium chloride CaCl 2 nH 2 O and number
of water molecules that is in solution. Now, it must form a liquid gas aerosol of calcium
chloride nH 2 O and upon heating it must forms solid. So, the water molecules will evaporate
and I have only calcium chloride here and then, the it must melt dissociate calcium
chloride will form and calcium atoms will form and then chlorine atoms.
And, this calcium atom from can form exiting it can get excited and then ionized or recombined
to form molecules. So, these are the waste techniques because, only the excitation is
the one process where the atomic absorption can occur, but ionization and recombination
are not concessive to good atomic absorption.
So, what are the types of the reactions? Here, it is CaO calcium atom and chlorine and it
can also react with the acid present in the solution to form calcium atoms and 2HCl that
is hydrochloric acid in the flame. All these reactions are occurring in the flame. So,
you can see that calcium oxide suppose it forms because there is air in the flame also,
and calcium oxide can get converted to calcium atom and oxygen atoms. So, when it is present
as calcium nitrate then, we can expect nitrogenous gases; for example, I have written here that
calcium nitrate CaNO 3 twice 3H 2 O gets dissociated into oxide calcium oxide plus NO 2 nitric
oxide nitrogen dioxide plus three water molecules. So, if we do this calcium oxide can again
decompose to calcium atoms and oxygen. So, I have shown you two types of reactions in
which calcium chloride or calcium nitrate may be present as the analyte.
So in chloride medium, it forms calcium atoms and chlorine atoms and the in nitric acid
medium, it forms calcium that is the mechanism. In chloride medium, it forms straight away
disassociates into calcium atoms. In any acid nitric acid medium that is an oxidant, its
gets converted to oxides and then it decompose. So, it is a two stage process. So as explained
earlier, the most common chemical interference is the compound formation. So most of the
elements in the alkaline earth elements such as beryllium, calcium, stroceium, barium etcetera;
they form highly refractory metal oxides in the flame resulting in the loss of these metal
atoms available for atomic absorption because, if they form the compounds, they are lost
to the atomic absorption.
So the dissociation of the metal oxides back into free atoms depends upon the temperature
of the flame. If the temperature of the flame is very high then, decomposition then always
occur. The higher the temperature of the flame, the more is the dissociation and hence better
sensitivity. So, it is a very simple logic that if a compound forms in the flame then,
we have to raise the temperature of the flame to get the atoms back. So, the air acetylene
gives a temperature of about 2300 to 2600 degree centigrade; that is depending upon
the composition of the acetylene as well as air mixture. Now, at this temperature most
of the metal oxides dissociate expect diffractions elements like alkali, alkaline earth elements
also to some extent; niobium, tantalum, aluminum, zirconium, etcetera they form refractory oxides.
So, a flame which gives a temperature higher than acetylene; that is 2600 degree is required
if your sample contains any of the elements like niobium, tantalum, aluminum, zirconium,
etcetera. Now, this is... How do you get this higher
temperature? Suppose, you want to analyze niobium only acetylene will not work, but
it will give you very less sensitivity. So, change over to nitrous oxide gas. That we
know that nitrous oxide gives you higher temperature up to 2900 centigrade. So, normally they are
elements whose dissociation energy is more than 5.0 volts, they cannot be determined
by acetylene. This is a very simple very simple logic and guideline for us. So, if the dissociation
energy is more than 5 volts, you need air acetylene you do not air acetylene, but what
you need is nitrous oxide acetylene gas. It is not always the temperature of the flame
that is important, we have to understand this effect because, it is not always the temperature.
Many times the carbon oxygen ratio in the flame also determines the sensitivity, not
only the high temperature, but adjust the carbon oxygen.
So, if you have a reducing flame that is the reddish flame. Now, if you have a reddish
flame that is oxidizing than you may get higher temperature even with the air acetylene also,
but there are always limitations of up to maximum you could get is around 2600 degrees.
But, higher than that, we have to go for no amount you have to go for nitrous oxides acetylene,
but no amount of flow ratio of acetylene and nitrous oxides will change the temperature
to the desired level. So, reducing flame providing more flue than the stoichiometric requirement
is desirable for refractory metal oxides. So, one should only optimize the flame condition
in the fuel to oxidant ratio, height etcetera to get maximum sensitivity. In our laboratory,
we have tried some investigations using sugar to alter the carbon oxygen ratio in the flame
in the flame for favorable for atomization because, it is very simple to use sugar along
because, sugar dissolves in water or acids to large extend and when you introduced sugar
in the flame; obviously, most of the sugar components will get decompose into carbon
atoms. So, this is one way of adjusting the carbon component also in the flame by just
by introducing the sugar.
And, we have observed that for elements like molybdenum, titanium, menedium, aluminum,
barium, atrium, dysprosium, aluminum, etcetera there is enhancement in the absorbance values,
while there is no enhancement in the case of less refractory elements like copper, cadmium,
cobalt, etcetera. A very this is a very interesting observation because, when we introduce sugar,
the large amount of carbon atoms in the flame and they give a enhanced signal for refractory
oxides; that is molybdenum, benydium, titanium, aluminum, all this form refractory oxides;
the what do you mean by refractory oxides is very stable oxides which are having, which
are stable up to 3000 degree centigrade etcetera. Now, such elements give you higher signal
when we put sugar, but when we do not put sugar, we have we have problems with ordinary
elements like copper, cadmium, cobalt, nickel, etcetera. Now, it is very funny because, why
there should not be a signal enhancement when we put sugar along with the solution. The
answer is very simple. Then, with the even without sugar the dissociation of the salts
is complete in the case of copper, cadmium, cobalt and nickel. So, any amount of addition
of sugar is not going to increase the concentration because, the dissociation is complete. But,
in the case of refractory oxides, we always get enhancement in the signal because, the
refractory oxides start decomposing when we put sugar because of the reducing atmosphere.
That is in addition to the gas and nitrous oxides flame.
So, the other type of compound formation is also possible where the elements can form
refractory phosphates and double oxides in the flame; for example, aluminum, silicon,
etcetera, strontium; they react with aluminum or silicon and forms refractory oxides such
as SrOAl 2 O 3 strontium oxide aluminum oxide compound and then SrOSiO 2 strontium oxide
silicon dioxide compounds. Similarly with phosphate, we have calcium phosphate during
the evaporation of the liquid drop lets in the flame. This compound is converted into
calcium pyrophosphate with heat and is very stable in the air acetylene flame. For the
sake of privity, I want you to read the slide the to get the ideas clear because, in most
of the systems, we do have a we do have a situation where refractory oxides give you
higher signal when sucrose is the added, and normal elements do not give you sucrose higher
significance, higher signal. So, we will continue our discussion in the next class.