(electronic tones)
- This is an introduction
to a technique called
inductively coupled plasma,
optical emission spectrometry.
We refer to this technique as ICP OES.
In this slide, what we could appreciate
is one of the hearts of this technique
which is the ICP torch
that is illustrated by very hot plasma,
about 10,000 degrees Kelvin
that is able to analyze and atomize
almost all the elements that
are present in any sample.
Two key characteristics of ICP OES will be
the ability to introduce these elements
into the atomic state and then obtain
emission spectra from these elements.
Analytically speaking, these elements
include almost all the
elements of the periodic table,
with the exceptions of the noble gases,
some of the low atomic weight elements,
and some of the internal transition
elements that we have over here.
So in general what we would like to do
is understand a little bit more about
the fundamentals that are behind
the use of this particular technique.
And we will start by looking
at the energy diagram
of an element, for example magnesium.
So what we have here is a plot of energy
versus different electronic states
of the magnesium atom.
What we can see here
is some quantum numbers
like three, four, five, six, and seven.
We also can see another quantum number
that is represented by S, P, D, or F.
Now if we look at this
energy level diagram,
we can see that some of these correspond
to the ground state, the
lowest energy possible,
and some of them correspond
to excited states
and some of them correspond
to ionization states.
In order to understand emission of light
or emission of light from an element,
we will focus first on excited states
and then we will appreciate
that when there is
emission of light, there
is going to be a transition
from one excited state for example, 4D,
to one that has less energy, such as 3P
in this particular example.
We notice as well that these transitions
are distributed by diagonal or a line.
The thicker the line, the most probable
that transition will occur.
We have here as well a number.
5528.
This corresponds to
the wavelength of light
that is emitted when
this transition occurs.
Another way to illustrate
these transitions
is by looking at an emission spectrum.
Here we have a representation
of the same line
that we saw before and as we can see
on the upper part of this slide,
we have several lines
indicating that for each element
there is going to be multiple transitions
that will lead to multiple wavelengths
of light being emitted.
We can see that 5528
angstroms is very intense
and there are some others
that are less intense.
If you would really like to appreciate
the complexity of these systems,
we can plot these
intensities in the low scale
and we can appreciate
that for each element,
we have multiple, many,
many, many different lines
that are part of this
particular emission system.
Another way to represent
each one of these lines
is by trying to blow up what happens
in terms of wavelengths.
We have here nominal wavelengths
for magnesium 5528 angstroms.
If you look at exactly
what this represents,
it represents a line profile.
Each line that we observe in a spectrum
has a line profile,
that is, we can plot the intensity,
or relative intensity, of
light versus wavelength.
Here's an example of a line profile
in which the nominal wavelengths
that we observed before from magnesium
could be represented by lambda naught.
At that point we have given intensity P.
This is, the intensity
is an important property
because it allows us to quantify
the abundance of an element.
The second component of the
line profile is the width,
usually represented as
the width of the profile
at half maximum, that is, at the halves,
and it's represented by
delta lambda one half.
Now, it is important to consider
what causes the width of these profiles.
The first cause of that is
the uncertainty principle,
which is Natural Broadening.
It's a fundamental limitation of nature.
So even if this, the
thinnest lines possible
will have a very thin,
will have some width.
The second component that
contributes to broadening
of a line profile is Doppler Broadening.
This results because in
all the collection of atoms
that are causing this emission,
there are different velocities,
resulting from the high temperatures
that we have in the system,
provided by the ICP torch.
Now this is gonna cause
some additional broadening
that is known as Doppler Broadening.
The third component is
Pressure Broadening.
Usually done because most
of this emission happens
at atmospheric pressure and indeed,
if the pressure was even higher,
the broadening would be even more.
Now that we have said that,
it's important to
understand a little bit more
of the process that will lead to ICP OES.
And for that, I will
be drawing on the board
a description of what happens
in this particular case.
The instrumentation that is used
for ICP OES will consist initially
of a sample reservoir,
usually in solution,
a way to bring that
sample into the gas phase,
through a nebulizer,
a way to take the aerosol
droplets into the ICP torch,
that as we saw in the initial slide
provides extremely high temperatures
and at the same time in that torch,
you're gonna have atomization
that reduces all the
components in the sample
to their atomic state, and when this,
when the atoms are produced,
these same high temperatures
will provide excitation,
and that will lead to emission of light.
Because we have multiple elements,
and we appreciate the
complexity of the spectrum
of each element, it is
important to do a selection
of the lambdas of interest.
And that selection leads then
to the measurement of light
that has been emitted.
It is the combination of these components
that we will be discussing next.
Now that we have described in general
the several components of
an ICP OES measurement,
let's analyze one component at a time.
The first component is the ICP torch.
The ICP torch is usually
a set of concentric tubes made of quartz,
a transparent material,
in which the sample is
introduced in an aerosol form
that we described before.
And this aerosol is carried by argon.
When the sample reaches,
or the aerosol reaches
this region, this is
exposed to the ICP torch.
Now let's talk about the
origin of the plasma,
resulting in the ICP torch.
The plasma results from the presence
of an electromagnetic
field provided by a coil
that basically surrounds this region.
This coil is subjected
to a very high frequency,
a very high current electromagnetic field,
which results in the
production of a magnetic field.
And anything that is
ionized in the process,
ions present in this
region will then be moved
due to the presence of the magnetic field.
It is the rapid movement of ions
that will basically cause friction
and will lead to very high temperatures,
so the characteristics of the ICP torch.
One of the basic
principles of the ICP torch
is the chemical reaction that leads
to the formation of argon ions.
This reaction consists
of argon in the gas phase
and provided by the high temperature,
an additional energy will result
in the production of an
argon ion plus an electron.
This basic reaction is the one that will
lead to the formation of plasma.
Now that we understand that
the basic reaction of this
is production of argon ions,
it's important to understand
how the temperatures
that result are distributed
in the ICP torch.
The ICP torch usually
has different regions
of different temperatures.
The hottest region is right at the point
where the plasma is emitted.
Plasma is gonna be a combination
of argon ions, electrons, and photons.
And as we can see, as we
move away from this region,
we observe lower and lower temperatures.
We also see changes in the plasma
that is important to describe
from the point of view
of its photon composition.
The photons that are
produced in the plasma
could be of three types.
One, they could be photons resulting
from the ion-electron recombination.
They provide a continuum spectrum.
Hence, one can imagine
that the white appearance
of the plasma is due to
this continuum spectrum.
There is also present
argon and argon ion lines,
that is, emission lines.
And finally, there is another phenomenon
called the Bremsstrahlung effect,
that results from having
ions going from
high velocity
to a velocity close to zero.
So the optical components, or the photons,
that are present in a plasma then,
would be the ones that will result
from this and will
contribute to the background
that we observe
in the analytical signal.
There are two possible configurations
to do an analytical
measurement by ICP OES.
The first one is the axial configuration.
This configuration
provides high sensitivity
because it allows us
to collect many photons
from the element of interest.
The second configuration is
the radial configuration.
This configuration is good
when one is interested
in obtaining high selectivity.
Because we can control, easily,
the region
from where photons are collected.
Hence we are able to
eliminate the plasma photons.
Another key component to
understand how ICP OES works
is the collection of emitted light.
As we see here, we have a representation
of the ICP torch.
And ultimately, all the
light that is emitted
is gonna be directed
through an optical system.
The main features to illustrate here
are those related to
the ability to separate
different wavelengths of light.
This is accomplished by
using a grating and a prism.
And they are, they have the ability
to disperse the light in
their particular orientations.
The ultimate result of that
is that when this light
is imaged onto a plane, represented here
by this particular
drawing, each wavelength
will have a unique position.
It is the ability to
recognize the unique position
that allows us to monitor the emission
of each element separately.
That said, it is important then to,
if we would like to
analyze several elements
and we know the wavelength
emission of each element,
we are able to simultaneously
analyze all these elements.
Here's an example of a
typical calibration curve
that demonstrates the ability to analyze
beryllium, chromium,
cadmium, zinc, and lead
in the same sample, in
the same measurement.
It's also important to appreciate
that in this particular plot,
we have intensity ratio versus
concentration of the analyte.
This is, this representation as a ratio
is because we are using
also an internal standard,
yttrium, that has a
characteristic wavelength
of emission as well,
and we are comparing the intensities
of each element with
respect to the intensity
of this particular element
used as an internal standard.
The third component or
feature of this plot
is that we have added to the sample
several potential interferences,
tap water, that has a
collection of multiple ions,
calcium plus magnesium, and sodium.
And as we can see in the
plot that we have here,
when these are present, illustrated
by the circles, the
triangles, and the squares,
there's no deviation or interference
due to the presence of
these interferences.
That is, this technique will be
quite robust for the
analysis of several elements,
even when some interference components
may be present in the sample.
For more information on this technique,
see the textbook, Skoog,
Holler, and Crouch,
Principles of Instrumental
Analysis, sixth edition.
