Atomization
There are two main types of atomizers:
discrete and continuous. Continuous atomizers introduce the analyte in a
steady manner whereas discrete atomizers introduce the analyte
discontinuously. The most common continuous atomizer in AAS is a flame,
and the most common discrete atomizer is the electrothermal atomizer.
Sample atomization limits the accuracy, precision, and limit of
detection of the analytical instrument. The purpose of the atomization
step is to convert the analyte to a reproducible amount of gaseous atoms
that appropriately represents the sample.
Diagram describing the process of atomization for continuous atomizers.
Electrothermal Atomization
During electrothermal atomization, a
sample goes through three phases to achieve atomization. First, the
sample is dried at a low temperature. Then the sample is ashed in a
graphite furnace (discussed below), followed by a rapid temperature
increase within the furnace where the sample becomes a vapor containing
atoms from the sample. Absorption is measured above the heated surface
where the sample was atomized.
A graphite furnace is made up of a
graphite tube open at both ends with a hole in the center for sample
introduction. The tube is encased within graphite electrical contacts at
both ends that serve to heat the sample. A supply of water is used to
keep the graphite furnace cool. An external stream of inert gas flows
around the tube to prevent outside air from entering the atomization
environment. Outside air can consume and destroy the tube. An internal
stream of inert gas flows through the tube, carrying away vapors from
the sample matrix.
Electrothermal atomizers provide enhanced
sensitivity because samples are atomized quickly and have a longer
residence time compared to flame AAS systems, which means more of the
sample is analyzed at once. This method can also be used for
quantitative determinations based on signal peak height and area.
Electrothermal atomization also offers the advantage of smaller sample
size and reduced spectral interferences because of the high temperature
of the graphite furnace. However, electrothermal atomizers have
disadvantages including slow measurement time because of the heating and
cooling required of the system and a limited analytical range.
Additionally, analyte and matrix diffuse into the graphite tube, and
over time, the tube needs replacing, increasing maintenance and cost
associated with electrothermal atomization.
Limits of Detection
For GFAA (gas furnace atomic absorption)
the range is between 100 ppb to 1ppb. This is because the matrix, even
though removed, still plays a role in the scale of detection.
Flame Atomization
After being nebulized by gaseous oxidant
and mixed with fuel, the sample is carried into a flame where the heat
allows atomization to occur. Once the sample reaches the flame, three
more steps occur, desolvation, volatilization, and dissociation. First a
molecular aerosol is produced when the solvent evaporates
(desolvation), then the aerosol is formed into gaseous molecules
(volatilization) and finally the molecules dissociate and produces
atomic gas (dissociation). During this process cations and electrons can
also be formed when the atomic gas is ionized.
Fuels and Oxidants
The table shown lists
the most common fuels and oxidants used to produce flames for AAS. A
mixture of different oxidants and fuels can be used to achieve a
specific temperature range. Because dissociation and breaking molecules
down to atoms is easier with more heat present, oxygen is the most
common oxidant used in flame atomization. To control the flow rate of an
oxidant and fuel a rotameter is used, this is a vertically placed
tapered tube. With the smallest end placed down, a float which is
located inside the tube determines the flow rate. Close control is vital
because the flame is very unstable outside of its specific flow rate
range. If the flow rate is not greater than the burning velocity
indicated, the flame will experience flashback and propagate back to the
burner. If the flow rate is too high, the flame will blow off the
burner. When the flow rate and burning velocity are equal, the flame is
stable. Usually the flame consists of an excess of fuel to prevent
oxides forming with the molecules of the sample.
Flame Structure
All
locations of a flame are not equal in temperature, and are not equal in
fuel to oxidant ratio. The three main zones of a flame include the
primary combustion zone, secondary combustion zone, and the interzonal
region. The interzonal region is prevalent in free atoms and is the
hottest area of the flame. It is therefore the region used for
spectroscopic analysis. The flame usually rises about 5 cm above the
burner tip, with 2.5cm being the max temperature point. The portion of
the flame used for AAS is specific as to what element is being analyzed.
Due to the formation of oxides, different elements achieve max
absorbance at different distances (cm) above the burner.Performance
Flame atomic atomization is the most
reproducible of all the liquid- sample introductions, however it has
many disadvantages. Oxides are easily formed which leads to a reduced
absorbance of samples, and flame atomization has a lower sensitivity
than electrothermal atomization. Samples could be drained as waste and
therefore have a low residence time, leading to low efficiency. Another
disadvantage of flame atomic atomization is the flame fluctuations which
can affect the absorbance of samples.
Flame from Atomic Absorption Spectrometer Instrument (AI 1200)
Limits of Detection
In Flame Atomic absorption Spectroscopy
the limit of detection is between 1 ppm for transition metals to 10 ppb
for alkali metals. Transition metals need more energy than alkali metals
to excite their outer electron which is why the higher detection limit
is needed
Other Atomization Methods
A variety of means are used to create the
vapor of atoms from the sample that will be analyzed by the AAS. In
addition to the methods previously discussed, glow-discharge
atomization, hydride atomization, and cold-vapor atomization are
techniques that can be very useful for AAS.
In a general glow-discharge atomization
system, the sample is placed on a cathode. Argon gas is ionized by an
applied voltage on the cell, causing the argon ions to accelerate to the
cathode where they interact with the sample and eject atoms. This
process is called sputtering, the ejection of atoms from a sample as a
result of bombardment by energetic species. Samples must either have
conducting qualities or be mixed with conducting materials like graphite
or copper. The sputtered atoms are then introduced to the path of
radiation for analysis by a vacuum; this is so outside air will not be
analyzed only the analyte of interest will be analyzed. This atomization
technique can be used in conjunction with a flame AAS system, and can
be used for bulk analysis and depth profiling of solids.
Glow discharge Atomization
A hydride generation and atomization system for AAS
Cold-vapor atomization is only used in the determination of mercury because mercury doesn’t atomize well in a flame or furnace. In this technique, mercury is acidified and reduced and then swept through by a stream of inert gas. Absorption of this gas is then determined.
Cold Vapor atomic fluorescence system
