Transcript Light source used for AA

Atomic Absorption Spectroscopy
Prof Dr Hisham E Abdellatef
2011
Light source used for AA
• What light source do we use with AA?
• Would it be a continuous light source or a line light
source?
• A line light source is used for AA
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Absorption of resonance line
from vapor lamp
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Hollow-Cathode Lamp, HCL
• The hollow cathode lamp (HCL) uses a cathode made of the
element of interest with a low internal pressure of an inert
gas.
• A low electrical current (~ 10 mA) is imposed in such a way
that the metal is excited and emits a few spectral lines
characteristic of that element (for instance, Cu 324.7 nm and
a couple of other lines; Se 196 nm and other lines, etc.).
• The light is emitted directionally through the lamp's window,
a window made of a glass transparent in the UV and visible
wavelengths.
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Hollow Cathode Lamp
• Most common source for AA.
• Ionization of inert gas at high potential.
• Gaseous cations cause metal atoms at cathode to
sputter, emitting characteristic radiation.
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Electrodeless discharge lamps
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A salt containing the metal of interest is sealed in a quartz tube along with an
inert gas
Discharge lamps, such as neon signs, pass an electric current through the inert
gas .
The electrons collide with inert gas atoms, ionizing them and accelerating their
cations to collide with the metal atoms which then will be excited and decay to
lower levels by emitting electromagnetic radiation with wavelengths
characteristic to the element.
Low-pressure lamps have sharp line emission characteristic of the atoms in the
lamp, and high-pressure lamps have broadened lines superimposed on a
continuum.
Common discharge lamps and their wavelength ranges are:
hydrogen or deuterium : 160 - 360 nm
mercury : 253.7 nm, and weaker lines in the near-uv and visible
Ne, Ar, Kr, Xe discharge lamps : many sharp lines throughout the near-uv to nearIR
xenon arc : 300 - 1300 nm
Deuterium lamps are the Uv source in Uv-Vis absorption spectrophotometers.
The sharp lines of the mercury and inert gas discharge lamps are useful for
wavelength calibration of optical instrumentation. Mercury and xenon arc lamps
are used to excite fluorescence.
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Electrodeless Discharge Lamp
•They sometimes provide superior performance to HCL.
•Their useful lifetime is longer than HCL
•They provide light intensity 10-100 times more than that of HCL.
•They are less stable than HCL
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ATOMIC ABSORPTION INSTRUMENTATION
Instruments
for
atomic
absorption
spectrometry (AAS) consist of a radiation
source, a sample holder, a wavelength
selector, a detector, and a signal processor
and readout. The sample holder in atomic
absorption instruments is the atomizer cell
that contains the gaseous atomized sample.
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1. Radiation Sources:
It is necessary that band width of the
radiation source must be narrow relative
to the width of an absorption peak. The
problem created by limited width of
atomic absorption peaks has been solved
by the use of line sources with bandwidths
even narrower than absorption peaks.
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Fig. 9-10a shows the emission spectrum of a
typical atomic lamp source. With a suitable filter
or monochromator, all but one of these lines are
removed. Fig. 9-10b shows the absorption
spectrum for the analyte between wavelengths 1
and 2. Passage of the line from the source
through the flame reduces its intensity from P0 to
P; the absorbance is then given by log(Po/P), which
is linearly related to the concentration of the
analyte in the sample. A disadvantage of the
procedure is that separate lamp source is needed
for each element.
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Hollow Cathode Lamps:
It is the most common source for atomic
absorption measurements. This lamp consists
of a tungsten anode and a cylindrical cathode
sealed in a glass tube that is filled with neon
or argon at a pressure of 1 to 5 torr. The
cathode is constructed of the metal whose
spectrum is desired. Ionization of the inert gas
occurs when a potential on the order of 300 V
is applied across the electrodes, which
generates a current of about 5 to 15 mA.
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… Hollow Cathode Lamps continued…
If the potential is sufficiently large, the
gaseous cation acquire enough kinetic energy
to dislodge some of the metal atoms from the
cathode surface and produce an atomic cloud
in a process called sputtering. A portion of the
sputtered metal atom are in excited states and
thus emit their characteristic radiation as they
return to the ground state. Eventually, the
metal atoms diffuse back to the cathode
surface or to the glass walls of the tube and
are redeposited.
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Electrodeless Discharge Lamps (EDLs):
These provide radiant intensities that are
usually one to two orders of magnitude
greater than hollow cathode lamps. A
typical lamp is constructed from a sealed
quartz tube containing a few torr of an
inert gas such as argon and a small quantity
of the metal (or its salt) whose spectrum is
of interest.
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…Electrodeless Discharge Lamps (EDLs) continued…
The lamp is energized by an intense field of
radio-frequency or microwave radiation.
Ionization of the argon occurs to give ions
that are accelerated by the high-frequency
component of the field until they gain
sufficient energy to excite the atoms of the
metal whose spectrum is sought.
Electrodeless discharge lamps are available
commercially for 15 or more elements.
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Atomic Absorption Instrumentation
• Hollow Cathode Lamp
– Ionization of inert gas
by potential
– Gas acceleration to
cathode
– Atoms on cathode into
gas state
• Some excited
• Deexcite with
photon emission
– Need to excite specific
elements for
measurement
Atomic Absorption Instrumentation
• Electrodeless Discharge
Lamps
– Inert gas in quartz tube
– Excite gas with RF
– Similar to cathode
expect excitation
How a HCL Works
• An applied potential of ~ 300 V DC ionizes the inert gas
Ne (g) -----> Ne+* (g) + e• The ionized gas generates a current flow in the lamp
• Metal cations (e.g. Fe, Mn, Ca) on the cathode acquire
(kinetic) energy from the ionized gas and dislodge into the
vacuum
• A “cation cloud” forms around the cathode (a process called
sputtering)
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How a HCL Works (cont.)
• Some of the sputtered cations are in excited states (M*) and
emit light (hn) as they return to ground state (M0)
M* -----> M0 + hn
• sputtered cations redeposit; this occurs mostly on the
cathode, but some also deposit on the inner glass surface
• Light intensity limitation –> self absorption
– As the current increases, M sputtered increases, but the % M*
decreases. Unexcited gaseous atoms (M0) absorb light produced
within the lamp, preventing it from exiting the lamp
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Radiation Sources for AAS
Electrodeless Discharge Lamps
Characteristics
• The EDL houses a sealed quartz
tube (lamp) containing argon gas
and a metal (or metal salt) of
interest
• The quartz tube is under high
vacuum
• A radio frequency (or microwave)
coil surrounds the lamp
• ~ 10 times more intense than a
hollow cathode lamp
• Unstable output
• Only available for about 15
elements
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How an EDL Works
• An intense RF (or microwave) field is applied to the sealed
quartz tube within the lamp
• Ar gas within the tube ionizes and gains kinetic energy from
the RF field
• Energy is transferred to the metal upon collision
• Excited metal returns to ground state, emitting light (hn)
Ar (g) ---------> Ar* (g) + M (s) -------> M* (g) + Ar (g)
hn
M (s)
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Spectral Line Sources
• Light sources emit spectral lines
– Lines in the line spectrum of the analyte being measured
• Preferred b/c they represent the precise wavelengths needed for
the absorption in the flame
– Flame contains this particular analyte
• Emitted b/c they contain the analyte to be measured
– When lamp is on
» Internal atoms are raised to the excited state
» Emit their line spectrum when they return to the ground state
» This is the light directed through the flame
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Hollow Cathode Lamp
• Hollow Cathode Lamp
– Most widely used spectral line source
– Cathode
• Negative electrode
• Contains the internal atoms
– Hollowed cup
– Internal excitation and emission process occurs inside this cup when
lamp is on
– Anode
• Positive electrode
– Connected with cathode to a high voltage
– Light emitted
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Hollow Cathode Lamp
• Hollow Cathode Lamp
– Sealed glass tube
– Filled with inert gas at low pressure
• Neon or argon
– How it works (fig.9.6, pg 251)
• Lamp turned on and argon atoms ionize
– Positively charged argon ions then crash into the negatively charged
cathode
– Causes sputtering
» Transfer of surface atoms in the solid phase to the gas phase
due to the collisions
• More collisions of argon ions with metal atoms cause metal atoms
to be raised to the excited state
– Light emitted with they drop back to the ground state
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Hollow Cathode Lamp
• Hollow Cathode Lamp
– Must contain the element being measured
• Usually have number of different lamps in stock
– Interchanged in the instrument
– Some are multi-elemental
• Several different specific atoms present in the lamp
– Separated by a monochromator after the flame to isolate the
specific spectral line of the analyte
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