Atomic Spectroscopy

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Chapter 28

• Atomic Spectroscopy

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28 A Origins of atomic spectra With gas-phase
atoms or ions, there are no vibrational or
rotational energy States, instead only
electronic transitions occur. Thus, atomic
emission, absorption, and fluorescence spectra
are made up of a limited number of narrow
spectral lines. In atomic emission
spectroscopy, analyte atoms are excited by heat
or electrical energy. A transition to or from
the ground state is called a resonance
transition, and the resulting spectral line is
called a resonance line.
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Absorption of radiation of 285, 330, and 590 nm
excites the single outer electron of sodium from
its ground state 3s energy level to the excited
3p, 4p, and 5p orbitals, respectively. After a
few nanoseconds, the excited atoms relax to
their ground state by transferring their excess
energy to other atoms or molecules in the medium.
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Atomic fluorescence is often measured at the same
wavelength as the source radiation and is called
resonance fluorescence. Atomic spectral lines
have finite widths. Several factors contribute
to atomic spectral line widths Natural
Broadening The natural width of an atomic
spectral line is determined by the lifetime of
the excited state and Heisenbergs uncertainty
principle. The shorter the lifetime, the broader
the line and vice versa. Collisional
Broadening Collisions between atoms and
molecules in the gas-phase lead to deactivation
of the excited state and thus broadening of the
spectral line. The amount of broadening
increases with the concentrations (partial
pressures) of the collision partners.
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Doppler Broadening
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28 B Production of atoms and ions The
atomization device must normally perform the
complex task of converting analyte species in
solution into gas-phase free atoms and/or
elementary ions. Atomization devices fall into
two classes continuous atomizers and discrete
atomizers. With continuous atomizers, such as
plasmas and flames, samples are introduced in a
steady, continuous stream. With discrete
atomizers, individual samples are injected by
means of a syringe or autosampler. The most
common discrete atomizer is the electrothermal
atomizer.
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• Plasma Sources
• A plasma is a hot, partially ionized gas.
• It contains relatively high concentrations of
  ions and electrons.
• Three power sources are used in argon plasma
  spectroscopy
• a dc arc source capable of maintaining a current
  of several amperes between electrodes immersed in
  the argon plasma.
• A radio-frequency, or inductively coupled plasma
  (ICP), source offers the greatest advantage in
  terms of sensitivity and freedom from
  interference.
• microwave-frequency generator through which the
  argon flows.

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When a nebulized sample is carried into a flame,
the droplets are desolvated in the primary
combustion zone, which is located just above the
tip of the burner. The center of the flame
called the inner cone is where particles are
vaporized and converted to gaseous atoms,
elementary ions, and molecular species
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Types of Flames Used in Atomic Spectroscopy
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Effects of Flame Temperature Flame temperature
determines to a large extent the efficiency of
atomization, which is the fraction of the
analyte that is desolvated, vaporized, and
converted to free atoms and/or ions. The flame
temperature also determines the relative number
of excited and unexcited atoms in a
flame. Emission methods require much closer
control of flame temperature than do absorption
procedures. Absorption methods should show
lower detection limits (DLs) than emission
methods. Several other variables also influence
detection limits.
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Absorption and Emission Spectra in Flames Both
atomic and molecular emission and absorption can
be measured when a sample is atomized in a
flame. Atomic emissions in this spectrum are
made up of narrow lines and are called line
spectra. Bands form when vibrational
transitions are superimposed on electronic
transitions to produce many closely spaced lines
that are not completely resolved by the
spectrometer. Because of this, molecular
spectra are often referred to as band spectra.
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• Electrothermal Atomizers
• Electrothermal atomizers are used for atomic
  absorption and atomic fluorescence
• measurements, but they have not been generally
  applied for emission work.
• With electrothermal atomizers, a few microliters
  of sample are deposited in the furnace by syringe
  or autosampler.
• A programmed series of heating events occurs
• drying,
• ashing, and
• atomization.

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28 C Atomic emission spectrometry Atomic
emission spectrometry is widely used in elemental
analysis.
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Wavelength Isolation Emission spectrometry is
often used for multielement determinations.
There are two types of instruments The
sequential spectrometer uses a monochromator and
scans to different emission lines in sequence.
The direct reading spectrometer uses a
polychromator with as many as 64 detectors
located at exit slits in the focal plane.
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Radiation Transducers Single-wavelength
instruments most often use photomultiplier
transducers as do direct reading spectrometers.
Computer Systems and Software Most of the
newer ICP emission systems provide software that
can assist in wavelength selection, calibration,
background correction, interelement correction,
spectral deconvolution, standard additions
calibration, quality control charts, and report
generation.
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Sources of Nonlinearity in Atomic Emission
Spectrometry Calibration curves that are linear
or at least follow a predicted relationship are
preferred. At high concentrations, the major
cause of nonlinearity when resonance transitions
are used is self-absorption. At low
concentrations, ionization of the analyte can
cause nonlinearity in calibration curves when
atomic lines are used.
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• Interferences in Plasma and Flame Atomic
  Emission Spectrometry
• The interference effects are classified as
• A blank, or additive, interference produces an
  effect that is independent of the analyte
  concentration.
• In emission spectroscopy, any element other than
  the analyte that emits radiation within the
  bandpass of the wavelength selection device or
  that causes stray light to appear within the
  bandpass causes a blank interference.
• Example, Spectral interferences produce an
  interference effect that is independent of the
  analyte concentration.

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• 2. Analyte, or multiplicative, interferences
  change the magnitude of the analyte signal
  itself.
• Examples, Chemical, physical, and ionization
  interferences.
• Physical interferences can alter the aspiration,
  nebulization, desolvation, or volatilization
  processes.
• Chemical interferences occur in the conversion of
  the solid or molten particle after desolvation
  into free atoms or elementary ions.
• Substances that alter the ionization of the
  analyte also cause ionization interferences.
  Example, the presence of an easily ionized
  element, such as K, can alter the extent of
  ionization of a less easily ionized element, such
  as Ca.
• An ionization suppressant is an easily ionized
  species that produces a high concentration of
  electrons in a flame and represses ionization of
  the analyte.

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28 D Atomic absorption spectrometry Flame atomic
absorption spectroscopy (AAS) is currently the
most widely used of all the atomic
methods. Line-Width Effects in Atomic
Absorption
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Instrumentation The instrumentation for a
single-beam AA spectrometer.
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Line Sources The most useful radiation source
for atomic absorption spectroscopy is the
hollow- cathode lamp. Hollow cathode lamps have
a cathode containing more than one element and
thus provide spectral lines for the determination
of several species.
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Source Modulation It is necessary to
discriminate between radiation from the
hollow-cathode or electrodeless-discharge lamp
and radiation from the atomizer. Most of the
atomizer radiation is eliminated by the
monochromator. The thermal excitation of a
fraction of the analyte atoms in a flame produces
radiation of the wavelength at which the
monochromator is set, causing interference. The
effect of analyte emission is overcome by
modulating the output from the hollow-cathode
lamp so that its intensity fluctuates at a
constant frequency.
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Modulation can be accomplished by placing a
motor-driven circular chopper between the source
and the flame.
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Complete AA Instrument An atomic absorption
instrument contains the same basic components as
an instrument designed for molecular absorption
measurements for a single-beam system. Photometer
s At a minimum, an instrument for atomic
absorption spectroscopy must be capable of
providing a sufficiently narrow bandwidth to
isolate the line chosen for a measurement from
other lines that may interfere with or diminish
the sensitivity of the method. Spectrophotometer
s Most measurements in AAS are made with
instruments equipped with an ultraviolet/visible
grating monochromator.
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Background Correction Absorption by the flame
atomizer as well as by concomitants introduced
into the flame or electrothermal atomizer can
cause serious problems in atomic absorption.
Molecular species can absorb the radiation and
cause errors in AA measurements. The total
measured absorbance, AT, in AA is the sum of the
analyte absorbance, AA, plus the background
absorbance, AB AT AA AB
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Continuum source background correction uses a
deuterium lamp to obtain an estimate of the
background absorbance. A hollow-cathode lamp
obtains the total absorbance. The corrected
absorbance is then obtained calculating the
difference between the two. Pulsed
Hollow-cathode lamp background correction or
Smith-Hieftje background correction uses a
single hollow-cathode lamp pulsed with first a
low current and then with a high current. The
low-current mode obtains the total absorbance,
while the background is estimated during the
high-current pulse. In Zeeman background
correction, a magnetic field splits spectral
lines that are normally of the same energy
(degenerate) into components with different
polarization characteristics. Analyte and
background absorption can be separated because of
their different magnetic and polarization
behaviors.
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Flame Atomic Absorption Flame AA provides a
sensitive means for determining some 60 to 70
elements. The optimum region of a flame must
change from element to element and that the
position of the flame with respect to the source
must be reproduced closely during calibration and
measurement. The flame position is adjusted to
give a maximum absorbance reading for the
element being determined.
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Interferences in Atomic Absorption Molecular
constituents and radiation scattering can cause
interferences If the source of interference is
known, an excess of the interferent (radiation
buffer) can be added to both the sample and the
standards. 28 E Atomic fluorescence
spectrometry Atomic fluorescence spectrometry
(AFS) is the newest of the optical atomic
spectroscopic methods. An external source is
used to excite the element of interest. The
radiation emitted as a result of absorption is
measured, often at right angles to avoid
measuring the source radiation. It is not
commercially successful partly owing to the lack
of reproducibility of the high- intensity sources
required and to the single-element nature of AFS.