Analytical Chemistry

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Analytical Chemistry Atomic absorption
Spectroscopy

• KR
• LSU

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IIntroduction to Atomic Spectroscopy   AAtoms
exhibit interactions with EM radiation. We will
discuss some of the spectroscopic methods used in
their determinations.   An absorption of a
photon causes an excited state, where an atom 's
electron goes from a lower energy state to one of
higher energy. As in molecular spectroscopy, we
can monitor the difference in the absorption or
monitor the subsequent photon emission.  
Let's look at the excitation of an atom by energy
absorption. As review from Freshman Chemistry
Remember the energy levels allowed in an atom
1s,2s,2p,3s,3p,4s,3d,4p, etc.   Shorthand
Notation 1H 1s1 6C 1s2 2s2 2p2 28Ni 1s2
2s2 2p6 3s2 3p6 4s2 3d8   Filled Shell
Configuration 1H 1s1 6C He 2s2
2p2 28Ni Ar 4s2 3d8
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• Every element has its own signature of energy
  levels where certain quantized energies are
  required to cause excitations, i.e. the energy
  spacings are different.
• Excited atoms may lose energy and emit a photon
  of light and return to a lower energy state.
• An atom's emission of the quantized wavelengths
  of light provide a series of narrow bands termed
  line spectra.
• Atomic instrumental methods typically monitor
  either the atomic specie's absorption or emission
  of light energy.

the emission spectra for Na and H in the visible
region are shown.
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ABSORPTION
 
EMISSION/FLUORESCENCE
Or laser
DETECTORS PMT, photodiode
Figures of Merit   Analyte concentrations part
per million (ppm) to part per trillion (ppt)
level
  Precision 1-2   Because spectrum is so
sharp(narrow), little overlap of different
elements (can do multiple analysis).    Able to
Measure over 60 elements in a sample. Atomic
spectra produce optical spectra of gaseous atoms
with bandwidth 0.001 nm whereas molecular
species have optical spectra with bandwidth 100
nm, SEE TEXTBOOK !!!
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Two methods used in atomic spectroscopy. i)
Atomic absorption spectroscopy (AAS)     AA
Instrumentation 1) hollow-cathode lamp 2)
monochromator 3) flame or graphite
furnace(sample cell) 4) Beam chopper not shown
(modulation) 5) detector 6) read out display  
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The flame converts the sample to atomic vapor and
the atoms absorb light produced from the Hollow
cathode lamp. The sample cell can be either the
burner flame or a graphite furnace! The
monochromator allows the selection of the
wavelength of light that reaches the detector.
There is no reference cell, the resonance line
light is modulated using a mechanical chopper
which alteranetly blocks the light beam reaching
the detector. Thus the burner flame light
reaching the detector can be compensated.     The
detector typically is a PMT.   A working
calibration curve involving standards is used to
determine analyte concentrations.
Can use a dual beam instrument, where the lamp
source light is split. The sample light beam is
directed through and reference beam around the
sample cell. In this setup, the baseline should
be more stable due to decrease in source
intensity fluctuations of the instrument.  
The AAS light source
Hollow cathode Lamp Since atomic absorption is
a Quantized process, we need light at specific
narrow line wavelength. Therefore, the HC lamp
must contain the element we are trying to
determine. The lamp consists of inert gas, an
anode and a cathode sealed in quartz glass. The
gas is ionized, which collide with the metal and
produce a sputtering effect. These excited metal
atoms give the light energy, which the sample
analyte can absorb.   - ? e- ?
---gt cathode anode
Ar---gtAr---gtM---gtM---gtM ?
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1) Flame burner head cell path--
AA sample cells
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        A liquid sample is introduced into an
atomizer whose temperature is 20003000K.        
Sample, oxidant, and fuel are combined and
nebulizedbroken into small droplets        
Droplets entering the flame lose their water
through evaporation then the remaining sample
vaporizes and decomposes into atoms         The
most common fuel-oxidizer combination is
acetylene and air (flame temp 24002700K)
        Hotter flames are needed for refractory
elements (those with high vaporization
temperatures) or to decompose species such as
metal oxides or hydroxides.         When a
hotter flame is required, the acetylenenitrous
oxide combination is usually used (flame temp
29003100K)         Sample amounts minimum
needed is 12 mL.
  Graphite furnace cell path--
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Furnaces

• Graphite furnace offers greater sensitivity than
  a flame and requires a smaller volume of sample.

• A graphite furnace (at 2000K) confines the
  atomized sample in the optical path for a
  residence time of several seconds resulting in
  high sensitivity.

• Sample amounts minimum needed is 12 µL.

        The GF Instrumental configuration is
difficult to operate.         Monochromator
wavelength selection diffraction
grating Detector photomultiplier tube

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Data Calibration Curve Usually make a "best
fit" plot using different concentrations of
standards and their absorbances. For
example Copper in sample using standards and
linear regression curve.
Standards Std1 ? 1.0 ppm Abs ?0.046 Std2 ? 3.0
ppm Abs ?0.168 Std3 ? 5.0 ppm Abs ?0.281 Std4
? 7.5 ppm Abs ?0.401 Std5 ? 10 ppm Abs
?0.529   unknown reading Abs 0.156 ? Conc ?

What if we had taken an absorbance reading of
0.769? Is this a problem? What would you suggest
we do?
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ii. Emission, Inductively Coupled Plasma
DETECTORS Any for fluorescence Also an
option ICP MS
SEE TEXTBOOK !!!
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Inductively Coupled Plasma AE (or MS, see
later) AE-atomic emission, excited atoms
emit -higher temperatures of plasma ( 6000 K ,
see Boltzman distribution textbook) -ultrasonic
nebulizer, better detection limit (ICP/AE 0.3
ng/g, ICP/MS 0.001ng/g, ultra trace) -better
stability for fluorescence -inert Ar
environment -lt50MHz induction coil Ar producing
plasma Ar coolant -
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(ii) Atomic emission spectroscopy (AES)
overview Similar to AAS but no light source (HC)
is needed. Some of the atomized atoms are
promoted to excited electronic states by
collisions with other atoms. The excited atoms
emit their characteristic radiation as they
return to their ground state.         an
emission technique         Allows simultaneous
analysis of different species.         Several
methods of excitation are possible flame, a
spark, or Inductively coupled Plasma (ICP).  
The other methods for excitation are not as
efficient in exciting the analyte (atoms) as ICP.
The analyte signal increases as the number of
same excited atoms increases. As According to
the Boltzman Distribution N/No (g/go)
e-?E/RT
COMPARISON OF AAS vs AES Atomic absorption (AAS)

• an absorption technique
• one analyte at a time
• cheaper analysis than AES
• requires analyte specific HC lamp source
• can use a flame or graphite furnace to heat
  sample
• works with metals and a few metalloids
• some matrix effects
• less signal variation due to temperature effects
  
• for a graphite furnace, it confines the sample to
  light path for longer times giving enhanced
  signal, greater sensitivity, less sample
  requirements

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Atomic Emission (AES)

• AES is more expensive
• ICP instrumentation somewhat more complicated to
  operate
• AES require less sample preparation and gives
  simultaneous analysis
• doesn't require different lamps( light sources).
  
• Uses PDA or a series of detectors to monitor
  different wavelengths (different element ID's)

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Interferences in Atomic Absorption
Spectroscopy   The detection limit is the
concentration of an element that gives a signal
equal to twice the peak-to-peak noise level of
the baseline.   IInterference is any effect that
changes the signal when analyte concentration
remains unchanged.  
Types of Interferences 1. Spectral unwanted
signals overlapping analyte signal 2. Chemical
 chemical reactions decreasing the concentration
of analyte atoms 3. Ionization ionization of
analyte atoms decreases the concentration of
neutral atoms  
What about interferences? How do we over come
interferences?  
Chemical interference -- most common in AA, due
to a thermally stable compound present, which
influences the analyte's energy absorption. Two
ways of controlling use hotter flame or add a
releasing agent (something that ties up the
competing species).
Ionization interference-- flame too hot causes
ionization of analyte. Add alkali metals, which
have lower ionization potentials to suppress
analyte ionization.   Matrix interferences -- due
to nature of sample, the junk in it! Dilute the
sample if possible to reduce effects or use
"method of standard addition", the sample is
spiked with a standard, readings plotted and the
concentration is extrapolated from plot.
SEE TEXTBOOK
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ICP MS Mass spectroscopy CHAPTER 21 - textbook