Title: Atomic Absorption and Atomic Fluorescence Spectrometry Section A
1(No Transcript)
2Atomic Absorption and Atomic Fluorescence
Spectrometry Section A
- By Matt Boyd, James Joseph, Jon Blizzard, Jackie
Freebery, Hunter Bodle
3Atomization Techniques
- AAS and AFS
- Two techniques
- Flame Atomization
- Electrothermal Atomization
4Flame Atomization
- Analyte is nebulized by flow of gaseous oxidants
- Desolvations
- Dissociation
- Volitalized
5Types Flames
Fuel oxidant Temperature (Celsius) Maximum Burning (cm s-1)
Natural Gas Air 1700-1900 39-43
Natural Gas Oxygen 2700-2800 370-390
Hydrogen Air 2000-2100 300-440
Hydrogen Oxygen 2550-2700 900-1400
Acetylene Air 2100-2400 158-266
Acetylene Oxygen 3050-3150 1100-2480
Acetylene Nitrous oxide 2600-2800 285
6Flame Structure
- Primary Combustion zone
- Blue region, rarely used for spectroscopy
- Interzonal region
- Most widely used part
- Secondary combustion zone
- Products of inner core disperse
7Flame Atomizers
- Uses
- Atomic Absorption
- Fluorescence
- Emission Spectroscopy
- Laminar-flow Burners are Commonly Used
- Aerosol, oxidant, and fuel are burned in flame
- Performance Characteristics
- Most reproducible
8Electrothermal Atomizers
- Provides enhanced sensitivity
- Operates evaporating sample at low temps
- Ashing at higher temp
- Measures absorption and fluorescence
- Used in the ICP
9Electrothermal Atomization
- Occurs in open ended cylindrical graphite tube
- Held between two contacts in water cooled housing
- Two inert gas streams are provided
10Output Signal
- Transducer output rises to a maximum
- Rapid decay back to zero
- Quantitative determinations
- Peak height
- Peak area
11Performance Characteristics
- Advantages
- Sensitivity
- Relative precision
- Disadvantages
- Furnace methods
- Analytical range
12Analysis of Solids with Electrothermal Atomizers
- 1st- weigh grounded sample into a graphite boat
and insert boat into furnace. - 2nd- prepare slurry of powdered sample by
ultrasonic agitation in an aqueous solution. The
slurry is then pipetted into furnace atomization.
13Specialized Atomization Techniques
- Glow-Discharge Atomization
- Hydride Atomization
- Cold-Vapor Atomization
14Chapter NineAtomic Absorption and Atomic
Fluorescence Spectrometry
- Section 9A Sample Atomization Technique
By Rachel Conroy Katie Payne
15Flame Atomization
- Sample is nebulized by a flow of gaseous oxidant
and fuel that carries it to a flame - Process
- Desolvation
- Volatilization
- Dissociation
- Ionization
- Excitation to form spectra
16At each phase of atomization spectra can be
obtained.
17Types of Flames
- Oxidants
- Air 1700oC to 2400oC
- Oxygen
- Nitrous oxide
- Burning velocity states when flame is stable
- Too low causes flashback
- Too high flame will blow off
18Table 9-1 Properties of Flames
Fuel Oxidant Temperature oC Maximum burning velocity. cm s -1
Natural Gas Air 1700 1900 39 43
Natural Gas Oxygen 2700 2800 370 390
Hydrogen Air 2000 2100 300 440
Hydrogen Oxygen 2550 2700 900 1400
Acetylene Air 2100 2400 159 266
Acetylene Oxygen 3050 3150 1100 2480
Acetylene Nitrous oxide 2600 2800 285
19Flame Structure
- Primary Combustion Zone
- Interzonal Area
- Secondary Reaction Zone
- Flame Profile
20Flame Atomizers Variables
- Fuel and Oxidant Regulators
- Double-diaphragm pressure regulators
- Rotameter
- Performance
- Most reproducible
- Low sensitivity
21Schematic of a laminar-flow burner, the typical
atomizer used in AAS.
22Electrothermal Atomization
- Long residence time
- Measurements and vaporization
- Evaporated at a low temperature
- Ashed at a higher temperature
23Electrothermal Atomizers
- Graphite tube
- 2 inert gas streams provided
- Transverse configuration
- Pyrolytic carbon seal
24Shown is the cross-sectional view of a graphite
furnace atomizer. The Lvov platform and its
position in the graphite furnace.
25Other info
- Output Signals
- Measures peak height
- Performance
- Slow because of cooling cycles
- Analytical range is narrow
- High sensitivity
- Analysis of Solids
- Finely ground samples, slurry
26Specialized Atomization Techniques
- Glow-discharge atomization
- Hydride atomization
- Cold-Vapor atomization
27Atomic Absorption Instrumentation9-B
- Brian May
- Mandi Kauffman
- Tyler MacPherson
- Carolyn Inga
- Ginny Harrison
28Atomic Absorption Instrumentation
- The AAS Consists of
- A radiation source
- Sample Holder
- Wavelength Selector
- Detector
- Signal Processor
- Read Out
29Radiation Sources
- Potentially highly specific because of narrow
absorption lines. - These narrow lines also cause problems because a
linear relationship between absorption and
concentration requires narrow source bandwidth
relative to the width of an absorption line, but
even good monochromators have bandwidths
significantly larger than the absorption lines.
30Problems Created
- Non-linear calibration curves are inevitable when
the AA is equipped with an ordinary
spectrophotometer and continuum radiation source. - Small calibration curves are obtained because
only a small amount of the radiation from the
monochromator slit is absorbed by the sample,
this gives poor sensitivity
31Solutions
- The use of bandwidths narrower than the
absorption lines. This is done by exciting the
atoms with a lamp, filtering the light, and
choosing appropriate operating conditions(source
temperature and pressure). - This disadvantage to this method is that it
require an additional source lamps for each
element, or group of elements.
32- Hollow Cathode Lamps (9B-1)Sample
33- -Most common source for atomic absorption
measurements - -Consists of a tungsten anode and a cylindrical
cathode sealed in a glass tube which is filled
with either argon or neon gas a pressure of
1-5torr - -Cathode is constructed from the metal whose
spectrum is desired (or, if not constructed from
the metal, it then serves to support a layer of
that metal)
34- -When a potential of about 300V is applied across
the electrodes, ionization occurs of the inert
gas (argon or neon). The current is generated (of
about 5-15 mA) as ions and electrons migrate to
the electrodes. - -if potential is large enough the gaseous cations
gather enough kinetic energy to dislodge the
metal atoms from the cathode surface and produce
an atomic cloud in the process known as
sputtering. - -The excited metal atoms (a portion of those
sputtered) emit their characteristic radiation as
they return to ground state
35- -Efficiency of the cathode depends on its
geometry and the operating potential - High potentials (and thus high currents) ?
greater intensities - -A down-fall to high currents is that they
produce an increased number of unexcited atoms in
the cloud which have the potential of absorbing
the radiation emitted from the excited atoms
(Self-absorption) - -This leads to lower intensities
36Electrodeless Discharge Lamps
37What are they made of?
- Sealed quartz tube
- Filled with an inert gas (Ar)
- Small amount of metal or its salt
38What does it use?
- Uses radio frequency
- Or microwave radiation to energize it
39What happens?
- The gas is ionized by the frequency
- Once enough energy is obtained it excites the
atoms of metal - The metal spectrum is the desired spectrum.
40What it provides
- Provides radiant intensities in greater supply
than a Hollow-Cathode Lamp (HCL) - Not as reliable as the (HCL)
- But better for elements such as
- Se, As, Cd, Sb
41Source modulation
- Emitted radiation is removed via the
monochromator - It is necessary to adjust the output the source
so intensity will fluctuate at a constant
frequency
42- Detector receives 2 signal
- An alternating from the source
- Continuous from the flame.
- These signals are then converted into electrical
responses - A high pass RC filter (section 2B-5) can be used
to remove unadjusted signals
43- Adjusting the emission can be done by inserting a
circular metal disc (chopper) into the system
between the source and the flame - Rotation of this disk at a constant rate will
create a beam that is chopped to the desired
frequency - Tuning forks with vanes attatched to alternately
allow the beam to pass and to not pass is another
technique - An alternative is the power supply being designed
for intermittent or ac operation so the source
can be switched oin and off at the desired
frequency
44AA SpectrophotometerSee Figure 9-13 for block
diagrams
- Instrument must be capable of providing a
sufficiently narrow bandwidth to isolate the line
chosen for the measurement - Glass filter alkali metals
- Only a few widely spaced resonance lines in the
visible region - Separate filter and light source for each element
- Most use photomultiplier tubes
45Single-Beam
- Several hollow- cathode sources
- Chopper or pulsed power supply
- Atomizer
- Simple grating spectrometer with a
photomultiplier transducer - 100 transmittance is set with a blank
- The blank is replaced with samples to determine
absorbance and transmittance
46Double Beam
- Beam from hollow-cathode source is split by a
mirrored chopper - One half passes through the flame and the other
half goes around it - 2 beams recombine by a half silvered mirror and
passed into a Czerny-Turner grating monochromator - Photomultiplier transducer
- Output input to a lock-in amplifier
- Ratio between reference and sample is amplified
and fed to the readout - Since reference beam is not passed through the
flame it cannot correct for loss of radiant power
due to absorption or scattering by the flame
47Chapter 9 Section C
- Megan Seeger, Andrea Lando, Joe Bailey, and Sarah
Duncan
48Spectral Interferences
- Can be caused by overlapping lines but is very
rare due to the emission lines of the
hollow-cathode sources being so narrow - Can also result from the presence of combustion
products that exhibit broadband absorption or
particle products that scatter radiation - Both reduce the power of the transmitted beam and
lead to positive analytical errors
49Continued
- A more troublesome problem occurs when the source
of absorption or scattering originates in the
sample matrix - Interferences because of scattering by products
of atomization is most often encountered when
concentrated solutions containing elements such
as Ti, Zr, and W are aspirated in the flame
50Continued
- Interferences caused by scattering may also be a
problem when the sample contains organic species
or if organic solvents are used to dissolve the
sample - Flame atomization spectral interferences by
matrix products are not widely seen and can be
avoided by variations in the analytical variables
51Radiation Buffer
- When an excess of the interfering substance is
added to the sample and standards - If the concentration added is large compared to
the concentration in the sample matrix then the
contribution from the sample matrix is
insignificant
52Two-Line Correction Method
- Uses a line from the source as a reference it
should be as close as possible to the analyte
line - This makes any decrease in power of the reference
line from that observed during the calibration
arises from absorption or scattering and is then
used to correct the absorbance of the analyte line
53The Continuum-Source Correction Method
- Deuterium lamp provides a source of continuum
radiation throughout the ultraviolet region - The radiation from the continuum source and the
hollow cathode lamp are passed alternately
through the electrothermal atomizer, the
absorbance from the deuterium radiation is then
subtracted from the analyte beam
54Chemical Interferences
- More common than spectral interferences
- Can be minimized by suitable operating conditions
55Most Common Interferences
- Occurs when anion form low-volatility compounds
with the analyte only a fraction of analyte is
atomized and the outcome is low results - Ex. Decrease in calcium absorbance with
increasing concentrations of sulfate or phosphate
56Common Interferences Cont
- Cation Interferences
- Outcome low results
- Ex Aluminum causes low results when determining
magnesium (forms a heat stable compound)
57Solutions to Interferences
- When caused by formation of species of low
volatility, interference can be eliminated by use
of higher temps - Releasing Agents cations that react preferably
with the interferent and prevent analyte
interaction - Protective Agents prevent interferences by
forming a stable, volatile species with the
analyte
58Background Correction Based on the Zeeman Effect
59Zeeman Effect
- When an atomic vapor is exposed to a strong
magnetic field, a splitting of electronic energy
levels of the atoms takes place that leads to the
formation of several absorption lines for each
electronic transition. The sum of the
absorbencies of the lines is equal to exactly the
value of the original line from which they were
formed. - A,B,C ? A
- --------------
--------- - B
- ---------
- C
- ---------
60Splitting Pattern
- Most common type of splitting
- Central line (p) and two equally spaced satellite
lines (s). This is observed with a singlet but
for more complex transitions these lines will be
split further. - The p line absorbs only plane polarized light in
a parallel direction to the magnetic field - The s lines absorb only polarized radiation at a
90 degree angle to the magnetic field
61How it works
- Turn to page 243 in textbooks
- Radiation from a cathode tube
- Rotating polarizer
- Separates the beam into two parts that are
polarized at 90 degrees to each other - These go into a graphite furnace that splits the
energy levels into three peaks (D) - This information then goes to a monochromator,
photomultiplier tube, and into a data analysis
system. - This system subtracts the perpendicular cycle
from the parallel half cycle giving a background
correction.
62Background Corrections with Source Self Reversal
- Also known as the Smith-Hieftje method
- Based on the self reversal or self absorption of
radiation from a cathode lamp - the absorbance is collected at periods where the
lamp is running at a low current - The background is collected when the lamp is at
high voltage - High currents high number of nonexcited
electrons that will absorb the radiation of the
excited species
63Dissociation Equilibria
- Dissociation reactions involving metal oxides and
hydroxides play an important role in determining
the emission and absorption spectra for an
element. - MO? M O
- The M is the analyte atom and the OH is the
hydroxyl radical. -
64Dissociation Equilibria
- Dissociation equilibria which involve anions
other than oxygen may also influence flame
emission and absorption. - Line intensity for Na is decreased by presence of
HCl - NaCl ? Na Cl
- Adding HCL decreases Na concentration thereby
lowering line intensity.
65Dissociation Equilibria
- V, Al, and Ti interact with such species as O and
OH. These are represents as Ox. These are always
present in flames. - VOx ? V Ox
- AlOx ? Al Ox
- TiOX ? Ti Ox
66Ionization Equilibria
- Ionization of atoms is small in combustion
mixtures that involve air as the oxidant, it is
often neglected. - Ionization is important in higher temp. flames
where oxygen or nitrous oxide is the oxidant.
There are free electrons produced by the
equilibrium. - M ? M e-
67Ionization Equilibria
- The equilibrium constant K for the reaction
-
- K Me- / M
- Degree of Ionization of metals at flame temps.
Table 9-2 pg. 246
68Ch. 9 Atomic Absorption Spectrometry
- Section D
- Atomic Absorption Analytical Techniques
69Sample Preparation
- 1. Flame Spectroscopic Methods
- Sample materials
- Soils
- Animal tissues
- Plants
- Petroleum products
- Minerals
- Common problem most are insoluble in aqueous
solutions so preliminary treatment to the sample
is required
70Preliminary Treatments
- Decomposition of material
- Rigorous treatment of the sample at high
temperatures - Con risk losing the analyte by volatilization or
as particulates in smoke - Treatment with specific reagents
- Con can cause chemical and spectral
interferences or can cause the analyte to appear
as in impurity in the solution - Common Decomp. Methods
- 1. Treatment with hot mineral acids
- 2. Oxidation with liquid reagents (sulfuric,
nitric, or perchloric acids wet ashing) - 3. Combustion in an oxygen bomb (or other
closed container) - 4. Ashing at high temperatures
- 5. High temperature fusion with reagents (
boric oxide, sodium carbonate, sodium peroxide,
and potassium pyrosulfate)
71Sample Preparation
- Electrothermal Atomization
- Sample Types
- 1. Liquid Samples blood, petroleum products,
and organic solvents. - liquid solvents can be pipetted directly into
the furnace for ashing and atomization. - 2. Solid Samples plant leaves, animal tissues,
and inorganic substances. - solids can be weighed directly into a
cup-type atomizer or into specific containers for
introduction into a tube type furnace.
72Sample Introduction by Flow Injection
- Introduce samples into a flame atomic absorption
spectrometer - Peristaltic pump and valve arrangements help
insure efficiency while conserving the sample - Carrier system Deionized water or diluted
electrolyte are used to provide continuous
flushing of the flame atomizer - This reduces build up from samples containing
high levels of salts or suspended solids
73Organic Solvents
- Low Molecular-weight organic solvents
- 1. Alcohols
- 2. Esters
- 3. Ketones
- Why Organic Solvents?
- Increased nebulizer efficiency- increases the
amount of sample that reaches the flame - Rapid evaporation of the solvent
- Solvent Ratios
- Leaner fuel-oxidant ratios must be used to offset
the presence of any added organic material - This produces lower flame temperatures, which can
increase the potential for chemical interferences
74Organic Solvents (cont.)
- Immiscible Solvents
- ex Methyl isobutyl ketone
- These solvents extract chelates of metallic ions
- The resulting extract in then nebulized directly
into a flame - Enhance absorption lines
- Only small amounts are required to extract from
relatively large volumes of aqueous solutions - Enhance the sensitivity of the sample, which
reduces interferences - Common Chelating Agents-
- Ammonium pyrrolidinedithiocarbamate
- Diphenylthicarbazone
- 8-hydroxyquinoline
- Acetylacetone
75Calibration Curves
- Should Follow Beers Law
- A abc
- A absorption (L/ g?cm)
- a absorptivity
- b path length through medium
- c concentration
76Calibration Curves (cont.)
- Should cover range of concentration found in the
sample - 1 standard solution should be measured after each
time an analysis is performed. Using 2 standards
that bracket the analyte concentration would be
more efficient in identifying any uncontrolled
variables that result from atomization and
absorbance measurements
77Application of AAS
- Sensitive men for the quantitative determination
of more than 60 metals or metalloid elements - Table 9-3 shows Detection Limits
- Columns 2 3 present detection limits for a
number of common elements by flame and
electrothermal atomic absorption - Detection Limits
- Flame Atomization 0.001 0.020 ppm
- Electrothermal Atomization 2 x 10-6 1 x 10-5
ppm - Accuracy
- Relative error
- Flame Analysis 1-2
- Electrothermal Analysis errors extend flame
errors by a factor of 5-10
789D ATOMIC ABSORPTION ANALYTICAL TECHNIQUES
- Roa Al-Qabbani
- and
- Ashley Appell
799D-1 Sample Preparation
- Sample has to be introduced into the excitation
in the form of a solution (disadvantage). - Many materials are not soluble in common
solvents extensive treatment is required. - Treatment with hot minerals, oxidation with
liquid reagent, ashing at high temperature, etc. - Some minerals can be atomized directly. Solid
samples are weighed into cup-type atomizers
(advantage).
809D-2 Sample Introduction by Flow Injection
- FIA Introduces samples into a flame atomic
absorption spectrometer. - Carrier stream of the FIA system provides
continuous flushing of the flame atomizer
(advantage).
819D-3 Organic Solvents
- The effect of organic solvents is attributable to
increased nebulizer efficiency. More rapid
solvent evaporation also contribute to the
effect. - Use of immiscible solvents is the most important
analytical application of organic solvents to
flame spectroscopy. - Resulting extract is nebulized into the flame
(sensitivity increases). - Part of the matrix components remain in the
solvent (advantage).
829D-4 Calibration Curves
- Theory is that calibration curves should follow
Beers Law which does not happen very often - Absorbance should be directly proportional to
concentration - Use two standards that bracket the concentration
of the analyte.
839D-5 Standard Addition Method
- Should use method found in Section 1D-3
- Need to compensate for chemical and spectral
interferences of the sample
849D-6 Application of AAS
- A sensitive way of determining 60 metals and
metalloid elements - Detection Limits
- Flame Atomization Atomic Absorption Spectroscopy
are in the range of 1-20ng/mL,or .001-.020ppm - Electrothermal Atomization are in the range of
.002-.01ng/mL or 2x10-6 - 1x10-5ppm - Accuracy
- Error in Flame Ionization Atomic Absorption
Spectroscopy 1-2 - Electrothermal Atomization increase by a factor
of 5-10
85Atomic Fluorescence Spectroscopy
- Keri Franz
- Kyle Howard
- Lauren Kaminsky
86Fluorescence
87Atomic Fluorescence Spectroscopy
- Useful and convenient for quantitative
determination of many elements - Not used as often as atomic emission and atomic
absorption - Useful for determining elements that form vapors
and hydrides- Pb, Hg, Cd, Zn - Fluorescence instruments are generally harder to
maintain and thus more expensive
88Instrumentation Sources
- Sample container is usually a flame or
electrothermal atomization cell, glow discharge,
or an inductively coupled plasma - Continuum source is ideal
- Hollow cathode lamps were used frequently but now
EDLs (electrodeless discharge lamps) are more
common - EDLs have greater intensity than hollow cathode
lamps - Lasers are good sources despite increased costs
and operational intricacies
89Dispersive Instrumentation
- Contains
- Modulated Source
- Atomizer (flame or nonflame)
- Monochromator or Interference Filter System
- Detector
- Signal Processor
- Readout
90Nondispersive Instrumentation
- Contains
- Source
- Atomizer
- Detector
- Advantages
- Low Cost Simplicity
- Multi-element Analysis Adaptability
- High Sensitivity
- Simultaneous Collection of Energy
- For accuracy, make sure source output lacks
elemental contamination and background radiation
should not be emitted
91Applications
- Determination of Metals
- Lubricating Oils
- Seawater
- Geological Samples
- Clinical Samples
- Environmental Samples
- Agricultural Samples