Reasons for the value of atomic absorption (AA) as analytical tool:
(1) Applicable to virtually all elements
(2) Inherently a quantitative technique
(3) Suitable for trace analysis
(4) All elements treated the same way
(5) Relatively great freedom from interferences: Interferences are well defined, easily
detected and easily eliminated.
1. Flame Emission
2. Atomic Absorption
3. Atomic Fluorescence
1. The Induction Coupled Plasma (ICP)
2. Cold Vapor Mercury
3. Hydride Generation
4. Graphite Furnace
(1) Flame Emission: a sample is subjected to a high energy, thermal environment in order to produce excited state atoms, capable emitting light.
a. Instrumentation
b. Calibration - arbitrary units
c. Advantages - high sensitivity for some elements
d. Disadvantages - background emissions, interelement effects.
e. A measure of the excited state population
(2) Atomic Absorption
a. The light scattering process
hn + M ® M* ®
M + hn
b. Instrumentation
c. Calibration
i. % Absorption
ii. Absorbance
iii. Normal deviation from linearity
d. A measure of ground state population
(3) Atomic Fluorescence
It incorporates both atomic absorption and atomic emission.
Ground state atoms created are excited by focusing a beam of light into the atomic vapor.
The emission resulting from the decay of the atoms excited by the source light is
measured.
a. A measure of scattered photons
b. Instrumentation
c. Advantages
i. measures signal above zero
ii. requires no special light source
iii. sensitivity depends on intensity of source
d. Disadvantages
i. expense
ii. correction for background emission
iii. correction for background scatter
(1) The Hollow Cathode Lamp
a. Construction and theory of operation
b. Current control and its effect on line width and sensitivity
c. Warm-up time
d. HC lamp failure
i. gassy lamps
ii. loss of intensity
iii. instability and noise
e. Multielement lamps
i. lower sensitivity and detection
limits
ii. equivalently shorter lifetime
(2) The Electrodeless Discharge Lamp (EDL)
a. Construction and theory of operation
b. Advantages
i. high intensity
ii. very pure spectral lines - sensitivity
iii. long life
c. Disadvantages
i. high cost and extra power supply
ii. relatively long warm-up time
(1) The nebulizer
a. Operation
b. Disassembly and cleaning
c. Optimization
d. Clogging
(2) The Mixing Chamber
a. Operation
b. Disassembly and cleaning
(3) The Burner Head
a. 10 cm slot
b. 5 cm slot
c. 3 cm slot
d. Cleaning
e. Removal of carbon deposits on 5 cm burner
f. Burner warm-up
i. for 10 cm slot burner
ii. for 5 cm slot burner
(4) The Drain Trap
(5) Burner Positioning in The Instrument
(1) Acetylene
a. Construction and contents of the cylinder
b. Evolution of acetone and 75 psig limit
c. Prohibition against Linde "Purified" grade
d. Handing and storage of cylinders before use
e. Maximum outlet pressure of psig
f. Color check for purity
g. Effects of purity on analytical results
h. Remote location of cylinders
i. Prohibition against copper tubing
(2) Air
a. The need for a compressor
i. uniform oxygen content
ii. low long term cost
iii. need for filtration and noise
isolation
b. The problems of bottled air
i. variability of oxygen content
ii. high long term cost
(3) Nitrous Oxide, N2O
a. Chemical properties - behaves like oxygen
b. Normal cylinder pressure, 750 psi
c. Cylinder contains liquid so no warning of low tank
d. Cool on expansion - may require
i. a heated regulator
ii. a heating tape
iii. an infrared space heater
(4) Hydrogen - No Longer Used Extensively
a. Principle problem: Check for leaks
(5) Argon
a. Inert gas
b. Follow normal precautions for handing high pressure gas
cylinders
(1) Stoichiometric flame: there are exactly chemically equivalent amount of fuel and
oxidant gases
(2) Lean flame: there is a chemical excess of oxidant in the flame
(3) Rich flame: there is a chemical excess of fuel in the flame
There are four basic types:
(1) Physical: Variations in viscosity or surface tension between samples and standards. Unequal rates of delivery of material to flame.
(2) Chemical: Due to the formation of a compound which cannot be decomposed in the
flame. The results will always be low.
a. General rule for suspecting chemical interferences
A chemical interference may exist if analyzing a polyvalent cation in the
presence of a polyvalent oxyanion or fluoride or fluoride containing substance.
b. Note the possibility of amphoteric element acting as polyvalent anion, e.g.,
aluminum, silicon, boron, chromium, iron, vanadium, molybdenum, etc.
(3) Ionization: Caused by ion formation in the flame. Lack of recognition and control
of this problem can produce either positive or negative errors in an analysis. This effect
is responsible for anomalous curvature in the calibration curve
a. All of the alkali metals show this interference in the acetylene-air
flame.
b. Most elements show is interference in the acetylene-nitrous oxide
flame.
(4) Background Absorption or Scatter: Caused by molecular absorption at reonance wavelength or by light scatter due to very small, unvolatilized particles in the flame. This phenomenon is wavelength dependent (greater at short wavelength) and always gives a positive error in the analysis.
(1) Physical Interferences
a. Dilution of samples and standards with a common solvent,
commonly used for oil analysis.
b. Calibration by the method of standard additions.
(2) Chemical Interferences - Use of a releasing agent
a. Tie up the interference chemically. Add between 0.1 and 0.2%
lanthanum chloride to samples and standards.
b. Tie up the element to be analyzed with EDTA. This reaction is
pH dependent so not as frequently used as lanthanum chloride.
Note: High purity La2O3
or LaCl3, specified for AA must be used.
EDTA should be recrystallized from acid several times before use.
c. Use the acetylene-nitrous oxide flame. Few chemical
interferences are ever found in this flame.
d. Isolate the element to be analyzed by extraction,
precipitation, or volatilization from the interfering agent.
(3) Ionization Interference.
a. Deuterium arc background correction (continuum source).
b. Non-absorbing line (or two-line) method.
(4) Background Interference - use of an ionization suppressor.
a. Add Between 0.1 and 0.2% of an alkali metal salt to all
samples and standards.
(1) General Definition of Sensitivity
a. Slope of the calibration curve
b. Variation of sensitivity with concentration
(2) Sensitivity Definition of AA - More Properly Called Characteristic Concentration
a. The reciprocal of general sensitivity
b. Defined in terms of % absorption: 1% absorption equals 0.0044
Absorbance.
c. Indicates concentration range for maximum photometric measuring
precision.
(3) Detection Limit
a. The concentration that gives a signal equal to twice the RMS value
(standard deviation) of the baseline noise.
b. A measure of S/N: signal quality
c. Used as a method of calculating uncertainty - significant figures in
result.
(1) Sample must be homogeneous solution at a concentration that will give a suitable
signal.
a. Methods of lowering sensitivity
1). dilution (dilution errors)
2). use of a secondary line
3). rotating the burner head
4). detuning the nebulizer
b. Methods of increasing signal
1). concentration by evaporation
2). concentration by extraction
3). concentration by co-precipitation
(2) Preparation of Mineral Samples
a. Acid solubilization
1). pressurized systems - the Teflon bomb
b. Fusion reactions
1). alkaline - carbonate, bicarbonate,
hydroxide, LiBO2
2) acidic - pyrosulfate
(3) Preparation of Organic Samples
a. dry ashing
b. wet ashing (digestion)
c. oxygen bomb
d. peroxide bomb
(1) Cold Vapor Mercury Analysis
(2) Hydride Generation
a. Gutzeit method
b. NaBH4 - sodium borohydride
(1) concentration 1000 times lower than flame AA: extremely sensitive
(2) Flameless sampling device: high electric current through a graphite tube
(3) Absorption signal is transient
Back Home »»»»»» This Way
Back to Water & Wastewater Page »»»»»» This Way