Chapter 21 Atomic Spectroscopy
p.453 Harris (7th)
Experiment 4. Determination of Copper and Zinc in Tap/River Water by Atomic Absorption
Spectroscopy.
Chapter 21 Atomic Spectroscopy
p. 494 Harris (6th)
1
Atomic Absorption Spectrophotometer
2
Light from the hollow
cathode lamp is partially
absorbed by the sample in
the flame, and then passes
through a monochromator .
Interference caused by
emission by the flame can
largely be removed by
locating the
monochromator between
the flame and the detector.
Advantages of atomic
spectroscopy
 High specificity (or selectivity)
o each element has its own characteristic spectrum;
o because the lines are so sharp, there is usually little overlap
o between the spectra of different elements (in the same sample) .
 High sensitivity
o limit of detection
= ppm flame atomization
= ppb graphite furnace atomiz.
 Wide applicability
o -70 elements can be determined .
 Precision
o %RSD -1-2%
 Easy analysis
o sample solution is aspirated (sucked) into the flame
o no optical ( cuvette) cells to clean.
Disadvantages of atomic spectroscopy
 equipment is expensive --$20,000 and up.
 many single-element hollow cathode lamps are required
o one lamp for each element; each $
o multi-element hollow cathode lamps are not available for all elements.
3
4
Selectivity
 any overlap of atomic absorption lines among different elements?
Applications of AAS
Widely used for metal analysis in environmental samples (air, water, and soil) and in biological fluids
and tissues :
 Mercury in "silver" dental fillings
o an amalgam of 50% Hg and 35% Ag
o Hg vapor from dental fillings may accumulate in our bodies
could have toxic effects on the nervous system, may be causing allergy problems for millions

Lead poisoning in blood
o U.S. Centers for Disease Control (CDC) has mandated that every child must be tested
for the presence of Pb in blood
allowable level of Pb in a child's blood = 10 ug /100 mL


Lead in gasoline, soil, wastes, paints, etc.
o lead-based paints were outlawed in 1978
Biological selenium (an essential nutrient)
o in human serum (0.01-5 ppm); in animals (0.1 ppm)
higher levels may cause toxic reactions


Trace element contents (Ba, B, Ga, Mn and Rb) of Florida and Brazil orange juice
Tungsten in ores and concentrates
5
6
22-1 Overview of Atomic Spectroscopy
p. 591 Harris (5th)
Spectra of gaseous atoms
 consists of very sharp lines (natural width < 0.01 nm)
 atoms cannot vibrate or rotate as molecules do
(vibrational and rotational quantum states cannot exist)
 unlike the spectra of condensed phases (liquid, solution or solid) with absorption band width of
~100 nm
Atomization
 water/organic solvent is evaporated.
 remaining solid (inorganic salts) is vaporised and broken (decomposed and reduced) into atoms.
3. The advantages of atomic spectroscopy do NOT include
(a) high selectivity.
(b) high sensitivity.
(c) wide applicability.
11
( d) no chemical reagents.
(e) many different hollow cathode lamps.
5. The absorption spectra of gaseous atoms consist of
20
( a) one sharp line.
(b) one broad band.
( c ) several sharp lines.
(d) several broad bands.
( e ) one sharp line on top of one broad band .
Quantitative AAS analysis p. 515 Harris (5th)
Atoms generated by the flame (in the light path) absorb part of the light coming from the hollow
cathode lamp.
The unabsorbed light is measured by the detector (after passing through the monochromator)
Absorbance = A = log Po /P
where Po
= power of light beam before entering the flame
p
= power of light beam emerging from the flame
Absorption signal (or absorbance) follows Beer's law:
A = €bc'
c' = k c

p.514-516 Harris (5th)
where € = molar absorptivity of element
b = path length
c' = concentration of atoms in the flame
where c = analyte concentration in sample solution being aspirated
k = efficiency of atomization (a variable)
A standard calibration curve of A vs. c must be prepared
7
8
Hollow Cathode Lamp
Emits the characteristic lines of the element of which the cathode is made.
 possess the correct/right wavelengths for the analyte atoms
o will not be absorbed by other elements in the flame to cause interference.
 narrower than the absorption lines of the element or atoms in the flame because of broadening
of the absorption lines at the high temperature and pressure of the flame
o monochromatic radiation
o no deviation from Beer's law.
Figure 21-15
Relative line widths or hollow-cathode
emission. atomic absorption. and monochromator. line widths are measured at
half the signal height.


no monochromator can isolate linewidths < 0.01 nm.
entire source line width is absorbed
o significant change in Po
o high sensitivity.
Hollow Cathode Lamp
Cathode
a hollow cylinder covered with the element or an alloy element to be determined
Anode
tungsten
Inert gas
Ar or Ne under vacuum ( 100-200 Pa)
Voltage
300 V
Current
1-50 mA
Principle
The inert gas is ionized and the positive gas ions are accelerated to the cathode. The
collision energy is sufficient to cause some atoms of the cathode to be sputtered into
gaseous atoms. These metal atoms are then excited by collisions with electrons and ions,
to emit the characteristic atomic emission lines.
9
7. The hollow cathode lamp
(a) emits the characteristic lines of all elements.
(b) can be operated in the current range from 1 to 50 ~A.
(c) produces light with a narrow line width which will not be absorbed by elements other than
the analyte.
(d) consists of a vacuum (without any gas) inside a glass envelope
10
(e) all of the above.
10. In the AAS instrument, a monochromator is used to
8
(a) to produce monochromatic radiation from the hollow cathode lamp.
(b) adjust the sample flow rate for maximum absorbance.
(c) measure the flame temperature.
(d) remove interference caused by flame emission.
12
(e) amplify the absorbance signal for accurate display.
1. It is crucial to atomize the metal compounds contained in a sample solution for AAS analysis
because
6
(a) ground and excited states cannot exist in the metal compound to absorb light.
6
(b) atoms cannot vibrate or rotate freely in the sample solution.
8
(c) the spectral lines of gaseous atoms are much sharper than those of compounds in the sample
solution.
(d) it is often desirable to measure the total content of a metal in a solution, irrespective of its
different compound forms.
(e) both (c) and (d).
3. Nebulization is essential for
3
(a) slow introduction of sample solution to the flame.
2
(b) minimal consumption of sample solution.
(c) oxidation of the metals in the sample solution.
(d) more efficient atomization of analytes in the flame.
6
(e) all of the above.
10
11
22-2 Atomization in a Flame
p. 619 Harris (5th)
Flame -combustion of a fuel in an oxidant
Fuel: oxidant ratio and flow rates -experimental variables
o maximum atomization and absorbance signal
o highest analytical sensitivity
 normally in stoichiometric amounts.
 for metals (AI, Ti, Mo, V) that form heat-stable or refractory oxides
o oxides do not have the same spectra as their free elements
→ lower atomic absorption signal
o the nitrous oxide-acetylene flame (2900°C) is useful
o a reducing flame (relatively rich in fuel) is more efficient in atomization.
 a lean flame has excess oxidant and is hotter than a rich flame.


use a fuel-rich name to reduce
oxidized analyte species to atoms.
higher name temperatures eliminate
many kinds of chemical
interferences.
e.g. Determination of Ca in the
presence of PO42- has poor sensitivity in
air-acetylene flame (2400-2700 K)
← Ca2P207 is refractory.

Use of nitrous oxide -acetylene
flame (2900-3100 K) improves the
sensitivity.
5. (a)(6pts.)
(b)(4pts.)
What criteria do you use to select a flame for atomic absorption measurements?
Despite the many drawbacks of the graphite furnace atomizer, under what circumstance
must one use it for AAS analysis?
(c)(10pts.) Draw a schematic diagram to illustrate the operation of a flameless atomic absorption
anal yzer for mercury .
(d)(5pts.) Explain why chemical vaporization is commonly performed in the AAS determination
of As, Bi, Ge, Sb, Se, Sn and Te?
5b.(a)(6pts.)
i) Temperature of flame must be sufficiently high to dissociate and atomize the analyte.
ii) Flame temperature is D;ot too high to cause ionization of the atoms.
iii) A reducing flame (excess fuel) .is more efficient for metals that form refractory
oxides.
(b)(4pts.) One must use the graphite furnace atomizer for the AAS analysis of samples at sub-ppb
concentrations.
(c)(10pts.) See lecture notes.
(d)(5pts.) Chemical vaporization is commonly performed in the AAS determination of As, Bi, Ge,
Sb, Se, Sn and Te to improve their atomization efficiency to ~100%.
12
6. The determination of Ca in a sample solution containing PO42- by AAS has poor sensitivity
17
(a) if an air-acetylene flame is used because Ca forms a refractory oxide.
(b) if an air-acetylene flame is used because Ca forms a heat-stable phosphate.
4
(c) if a nitrous oxide-acetylene flame is used because Ca forms a refractory oxide.
(d) if a nitrous oxide-acetylene flame is used because Ca forms a heat-stable phosphate.
7
(e) both (a) and (d).
10. Temperature programming is essential to the success of graphite furnace AAS analysis because
(a) background absorption due to water vapour, smoke and matrix will not interfere with the
analyte absorption signal.
2
(b) uniform heating of the furnace up to 2000-3000°C by electric current cannot be attained in
less than 1 min.
16
(c) atomization of the analyte is most efficiently done in several heating steps.
(d) drying and ashing requires the same furnace temperature.
(e) the analyte should be atomized before the drying and ashing steps.
4. Matrix interference
 absorption of a given concentration of an element in organic solvents can be 4 - 5 times higher
than an aqueous solution of the same concentration.
← increased atomization efficiency due to lower viscosity
→ increased rate of aspiration
→ finer droplets → more efficient evaporation of the solvent.
 Matrix of the sample must always match that of the standards .
e.g. In the determination of Pb in gasoline, a similated solvent matrix must be used for standards,
not water .
2. Chemical interference
 sample components decrease the extent of analyte atomization.
e.g. SO42- and PO42- hinder the atomization of Ca2+, perhaps by forming involatile salts.
 releasing agents can be added to the sample to protect or free the analyte.
e.g. EDT A or 8-hydroxyquinoline protects Ca2+ from SO42- and PO42e.g. La3+ preferentially reacts with PO42- and frees Ca2+.
e.g. In the determination of Ca in serum, Na and K (in concentrations equal to those in serum) are
added to Ca standards to prevent ionization interference .
3. Ionization interference
M(g) ↔ M+(g) + e-(g)
Ionized atoms have different energy levels (and absorb at a different wavelength) from neutral atoms
- the desired atomic signal is decreased.
Alkali metals have the lowest ionization energies
- extensively ionized in a flame.
e.g. At 2450 K Na is -5% ionized; K is 33% ionized.
** ionization suppressor can be added to the sample to decrease the extent of ionization of the analyte.
13
e.g. In the AAS of K, 1<XX> ppm of CsCl is added (CsCl is more easily ionized than K)
Cs(g) ↔ Cs+(g) + e-(g)
high concentration of e- in flame
+
K(g) ← K (g) + e (g)
ionization of K is suppressed.
e.g. In the AAS of Sr, 1000-25000 ppm of K or Na is added AL, as the suppressor .
The absorbance by the sample is dependent on,
 the sample flow rate - decreasing absorbance at higher flow rates
o cooling of the flame by water/solvent from the sample.
Profile or Ca emission line in a cyanoaen/oxylen name.
 height of observation in the flame
o different elements have different absorption profiles:
o two opposing effects: atomization vs. oxidation; oxides are non-absorbing at the
analytical (atomic)
o to obtain maximum analytical sensitivity, flame must be adjusted w.r.t. light beam
maximum absorbance.
Summary
Advantages of Flames for atomization in AAS
 Reproducible
o precision = 1-2% RSD
 Continuous reading
o aspiration of sample solution into the flame is convenient
Disadvantages of Flames for atomization in AAS
 >1 mL of sample solution volume
 Detection limit = ppm
o a small fraction (-5%) of sample solution enters the flame and becomes atomised
o small residence time of atoms in optical path (-0.1 ms)
o they rise rapidly through the flame (depending on flow rates of fuel and oxidant)
14
22-2 Electrically-heated Graphite Furnace for
Atomic Spectroscopy
p.620 Harris (5th)
Figure 22-7
Graphite furnace (electrothermal) for atomization
in AAS
Advantages
 A few JJL of sample solution volume
 Sensitivity = 100-1000 X higher than flames
- entire sample is atomised and confined in
the light path
- residence time > 1 s
 Formation of refractory oxides is prevented
- atomization occurs in an inert
atmosphere
22-2 Atomization in a Graphite Furnace
p. 620 Harris (5th)
Electrically heated in three steps (programmed) after 10 uL of sample injected into the cold graphite
tube or cup:
Temperature
Time
1. Drying to evaporate solvent
100-200 °C
20 s
2. Charring (ashing) to destroy organic matter
500-1400 °C
60 s
- otherwise creates smoke interference
3. Atomization
2000-3000 °C
10 s
- Measure either the maximum absorbance (peak height) or the time-integrated absorbance (peak area).
15. Does atomic absorption spectrometry (AAS) follow Beer's law? What is the path length, b, in flame
AAS?
(a) No, AAS does NOT follow Beer's law.
(b) Yes. b = 1.000 cm.
(c) Yes. b is given by the dimensions of the sample cell.
(d) Yes. b is equal to the dimension of the burning flame.
(e) Yes. b is determined by widths of the entrance and exit slits.
16. What criteria do you use to select a light source for atomic absorption measurement?
(a) It should emit the right wavelength for absorption by the analyte atoms.
(b) Its emission linewidth must be narrower than the absorption linewidth for both high
sensitivity and adhesion to Beer's law.
(c) Its emission linewidth can be isolated by a monochromator from the absorption wavelengths
of other elements.
(d) All of the above.
(e) Only (a) and (b).
15
17. The nebulizer in a flame atomizer is crucial for quantitative AAS analysis because
(a) it determines the atomization efficiency which relates the concentration of atomic vapour in
the flame to the analyte concentration in the sample solution.
(b) it prepares a standard calibration curve of absorbance versus analyte concentration.
(c) it forms volatile hydrides of As, Bi, Ge, Sb, Se, Sn and Te for AAS measurement.
(d) it provides higher heating rates for refractory elements.
(e) it requires only a few III of sample for analysis.
18. What criteria do you use to select a flame for AAS analysis?
(a) Temperature of flame must be sufficiently high to dissociate and atomize the desired
compound.
(b) Flame temperature is not too high to cause ionization of the atoms; flame should not be too
oxidizing.
(c) A reducing flame (usin& excess fuel) is more efficient for metals that form stable refractory
oxides.
(d) All of the above.
(e) Only (b) and (c).
19. What criteria do you use to select a Iraphite furnace. instead of a flame atomizer. for atomic
absorption measurements?
(a) Precision of 1-2 % .
(b) ≥ 1 ml of sample volume.
(c) Detection limits of ppb or sub-ppb.
(d) All of the above.
(e) Both (a) and (b).
20. Write a three-step temperature-versus-time pro&ram for a typicallraphite furnace AAS analysis.
(a) After the sample is injected into the cold furnace. the furnace is programmed to dry the
sample (at 125-C for 20 s to remove solvent).
(b) This is followed by charrin& (at 1200-c for ro s) to destroy orlanic matter (in order to
prevent smoke interference).
(c) Atomization is accomplished by heatinl (to 2700-c for 10 s) to reach a maximum in
absorbance signal.
(d) First (a), next (b), and finally (c).
(e) First (a), next (c), and finally (b).
16
Figure
Temperature
programming
scheme with
electrothermal
atomization
and
resulting signal
Graphite furnace (electrothermal) for atomization in AAS
Disadvantages
 Not very reproducible
precision = 5-10% RSD
 Single-shot reading (~2 min)
replicate analyses can be time consuming
 Frequent calibration is required
electrical connections get corroded over multiple firings
17
Taken from http://www.bioprobe.com/faq.asp
Question: WHAT IS DENTAL AMALGAM?
Answer: Dental amalgam is the most commonly used dental filling material. The dental profession refers to this
material as ''Silver'' amalgam, ''Silver'' fillings, or just ''amalgam'' fillings. Dental amalgam is a mixture of mercury
and a metal alloy. The normal composition is 45-55% mercury; approximately 30% silver, and other metals such
as copper, tin and zinc, dependent upon each manufacturers specific formula. It is obvious from the above
composition that his material should rightfully be called ''Mercury'' fillings or ''Mercury/Silver'' fillings. However,
since the beginning of its use as a dental filling material until the present time, some 150 years or more, the use
of the word ''mercury'' in describing this type of filling has been studiously avoided. Could it be that the ADA and
the dental profession did not want the patient to know that approximately 1/2 of the material implanted in their
teeth was actually one of the most toxic metals known to man? The guise is so effective that most physicians do
not know that they have had mercury implanted in their teeth. Mercury is more toxic than arsenic, lead and
cadmium.
18
Cold Vapour AAS
Mercury does not have a favourable flame MS detection limit and is difficult to determine with the use
of a graphite furnace atomizer. Around 1970, a special technique was developed for the flameless AA~
determination of mercury.
Chemical vaporization of Hg - involves the room-temperature reduction of mercury salts in a sample
so to the elemental state by treatment with an acidic solution of stannous chloride
Hg2+ + Sn2+ → HgO + Sn4+
The finely divided particles of mercury are volatilized by passing air through the sample solution.
This air stream sweeps the mercury atoms as a vapour into a long optical cell for direct absorption
measurement at the 253.7 nm mercury line (without further atomization).
Detection limit = 10-9 g
19
Taken From http://www.bucksci.com/atomic.htm
Atomic Absorption Spectrophotometer
Part Number:
Description:
BUCK 210VGP
Atomic Absorption Spectrophotometer
Detailed Product Information
210VGP Atomic Absorption Spectrophotometer
Buck Scientifics 210VGP Atomic Absorption Spectrophotometer is the culmination of more than 20
years success and experience as a manufacturer of spectroscopy instruments. The 210VGP is a
powerful, compact and cost effective solution to all atomic analyses. It has been designed to meet
the performance and flexibility your laboratory requiresnow and in the future.
Standard Features
The 210VGP is shipped ready for use. All operating conditions are preloaded in the internal
computer, including lamp settings, secondary wavelengths, and alternate methods of analysis for
over 60 elements by flame, furnace, or hydride techniques. The three lamp turret has individual
controls for alignment and staywarm/operating facilities for each lamp. Direct report generation to
a printer or data linking to the optional AAnalyze software package is easily done using the
parallel and RS232C ports.
Analytical Performance
The Buck 210VGP is a high energy, microprocessor controlled single beam atomic absorption
spectrophotometer. Solid state electronics and simple optics provide the basis for our superior
stability and sensitivity. The Ebert mount monochromator and user-selectable bandwidth give the
system maximum flexibility.
Our short-path dynamic nebulizer/burner configuration is highly efficient. An inert needle,
precisely positioned in a high flow venturi, delivers sample to the corrosion proof impact bead.
This results in a tremendously high nebulization effect for all types of sample matrices.
On-board Microprocessing
The completely integrated firmware found in the Buck 210VGP gives absolute control of operation
and data manipulation. From setting lamp parameters to optimizing calibration curves and furnace
temperature/time programs; the internal firmware provides access to all of the system settings.
20
Cost Effectiveness
The 210VGP is designed for minimal maintenance. The burner assembly is easily accessible for
quick cleaning. The microprocessor uses state-of-the-art components and is machine assembled
for quality and reliability. With the best performance-to-price ratio in the market, the 210VGP is
truly an affordable instrument that will maximize your return on investment.
Optical System
Buck Scientific's Stable Beam System creates an optimum optical alignment. We have the lowest
number of energy reducing optics (no energy wasting beam splitters) for the greatest throughput
of sample energy.
Background Correction
The Model 210VGP offers two exceptional background correction systems; a unique, in-line D2
system and Variable Giant Pulse (VGP) correction.
Deuterium (D2) Continuum Lamp Correction
A D2 lamp emits radiation from the far-UV region (<190nm through approximately 350nm). With
our proprietary in-line system, the D2 emission corresponds precisely with the spectrum of the
analyte. Using modulated signals, the absorbance of analyte and background interferences are
ratioed resulting in a clean, unbiased
absorbance signal.
Variable Giant Pulse (VGP) Correction
Hollow cathode lamps normally operate at currents of 3-15 mA. If the applied power is raised to
several hundred mA, they exhibit a phenomenon called self-reversal. This giant pulse of current
changes the nature of the analyte absorption line so it will only measure the background
absorbance. Like D2 correction, the background absorbance is subtracted from the total signal to
give the corrected sample reading. The VGP system removes interferences for elements outside
the normal D2-UV region.Bucks unique internal software allows the user to vary both modes of
the background correction to optimize the analysis for selected elements.
Options and Accessories
Model 420 Continuous Flow Hydride
The Model 420 hydride accessory provides virtually instant access to PPT detect ability for hydride
metals in a semi-automated, continuous flow system. Low level determinations of As, Se, Sb, Bi,
Te, Sn, and Ge are easily performed. The cold vapor mercury model achieves trace level analysis.
Model 220AS Furnace Auto sampler
Complete system automation can be achieved using the Model 220AS Auto sampler. This fully
programmable, random access system provides 40 sample cups and eight calibration/QC cups. It
can also perform automatic matrix modifier additions and sample dilutions. Sample cups can be
identified in the built-in table, and method development time can be reduced substantially.
Model 240 Flame Auto sampler
To enhance laboratory productivity, this 150 position auto sampler features a fast access rate for
unattended operation. Users can program various dilution and sample volumes with automated
spike addition. A high quality coating resists acids and bases. Full random access allows for
complete flexibility.
AAnalyze Data Acquisition Software
Provides automated data collection via RS232C, conforming to all EPA protocols. Data handling
capabilities include auto calibration, spike recoveries, drift corrections, and QC charting.
Calibrations are performed using linear or polynomial regressions in a Windows-based, easy to
use format. Full report generation with calibration curves and QC chart printouts as well.
21
AA Accessories
Buck Scientific offers high quality hollow cathode lamps, and aqueous and organic standard
solutions. For furnace operation, we offer matrix modifiers and buffers, plus three styles of
graphite tubes. Buck has all the accessories and supplies needed for atomic absorption
spectroscopy.
p. 614, Harris 4th
21- 5 INTERFERENCE
any effect that changes the
signal when analyte
concentration remains
unchanged.
1. Spectral interference overlap of analyte signal with
signals due to other elements,
molecules, the flame or furnace.
The spectrum of a molecule is
much broader due to vibrational
and rotational transitions.
** choose another wavelength
for analysis, or use D2 or
Zeeman background
subtraction.
22