AAS.ppt

Air, Water and Land Pollution
Chapter 9:
Atomic Spectroscopy for Metal Analysis
Copyright © 2010 by DBS

Contents
• Introduction to the Principles of Atomic Spectroscopy
• Instruments for Atomic Spectroscopy
• Selection of the Proper Atomic Spectroscopic Technique
• Practical Tips to Sampling, Sample Preparation, and Metal Analysis

Introduction to the Principles of Atomic Spectroscopy
• Molecular (UV-VIS and IR) spectroscopy:
– deals with inorganic or organic molecules in solution
• Atomic spectroscopy:
– mainly deals with high energy absorption/emission of individual atoms

Introduction to the principles of Atomic Spectroscopy
• In molecular spectroscopy, low-energy radiation (IR, VIS, UV) causes molecules to
vibrate/rotate or outer electrons to transit from low to high energy states
• In atomic spectroscopy, higher energy radiation is used to transit inner electrons from
low to high energy states
• High energy radiation is provided by:
(a) flame in flame atomic absorption spectroscopy (FAA)
(b) electrothermal furnace in flameless graphite furnace atomic absorption
spectroscopy (GFAA)
(c) plasma in inductively coupled plasma-optical emission spectroscopy (ICP-OES)
(d) X-rays in X-ray fluorescence spectroscopy (XRF)

3 major types of atomic spectroscopy:
• Absorption – light of a wavelength characteristic of the element of interest radiates
through the atom vapor. The atoms absorb some of the light. The amount absorbed
is measured.
• Emission – sample is heated to excitation/ionization of the sample atoms. Excited and
ionized atoms decay to a lower energy state through emission. Intensity of the light
emitted is measured.
• Fluorescence – a short wavelength is absorbed by the sample atoms, a longer
wavelength (lower energy) radiation characteristic of the element is emitted and
measured

Introduction to the principles of Atomic Spectroscopy
3 major types of atomic spectroscopy:
• Absorption – light of a wavelength characteristic of the element of interest radiates
through the atom vapor. The atoms absorb some of the light. The amount absorbed
is measured.
• Emission – sample is heated to excitation/ionization of the sample atoms. Excited and
ionized atoms decay to a lower energy state through emission. Intensity of the light
emitted is measured.
• Fluorescence – a short wavelength is absorbed by the sample atoms, a longer
wavelength (lower energy) radiation characteristic of the element is emitted and
measured

• Liquid sample is aspirated to become aerosols of fine particles (nebulization)
• Flame vaporizes the aerosols (atomization)
• Elevated temperatures in a flame or furnace changes the chemistry of atoms
• Temperature affects the ratio of excited and unexcited atoms
• Beer’s law is used to calculate concentration

Flame and Flameless Atomic Absorption
Processes Occurring in Flame and Flameless Furnace
• Solution is introduced into a high-temperature flame or furnace, molecules containing
the elemental atoms become gaseous atoms through a series of reactions
• Flame and flameless furnaces are two common radiation sources used in atomic
spectroscopy

• e.g. Calcium present as a salt (CaCl2):
1. removal of water produces gaseous CaCl2
2. gaseous CaCl2 is further dissociated into gaseous Ca0 atoms
At elevated temperatures Ca can have other electronic states:
3/4. Ca0* (excited Ca atom),
5. Oxide/Hydroxide formation
6. Ca+ (ionic Ca),
7. Ca+* (ionic Ca with excited e-)

• Ca analyzed by atomic absorption spectroscopy the radiation absorption of gaseous
atom (Ca0) is measured (Fig. 9.1 reaction 3)
• Ca can also be analyzed by atomic emission spectroscopy (Fig. 9.1 reaction 4)

• If M is used to denote the vapor form of any atom (metals),
M + hν → M* (for FAAS)
M * → M + hν (for FAES)
• Note: Formation of metal oxide/hydroxide (5) and ionization of gaseous atom (6) are
common interferences that must be miimized

• Flame used in abs/emission spectroscopy is around 2000-3000 K
e.g. air-acetylene 2250 ºC, nitrous oxide-acetylene 2955 ºC
• Flameless graphite furnace – electrically heated graphite boat

Understanding “Nebulization” and “Atomization” Process: Why Higher
Sensitivity is Achieved in Flameless GFAA Than Flame FAA
• Nebulization
– In FAAS a liquid sample is nebulized – aspirated into small liquid particles
(aerosols), remaining larger droplets condense out (only around 10 % of fine
aerosols reach the burner)
• Atomization = conversion of element into atomic vapor
– In FAAS nebulization takes place prior to atomization making the process far less
efficient than GFAAS
– In GFAAS the entire sample is atomized inside the graphite boat leading to lower
detection limits

• Graphite furnace AAS
• Sample injection into graphite tube
– Drying
– Decomposition
– Atomization
• Absorbance is measured during
atomization
Advantages of
FAAS
Advantages of
GFAAS
Simple technique Increased
sensitivity (μg L-1)
Solvent extraction
removes
interferences
Not needed
Readily available
equipment
Smaller samples
Shorter instrument
time
Unattended
operation possible
Lower instrument
cost
Reduced
contamination

Quantitation and Qualification of Atomic Spectroscopy
• Concentration of an element present in the sample is described by Beer’s law
• Absorption depends on the number of ground state atoms in the optical path
A = klC
Where A = absorbance, C = concentration, l = path length of the flame, k = coefficient
unique to each element
• For emission spectroscopy, the emitted light intensity I of a population of n excited
atoms depends on the number of atoms dn that return to the ground state during an
interval time dt (dn/dt = kn). As n is proportional to C the concentration of the
element, the emitted light intensity I, is also proportional to concentration:
I = klC

Inductively Coupled Plasma (ICP) Atomic Emission
• Same process occur as with flame atomic emission (Fig. 9.1)
• ICP (plasma) is an ionized gas at extremely high
temperature
Ar → Ar+ + e-
• The energy in the plasma is transferred by collision of
Ar+ with the atoms of interest
• Enough to ionize many metals with I.E. 7-8 eV
• Most metals are ionizable emitting in the UV range, non-
metals do not form ions in the ICP (require a vacuum)

Inductively Coupled Plasma (ICP) Atomic Emission
• ICP can theoretically analyze almost all elements
• FAAS and FES can only measure around 70 elements
• ICP can also measure multiple elements simultaneously, whereas flame techniques
can only measure one at a time
Note: ICP-AES (Atomic Emission Spectroscopy) is used interchangeably with ICP-
OES (Optical Emission Spectroscopy)

Atomic X-Ray Fluorescence
• X-ray fluorescence is a two-step process
1. Excitation of inner electrons via X-rays
2. “jump ins” of the electrons from higher energy levels to fill vacancies
• Atom is stabilized – emits characteristic X-ray fluorescence unique to the element
• XRF instrument measures the photon energy from the fluorescence to identify the
element and the intensity of the photon to measure the amount of element in the
sample

Instruments for Atomic Spectroscopy
• Basic instrument components:

1. Light source: hollow cathode lamp (HCL) of the element being measured.
Provides the spectral line for the element of interest.
Inside the lamp, filled with argon or neon gas, is a cylindrical metal cathode
containing the metal for excitation, and an anode. When a high voltage is
applied across the anode and cathode, gas particles are ionized. As voltage is
increased, gaseous ions acquire enough energy to eject metal atoms from the
cathode. Some of these atoms are in an excited states and emit light with the
frequency characteristic to the metal.

Atomic Spectroscopy
2. Nebulizer and atomizer:
In a flame system (a), the nebulizer sucks up the liquid sample, creates a fine
aerosol, mixes the aerosol with fuel/air. Flame creates vaporized atoms.
In a flameless graphite furnace system (b) both liquid and solid
samples are deposited into a graphite boat using a syringe inserted
through a cavity. Graphite furnace can hold an atomized sample in the
optical path for several seconds, compared with a fraction of a second
for a flame system
– results in higher sensitivity of the GFAA compared to FAA

Atomic Spectroscopy
2. Monochromator:
Isolates photons of various wavelengths that pass through the flame or furnace.
Similar to the monochromator in UV-VIS spectroscopy in that it uses slits, lenses,
mirrors and gratings/prisms.
3. Detector:
The PMT detector determines the intensity of photons in the analytical line exiting
the monochromator

Atomic Spectroscopy
3. Detector:
The PMT detector determines the intensity of photons in the analytical line exiting
the monochromator.
Before an analyte is atomized, a measured signal is generated by the PMT as
light from the HCL passes through the flame/furnace. When analyte atoms are
present – some part of that light is absorbed by those atoms. This causes a
decrease in PMT signal that is proportional to the amount of analyte.

Cold Vapor and Hydride generation Atomic Absorption
• FAAS and GFAAS can measure most elements
• Cannot measure: mercury, selenium and arsenic
• All too volatile to be measured by flame or furnace techniques

Cold Vapor Atomic Absorption (CVAA) Spectroscopy for Hg
• Free mercury atoms exist at room temperature, no requirement for heating
• Sample may contain Hg0, Hg2
2+ or Hg2+
• In CVAA Hg is chemically reduced to the atomic state by reaction with a strong
reducing agent (e.g. SnCl2 or NaBH4) in a reaction flask
• Hg is then carried via gas stream to the absorption cell

Hydride Generation Atomic Absorption (HGAA) Spectroscopy for As and Se
• AsH3 and SeH3 generated by reaction samples containing As and Se with NaBH4
• Uses same setup as FAAS except it switches nebulizer for the hydride generation
module

Hydride Generation Atomic Absorption (HGAA) Spectroscopy for As and Se
• Sample is reacted in the external hydride generator with reducing agent (NaBH4)
• Hydride generated is then carried via inert gas to the sample cell in the light path of
the FAAS
• Unlike CVAA product is not free atoms but AsH3 / SeH3 which are not measurable
• Sample cell must be heated to dissociate the hydride into free atoms (As0) and Se0)
• Higher sampling efficiency leads to
lower detection limits:
ppb vs ppm for regular FAAS and
GFAAS

Inductively Coupled Plasma Atomic Emission (ICP-OES)

• Sample is nebulized and entrained in the flow of plasma support gas (Ar)

• Plasma torch inner tube contains the sample aerosol and Ar support gas
• Radio frequency generator produces a magnetic field which sets up
an oscillating current in the ions and electrons of the support gas (Ar)
• Produces high temperatures (up to 10,000 K)

• Atomizes the sample and promotes atomic and ionic transitions which are observable
at UV and visible wavelengths
• Excited atoms and ions emit their characteristic radiation, which are collected by a
device that sorts the radiation by wavelength
• Intensity of the emission is detected and turned into a signal that is output as
concentration

• Three main types:
– wavelength dispersive
– energy dispersive
• Consists of a polychromatic source (X-ray tube or radioactive material), sample
holder, photon detector (Si-semiconductor)

• Wavelength dispersive XRF (WDX) – fluorescence radiation is separated according
to wavelength by diffraction on an analyzer crystal before being detected, can detect
multiple elements at the same time
• Energy dispersive XRF (EDX) – energy of a photon of a specific wavelength is
detected

• Energy dispersive XRF – consists of a polychromatic source (X-ray tube or
radioactive material), sample holder, detector
– Smaller and cheaper than wavelength dispersive XRF
– Ideal for field investigations
– No moving parts

• XRF is unique among all atomic spectroscopic techniques in that it is non-destructive
• Good for elemental composition analysis
• For quantitative analysis require reference standard with similar matrix to that of the
sample
• Detection limits are in the ppm (mg/kg) range

Selection of the Proper Atomic Spectroscopic Techniques
• Important factors:
– Detection limit
– Working range
– Sample throughput
– Cost
– Interferences
– Ease of use
– Availability of proven methodology

Ions Found in Natural Waters
Conc. Range
(mg L-1)
Cations Anions
0-100 Ca2+, Na+ Cl-, SO4
2
-, HCO3
-
0-25 Mg2+, K+ NO3
-
0-1 Fe2+, Mn2+, Zn2+ PO4
3-
0-0.1 Other metal ions NO2
-
Reeve, 2002

Comparison of Detection Limits and Working Range
• Low detection limit is essential for trace analysis
• Without low level capability – sample pre-concentration is required
FAAS > ICP-OES > HGAAS > GFAAS > ICP-MS

• Analytical range is the concentration range over which quantitative results can be
obtained without the need for recalibration
• Ideal working range minimizes analytical effort (e.g. dilutions, pre-concentration)

Comparison of Interferences and Other Considerations
Interference
• Three types:
(i) spectral,
(ii) chemical,
(iii) physical

Interference
• Spectral: in spectroscopy, interference occurs when another emission line (e.g. from
other elements in the sample) is close to the emitted line of the test element and is
not resolved by the monochromator
• Chemical: formation of undesired species during atomization
• Physical: variation of instrument parameters such as uptake in the burner and
atomization efficiency (gas flow rate, sample viscosity etc.)

Other Considerations
• Sample throughput, cost, ease of use, availability of proven methodology
• Single-element (FAAS and GFAA) vs. multi-element (ICP-OES/MS)
• Single:
– Change of lamp
– Run time ~1 min
• Multi:
– 10-40 elements per minute

• Cost:
FAAS < GFAAS > ICP-OES << ICP-MS

• ICP-OES and ICP-MS are multi-element techniques favored when there is a large
number of samples and cost is not a concern

• ICP-OES has become the dominant instrument for routine analysis of metals
• Compared to FAAS:
– Lower interferences (due to higher temperatures)
– Spectra for most elements can be recorded simultaneously under the same
conditions
– Higher temperature allows compounds (e.g. metal oxides) to be measured
– Determination of non metals (e.g. Cl, Br, I, S)
– Wider linear working range
• Cons: cost, carrier gas consumption (runs overnight), slightly more complicated to
run, limited use for group one metals (Li, Na, K, etc.) (emission lines are near IR)

Practical Tips to Sampling
Correction for Sample Moisture and Dilution
• For solid samples moisture content (w), dry weight of sample (m), and volume of
digestate (V) is required in order to calculate concentration of element:

Instrumental Drift and Run Sequence QA/QC
• Instrument drift is a common problem
• Inexperienced analysts are often frustrated by broad variation of results for the same
sample tested in different batches
• Very unlikely to get the exact same results for the same sample
• Follow QA/QC protocols and report margin of error (SD)
• Usual for operator to run the instrument blank and standards several times between
samples

Erroneous Data and Methods of Compensation
• Erroneous results arise due to one or more of the sources of interference described
above
• Compensation methods: background correction, higher temperatures, release agent,
alternative wavelength, internal standard, matrix spike, etc.
• e.g. Chemical interferences
– Refractory salts e.g. PO4
3-, SO4
2- and silicate ion
e.g. Ca2+ forms refractory insoluble Ca3(PO4)2
– Add release agent (10% lanthanum solution or EDTA)
• Complex solutions (matrix) require method of standard additions
– Add small volumes higher concentration standards (change in volume is
negligable)
– Graph of concentration vs. absorbance
– Concentration of sample is x-intercept
– Overcomes problem of matrix effects

Erroneous Data and Methods of Compensation
• Simple solutions (e.g. water) use standard curve technique to find unknown
concentration
• Complex solutions (matrix) require method of standard additions
– Add small volumes higher concentration standards (change in volume is
considered negligible)
– Graph of concentration vs. absorbance
– Concentration of sample is x-intercept
– Overcomes problem of matrix effects

Question
A series of solutions is made up by adding 0.1, 0.2, 0.3, 0.4 and 0.5 mL of a
10 mg L-1 lead standard to 100 mL aliquots of the unknown solution. The following
results were obtained:
Volume std. (mL) 0 0.1 0.2 0.3 0.4 0.5
Abs 0.27 0.37 0.53 0.65 0.75 0.88
Plot a calibration graph and determine the concentration of the unknown
Assuming constant volume of 100 mL, the concentration increase in the
5 solutions are 10, 20, 30, 40, and 50 μg L-1.
(e.g. 0.5 mL of 10 mg/L = 5 x 10-3 mg in 100 mL = 5 μg/0.1 L = 50 μg L-1)
Absorbance = (0.01235 x conc) + 0.2694
Unknown = 21.8 g μL-1 lead

References
• Csuros, M. and Csuros, C. (2002) Environmental Sampling and Analysis for Metals. CRC
press, Boca Raton, Fl.
• Tatro, M.E. (2000) Optical Emission Inductively Coupled Plasma in Environmental
Analysis. Encyclopedia of Analytical Chemistry, Edited by Meyers, R.A. John Wiley &
Sons, West Sussex, UK.

Questions
1. Explain: (a) The difference in the electronic configuration among various species of
calcium, that is, Ca in CaCl2, Ca0, Ca0*, and Ca2+; (b) of these species, which one(s) are
desired for AAS measurement of Ca and which one(s) are unwanted species that may
cause interference for AES measurement of Ca?
2. For the following atomic absorption spectrometers – FAA and ICP-OES: (a) Sketch the
schematic diagram; (b) Describe the principles (functions) of major components.
15. Explain why NH4NO3 is added to seawater when Pb and Ca are analyzed by FGAA.
(hint: removes interference due to high salinity – show chemistry)
23. A groundwater sample is analyzed for its K by FAA using the method of standard
additions. Two 500 µL aliquots of this groundwater sample are added to 10.0 mL DI water.
To one portion, 10.0 µL of 10 mM KCl is added. The net emission signals in arbitrary units
are 20.2 and 75.1. What is the concentration of K in this groundwater in mg/L? (hint: use
example 9.2)
24. A 5-point calibration curve was made for the determination of Pb via FAAS. The
regression equation was: y = 0.155x + 0.0016, where y is the signal output as absorbance,
and x is the Pb concentration in mg/L.
(a) A contaminated groundwater sample was collected, diluted from 10 to 50 mL, and
analyzed without digestion. The absorbance reading was 0.203 for the sample. Calculate
the concentration of Pb in this groundwater sample.

Questions
23. A groundwater sample is analyzed for its K by FAA using the method of standard
additions. Two 500 µL aliquots of this groundwater sample are added to 10.0 mL DI water.
To one portion, 10.0 µL of 10 mM KCl is added. The net emission signals in arbitrary units
are 20.2 and 75.1. What is the concentration of K in this groundwater in mg/L? (hint: use
example 9.2)
Assume x millimols (mmols) of K in 500 µL sample
Total mmols K in spiked sample = x + 10 mmol/L x 10µL x 1 L / 106 µL
= x + 1 x 10-4 mmols
Using ratio technique:
x/20.2 = (x + 1 x 10-4 mmols) / 75.1
Solve for x, x = 3.89 x 10-5 mmols in 500 µL aliquot
x = 3.89 x 10-5 mmols / 500 x 10-6 L x 1 mol/1000 mmols = 7.78 x 10-5 mols/L
7.78 x 10-5 mols/L x 39.10 g/mol x 1000 mg/g = 3.04 mg/L = 3.04 ppm

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