Transcript Field Methods of Monitoring Atmospheric Systems

Field Methods of Monitoring
Aquatic Systems
Unit 12 – Metal Ions: AAS
Copyright © 2006 by DBS
NATURAL
ANTHROPOGENIC
Sources
LEACHING OF
ORE DEPOSITS
FOSSIL-FUEL
COMBUSTION
WIND-BLOWN
DUST
MINING &
SMELTING
ATMOSPHERE
VOLCANOES
IRON & STEEL
PRODUCTION
FOREST FIRES
WASTE
INCINERATION &
DISPOSAL
SEA SPRAY
AQUATIC ENVIRONMENT
Techniques
• Atomic Spectrometry
– Flame Atomic Absorption Spectrometry (Flame AAS)
– Graphite Furnace Atomic Absorption Spectrometry (GFAAS)
– Inductively Coupled Plasma-Optical Emission Spectrometry (ICPOES)
– Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)
• Visible Absorption Spectrometry (CHEM3550)
• Anodic Stripping Voltammetry
Storage
• Polyethylene bottles – less likely to contaminate sample than
glass (except Hg analysis)
• Acidification – minimizes precipitation of metal ions
(2 mL 5 M HCl per L of sample)
• Cleaning – acid washing ensures complete removal of metal
ions. Reserve glassware for metal analyses (e.g. Al analysis
pre-leach with dilute HNO3)
Pretreatment
(Not required for GFAAS + ICP)
•
Pretreatment step
– Evaporation to dryness, redissolution in acid
– Partial evaporation with acid
– Digestion with acid
•
Extraction/Concentration step
– Solvent extraction – FAAS/UV-VIS
– Concentration step – IC/ASV
Dissolves suspended
material ensures metal
present as free ion
Removes interfering ions
(i) Formation of neutral complex with organic ion and
extraction into organic solvent, (ii) chelating or IE columns
FAAS
•
•
•
A light beam of the correct
wavelength specific to a
particular metal is directed
through a flame
The flame atomizes the sample
producing atoms in the ground
state. These atoms absorb
radiation from the lamp
Absorption is related linearly to
concentration (0 – 5 mg L-1)
Question
From your previous knowledge of FAAS can you think of some
of the advantages of this technique?
It is a rapid technique and can be easily automated
It is a simple method for routine use
Standard procedures are available for all metals
The analyses are generally free from interferences, known
interferences can be overcome
Apart from the pretreatment stage little or no sample preparatio
is needed for aqueous samples
Major Cations: Na, K, Ca and Mg
• Atomic emission (flame photometry) is the preferred
technique for Na and K
• Intensity of light emitted from electronically excited
atom is proportional to the concentration of the
excited species
• Mg must be measured via AA
Metals via FAAS
• Zn, Fe, Mn: partial evaporation
• Others: adjust pH, chelation with ammonium
pyrrolidinedithiocarbamate (APDC) followed by
solvent extraction (MIBK)
Disadvantages:
Time consuming
Insufficient sensitivity for lowconcentrations
NH4+
Risk of contamination
SM 3-19
Flameless AA
•
•
•
•
Graphite furnace AAS
Sample injection into graphite tube
– Drying
– Decomposition
– Atomization
Absorbance is measured during
atomization
Error due to background
interference (light scattering), can
be corrected
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
Quantification
•
•
•
External standards and calibration graph
Chemical interferences
– Refractory salts e.g. PO43-, SO42- and silicate ion
e.g. Ca2+ forms refractory insoluble Ca3(PO4)2
– Add release agent (10% lanthanum solution or EDTA)
Complex solutions 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
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 unknonw solution. The following
results were obtained:
Volume std. (mL) 0
Abs
0.27
0.1
0.37
0.2
0.53
0.3
0.65
0.4
0.75
0.5
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 μ g L-1.
Absorbance = (0.01235 x conc) + 0.2694
Unknown = 21.8 g μL-1 lead
Quantification
Iductively Coupled Plasma Techniques
• Excellent for analyzing large
numbers of samples of varying
composition
– Does not require
preconcentration
– Does not use flammable gases
– May be operated unattended
• Sample is atomized in an ionized
argon plasma flame 6000-1000 K
• ICP-OES and ICP-MS
Visible Spectrometry
• Common before use of
atomic spectrometric
techniques
• Now used for portable
devices (e.g. Fe, Mn, Cr, Cu)
Anodic Stripping Voltammetry (ASV)
•
•
•
•
•
•
Electrochemical method
Electrolytic cell consisting of 3 electrodes
– Working electrode (mercury drop or film)
– reference electrode
– counter cell
Sample is placed in a cell containing electrolyte
Quantity of metal deposited on working electrode (-ve) is
proportional to concentration
M2+ + 2e- → M
Potential of electrode is changed (+ve) metal is oxidized
M → M2+ + 2eHeight of peak current is proportional to concentration
Metal Speciation
•
•
•
Speciation – the
different physical and
cheical forms of a
substance
Transport and
toxicology different
for each!
e.g. Cr2O72- > Cr3+
Combination of
analytical techniques
may be used
Species
Example
Physical Form
Free metal
Pb2+
Solution
Ion-pair
PbHCO3+
Solution
Complexes with
organics
Pb2+/EDTA
Solution
Complexes with
natural acids
Pb2+/fulvic acid
Suspension
Ion absorbed onto
colloids
Pb2+/Fe(OH)3
Colloidal
Metal within
decomposing OM
Pb in organic solids
Solid
Ionic solids
Pb2+ held within
clays
PbCO3
Solid
Solid
Response of Analytical techniques to Metal Species
Technique
Response
Atomic spectrometry
All metal species (total metal)
Visible absorption spectrometry
Free ions + ions from complexes
ASV
Free ions + ions from complexes (total ASVlabile content)
LC
Non-labile (interconverting) species can
sometimes be determined separately
GC
Organic derivatives
Text Books
•
•
•
•
•
•
•
•
Rump, H.H. (2000) Laboratory Manual for the Examination of Water, Waste Water and Soil.
Wiley-VCH.
Nollet, L.M. and Nollet, M.L. (2000) Handbook of Water Analysis. Marcel Dekker.
Keith, L.H. and Keith, K.H. (1996) Compilation of Epa's Sampling and Analysis Methods.
CRC Press.
Van der Leeden, F., Troise, F.L., and Todd, D.K. (1991) The Water Encyclopedia. Lewis
Publishers.
Kegley, S.E. and Andrews, J. (1998) The Chemistry of Water. University Science Books.
Narayanan, P. (2003) Analysis of environmental pollutants : principles and quantitative
methods. Taylor & Francis.
Reeve, R.N. (2002) Introduction to environmental analysis. Wiley.
Clesceri, L.S., Greenberg, A.E., and Eaton, A.D., eds. (1998) Standard Methods for the
Examination of Water and Wastewater, 20th Edition. Published by American Public Health
Association, American Water Works Association and Water Environment Federation.