field methods of monitoring aquatic systems unit 12 – metal ions: aas copyright © 2006 by dbs
TRANSCRIPT
Field Methods of Monitoring Aquatic Systems
Unit 12 – Metal Ions: AAS
Copyright © 2006 by DBS
SourcesNATURAL ANTHROPOGENIC
FOSSIL-FUEL COMBUSTION
MINING & SMELTING
IRON & STEEL PRODUCTION
WASTE INCINERATION
& DISPOSAL
AQUATIC ENVIRONMENT
ATMOSPHERE
WIND-BLOWN DUST
VOLCANOES
FOREST FIRES
LEACHING OF ORE DEPOSITS
SEA SPRAY
Techniques
• Atomic Spectrometry– Flame Atomic Absorption Spectrometry (Flame AAS)– Graphite Furnace Atomic Absorption Spectrometry (GFAAS)– Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-
OES)– 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
(i) Formation of neutral complex with organic ion and extraction into organic solvent, (ii) chelating or IE columns
Dissolves suspended material ensures metal present as free ion
Removes interfering ions
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 low-concentrations
Risk of contamination
SM 3-19
NH4+
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-, SO4
2- 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 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 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+ + 2e-
• Height 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 ASV-labile 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.