Field Methods of Monitoring Aquatic Systems Unit 12 – Metal Ions: AAS Copyright © 2006 by DBS

Sources NATURALANTHROPOGENIC 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 HNO 3 )

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 NH 4 +

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 techniqueIncreased 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. PO 4 3-, SO 4 2- and silicate ion e.g. Ca 2+ forms refractory insoluble Ca 3 (PO 4 ) 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) Abs 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 = ( x conc) 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 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 M e - → M Potential of electrode is changed (+ve) metal is oxidized M → M e - 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. Cr 2 O 7 2- > Cr 3+ Combination of analytical techniques may be used SpeciesExamplePhysical Form Free metalPb 2+ Solution Ion-pairPbHCO 3 + Solution Complexes with organics Pb 2+ /EDTASolution Complexes with natural acids Pb 2+ /fulvic acidSuspension Ion absorbed onto colloids Pb 2+ /Fe(OH) 3 Colloidal Metal within decomposing OM Pb in organic solidsSolid Ionic solidsPb 2+ held within clays PbCO 3 Solid

Response of Analytical techniques to Metal Species TechniqueResponse Atomic spectrometryAll metal species (total metal) Visible absorption spectrometryFree ions + ions from complexes ASVFree ions + ions from complexes (total ASV- labile content) LCNon-labile (interconverting) species can sometimes be determined separately GCOrganic 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.