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.