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Analytical methods
for studying trace metal speciation
in the natural environment
(Analytiske metoder for speciering av spormetaller i naturen)
As Hg Cr Sn Se Pb Cd Fe Cu Zn Ni Co Al …..
Date: 8 February 2006
Duration: 45 minutes
Target groups: 1st and 2nd year Chemistry students
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Contents
1. Chemical speciation and fractionation
2. Analytical strategies and methodological
approaches
3. In-situ (on-field) speciation analysis in aquatic
systems
4. Conclusions
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1.
Chemical Speciation and Fractionation
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Chemical Speciation and Fractionation
•
•
•
•
- Specific forms of an element
Chemical species
Fe(II)/Fe(III);
As(III)/As(V)
- Activity
of identifying
and
Speciation analysis
measuring
species
CuCl2/CuCO
3; Hg/CH3HgCl
Distribution
amongst
Speciation of an element
chemical species in a
Fractionation - Analytes classification
system
according
to physical or chemical properties
Acc. to “Guidelines for terms related to chemical speciation and fractionation of
elements. Definitions, structural aspects, and methodological approaches”
(IUPAC Recommendations, Pure Appl. Chem. 2000)
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Speciation of Metals
•
•
•
•
•
Isotopic composition
Electronic and oxidation state
Inorganic compounds and complexes
Organometalic compounds
Organic and macromolecular complexes
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Benefits and Fields of Interest
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What is determining the biogeochemical impact
of the metal ions species in aqueous systems?
• Concentration
• Nature of considered organism
• Physico-chemical form:
• Particulate ( > 1 µm)
• Colloidal (1 nm – 1 µm)
• Dissolved (< 1 nm):
Free metal ions
Simple inorganic complexes
Complexes with anthropogenic and natural
ligands



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2.
Analytical Strategies
and Methodological Approaches
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Analytical Strategy – the Main Steps
1. Formulation of the problem
2. Sampling and sample preparation
3. Measurement of the analytical signal
4. Analytical signal interpretation (quantitative and qualitative)
5. Critical evaluation of the analytical performances and
method validation
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What is relevant in a metal speciation study?
Individual species
Groups
of different species
with similar properties
1. Formulation of the problem
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Bio-uptake of Metal Species in Soils
• Exchangeable fraction: species most available for biouptake (reagent used: acetic acid 0.11 M)
• Reducible fraction: potentially available for plants
(reagent used: a reducing agent like hydroxylamine chloride)
• Oxidizable fraction: potentially available for plants
(reagent used: an oxidizing agent like H2O2 and NH4COOCH3)
• Rezidual fraction: contains naturally occurring minerals
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Bio-uptake of Metal Species
in Aquatic Systems
• Free metal ions – related to biological uptake
• Dynamic metal species (free metal ions and small
labile complexes) – potentially availably for organisms
• The particulate and colloidal species – role in
transport and residence time
The total extractable metals –
the reservoir of metal in the test solution
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The Baia Mare Accident
Surface-water
sampling sites
!! Higher toxicity due to heavy metals (Cu(II),
Zn(II))
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Metal Species in Aquatic Systems
• Free metal ions – related to biological uptake
• Dynamic metal species (free metal ions and small
labile complexes) – potentially availably for organisms
• The particulate and colloidal species – role in
transport and residence time
The total extractable metals –
the reservoir of metal in the test solution
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Sampling – Sample preparation
A major challenge in speciation analysis
2. Sampling and Sample Preparation
Sampling
Transport
Pre-treatment
Preservation
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Contamination
Loss of analyte
Species transformation
16
What is influencing the species stability?
• Chemical factors
• Physical factors
• Biological factors
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Treatments for Sample Preservation
•
•
•
•
•
•
•
Acidification
Low-temperature
Drying
Freezing
Pasteurization
Lyophilization
Adsorption on cartridges or
solid-phase micro-columns
• Storage in the dark
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Preservation of Sb samples:
- Acidification to prevent
hydrolysis
- Extraction on solid-phase
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Artifacts in Sample Speciation
0.2 mM Pb2+, pH 6
Pb2+(aq)
Pb2+(ads)
Fe(OH)
Fe2+ 3
O2
• Precipitation
• Sorption onto the container walls
% total content of Pb(II)
100
80
60
PS film coated glass
40
20
Glass cell
0
0
4
8
12
16
20
24
time / h
Adapted from J. P. Pinheiro et al., Anal. Bioanal. Chem., 2004
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Analytical Methods
for Trace Metal Analysis and Speciation Analysis
Separation/ extraction
• Chromatography (GC, LC, HPLC)
• Capillary electrophoresis
• Ion-chromatography
3. Measurement of
• L-L extraction;
• Ion-exchange; co-precipitation….
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Measuring techniques
• Atomic spectroscopy
• Inductively coupled plasma-mass
the
analytical
signal
spectroscopy
(ICP-MS)
• Neutron activation analysis (NAA)
• X-ray photoelectron spectroscopy
• Electroanalytical methods
20
IC-ICP-MS Chromatogram of 50 mg/L Arsenic Species
Dimethylarsinic acid (DMA, cacodylic acid)
O
||
H3C-As-CH3
|
OH
Hyphenated techniques for speciation analysis
Separation – Excitation – Detection
Arsenious acid (As+3)
HO-As-OH
|
OH
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HPLC – ICP – MS
GC – ICP – MS
IC – ICP – MS
Monomethylarsonic acid (MMA)
O
||
H3C-As-OH
|
OH
Arsenic acid (As+5)
O
||
HO-As-OH
|
OH
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Metals speciation analysis
by electroanalytical techniques
Potentiometry
Free metal ion
and labile forms
concentration
Sample
Free metal ion
activity
Voltammetry
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Ion-selective electrodes
RT
EMF  k 
ln aI
zF
Free metal ion activity, aI
Bioavailability
Speciation
• Speciation of Pb(II) and Cd(II) in drinking water
• Detection of free Cu(II) in sea water
• The uptake of Cd(II) species by plant roots
LOD: 10-7 – 10-6 M
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10-11 – 10-8 M
Acc. E. Bakker, E. Pretsch, Trends Anal. Chem., 2005.
23
Trace metal speciation analysis by voltammetry
Anodic stripping voltammetry (ASV)
Anodic
stripping
voltammetry
(ASV)
Adsorptive
stripping
voltammetry
(AdSV)
Potentiometric stripping analysis (PSA)
Current – potential curves
i-E
Potentiostat
E
E
E
WE
12
Measured
signal
in SWASV
8
4
RE
AE
Current, µA
Voltammetric cell
time
(A)
time
(B)
time
0
-4
(C)
0,4
0,2
0
E ,V
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How is an ASV experiment working ?
3 steps experiment:
I. Accumulation/Preconcentration (Mz+ + ze-
M0(Hg))
II. Equilibration
III. Measurement of the analytical signal (M0(Hg))
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Mz+ + ze-)
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Strengths and advantages
of the voltammetric techniques
• Accuracy
• Sensitivity
• Simplicity
• Low detection limit (ppm - ppt)
• Well-suited for automatic in situ speciation
• Allow to determine the complexing properties of
model or naturally occurring complexants
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5. Critical evaluation of the analytical performances
and methods validation

To test accuracy and traceability

Use of CRM (Certified Reference Materials)
Example of simple certified reference materials
for speciation analysis of arsenium and chromium in water samples
Supplier
Name
Material
Certificate
BCR
CRM 626
Solution
Arsenobetaine
BCR
CRM 544
Lyophilized solution
Cr(III), Cr(VI)
NIST
SRM 2108
Solution
Cr(III)
NIST
SRM 2109
Solution
Cr(VI)
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Why validation methods are important?
In situ detection of O2, Fe(II), Mn(II) in
sediments porewaters with
unprotected Au/Hg WE (100 μm)
Acc. G. W. Luther III et al., Environ. Sci. Technol. 33 (1999) 4352
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3.
In-situ (on-field) speciation analysis
in aquatic systems
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ATMA (Automated Trace Metal Analyzer)
VIP (Voltammetric
In situ
Profiler)
In-situ
voltammetric
analyzers
MPCP (Multi Physical-Chemical Profiler)
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ATMA
• PSA measurements
• Electrodes: a mercury film deposited on a glassy carbon
rod or a thin gold electrode
• Used for measuring As(III); Cr(VI); Cu(II); Hg(II); Se(IV).
Automated Trace Metal Analyzer (ATMA)
Interferences from matrix (organics, solids, other metals) can
Acc.
to a
Space
and Naval
Warfare
Systems
Center,
USA (2002)
have
dramatic
effect
on the
accuracy
of San
the Diego,
instrument
Caution in use on unpredictable or
unknown effluents
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Gel protected voltammetric microelectrodes
- GIME
- speciation
In situ trace
metal
Gel Integrated Microelectrode
based
on bioanalogical sensors
- CGIME Complexing
Gel Integrated
VIP (Voltammetric
InMicroelectrode
situ Profiler)
MPCP (Multi Physical-Chemical Profiler)
- PLM – mTAS Permeation Liquid Membrane – Total Analytical System
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Model of metal uptake by a biological cell
Bulk solution
Diffusive boundary layer
Cell wall layer
kint, M
Cell membrane
Cell interior
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GIME (gel-integrated micro-sensor)
MX
Test solution
(volume)
M
ML
+ Y (KMY)
MY
+ L (KML)
MY
M
ML
Agarose gel
0
M
Ir
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Si3N4
Hg (5 μm)
Silicon
34
CGIME
(complexing gel-integrated micro-sensor)
MX
Test solution
(volume)
+ Y (KMY)
ML
M
MY
M
MY
+ L (KML)
Agarose gel
ML
Resin
M-R
M0
Ir
Si3N4
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Silicon
Hg (5 μm)
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From voltammetric sensors to in situ probes
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VIP – Voltammetric In situ Profiler
www.idronaut.it
• Based on an array of interconnected GIME sensors
covered with a 300 mm thick agarose antifouling gel
• Measures the concentration of dynamic fraction of trace
metals (Cu(II), Pb(II), Cd(II), Zn(II), Mn(II), Fe(II))
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Voltammetric in situ profiler (VIP)
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MPCP
Speciation of Cu, Cd, Pb
 Pressure
 pH
Environmental monitoring
 Temperature
and pollution control
 O
a system for in-situ trace2 metal speciation
 Conductivity
Biogeochemical studies
Acc. to M.-L. Tercier-Waeber/
Marine Chemistry 2005
 Salinity
 Redox potential
 Turbidity
 Chlorophyll a

MPCP (Multi Physical-Chemical Profiler)
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MPCP – 3 channels configuration
Channel 1
Channel 2
Channel 3
GIME
CGIME
FIA - GIME
Dynamic fraction
of trace metals
Free metal ion
Total extractable
metal concentration
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Speciation analysis of copper with MPCP
120
Cu in Breakwater
marine costal area (UK)
at 3 different times
% of lab total conc.
100
80
60
40
20
0
FIA
1
4
7
10
2
5
8
11
GIME
CGIME
3
6
9
Acc. to M.-L. Tercier-Waeber/ Marine Chemistry 2005
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Conclusions
- In situ voltammetric speciation analysis have many
advantages
- It demands improvement of the voltammetric devices
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4.
Conclusions
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Interdisciplinarity and Speciation Analysis
Analytical
Chemistry
Environmental
Chemistry
•Transport processes
•Consumptive processes
(chemical reactions and biological uptake)
Geochemistr
y
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Biology and
Biochemistry
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Summary
Analysis of trace metals
in the natural environment
Speciation analysis
Fractionation
Location
Laboratory
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In situ
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Suggestions for further reading
•
R. Cornelis, J. Caruso, H. Crews, K. Heuman (Eds.), Handbook of Elemental Speciation:
Techniques and Methodology, Wiley, N. Y., 2005.
•
J. Buffle, G. Hoarvai (Eds.), In situ Monitoring of Aquatic Systems: Chemical Analysis and
Speciation, IUPAC Ser. Anal. Phys. Chem. Environ. Syst., Vol. 6, Wiley, Chichester, UK, 2000.
•
D. M. Templeton, F. Ariese, R. Cornelis, L-G. Danielsson, H. Muntau, H. P. van Leeuwen, R.
Łobiński, “Guidelines for terms related to chemical speciation and fractionation of elements.
Definitions, structural aspects, and methodological approaches”, Pure Appl. Chem. 72 (2000) 14531470.
•
E. Prichard, G. M. MacKay, J. Points (Eds.), “Trace Analysis: a structured approach to obtaining
reliable results”, Royal Society of Chemistry, Cambridge, 1996.
•
Institute for Reference Materials and Measurements (http://www.irmm.jrc.be/html/homepage.htm)
•
The European Virtual Institute for Speciation Analysis (EVISA) (http://www.speciation.net/)
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‘Many shall pass through and learning shall be increased’
Multi pertransibunt et augebitur scientia
The Great Instauration, Francis Bacon
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