Download Introduction into Element Speciation

Introduction into Element
Speciation
Jörg Feldmann
University of Aberdeen
Scotland, UK
Content: Element Speciation
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Definitions
Why element speciation
Which are the elements of interest?
Some popular Examples
Some Considerations towards Toxicity
Biogeochemical Cycling of Elements
Species Stability
Summary
Definitions
• What is speciation ?
– Speciation is the distribution of an element amongst
defined chemical species in a sample.
• What is a chemical species ?
– A species isAa specific
form species
of an element
chemical
is adefined as to
nuclear composition, electronic or oxidation state, and/or
well-defined molecular form of
complex or molecular structure.
an element
(IUPAC 1999)
occurring in a sample.
Element speciation vs
Sequential Extraction
Definition of Sequential extraction:
“Operationally defined fractionation of an element
in a sample with subsequent determination of the total
element content in the different fractions”
Example: Zn extraction from soil
- with EDTA or CaCl2: “bioavailable” fraction
- with NH2OH.HCl: Oxide bound Zn
- with Aqua Regia: Zn bound to sulfides, oxides...
Sequential extraction ≠ Speciation!
Which extraction method for speciation?
H2O/MeOH
Water-soluble species
Speciation
hydrophilic
fraction
HNO3/H2O2
ICP-MS
Total [As]
blender
Fat soluble species
HPLCICP-MS
MeOH/
Speciation
CHCl3
lipophilic
fraction..
RPHPLCICP-MS
Direct element speciation in the
solid, or in the original sample?
X-ray Spectroscopic methods: XANES, EXAFS
 Information on the directly bond atom:
 As-O, As-S, Hg-S, Hg-C..
 EXAFS and XANES are extremely valuable where
“labile” bonds are concerned, or where non-destructive
species extraction is impossible and bulk analysis.
X-ray photoelectron spectroscopy (XPS) for surface
analysis only – no information on speciation in bulk.
Information of redox state mainly
Speciation analysis with XAS
XANES - X-ray absorption near edge structure.
EXAFS - Extended X-ray absorption fine structure spectroscopy.
• Non-destructive techniques, measurement in-situ
• Species integrity is preserved
• Information on oxidation state of element ions (mainly XANES)
• Bond length and coordination number (mainly EXAFS)
• Limitation on sensitivity (LOD > 10mg/kg)
Uses:
• Studies of the binding of ionic species to matrix components
• Whether extraction procedure alters species or not?
X-ray Absorption Spectroscopy (XAS)
a speciation method for solid samples
XANES
X-ray
Absorption
Near edge
spectroscopy
EXAFS
Extended x-ray Absorption
Fine Structure
 Gives information about
the bond distances !!
X-ray Absorption
Spectroscopy
(XAS) can provide
information
about the short
range chemical
environment
Samples can either
be crystalline or
amorphous.
(T. alata roots exposed to 1 ppm As(V) for 24 h)
Use XANES for As-S bond with freshly exposed
plant
1.0
Arsenate
Arsenite
As(GS)3
Intensity
0.5
0.0
Bottleneck:
-0.5
-1.0
11860
11865
11870
11875
LODs usually in ppm range
only info on next
atom
T. alata
+ As(V)
11880
Energy (eV)
11885
11890
11895
11900
XANES/EXAFS
• Advantages
– Direct speciation (no sample preparation)
– Species integrity guaranteed (almost)
• Disadvantages
– High detection limits (ppm range)
– Synchrotron sources necessary (no routine
analysis)
– Only major species can be detected - minor
species of an elements undetectable
Why Element Speciation ?
Do organic chemists only use total C determination?
Organic Analysis:
Inorganic Analysis:
C,N,O,S:
Total =
Sum parameter!
HO
O
HS
NH2
Cysteine
Hg, Pb, Sn, As:
Total =
Sum parameter!
Information on the
Molecular
level
Bu
Bu
Sn
Bu
X
Tributyltin
Elements of interest for speciation
in environmental/biological sciences
beneficial/essential
H
He
toxic
Li
Be
B
C
N
O
F
Ne
Al
Si
P
S
Cl
Ar
Cu Zn Ga Ge As
Se
Br
Kr
metal probes and drugs
Na Mg
K
Ca
Sc
Ti
V
Cr Mn Fe
Rb
Sr
Y
Zr
Nb Mo Tc
Cs
Ba
L
Hf
Ta
Fr
Ra
A
W
Co
Ni
Ru Rh Pd Ag Cd
Re Os
Ir
Pt
Au Hg
In
Sn
Sb
Te
I
Xe
Tl
Pb
Bi
Po
At
Rn
 Speciation is not restricted to toxic elements
Parameter with determines the
diversity and type molecular species
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Redox state
Complexing ligands
Bond energies
Concentrations
Which Element Species?
Redox
Organometallics
Biomolecules
As III/V
Se IV/VI
Fe II/III
Cr III/VI
I (I-/IO3-/I2)
Methyl- Hg, Sn, Se,
As, Bi (Cd, Te,...?)
Butyl- Phenyl- Sn
Alkyl- Pb
As sugars,...
Se amino acids,..
Metalloproteines
(Cu, Cd, Zn..)
Physico-chemical properties:
Fate, Formation, Stability, Kinetics, Toxicity,
bioavaliability, ...
Knowing speciation = Understanding of processes!
Example: Organotin compounds
1980s: France, Arcachon Bay
 Tributyltin (TBT) used as a pesticide in anti-fouling boat paint
 TBT enters the water column and accumulates in sediment
 Induces inhibited growth, malformation (mussels), imposex (Dogwhelk)
Organotins are endocrine disruptors, which interfere with the
hormonal system! (see: « our stolen future »)
Bu
Bu
Sn
Bu
X
Oyster shell malformation
Imposex
Example: Cr III / Cr VI
Hexavalent Chromium (CrVI): extremely toxic, carcinogen
Trivalent Chromium (CrIII): essential?
The movie (a true story): Erin Brockovich – with Julia Roberts !
« The real » Erin Brockovich
Julia Roberts
Hollywood and metal speciation
1. People are exposed to normal
level of chromium in drinking
water.
2. But: instead of essential Cr(III) it
is in the form of Cr(VI).
3. Biological Respons: leukaemia,
miscarriages, nose bleeding, etc.
Erin
Brockovich
Cr(III) / Cr(VI)
Toxicity is dependent on the speciation!
What happens with a chemical when it enters the body?
Metabolic transformation
• Most metabolic reactions take place in
hepatic cells in the liver.
– Metabolic transformation (Phase 1)
– Conjugation (Phase 2)
Chromate (Cr(VI))
toxicity
• Mode of action is that
Cr(III) binds to DNA.
• BUT: toxicokinetics does
not allow Cr(III) to enter
the cell.
• Toxicokinetic of Cr(VI)
allows Cr to enter and
to metabolise to Cr(III)
inside the cell (most
potent metal).
Cr(OH)2(H2O)+
CrO42-
Cr(III)
Cr(VI)
CrO42Reduction
+GSH
Cr(v)-GS
Adduct formation
+DNA
GS-Cr(v)-DNA
Reduction
- GSH
Cr(III)-DNA
-carcinogenic ?
J. Feldmann, Bull. Environ. Chem. 2006
Uptake and translocation of As in plants
DMA(V)  MA(V)  As(III)  As(V)
accumulation
in shoots
Shoots
vacuoles
DMA(V)
MA(III/V)
translocation
As(III/V)
Xylem sap
accumulation
in roots
Root
vacuoles
DMA(V)  MA(III)  MA(V)  As(III)  As(V)
Roots cytosol
uptake
Acer3p
Fps1p
Pho84p
DMA(V)  MA(V)  As(III)  As(V)
Soil porewater
transformation
Abedin, Feldmann, Meharg, Plant Physiol. 128, 1120 (2002).
As uptake, accumulation
and excretion mechanism
180
As influx (nmol g-1 f. wt h-1)
160
Acer3p
140
120
Aquaporin
channel
100
80
60
40
As(OH)3
HAsO42-
Fps1p
Pho84p
20
0
0.00
0.01
0.02
0.03
0.04
0.05
0.06
As concentration (mM)
excretion
translocation
As(OH)3
As(GS)3
As(OH)3
HAsO42Ycf1p
?
As-peptides
?
DMA(V), MA(V)
adopted model from yeast (B. Rosen)
Vacuole
Cytosol
Phosphate
channel
Environmental Metal Cycling
Aerosols
Sn
Se
As
Cd
Hg
Gas Phase and
Gas-Aerosol
Réactions
Dry
Deposition
Wet
Deposition
Evaporation
Fabrication
Transport
Volatilisation
Source
Automobile
Sb
Pb
Effluents
Surface Water
Runoff
Rivers
bioaccumulation
Chemical cycles:
Formations, Transformations, Reactivity, Stability, Kinetics, Toxicity
Example:
The Global Mercury Cycle
Air
Hg0
Hg2+
Coal-fired
power plants
Hg0
Me2Hg
MeHg+
MeHg+
MeHg+
Hg0
Hg2+
Me2Hg
Water
MeHg+
Sediment
Hg2+, HgS
MeHg+
Me2Hg
MeHg+:
90% uptake,
neurotoxin
Different Hg Species = different physico-chemical properties:
Stability, Toxicity, Bioavaliability,...
Impact assessment only through speciation analysis!
Mercury Cycle
E.B. Swain et al. Ambio Vol. 36 No 1 Feb 2007
Transport of metal species into the
atmosphere: volatile metal species
Which Element Species?
Redox
Organometallics
Biomolecules
As III/V
Se IV/VI
Fe II/III
Cr III/VI
I (I-/IO3-/I2)
Methyl- Hg, Sn, Se,
As, Bi (Cd, Te,...?)
Butyl- Phenyl- Sn
Alkyl- Pb
As sugars,...
Se amino acids,..
Metalloproteines
(Cu, Cd, Zn..)
Some considerations on the species’ stability are
helpful to plan sampling and analysis strategies!
Redox species: Occurrence in the Environment
(Cr(III) / Cr(VI) and I-/IO3-/I2)
• What is the redox potential?
• Are the species easily interchanging forms?
For sample prep:
Do we need to exclude air (O2) or add a redox
buffer?
Check out the thermodynamic stabilities using a
Pourbaix diagram
How can the species be (kinetically) stabilised?
Pourbaix diagram
• Cr(VI) only stable at
alkaline pH
• Oxygenated
• Needs stablisation of
Cr(III), maybe chelation
with EDTA ?
Sample preparation for
iodine redox speciation
• I- or IO3- or I2
• If iodine species
digested with acid, I2 is
generated and lost due
to its volatility.
 Iodine speciation
always in alkaline pH
(TMAH for extraction of
I species from biological
tissues)
Some thoughts on Arsenic…
Redox
Organometallics
Biomolecules
As III/V
Se IV/VI
Fe II/III
Cr III/VI
I (I-/IO3-/I2)
Methyl- Hg, Sn, Se,
As, Bi (Cd, Te,...?)
Butyl- Phenyl- Sn
Alkyl- Pb
As sugars, lipids..
Se amino acids,..
Metalloproteines
(Cu, Cd, Zn..)
Which arsenic species are relevant ?
As speciation: As (III) vs As (V) in water
As(V)
As(III)
O
OH
HO
As
HO As
OH
OH
OH
Quick interchange of redox forms!
As-species in the environment
Delicate redox system
As(III) difficult to
do chromatography
OH
HO
Advisable to report only
inorganic arsenic
MA(V)
O
As(III)
HO As
As
OH
OH
O
HO As
OH
O
CH3
OH
Methylated As compounds are very
stable and can easily be separated
HO
As
As(V)
CH3
CH3DMA(V)
Main arsenic in fish occurs as
arsenobetaine
CH3
H3C As
+
COO-
CH3
Very stable organoarsenicals
Only occurs in biota
Mainly in fish and seafood
Unstable arsenic metabolites in biota
need special attention
O
S
O
S
H3C As
H3C
O
O
OH
OH
HO
OH
O
O
O
SO3H
OH
HO
OH
OS
S
H3C As
H3C
H3C As
H3C
COOH
Only discovered
by mass balance
H3C As OH
H3C
Speciation of unstable metabolites
Direct analysis of urine or extracts
No fraction collection is possible
Check the stability with ES-MS
Check the column recovery by mass balance
Use of H2O2
Transfer all
Thioarsenicals into
oxoarsenicals
Metal binding to proteins
Metal complexed
Non-covalently
Increase in
metal - protein
Bond strength
Metal covalently
Bound to heme
Group
e.g., Cu/Zn in
superoxide
dismutase
e.g., Fe in
hemoglobin
Heme complexed
Or covalently bound
Metal covalently bound
Se replaces S in cys
e.g., Se in
Selenoprotein
Metal binding influences
the choice for analytical methods
• As(III/V)
• MeHg
• TBT
• Cr(VI)
GLU
CYS GLU
S
S
As
• org. As species
• org. Se species
Species stability decreases
S
CYS
GLU
CY
S
Gly
• As-PCs, Hg-PCs
• Fe-eudistoma
Compounds
With large species
Diversity of one element
Compounds
Stability: = f( pH,
[ligand], Eh)
HPLC-ICPMS followed by
fraction collection
and ES-MS identification
Parallel online
HPLC-ICPMS/ESMS
Covalently bound
metalloproteins
Non-covalently
bound metals
GE-LA-ICPMS
followed by
MALDI-TOFMS
??????????????
??????????????
…and what about these species?
• M-Ln + M-L’
M-L’Ln-1
• e.g., CdCl42-, Cu-humate, Sbcitrate, As-oxalate
• Metal complexes in aqueous media
(water, biological fluids, etc.)
 Chromatography not possible
t(separation) > t1/2 (complex)
J Feldmann, P.Salaun, E. Lombi, Environ. Chem. (2009)
Electrochemical (EC) detection
but only for liquid samples.
… or X-ray Absorption Spectroscopy (XAS)
•
•
•
•
•
Solid samples (biological)
In-situ speciation without
separation
Identification of electronic
environments of metals
Oxidation state
Bond length (M-L)
Outgoing
photoelectron
wave
Absorption
edge
Backscattered
photoelectron
wave
As
S
S
S
XANES
11800
11900
EXAFS
12000
Energy (eV)
J Feldmann, P.Salaun, E. Lombi, Environ. Chem. (2009)
12100
12200
Elemental speciation –
a question of taste or upbringing
A species is a specific form of an element defined as to nuclear composition, electronic or oxidation state,
and/or complex or molecular structure.
(IUPAC 2000)
- MS community: elemental species are defined by their entire
molecular structures
- EC community: element species are defined by their element
chemistry (e.g. free metal ion, labile fast equilibrating
complexes, mobile complexes, redox species)
- XAS community: elemental species are defined by their redox
state, metal-ligand information, etc., but never as to the entire
molecule.
Elemental species detectable by different methods
species
information
molecular
information
reactive
intermediates
(Cr(V) or DMA(III)
in cells)
thio-organo
arsenicals and
Se species
organometallic
species of As, Se
MS
Ligand/
metal
information
intermediates
in solid
samples
(Hg-cysteine)
biomolecular
species
(Cd-PC)
stable
biomolecular
species
(As-PC)
redox state
of metal
labile redox
species
in solids
(Cu (I/II)
redox active
metals in water
vanadium
redox active
metals in water
(As(III/V)
complexation
capacity
Detection
Free Metal Ion
(AGNES, ISE,
CSV)
EC
XAS
molecular
modelling
stable
fast-equilibrating species
complexes
in water (complexes with FA
in water
or OH, Cl)
Fe siderophores
Small and labile
Inert organic complexes
inorganic or organic
organometallic
Directly
measured by PP
complexes
species
of As, Se
or indirectly by CSV)
ASV
J Feldmann, P.Salaun, E. Lombi, Environ. Chem. (2009)
stability of species
(time scale of equilibration)
Speciation in foodstuff (due to toxicity)
 ”Fish arsenic”
(Chapman 1926)
”…a different (and non-toxic) compound than As2O3”
As
 Minamata, Japan
MeHg+
(1950s)
emissions from industry -> fish -> man
 Arcachon Bay, France
(1980s)
TBT from antifouling agents -> oysters
Cr(VI) in drinking water
(1990s)
Import restriction for inorganic As in grain
Chinese Mandatory Hygiene regulation
(2006)
Summary Intro Element Speciation
 Main driver for Element Speciation analysis are
toxicity / environmental and health implications
 A huge variety of Elements is present in different
chemical forms
 The fate and action of Elements depends on their
speciation, i.e. their physico-chemical properties
 Some elements (As, Hg, Se...) can occur in an
enormous variety of species and concentrations
As