Some Aspects of the Determination ofMetals in Environmental Samples
Prof. dr. Marjan Veber
• Intoduction
• Survey of most commonly used analyticaltechniques for trace metal determination
• Sample preparation
• Importance of speciation analysis
• Method validation
• Conclusions
Heavy metals – an introduction
Metals occur naturally in the environment. They are present in minerals, rocks, soil, water and livingorganisms. Metals can be present in different forms: as ions dissolved in water, as vapors, or as salts or minerals in rock, sand, and soil. They can also bebound in organic or inorganic molecules, or attached to particles in the air. Both natural andanthropogenic processes and sources emit metalsinto air and water.
•
Heavy metals are considered to be one of the mainsources of pollution in the environment, since theyhave a significant effect on its ecological quality. Human activity leads to increasing levels of heavymetal contamination in the environment.
The determination of heavy metal in environmentalsamples is therefore very important in monitoring
the pollution of eco systems.
Essential(Cu,Zn,Co,Cr,
Mn,Fe)
Nonessential(B, Li, Zr)
Less toxic(Sn, Al)
Highly toxic(Pb, Hg,Cd)
Mukesh K. Raikwar et al., 2008)
Hollemen and Wiverd, 1985
1. As: pesticices, wood preservatives, biosolids, ore mining and smelting2. Cd: Paints and pigments, plasticstabilizers, electroplating, 3. Cr: Tanneries, steel industry4. Cu: pesticides, fertilizers, biosolids, ore mining and smelting5. Hg: Au, Ag mining, coal combustion, medical waste6. Ni: effluent, kitchen appliances, surgical instruments, batteries7. Pb: batteries waste, insecticides andherbicides
1. Weathering of minerals2. Erosion and volcanic activities3. Forest fires and biogenic
sources4. ………..
Some antropogenic sources of metals in aqueous environment
https://embryology.med.unsw.edu.au/embryology/index.php/Abnormal_Development_-_Heavy_Metals
Analytical approach for the determination ofheavy metals in environmetal samples
3 steps:
• Sampling
• Sample pretreatment
• Measurement (analysis)
SamplingSampling depends on type of the material to be sampled
• Soil
• Water (Sea water, River water, Ground water, Rain water)
• Sediments
• Sludges
• Air particulates
• Plant material
• Animal tissues
• ………..
Sampling-Sample preparation
Sampling
Transport
Pretreatment
Preservation
Contamination
Loss of analyte
Species transformation
• Spectroscopic (Atomic absorption spectrometry -AAS, Inductively Coupled Plasma Atomic Emission Spectrometry -ICP-OES, Inductively Coupled Plasma Mass Spectrometry ICP-MS, X-ray specrometry, Neutron activation analysis…)
• Electroanalytical (Potentiometry, Voltammetry)
• Chromatographic (Ion chromatography)
• Hyphenated techniques! HPLC- ICP-MS
Most commonly applied analytical techniques for the determination of trace concentrations of metals:
Atomic/elemental/ spectrometry
Atom cell Data system
Emission spectrometry
Atomic absorption spectrometry
monochromator and detector
Atom cell
Source
monochromator and detector
ion source
mass filter and detector
Atomic fluorescence
Elemental mass spectrometry
Data system
Data system
monochromator and detector
light source
Atom cell
Data system
Atomic absorption spectrometry; principles
Atomic absorption spectroscopy: sample introduction(I)
SAMPLE
NEBULIZATION
AEROSOL
DESSOLVATION VOLATILIZATION
FREE ATOMS
IONSMOLECULES
DISSOCIATION
HH
Atomic absorption spectroscopy: sample introduction (II)
VOLATILIZATION
FREE ATOMS
IONICSPECIES
MOLECULARSPECIES
DISSOCIATION
H
SOLID PARTICLES
SAMPLE
SAMPLE TRANSFER DISCRETEDROPLET
H
DESOLVATION
VAPOR
Lambert-Beer‘s law
Transmitance T= P/P0
Absorbance
A = -log(T)
A= abc = k.c (linear function of concentration!)
AAS – types of flames
Air/C2H2
3000oC 2300oC
N20/C2H2
Nitrous-oxide/Acetylene Air/Acetylene
6 cm 10 cm
Flame (FAAS)• Typical detection limits:
– 10 – 100 mg/l (sub ppm)
• Advantages:
– High throughput for few elements
– High dissolved solids tolerance
– Low capital and running costs
• Disadvantages:
– Typically single element technique
– Limited dynamic range (2 to 3 decades)
– Poor detection limits (application dependent)
Furnace AAS- Thermal Stages
Dry
Ash
Atomize TEMP
T I M E
Clean Out
CoolDown
Graphite Furnace AAS (GFAAS)• Detection limits:
– 10 – 100 ng/l (ppt)
• Advantages:
– Excellent detection limits
– Approaching ICP-MS for some elements
– Low sample consumption
– Relatively low capital and running costs
• Disadvantages:
– Single element technique
– Limited dynamic range (2 to 3 decades)
– Many chemical interferences
Atomic absorption spectrometer
Žarnica z votlo katodo Leči
MONOKROMATOR
DETEKTOR
OJAČEVALECZAPIS SIGNALA
Hollow cathode lamp atomizer
Monochromator
Detector
Readout
Atomic emission spectrometry
• Emission spectrometry is based on formation and detection of spectra which are formed due to deexcitation of excited electrons.
• In these processes outer, so called optical electrons are involved.
• Optical emission spectrometry is multielemental technique.
Excitation of atom
Mg Mg*
Mg K LM
e
e
Atomic emission spectrometry
Atomic emission spectroscopy: Sample introduction
PLASMA
HYDRIDEGENERATORNEBULIZER
ELELECTROTHERMALATOMIZATION
FIA
SAMPLE
PROBE
HPLC
Definition of plasma
Plasma is gas consisting of charged particles (ions, electrons). Every ionized gass can be considered as plasma.
In plasma equilibrium between ions and electrons exists:
X <=> X+ + e
The role of plasma
Plasma is energy reservoir for dissociation (atomization), excitations and ionization.
Requirements for excitation:
High temperature
high energy
Efficient sample introduction
Proper selection of plasma gas is necessary.
Inductively coupled plasma (ICP)
Properties of ICP source
• High temperature
• High electronic density
• Inert environment
• Low concentration of molecular species
• No electrodes
• No explosion hazards
• Plasma is optically thin (no self- absorption)
Thermal excitation/ionization:
Collisions between atoms and electrons
X + e-(fast) X* + e-
(slow)
M + Ar (fast) M* + Ar(slow)
Spectral interferences in emission spectrometry
• Ga Mn
• 403,298 403,307
• Na Mg
• 285,28 285,21
• Solution: echelle monochromator
Typical spectral interferents
• Co, Fe, Mo, Nb, Ta, U, V, W.
• Dy, Er, Ho, Sm, Tb, Tm.
Optical emission spectrometer
Elemental mass spectrometry (ICP-mass spectrometry)
Ionization process
Mg K LM
e
e
Ionization
eMM
Saha equation:
M
eMi
n
nnK
ne....electron density
T
ET
Z
ZK ion
M
Mi 5040log
2
3log684,15log
Ionization in ICP-MS
0 5 10 15 20 25 300
0,2
0,4
0,6
0,8
1,0D
eg
ree
of
ion
iza
tion
First ionization energy (eV)
IONIZATION MECHANISMS:
M + A+ M+ + A+ + e- collisions ion-atom
M + A M+ + A + e- collisions atom-atom
M + e- M+ + 2e- electron impact
Ar+ + X Ar + X+ charge transfer
Arm + X Ar + X+ + e- »Penning« ionization
ICP-MS “SPECTRAL” INTERFERENCES
Species with similar masses than analyte
(i) isobaric e.g..: 58Ni interferes 58Fe 40Ar interferes 40Ca
(ii) poliatomic: 40Ar-Ar interferes 80Se35Cl40Ar interferes 74Se35Cl16O interferes 51V
(III) double charged ions: 138Ba++ interferes 69Ga+
208Pb++ interferes 104Ru+
ICP-MS instrument
ICP-MS: detection limits
Spectrochemical measurement process
Spectrochemicalencoder
Sample
introduction
system
Opticalinformation
selectorRadiationtransducer
Signal processing
Number
Control system
Samplec1, c2, c3,….
c1‘, c2‘, c3‘
O1,O2,O3O1
SPEKTROCHEMICAL ANALYSIS
),,( ia XCfS
)(SgCa
Standard addition technique !
Calibration function (Matrix matched standard solutions should be used)
Analytical function
Electroanalyticalmethods
Interfacial methods
Staticmethods
(I=0)
PotentiometryPotentiometric
titrations
Dynamicmethods
(I>0)
Controlledpotential
Constant electrodepotential coulometry
VoltammetryAmperometric
titrationsElectrogravimetr
y
Constantcurrent
Coulometrictitrations
Electrogravimetry
Bulk methods
ConductometryConductometric
titrations
Voltammetry
Enables determination of selected metal ions in nM concentration levels.
Polarography, Heyrovsky (1927)Dropping mercury electrode
Anodic Stripping voltammetry
Anodic Stripping voltammogram
Anodic Stripping voltammetry
Potentiometry
Sample pretreatment
• Depends on the aim of the analysis (total metal determination, speciation analysis)
• Some pretreatment procedures:
acidification, filtration, acid digestion,
preconcentration
Determination of total metal concentration: Acid digestion (microwave assisted pressurized system)
Most frequently used acids:
• Nitric acid
• Nitric acid/hydrogen peroxide (typical mixture HNO3:H2O2 = 4:1
Less favorable for ICP techniques:
• Hydrochloric acid (insoluble oxides: Al, Be, Cr, Sb, Sn, Si, Ti, Zr)
• Nitro hydrochloric acid HCl:HNO3 = 3:1 (Digestion of precieous metals, sulfides)
• Hydrofluoric acid (complexing with H3BO3 is required prior further use!)
• Sulfuric acid (insoluble sulfates Ba, Pb, Sr)
Vessels for pressurized decomposition
Interaction of microwaves with matter
Biochemical impacts on metal species in aqueous system depend on:
• Concentration• Nature of organisms• Physico-chemical form of metal
Particulate (< 1mm)Colloidal (1 nm-1 mm)Dissolved
-Free metal ions-Simple inorganic complexes-Complexes with different ligands
Analysis of trace metals in the natural environment
Speciationanalysis
Fractionation
Location
Laboratory In situ
Speciation analysis
The recognition of the fact that in environmental chemistry the chemical, biological and toxicological properties of an element are dependent on the form in which an element occurs has stimulated a development of an area of analytical chemistry referred to as speciation analysis.
Speciation analysis
The term speciation refers to the distribution of an element among different chemical species
• Isotopic composition
• Electronic and oxidation state
• Inorganic compounds and complexes
• Organometalic compounds
• Organic and macromolecular complexes
Speciation analysis
The area of speciation analysis has been undergoing a continual evolution and development since 40 years. The area most frequently referred to its speciation of anthropogenic organometallic compounds and the products of their degradation in environment such as methyl mercury, alkyllated, buthyl and phenyltin compound and organoarsenic or organoselenium species.
Metal speciation using electroanalytical techniques
Free metal ion and labile forms
Sample Free metal ion (activity)
Voltammetry Potentiometry
SpeciationThe use of hyphenated techniques
Recent impressive progress toward lower detection limits in ICP-MS toward higher resolution in separation techniques e.g. capillary electrophoresis and electrochromatography and toward higher sensitivity in electrospray mass spectrometry for molecule specific detection at trace levels in complex matrices allows new frontiers to be crossed
Today a fundamental tool for speciation analysis is the combination of a chromatographic separation technique which ensures that the analytical compound leaves the column unaccompanied by other species of the analyte element with atomic spectrometry permitting a sensitive and specific detection of the target element.
Hyphenated techniques
Chromatography
(GC or HPLC)Inductively coupled
plasma
Mass spectrometry (ICP-MS)
detectionseparation
Analytical Approaches for Speciation Analysis
Separation /extraction
Chromatography (GC; LC ; HPLC)Capillary electrophoresisIon-chromatographyL-L extractionIon exchange, co-precipitation…
Measuring techniques
Atomic spectroscopyElemental Mass spectrometry (ICP-MS)Neutron activation analysis (NAA)X-ray photoelectron spectroscopyElectroanalytical methods
Why ICP-MS is mostly used in speciation analysis?
ICP ion source
mass spectrometer separation with respect to m/z
→ favorable limits of detection
→ relative simple spectra
→ multielemental analysis
→ isotopes ratios
→ relative simple coupling with separation techniques
Coupling SEC, UV/VIS in ICP-MS
HPLC
system
columnT-jun.
peristaltic
pump
Solution with internal
standards
ICP-MS
system
DAD
Chemical speciation: Hg
Inorganic Hg, Organomercury compounds
Mercury is most toxic in the alkylated state
• Alkylmercury compounds are extremely stable within the body
• Due to their lipophilic nature, they may readily cross biological membranes thereby accumulating in body tissues
Chemical speciation- Pb
Different species of lead in environment
Inorganic Pb
Trimethyl Pb
Triethyl Pb
Chemical speciation: As
Species of Arsenic that can be found in environment:
– Arsenobetain
– Dimetylarsinat
– Monometylarsonat
– Arsenate (III)
– Arsenite (V)
Chemical speciation: Sn
Species if Sn in environment:
• Inorganic Sn
• Organotin compounds
• alkyltin and aryltin compounds.
The trimethyl and triethyltin are the most toxic in this group.
Speciation analysis
AnalyticalChemistry
EnvironmentalChemistry
GeochemistryBiology and
Biochemistry
Transport processesChemical reactions
and uptake
Method validation
Definition:
Method validation is the process used to confirm thatthe analytical procedure employed for a specific test is suitable for its intended use. Results from methodvalidation can be used to judge the quality, reliabilityand consistency of analytical results; it is an integral part of any good analytical practice.
Validation of the analytical procedure:
Parameters of validation:• Accuracy (Bias testing)
• Precision (Repetability and Reproducibility)
• Limit of detection
• Limit of quantification
• Selectivity
• Working and linear range
• Measurement uncertainity
• Ruggedness testing
• ………………………….
Validation of the analytical procedure:
Tools of validation:
• Blanks( reagent blanks, sample blanks)
• Samples/test materials
• Fortified materials
• Spiked materials
• Independently characterized materials
• Measurement Standards
• Reference materials
• Cerified reference materials
• Statistics
Critical Evaluation of the analytical performances and metod validation
• To test accuracy and traceability
• Use of CRM (Certtified Reference Materials)
Supplier Name Material Certifiedspecies
BCR CRM 626 Solution Arsenobetaine
BCR CRM 544 Lyophilizedsolution
Cr (III), Cr (VI)
NIST SRM 2108 Solution Cr (III)
NIST SRM 2109 Solution Cr(VI)
ConclusionsThere are several techniques available for the determination of heavy metals in environmental samples. Among them spectroscopic techniques based on inductively coupled plasma prevail. Selection of the technique depends on:
• Analytes to be determined (necessity for speciation)
• Type of Samples
• Concentration range (Limit of detection!)
• Number of samples
• Available instrumentation
The quality control in analytical measurements is extremely important!