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!