Heavy Metals in the Environment - Chapter 2 pot

Most of the new information about the environmental chemistry of heavymetals results from continuing improvements in trace element analytical research.This is particularly true in the fie

Analytical Methods for Heavy Metals in the Environment: Quantitative Determination, Speciation, and Microscopic Analysis

Li, Al, Ti, V, Cr, Mn, Co, Ni, Cu, As, Se, Sr, Mo, Pd, Ag, Cd, Sn, Sb, Te, Cs,

Ba, W, Pt, Au, Hg, Pb, and Bi

Analytical measurements are an integral part of environmental ment Quantitative determination techniques, described below, are required andmust provide valid and affordable element analysis They are used to assess healtheffects, which are important in prioritizing contaminants for regulation Routinemonitoring of regulated contaminants ensures compliance with allowed levelsand can indicate a hazardous situation In addition, cleanups of contaminatedsites are driven by measurements indicating the location and extent of contamina-

manage-T ABLE 1 Some Useful Internet Links for Environmental Analysis

http:/ /www.who.int/peh/site map.htm World Health Organization

Pro-tection of the Human ronment

(EEA)

Pro-tection Agency (USEPA)

http:/ /www.epa.gov/epahome/index/sources.htm Sources of USEPA test methods

Information Web Site The site

is managed by USEPA’s nology Innovation Office

Tech-http:/ /www.osha.gov/ Occupational Safety and Health

Administration (OSHA)

http:/ /www.cdc.gov/niosh/homepage.html The National Institute for

Occupa-tional Safety and Health (NIOSH)

http:/ /nvl.nist.gov/ National Institute of Standards

and Technology (NIST)

tion Most of the new information about the environmental chemistry of heavymetals results from continuing improvements in trace element analytical research.This is particularly true in the fields of heavy metals speciation analysis andmicroscopic analysis, reviewed below

The problems associated with the collection, preservation, and storage ofsamples as well as sample preparation and pretreatment will not be detailed inthis chapter The reader is referred to specialized textbooks and monographs forheavy metals water analysis (1), soils and sediments analysis (2), and dust sam-pling (3) Finally, some Internet links on environmental analysis are given inTable 1, they provide valuable complementary information to this chapter

2 HEAVY METALS QUANTITATIVE ANALYSIS

This section will only focus on quantitative determination techniques It couldhave been organized either by analytical methods, analytes, or matrices (air, wa-ter, soil, and sediments) In an effort of concision the first type of presentationhas been selected as analytical methods used for heavy metals quantitative deter-mination are often multielemental and applicable to various type of matrices.The element-specific methodologies for individual determination of metals andmetalloids, from lithium to transuranium elements, have been recently reviewed

in the excellent textbook of Lobinski and Marczenko (4) The detailed description

for individual analysis of heavy metals can also be found in other books (5,6),

or monographs, such as for mercury determination in the environment (7), or forlead analysis (8) On the other hand, the very useful articles of Clement and Yang(9,10) reviewed the developments in applied environmental analytical chemistry

in recent years, including inorganic analysis, ordered by matrix type: air, water,soil, and sediments Detailed protocols depending on matrix type are also given

in a number of books, for the examination of heavy metals in soils (2), waterand wastewater (1), or workplace atmosphere (11)

2.1 Atomic Absorption and Emission Spectrometry

Atomic absorption spectrometry (AAS) and atomic emission spectrometry (AES)are the most widely used techniques for heavy metals quantitative analysis inenvironmental samples These two techniques, and their environmental applica-tions, will be briefly described in this section For greater depth description than

is possible in this chapter, there are many books and articles on analytical atomicspectrometry and these should be consulted (12–14) AAS and AES are particu-larly applicable where the sample is in solution or readily solubilized The U.S.EPA has published a sample preparation procedure for spectrochemical determi-nation of total recoverable elements, method 200.2 (15) This method providessample preparation procedures for the determination of total recoverable analytes

in groundwaters, surface waters, drinking waters, wastewaters, and in solid-typesamples such as sediments, sludge, and soils This method is applicable for thefollowing analytes: Li, Be, B, Na, Mg, Al, P, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu,

Zn, As, Se, Sr, Mo, Ag, Cd, Sn, Sb, Ba, Hg, Tl, Pb, Th, and U U.S EPA method200.3 describes sample preparation procedure for spectrochemical determination

of total recoverable elements in biological tissues (16)

2.1.1 AAS

AAS is one of the most valuable technique for environmental heavy metals sis (for review see ref 13) AAS is very simple to use, reliable, and cost effective.Although flame AAS has now largely been superseded by inductively coupledAES (see below), electrothermal (ET) AAS, hydride generation (HG) AAS, andcold vapor (CV) AAS, still present very interesting features for heavy metalsanalysis

analy-Description. AAS involves the absorption of radiant energy produced by

a special radiation source (lamp), by atoms in their electronic ground state Thelamp emits the atomic spectrum of the analyte elements, i.e., just the energy thatcan be absorbed in a resonance manner The analyte elements are transformed

in atoms in an atomizer When light passes through the atom cloud, the atomsabsorb ultraviolet or visible light and make transitions to higher electronic energylevels A monochromator is used for selecting only one of the characteristic wave-

lengths of the element being determined, and a detector, generally a plier tube, measures the amount of absorption The amount of light absorbedindicates the amount of analyte initially present.

photomulti-Since samples are usually liquids or solids, the analyte atoms or ions must

be vaporized and atomized Several AAS can be distinguished depending on themode of sample introduction and atomization Flame (FAAS), electrothermal at-omizers (ETAAS), hydride generation (HGAAS), and cold vapor (CVAAS) sys-tems have been described extensively (12,14,17) In FAAS, the liquid sample ispneumatically nebulized, the aerosol is mixed with acetylene, and then introduced

in a flame atomizer FAAS is applicable for quantitative analysis of nearly 70elements In ETAAS, which includes graphite furnace AAS (GFAAS), as theatoms are concentrated in a smaller volume than a flame, more light absorptiontakes place, resulting in detection limits approximately 100 times lower than thosefor FAAS However, GFAAS generally requires time to heat the furnace, whichmakes it slower than flame AAS ETAAS is applicable to nearly 60 elements

In HGAAS, the analyte is reduced to its volatile hydride (AsH3, SeH2, etc) Thehydride is stripped-out from solution by an inert purge gas Ar and atomized ineither a flame, an electrically heated tube, or a plasma This technique is onlyapplicable for the elements forming covalent gaseous hydrides, Ge, As, Se, Sn,

Sb, Te, Bi, and Pb Finally, CVAAS applies solely to Hg as it is the only analytethat has an appreciable atomic vapor pressure at room temperature

Multielement Capability. AAS is predominantly a single-element nique Although there is a potential for simultaneous multielement analysis (two

tech-to six elements), AAS is, however, seriously rivaled by other truly multielementtechniques such as ICP-AES and ICP-MS

Detection Limits.

FAAS⬍10 µg/L for Li, Be, Na, Mg, K, Ca, Mn, Cu, Zn, Ag, Cd 10–100µg/L for Al, Ti, V, Fe, Co, Ni, As, Rb, Sr, Rh, Pd, Te, Cs, Au, Tl, Pb100–1000µg/L for Si, Sc, Cr, Ga, Ge, Se, Y, Ru, In, Sn, Sb, Ba, Ta,

Os, Pt, Hg, Bi

GFAAS⬍0.01 µg/L for Be, Mg, K, Cr, Mn, Fe, Cu, Co, Zn, Sr, Ag, Cd0.01–0.1µg/L for Li, Na, Al, Ca, Sc, Ni, Ga, Rb, Mo, In, Cs, Ba, Au,

Tl, Pb, Bi 0.1–0.5µg/L for B, Si, Ti, V, Ge, As, Se, Y, Zr, Nb, Tc, Ru,

Rh, Pd, Sn, Sb, Te, La, Hf, Ta, W, Re, Os, Ir, Pt, Hg

HGAAS⬍0.1 µg/L for As and Se

CVAAS⬃0.02 µg/L for Hg

Environmental Applications. AAS can be applied to a wide range of ments, provided a suitable light source is available In choosing among AAStechniques, FAAS should be considered first, or second if simultaneous ICP-AES

ele-is available, in the determination of Li, Na, Mg, Al, K, Ca, Mn, Fe, Ni, Cu, Zn,

Cd, Ba, and Pb FAAS has been widely used with adapters for flame gases andatom traps for the measurement of toxic metals such as Cd and Pb, with respectivedetection limits of 0.1 and 1µg/L U.S EPA 7000 series methods detail FAASprotocols for 25 metals and metalloids (18) Owing to its better sensitivity,ETAAS is a technique of choice for the following elements: Be, Si, Cr, Co, Mo,

Ag, In, Sn, Sb, Au, Tl, and Bi It is probably the most commonly used techniquefor measuring ambient levels of chromium in environmental samples (19) U.S.EPA method 200.9 provides a procedure for the determination of dissolved andtotal recoverable elements by GFAAS in groundwater, surface water, drinkingwater, storm runoff, industrial and domestic wastewater, sludge, and soil (15).This method is applicable to the following elements: Be, Al, Cr, Mn, Fe, Co, Ni,

Cu, As, Se, Ag, Cd, Sb, Sn, Tl, and Pb HGAAS can be used to determine virtuallyall elements forming volatile hydrides, such as Ge, As, Se, Sn, Sb, Te, Bi, and

Pb, to overcome problems associated with flame AAS determinations CVAAS

is the technique of choice for mercury with limits of detection down to 0.02µg/L.U.S EPA method 245.1 describes the determination of total mercury in drinking,surface, ground, sea, brackish waters, and industrial and domestic wastewater byCVAAS (15) U.S EPA methods 245.5 and 245.6 describe the determination ofmercury by CVAAS, respectively, in sediments, and tissues (16)

2.1.2 Inductively Coupled Plasma Atomic Emission

Spectrometry (ICP-AES)

Flame AAS was until recently the most widely used method for environmentaltrace metal analysis However, it has now largely been superseded by ICP-AES(for review see ref 20)

Description. AES measures the optical emission from excited atoms todetermine analyte concentration High-temperature atomization sources are used

to promote the atoms into high energy levels causing them to decay back to lowerlevels by emitting light Inductively coupled plasma is a very high excitationsource (7000–8000 K) that efficiently desolvates, vaporizes, excites, and ionizesatoms (21) The wavelengths of photons emitted are element specific The inten-sity of emission is generally linearly proportional to the number of atoms of thatelement in the original sample ICP-AES and the other atomic emission tech-niques simultaneously or sequentially measure the concentrations of 20 elements

or more at sensitivities equivalent to those of AAS A second advantage of AES is its broad dynamic range; ICP-AES calibration curves can be linear overseveral orders of magnitude In addition, ICP-AES quantifies some nonmetals;phosphorus in particular is an example

ICP-Multielement Capability. Since all atoms in a sample are excited, theycan be detected simultaneously, which is the major advantage of AES compared

to AAS

Detection Limits. 0.1–10µg/g (solids); 1–50 µg/L (aqueous).

Environmental Applications. Environmental applications utilizing AES for metal determination encompass a wide range of materials, such as natu-ral waters, seawater, soils, sediments, biological tissues, and air particulate Wa-ters, wastewaters, and solid samples should be prepared as described in U.S EPAmethod 200.2 (15) U.S EPA method 200.7 describes ICP-AES measurement ofmetals and some nonmetals (15) This method is applicable to the following ana-lytes: Li, Be, B, Na, Mg, Al, P, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As,

ICP-Se, Sr, Mo, Ag, Cd, Sn, Sb, Ba, Ce, Hg, Tl, and Pb OSHA method ID-125G(11) describes ICP-AES analysis procedure for metal and metalloid particulate

in workplace atmospheres It is applicable for the quantitative analysis of 13elements found in welding fume: Be, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Sb,

by the rods Depending on the RF/DC ratio, the electric field between the rodswill allow ions in a narrow m/z range to pass, typically 0.8 m/z Hence by chang-ing the RF/DC ratio in a controlled manner, the quadrupole can be scannedthrough the range allowing ions of consecutively higher m/z to pass through.Therefore, the quadrupole mass analyzer can only be operated in sequential mode,although the speed with which this can be achieved makes it seem almost likesimultaneous mass analysis The quadrupole mass analyzer has the advantage ofbeing cheap, reliable, and compact, with mass resolution that is sufficient forelemental analysis It is the most commonly used mass analyzer However, if anextremely high degree of resolution or true simultaneous mass analysis is re-quired, then a magnetic sector must be used Magnetic sector mass analyzers rely

on the fact that ions are deflected by a magnetic field In typical commercialinstruments, the ions are accelerated after they are skimmed from the plasma,then travel through an electric sector, which acts as an energy filter The ions

are then deflected in a single plane by the magnetic field, with the degree ofdeflection increasing with increasing m/z A mass spectrum can be generated byscanning the magnetic field Alternatively, the magnetic and electric fields can

be held constant and several detectors arranged in an array, thereby allowingtruly simultaneous mass analysis Magnetic mass analyzers are more expensive,less common, and less easy to operate than quadrupoles Magnetic sectors cannot

be scanned as rapidly as quadrupoles, and they are also capable of simultaneousoperation for a limited number of masses The main advantage of a magneticsector is the high degree of resolution obtainable (R⫽ M/∆M) The resolutionobtainable with quadrupoles used in ICP-MS is typically between 12 and 350,depending on m/z, which corresponds to peak width between 0.7 and 0.8 Incomparison, magnetic sectors are capable of resolution exceeding 10,000 Formost applications the resolution provided by quadrupoles is sufficient; however,for applications where spectroscopic interferences cause a major problem, theresolution afforded by magnetic sector may be desirable For example, a particu-lar problem is the determination of arsenic, m/z⫽ 75, in a matrix that containschloride because of interference with40Ar35Cl⫹

Multielement Capability. If many elements must be determined in a ple, ICP-MS is fast, many times faster than GFAAS and comparable to ICP-AES A major advantage over any other spectrometric technique is the access toisotope determination ICP-MS offers rapid multielement capability but suffersfrom a number of interferences Spectroscopic interferences arise when an in-terfering species has the same nominal m/z as the analyte of interest

sam-Detection Limits. Quadrupole 1–10 ng/L Magnetic sector 0.01–0.1 ng/L

ICP-MS is more sensitive than the GFAAS by more than one order ofmagnitude By comparison with ICP-AES, it is more sensitive by almost threeorders of magnitude

Environmental Applications. The applications of ICP-MS are broadlysimilar to those for ICP-AES, although the better sensitivity of the former hasresulted in applications such as the determination of ultralow levels of trace ele-ments (23) ICP-MS technique has been employed to determine a large number

of elements in environmental samples (for review see refs 20,24), and it is cially suited for heavy metals analysis in groundwater samples (25) U.S EPAmethod 200.8 provides procedures for determination of dissolved elements ingroundwaters, surface waters, and drinking water using a quadrupole mass ana-lyzer in scanning mode (15) It may also be used for determination of total recov-erable element concentrations in these waters as well as wastewaters, sludge, andsoil samples This method is applicable to the following elements: Be, Al, V, Cr,

espe-Mn, Co, Ni, Cu, Zn, As, Se, Mo, Ag, Cd, Sb, Ba, Tl, Pb, Th, and U The majordrawback of ICP-MS is its expense, and that is gradually reducing.

2.3 X-Ray Fluorescence Methods

The English physicist H G J Moseley discovered, in 1914, that the elements

in any solid sample could be identified by measuring the spectrum of the ary X-ray they emit when excited with a X-ray source This result was of theutmost significance because it gave the periodic classification of elements itsdefinite form; moreover, this technique is now widely used as a method of nonde-structive analysis When atoms are subjected to radiation of appropriate energy,provided by electrons, ions, or photons bombardment, electrons from the innerorbital shells are removed The orbital vacancies formed are filled with outerorbital electrons producing X-ray radiation The measurement of their energy andintensity forms the basis of X-ray fluorescence spectrometry

second-2.3.1 X-Ray Fluorescence (XRF)

Description. XRF spectrometry uses X-rays as primary excitation source,usually provided by X-ray tubes, or radioisotopes, which cause elements in thesample to emit secondary X-rays of a characteristic wavelength The elements

in the sample are identified by the wavelength/energy of the emitted X-rays whilethe concentrations are determined by the intensity of the X-rays Two basic types

of detectors are used to detect and analyze the secondary radiation dispersive XRF spectrometry uses a crystal to diffract the X-rays, as the ranges

Wavelength-of angular positions are scanned using a proportional detector Energy-dispersiveXRF spectrometry uses a solid-state detector from which peaks representingpulse-height distributions of the X-ray spectra can be analyzed Usually, samplepreparation required for XRF analysis is minimal compared to conventional ana-lytical techniques However, for solid samples, since particle size, composition,and element form may affect the analysis, a homogeneous sample is usually pre-pared for quantitative analysis by fusion with a borate flux (2)

Multielement Capability. XRF spectrometry allows simultaneous mination of most elements with the exception of those with atomic numberbelow 8

deter-Detection Limits. 10–100µg/g (soil); 0.5–10 mg/L (water)

Environmental Applications. Energy-dispersive XRF has been fully applied to determine the major constituents of soils but its poor sensitivitymakes it less suitable for analysis of minor and trace elements Wavelength-dispersive XRF is therefore the technique most used in soil analysis (2) XRFcan be applied for elemental and trace metals analysis of ambient air particles.OSHA method ID-185 describes a protocol for vanadium pentoxide determina-

success-tion in workplace atmosphere using XRF analysis of PVC filters (26) Portablefield X-ray fluorescence spectrometry is becoming a common analytical tech-nique for on-site screening and fast analysis of elements in hazardous waste sam-ples (for review see ref 27) U.S EPA has published a standard operating proce-dure for elemental analysis using a field X-ray fluorescence analyzer (28).

Applications include the in situ analysis of metals in soil, sediments, air

monitor-ing filters, and lead in paint The portable energy-dispersive XRF instrumentscan be used for scanning the ground surface to determine the presence of metalswithout collecting a sample for analysis However, portable XRF instruments arerelatively limited in sensitivity and accuracy

2.3.2 Particle-Induced X-Ray Emission (PIXE)

Description. PIXE is a variant of the broad family of X-ray emissiontechniques; heavy charged particles, typically protons of 1–4 MeV, are used toproduce the generated X-rays of the analyte in the sample The emitted X-raysare virtually always measured with an energy-dispersive detector For detailed

information on the technique, the book of Johansson et al (29) is highly

recom-mended An excellent review compares PIXE spectrometry to the other atomicand nuclear spectrometric techniques (30) Compared to conventional energy-dispersive XRF, PIXE offers detection limits that are often one order of magni-tude better, it is faster, and also allows analysis of a smaller sample mass Themicrobeam variant of PIXE, the micro-PIXE, offers the possibility of spatiallyresolved analysis with micrometer resolution (cf section 4.2) The major draw-backs of PIXE are that it requires a MeV particle accelerator and that commercialPIXE apparatus are not readily available

Multielement Capability. All elements from Na to U can in principle bemeasured simultaneously

Detection Limits. 0.1–10µg/g

Environmental Applications. The major part of PIXE applications in vironmental sciences are related to heavy metals measurement in aerosols and

en-in biological samples (31–33)

2.3.3 Total-Reflection X-Ray Fluorescence (TXRF)

Description. The principle of TXRF is the use of the total reflection ofthe exciting beam from conventional radiation sources at a flat support (for reviewsee ref 34) TXRF analysis requires excitation with a very narrow beam at anangular divergence of less than 1 mrad Due to a remarkable improvement of thesignal-to-background ratio, absolute detection limits can be two or three orders

of magnitude lower than that of conventional X-ray fluorescence techniques Forliquid samples, the classical sample preparation technique consists in deposition

of a droplet of solution on a pure substrate, after evaporation of the solvent, theresidue can be irradiated and analyzed.

Multielement Capability. Yes

Detection Limits. 0.1µg/L (water)

Environmental Applications. TXRF has been increasingly applied formultielement analysis in a wide range of environmental and biological materials.These specimens ranged from river water, seawater, rain, ice, and to a number

of solid materials such as airborne particles, aerosols, biopsy samples, food, andhumic substances (for review see refs 34,35)

2.4 Neutron Activation Analysis (NAA)

Description. NAA is a highly sensitive procedure for determining theconcentrations of chemical elements in the most varied substances (for reviewsee ref 36) NAA is based on conversion of stable nuclei of atoms into radioactiveones and subsequent measurement of characteristic nuclear radiation emitted bythe radioactive nuclei When a nuclear reaction results at a radioactive nucleus,the process is denoted as activation The incident neutrons required for activationcan be obtained by various means; fast neutrons with energies of several MeVcan be produced with a neutron generator or in an isotopic neutron source Theproduced radionuclide decays to a stable atomic nucleus under emission of char-acteristic radiation (often gamma-radiation) By determining the energy of thegamma-radiation and using the decay schemes, the emitting radionuclide can beidentified as well as the nature of the activated element Quantitative activationanalysis is based on measurement of the intensity of the radiation The radioactiv-ity is proportional to the number of target nuclei in the irradiated sample NAAhas the advantage of requiring little, if any, pretreatment of the sample The maindrawback of NAA is probably the high cost and limited access to the facilities.When the sensitivity of instrumental activation analysis is insufficient, ra-diochemical neutron activation analysis may be used In this case, the radio-nuclides corresponding to the elements of interest are chemically separated post-irradiation Various separation techniques can be used including ion exchange,chromatography, precipitation, electrolysis, and distillation The separationschemes are specific not only for the elements to be measured, but also for thematrix composition of the material Generally, the schemes cover a few elements

in which the radionuclides are grouped in such a way that they can be determinedwithout mutual interference (37)

Multielement Capability. Most elements can be determined with somelimitations such as for Pb Interferences occur when radionuclides emit gamma-rays of similar energy

Detection Limits. Depending upon the kind of material under tion the limits of detection may be as low as 0.1 ng/g.

investiga-Environmental Applications. NAA can be performed for a number ofheavy metals by measuring the gamma activities of their activated radioisotopessuch as:51Cr (but Hg might interfere); 52V;56Mn; 60Co (after59Fe separation);

65Ni (after separation from other short-lived nuclides); 65Zn; 75Se; 76As;99mTc(for Mo determination from activation of99Mo);110mAg (radiochemical separationrequired);115Cd;122Sb (interference with76As) or124Sb;131Ba (low sensitivity);

198Au (high sensitivity);199Au (for Pt determination from activation of198Pt); and

203Hg (4) The sensitivity for chromium is, for example, 0.3µg/L by instrumentalNAA on an interference-free basis, and a 100-fold enhancement with radiochemi-cal NAA is possible (19) Typical applications of radiochemical NAA are oftenrelated with trace elements in human or animal tissues and body fluids, foodstuffsand other material of nutritional interest, and waters The radiochemical NAA isless popular but produces very interesting data on less documented elements such

as V and Mo for example

2.5 Miscellaneous Measurement Techniques

2.5.1 Atomic Fluorescence Spectrometry (AFS)

Description. AFS is a complementary technique to AAS in that it sures the light that is reemitted after absorption (for details see refs 4,14) Thefluorescence is normally measured at a 90° angle to the exciting line source tokeep transmitted line source radiation out of the detector Air/acetylene and ni-trous oxide/acetylene flames are the most commonly used atomizers but sufferfrom chemical interferences ICP plasma is an efficient atomizer offering mini-mum light scatter and chemical interferences The intensity of the fluorescence

mea-is directly related to the intensity of the light source, so high-intensity sourcesare sought and used These include electrodeless discharge lamps, tuneable dyelasers, and pulsed hollow cathode lamps

Multielement Capability. AFS is essentially a single-element techniquebut multielement analysis can be achieved using a continuum source, or a lasersequentially tuned to different wavelengths

Detection Limits. 0.5µg/L

Environmental Applications. The recent availability of a commercial AFSinstrument with hydride generation enables the analysis of some environmentallyimportant elements, including mercury, arsenic, and selenium (38)

2.5.2 Ion Chromatography (IC)

Description. IC is a form of liquid chromatography that uses change resins to separate atomic and molecular ions based on their interaction

ion-ex-in the resion-ex-in It is the most convenient analytical approach for the determion-ex-ination

of environmentally important inorganic anions such as NO3⫺, NO2⫺, PO4 ⫺, etc.However, it has gained in importance for metal determination (for review seeref 39) For cation separation, the cation exchange resin is usually a sulfonic orcarboxylic acid For anion separation, the anion exchange resin is usually a qua-ternary ammonium group Metals in solution are generally detected by measuringthe conductivity of the solution Postcolumn reactions can be employed to en-hance the specificity and selectivity of the detection, 4-(2-pyridyzalo)resorcinol(PAR) being the preferred reagent for most metal ions (40)

The lack of selectivity control limits the versatility of IC methods, larly if there is interest in trace metals eluting in the presence of massive amounts

particu-of other metals, e.g., seawater samples However, there is a solution to this lem, that is, to use a metal chelating ion-exchange rather than a simple ion-ex-change substrate High-performance substrates can be used in analytical separa-tion columns in a IC system just like ion-exchange columns, but have the addedadvantages of selectivity control and insensitivity to changes in ionic strength(41)

prob-Multielement Capability. Yes

Detection Limits. 0.1–1µg/g (soil); 1–50 µg/L (water)

Environmental Applications. Although IC can be used for the direct mination of some heavy metals, it is more often combined with atomic spec-trometries for metal speciation analysis (cf Section 3) U.S EPA method 218.6describes the procedure for Cr(VI) determination in water (15)

deter-2.5.3 Electrochemical Methods

Description. Electroanalysis is a broad spectrum of techniques that can

be distinguished by the variable that is controlled: voltage or current The usualpractice is to apply one of these variables to a solution containing the analytespecies and measure one of the other variables From a plot of the measuredvariable versus the applied variable, information regarding the concentration andidentity of electroactive species in solution is determined Of the many electro-chemical techniques, only a few are routinely used for environmental analysis:voltammetry, direct-current DC, polarography, and potentiometry

Voltammetry is the name usually given to the family of techniques in whichcurrent is measured in function of applied potential Anodic stripping voltamme-try (ASV) is used for the determination of metals in soils and water The measure-ment is performed in an electrochemical cell under polarizing conditions on aworking electrode Analysis involves a two-step process consisting of electrolysisand stripping The analyte of interest is reduced and collected at the workingelectrode, then stripped off and measured Electroanalytical techniques can pro-

vide quantitative and qualitative information They are also among the most effective methods to do environmental analysis.

cost-Multielement Capability. Only consecutive analysis of distinct metal ions

is possible However, the advantage of ASV is the ability to distinguish betweendifferent oxidation states of the same metal

Detection Limits. ASV 100–1000µg/g (soil); 1–50 µg/L (water)

Environmental Applications. Electrochemical determination of metalions has been widely applied to seawater, natural and potable water, wastewater,air, soil, sewage, sediment, dust, ash, and many other matrices Electrochemicaltechniques are able to determine most transition metals and metalloids, from Ti

to Bi (for review see ref 42) Seawater, as a matrix, may cause problems formany analytical techniques because of its high salt content and corrosivity Thedetermination of metal ions in seawater is done almost exclusively with strippingtechniques ASV was the first stripping technique used extensively to analyzeseawater for metal ions such as Cu, Zn, Cd, and Pb ASV is sensitive enough todetermine these elements at their natural concentrations, typically⬍20 µg/L inunpolluted seawater, without any sample pretreatment U.S EPA methods 7063and 7472 describe a protocol for, respectively, arsenic, and mercury, determina-tion in aqueous samples and extracts using ASV (18)

2.5.4 Spectrophotometry

Description. Spectrophotometry is based on the simple relationship tween the molecular absorption of UV-VIS radiation by a solution and the con-centration of the colored species in solution A good theoretical and technicaldescription of spectrophotometry with special reference to environmental analy-sis is given by Gauglitz (43) The basic components of a spectrophotometer in-clude a light source, a monochromator, which isolates the desired source emissionline, a sample cell, a detector-readout system, and a data-processing unit Spectro-photometric measurements are based on the Beer-Lambert law, which describes

be-a linebe-ar dependence of be-absorbbe-ance on the concentrbe-ation Hebe-avy metbe-als photometric methods rely on reactions of analytes with color-forming reagentssuch as dithizone (for Co, Ni, Cu, Zn, Ga, Pd, Ag, Cd, In, Pt, Au, Hg, Tl, Pb,Bi) and thiocyanates (for Ti, Fe(III), Co, Mo, W, Re, U)

spectro-Multielement Capability. Spectrophotometry is not a multielement nique Moreover, it often suffers from a poor selectivity and requires a priorseparation of the element to be determined

tech-Detection Limits. About 1µg/L after preconcentration

Environmental Applications. Antimony can be determined by extraction

of SbCl6 ⫺with a basic dye, e.g., Rhodamine B or Crystal Violet For arsenic,

the arsenomolybenum blue method is the most widely used Bismuth forms anorange-brown dithizonate with dithizone; cadmium forms a pink dithizonate.Mercury, lead, and zinc are also determined by the dithizone method Reaction

of Cr(VI) with 1,5-diphenylcarbazide at pH 1 is the basis of a sensitive and fairlyselective method Tin(IV) reacts with phenylfluorone at pH 1 to form an orange-red solution The excellent book of Lobinski and Marczenko (4) gives a briefindividual protocol for spectrophotometric determination of most metal ions, aswell as complementary references

2.5.5 Biomethods

Description. Immunoassay technology relies on an antibody that is oped to have a high degree of sensitivity to the target compound This antibody’shigh specificity is coupled within a sensitive colorimetric reaction that provides

devel-a visudevel-al result Immunodevel-assdevel-ays offer significdevel-ant devel-advdevel-antdevel-ages over more trdevel-aditiondevel-almethods of metal detection; they are quick, inexpensive, simple to perform, andcan be both highly sensitive and selective

Detection Limits. 0.1–1µg/g (soil); 1–50 µg/L (water)

Environmental Applications. Although most environmental says are directed toward halogenated aromatic compounds and pesticides, thetechnique is theoretically applicable to any pollutant for which a suitable antibodycan be generated Antibodies that recognize chelated forms of metal ions havebeen used to construct immunoassays for Ni(II), Cd(II), Hg(II), and Pb(II) (44)

immunoas-3 SPECIATION ANALYSIS

The IUPAC definition of the speciation of an element is the distribution of definedchemical species of an element in a system, speciation analysis being the analyti-cal activity of identifying and measuring the quantities of one or more individualchemical species in a sample The chemical species of an element are the specificforms of an element defined as to molecular, complex, or nuclear structure, oroxidation state The chemical and physical associations of toxic elements withtheir environment can strongly influence their distribution, mobility, and bio-logical availability; therefore, there is an increasing need for metal speciationanalysis in environmental samples (for review see refs 45–50) The main envi-ronmental applications involve speciation analysis of redox and organometallicforms of antimony and arsenic, redox forms of chromium, protein-bound cad-mium, organic forms of lead such as alkyllead compounds, organomercurycompounds, inorganic platinum compounds, inorganic and organometallic com-pounds of selenium, organometallic forms of tin, and redox states of vanadium.Appropriate methods for speciation analysis of individual elements will be re-viewed later

Species stability in samples is an important issue, since natural tal samples are not usually analyzed immediately after sampling and long-termstorage can produce a significant alteration of the chemical species The use ofdifferent preservation treatments such as acidification, low temperature, drying,freezing, pasteurization, freeze-drying, adsorption on cartridges or solid-phasemicrocolumns, and storage in the dark makes the species stabilization in someenvironmental samples possible The stabilization methods have been recentlyreviewed for Cr, As, Se, Sn, Sb, Hg, and Pb species in environmental samples(51) The atomic and nuclear spectrometric methods (AAS, ICP-AES, XRF,PIXE, MS, NAA) are techniques for elemental analysis independent of the chemi-cal form Without selective chemical pretreatments, the results obtained do notprovide information about the chemical speciation of the elements Some meth-ods, however, are able to provide direct and selective determination of chemicalspecies of an element; they are discussed in the next section Coupled methods forspeciation analysis, which generally involve a separation method and an element-specific analytical technique, are presented below.

environmen-3.1 Direct Methods for Speciation Analysis

3.1.1 Spectrophotometry and Colorimetry

Colorimetry and spectrophotometry determination originate with changes in lecular rather than atomic or nuclear energy levels and therefore depend on thechemical form of the element providing information about its speciation Oxida-tion state determination using colored reagents has been reported for a number

mo-of metals and metalloids, such as Al(III), V(V), Cr(VI), Fe(II), Se(IV), Sn(IV),Pt(II), Pt(IV), Tl(III) (for review see ref 4) The best-known example is the deter-mination of hexavalent chromium by reaction with diphenylcarbazide in acidsolution Spectrophotometric determination of V(IV) in the presence of V(V),based on the catalytic oxidation of aniline blue by bromate, is a more recentlystudied example (52) In addition, these procedures frequently offer the advantage

of speed, simplicity, and low cost of instrumentation These advantages are times compromised by a lack of specificity and sensitivity

some-3.1.2 Electrochemical Methods

Electrochemical techniques can be used to study speciation of metal ions in ral, drinking, and seawater They are useful for the measurement of the effect ofionic species and oxidation states on bioaccumulation and geochemical cycling.The ability to provide speciation information in seawater is an important area

natu-in which electrochemistry is used Adsorptive strippnatu-ing voltammetry is used natu-inseawater analysis for the measurement of metal ions such as Al3 ⫹, Ti4 ⫹, V5 ⫹,

Mn2 ⫹, Fe2 ⫹, Fe3 ⫹, Cu2 ⫹, Zn2 ⫹, Se4 ⫹, Se6 ⫹, and Mo6 ⫹; adsorptive stripping ometry for Co2 ⫹, Ni2 ⫹, and Zn2 ⫹; potentiometric stripping analysis for Cd2 ⫹and

potenti-Pb⫹; anodic stripping voltammetry for Hg⫹; and differential pulse polarographyfor Cr6⫹(for review see ref 42) The determination of metal ions in seawater

is done almost exclusively with stripping methods because of their appropriatesensitivity Natural concentrations of metal ions in seawater are typically⬍20µg/L and the in situ preconcentration, deposition step of stripping techniquesmakes these techniques fast, sensitive, inexpensive, and portable

3.1.3 X-Ray Absorption Fine-Structure Spectroscopy

eV, the EXAFS (extended XAFS) region commences The fine structure is caused

by the interference of the outgoing photoelectric wavefront with the waves scattered from neighboring atoms From the fine structure, the interatomic dis-tances and coordination numbers around the absorbing atom can be determined

back-As such, the XAFS is a very important structural investigation method for ing noncrystalline materials Because of the requirement for a highly monoener-getic X-ray beam, XAFS measurements are almost exclusively performed withsynchrotron radiation sources

study-XANES can be applied to the determination of the oxidation state of heavymetals in solid samples For example, the ratios between the different oxidationsates of chromium and arsenic during the deposition of fly ash could be deter-mined quantitatively using XANES spectrometry (53), as well as Cr(VI)/Cr(III)ratios in chromium-contaminated soils (54) On the other hand, EXAFS spectracontain structural information such as the central atom-neighbors’ atom distance,the nature of the neighbors, the local disorder, and the number of neighbors.EXAFS have been applied to direct determination of heavy metals speciation invarious type of matrices The molecular-level speciation analysis of arsenic andlead in mine tailings, and selenium in contaminated soils, has been recently re-ported (49); EXAFS was proposed as an efficient tool for evaluating chemicalremediation strategies in chromium-contaminated soils (55); lead speciation anal-ysis in contaminated soils enabled the differentiation between sources of leadpollution (56,57); chemical speciation analysis of lead and copper in potable wa-ter was also investigated (58) EXAFS is also a useful tool for metal cations

complexation studies with natural molecules such as for the structure tion of metal ions complexes with humic substances (59,60).

determina-3.1.4 Other Techniques for Direct Metal Speciation

Hg, and Pb compounds

Direct speciation analysis can also be carried out using separation and concentration of particular metal species by either chromatographic methods, co-precipitation, ion exchange, separation with chelating resins, or solvent extrac-tion A comprehensive review of these methods has been recently accomplished

pre-by Lobinski and Marczenko (4) for each metals and metalloids These separationand preconcentration protocols are usually poorly sensitive and/or specific, bythemselves To gain in specificity and sensitivity, the chemical or chromato-graphic separation of metal species is often coupled, off-line, or on-line, to ele-ment-specific analytical methods such as AAS, ICP-AES, ICP-MS, XRF, PIXE,and NAA For example, As(III), As(V), Se(IV), Se(VI), Sb(III), and Sb(V) weresimultaneously determined in natural water by coprecipitation and neutron activa-tion analysis (62) The detection limits were 1 ng/L for arsenic and selenium and0.1 ng/L for antimony On-line chromatographic combinations will be described

in the next section

Finally, biological substrates such as algae, plant-derived materials, ria, yeast, fungi, and erythrocytes can be used for metal preconcentration anddirect speciation analysis (63) The sorption properties of living or dead organ-isms can be used to differentiate metal species: red blood cells for specific sam-pling of chromate even at high Cr(III) levels; baker’s yeast cells to separate,respectively, Hg(II) from CH3Hg, Sb(III) from Sb(V), and Se(IV) from Se(VI)

bacte-3.2 On-Line Coupled Methods for Metal Speciation

Analysis

Directly coupled systems that utilize the separatory powers of chromatographyand the sensitive detection of atomic spectroscopy are increasingly used for envi-ronmental speciation studies In a hybrid technique the separation process andelemental detection occurs on-line (for review see refs 64,65) The state of devel-opment and various problems that occur in determination of organometallic com-