等离子体质谱ICP-MS课件 英文版.pdf

LECTURE 1Introduction on Isotopes and Sample preparation1-Isotopes:Definitions:Atom:Isotope:Isotone:IsobareIsomereElemental and isotopic abundancesMethodResultsModelNuclear stabilityNuclear forcesVariations in isotopic abundancesRadiogenic isotopes-Nuclear processesStable isotopes-Chemical fractionation2-Definitions3-Sample preparationSamplingCrushingMineral separationDissolutionDilutionIsotope dilutionChromatography4-Suggested reading:1-Isotopes:Definitions:Atom:Identified and labeled according to the number of protons in itsnucleus:atomic number:Z.The great importance of the atomic number derives fromthe observation that all atoms with the same atomic number have identical chemicalproperties.The periodic table of the elements assigns one place to every atomicnumber,and each of these places is labelled with the common name of the element.Isotope:One of two or more species of atoms of a chemical element with thesame atomic number(Z,figure 1.3,table 1.1)and position in the periodic table andnearly identical chemical behaviour but with different atomic masses and physicalproperties.Every chemical element has one or more isotopes.Many importantproperties of an isotope depend on its mass.The total number of neutrons and protons(mass number,symbol A),of the nucleus gives approximately the mass measured onthe so-called atomic-mass-unit(amu)scaleThe standard notation,1/1H refers to the simplest isotope of hydrogen and 235/92Uto an isotope of uranium widely used for nuclear power generation and nuclearweapons fabrication(Figure 1.1:isotope notation).Figure1.1:Standard notation for isotpes.A-Z=numbers of neutrons=NElements are divided into nuclides and classified according to their atomicnumber(Z,vertical axe)and number of neutron(N,horizontal axe).See figure 1.2:the lower part of the chart of nuclide.Figure 1.2:The lower part of the chart of nuclides.Isotone:Nuclide with the same N but different Z or A(figure 1.3,table 1.1)Isobare:Nuclide with the same masse (A)but different Z(figure 1.3,table1.1)Isomere:Same two nuclide but with different energy state(fundamental andexcited state)Figure 1.3:Classification of nuclideFigure 1.4:Example of isomere:40K-40Ar electron capture:Isotopes Z=20NZA%Calcium20204096.97Calcium2220420.64Calcium2320430.145Calcium2420442.06Calcium2620460.0033Calcium2820480.185Isotones N=20Sulphur2016360.0136Chlorine20173724.471Argon2018380.063Potassium20193993.1Calcium20204096.97Isobars A=40Argon221840Potassium211940Calcium202040Table 1.1:Examples of isotopes,isotones and isobars:There are 264 stable isotopes in nature(with Z 82 arealways radioactive(Bi,Th,U).The isotopic abundance depand on wether the A,Z orN number are odd or even.AZNNumber of Nuclide(stable isotope)even even even157Oddeven Odd53OddOddeven50even OddOdd4Total264Table 1.2:Stability of isotopes as a function of their A,Z and N numbersElemental and isotopic abundances*MethodTo understand how the elements formed we need to understand a fewastronomical observations and concepts.The universe began some 10 to 20 Ga agowith the Big Bang.Since then the universe has been expanding,cooling,andevolving.Our present understanding of nucleosynthesis comes mainly from two sorts ofobservations:a-The abundance of isotopes and elements in:-the Sun&Stars,gaseous nebula,interstellar fields(observations of starsusing spectroscopy),-Cosmic Ray:much harder to measure.It is mainly made of protons,He andother elements such as Li,B etc-Earth,moon,meteorites etc:The composition of the earth is only wellknown for the continental and oceanic crust but the mantle isnt still wellcharacterized-Other planets,asteroids,comets etc(in the solar system):Theircomposition isnt well characterized.For example we only know the composition ofthe atmosphere of Venus,rich in CO2.b-Experiments on nuclear reactions that determine what reactions arepossible(or probable)under given conditions.*ResultThe total mass of the earth is negligible in comparison with the total mass of thesolar system.Ninety-height percent of the total mass of the solar system is located inthe sun.The rest is shared between other planets,asteroids,comets,moonetcTherefore,if we want to know the composition of the solar system,we need tolook at the sun and other stars rather than the Earth(figure1.5,table 1.3).Figure 1.6represent the different nucleosynthetic processes responsible for the formation of thedifferent element in function of their atomic number.Figure 1.5:Cosmic abundances of theElementsThe total mass of the earth is negligible incomparison with the total mass of thesolar system.98%of the total mass of thesolar system is located in the sun,the restI shared between other planets,asteroids,comets,moon etcTherefor,if we wantto know the composition of the solarsystem,we need to look at the sun andother stars rather than the Earth.Figure 1.5 shows that H,He and elementsup to Ti represent nearly 100%of thesolar system composition.ZElementConcentrationZElementConcentration1H2.79E+1044Ru1.862He2.72E+0945Rh0.3443Li57.146Pd1.394Be0.7347Ag0.4865B21.248Cd1.616C1.01E+0749In0.1847N3.13E+0650Sn3.828O2.38E+0751Sb0.3099F84352Te4.8110Ne3.44E+0653I0.911Na5.74E+0454Xe4.712Mg1.07E+0655Cs0.37213Al8.49E+0456Ba4.4914Si1.00E+0657La0.44615P1.04E+0458Ce1.13616S5.15E+0559Pr0.166917Cl524060Nd0.827918Ar1.01E+0562Sm0.258219K377063Eu0.097320Ca6.11E+0464Gd0.3321Sc34.265Tb0.060322Ti240066Dy0.394223V29367Ho0.088924Cr1.35E+0468Er0.25325Mn955069Tm0.038626Fe9.00E+0570Yb0.24327Co225071Lu0.036928Ni4.93E+0472Hf0.17629Cu52273Ta0.022630Zn126074W0.13731Ga37.875Re0.050732Ge11976Os0.71733As6.5677Ir0.6634Se62.178Pt1.3735Br11.879Au0.18636Kr4580Hg0.5237Rb7.0981Tl0.18438Sr23.582Pb3.1539Y4.6483Bi0.14440Zr11.490Th0.033541Nb0.69892U0.00942Mo2.55Table 1.3:Cosmic abundances of the Elements in Atoms per 104 Atoms Si.Figure 1.6 Nucleosynthetic processes as a function of the atomic number of theelement.*ModelA polygenetic hypothesis with four phases of nucleosynthesis has been proposedto explain the abundances of the elements.a-COSMOLOGICAL NUCLEOSYNTHESIS:Cosmological nucleosynthesisoccurred shortly after the universe began and is responsible for the cosmicinventory of H and He,and some of the Li.Helium is the main product ofnucleosynthesis in the interiors of normal,or“main sequence”stars(figure 1.7).b-STELLAR NUCLEOSYNTHESIS:The lighter elements,up to and includingSi,but excluding Li and Be,and a fraction of the heavier elements may besynthesized in the interiors of larger stars during the final stages of theirevolution.Figure 1.7 the represent the sequence of light element production inan hypothetical star.c-EXPLOSIVE NUCLEOSYNTHESIS:The synthesis of the remainingelements occurs as large stars exhaust the nuclear fuel in their interiors andexplode during a supernova.Figure 1.8 illustrate the r and s process duringsupernova and their role in producing heavier isotopes.d-NUCLEOSYNTHESIS IN INTERSTELLAR SPACE:Li and Be arecontinually produced in interstellar space by interaction of cosmic rays withmatter.Figure 1.7:Schematic representation of the different stage of the evolution ofstarFigure 1.8:Role of rapid(r)and slow(s)neuton capture process in theformation of heavier isotopes.Note the competitive role of radioactivity-decay to produce stable nuclides.Nuclear stabilityFigure 1.9:Variation in binding energy per nucleon versus the atomic number.Isotopes are said to be stable if,when left alone,they show no perceptibletendency to change spontaneouslyScale of nuclear stability is based on a comparison of measured masses with themasses of their constituent electrons,protons,and neutrons.The actual masses of all the stable isotopes differ appreciably from the sums oftheir individual particle masses.For example,the isotope 12/6 C,which has aparticularly stable nucleus,has an atomic mass defined to be exactly 12 amu.Thetotal separate masses of 6 electrons and 6 protons,treated as 6 hydrogen atoms and 6neutrons,add up to 12.09894 amu.The difference,_m,between the actual mass ofthe assembled isotope and the masses of the particles gives a measure of the stabilityof the isotope:the larger and more negative the value of _m,the greater the stabilityof the isotope.The quantity of energy calculated is called the nuclear binding energy (EB).Division of the binding energy EB by A,the mass number,yields the bindingenergy per nucleon.This important quantity reaches a maximum value for nuclei inthe vicinity of 56Fe.Nuclear forcesAll nuclides are stable.For some combinations of N and Z,a nucleus forms,butis unstable,with half-lives from 10 15 yrs to 1%by massMinor element1 0.1%by massTrace element 0.1%by massBulk(large volume)versus spatially resolved(high-resolution,in-situ)analysisSpectrometry(information on abundances)versus Spectroscopy(spectralwavelength after excitation of an element.Provide information on chemical boundingand environment of the element).Precision:Errors on the repetition of an analytical procedure several time under thesame conditionAccuracy:Deviation of a single observation from the TRUE valueBias:Deviation of the mean of a series of analysis from the TRUE valueFigure 1.14:Difference between precision and accuracy.3-Sample preparationa Sampling(sample number and sampling procedure)Not an exact scienceDepands on;-the geochemical objective-the heterogeneity of the studied area(mineral,rock,batholith etc.)-the number of classes which will define the population-the cost of the field expedition-the type of sample:?water:filtration of the sample(0.45 m)on site acidification of the sample for cation determination storage in plastic bottle?sediment in plastic bags?rocks in plastic bagsbCrushingBe careful about contamination.See table 1.5.cMinerals separationBe careful about contamination when using sieves(Table 1.5).Also theheavy liquids are not purified and may be a source of trace elementcontamination.o Density Separation(see figure 1.16)Density separation is based on the fact that different minerals have differentdensities.Thus,if a mixture of minerals with different densities can be placedin a liquid with an intermediate density,the grains with densities less than thatof the liquid will float and grains with densities greater than the liquid willsink.Typical mineral densities range from about 2.2 g/cc to as much as 8 g/cc,but are generally between 2.5 and 3.5 g/cc for silicate minerals.Suitableliquids for density separation include bromoform(density=2.84 g/cc)anddiiodomethane(density=3.31 g/cc).High density liquids with a range ofdensities can also be prepared by dissolving sodium tungstate powder in water.Because of the use of high density liquids,density separation is often referredto as heavy liquids separation.Heavy liquids separations are generally done inseparatory funnels.The procedure is very simple.The sample is placed in theseparatory funnel and the heavy liquid is added.The funnel is then left forsome time to permit light minerals to float and heavy minerals to sink.Whenthe minerals have been separated,the separatory funnel is opened and theheavier minerals are transferred onto a piece of weighing paper in a funnel(toallow the heavy liquid to drain away).The mineral separate is then washed andexamined optically for purity(the use of ethanol improve the opticalexamination).The densities of bromoform and diiodomethane can be adjustedby adding acetone(density about 0.7 g/cc)and the densities of sodiumtungstate solutions can be varied by adding water or more sodium tungstate.Thus,minerals with only slightly different densities can be separated byadjusting the density of the heavy liquid until it lies between those of theminerals.The negatives of density separation are based on two factors.First,bromoform and diiodomethane are halogenated organic liquids and,as such,present significant health hazards.Sodium tungstate solutions are safer,but arequite expensive.Second,when density differences or mineral grain sizes aresmall,heavy liquids separation can take many hours.These latter effects canbe reduced if the separation is done in a centrifuge.Figure 1.15:Schematic representation of the different steps required to analyse wholerocks on mineral separates.Table 1.5:Contamination using different grinding and sieving equipements,based onthe analysis of pure SiO2 material.Data from Thompson and Barkston(1970).Figure 1.16:Schematic of a heavy liquid separation aparatuso Magnetic Separation(see figure 19)Figure 1.17:Schematic of a Frantz Isodynamic Magnetic Separator.Magnetic separation takes advantage of differences in the magneticproperties of minerals.Minerals fall into one of three magnetic properties:ferromagnetic,paramagnetic and diamagnetic.Ferromagnetic minerals arethemselves magnetic(i.e.,magnetite and pyrrhotite)and can be easilyseparated from other minerals with a magnet since they will stick to thepoles of the magnet.These minerals can be separated by wrapping thepoles of a magnet in paper,passing the magnet over the mineral mixture.The ferromagnetic minerals will stick to the magnet and may be easilyseparated by removing the paper covering the magnet.Paramagnetic anddiamagnetic minerals are not magnetic,but they differ in how they interactwith a magnetic field(figure 1.17).Paramagnetic minerals are weaklyattracted into a magnetic field and diamagnetic minerals are weaklyrepelled by a magnetic field.Thus,if a mixture of paramagnetic anddiamagnetic minerals is passed through a magnetic field,they will bepulled into the field(paramagnetic)or repelled from the field(diamagnetic)and may be separated.Furthermore,paramagnetic mineralswith different degrees of paramagnetism can be separated from oneanother in the same way.The device used to separate minerals based ontheir magnetic properties is called a Frantz Isodynamic Magnetic Separator(figure 1.17).The magnetic separator consists of a large electromagnetthrough which mineral mixtures can be passed on a metal trough which isdivided near its exit end.Varying the strength of the magnetic field and/orslope of the separation trough is used to separate minerals.Figure 1.18:Schematic representation of the different steps required before analysisof the sample by ICP-MS.d Dissolutiono Most solid geological samples cant be dissolved in water and strongacid is required as a preliminary to using solution methods of analysis.o Hydrofluoridric(HF)acid digestion combine with strong mineralacid having a higher boiling point(HNO3,HClO4)in Teflon beaker,ona hotplate.Insolubles fluorides are converted into more soluble slats(nitrates or perchlorates).HF attacks the silicate matrix and the excessSi is removed during evaporation of volatile SiF4.The remaining solidis dissolve in nitric or hydrochloridric acid.o Sodium Peroxide Decomposition:For samples containing refractoryminerals,such as zircon,which are difficult to dissolve completely inacid,Na2O2 sinter technique is useful.It requires a relatively low fluxto sample ratio,around 4.It is most appropriate for rocks carrying asignificant fraction of their trace elem