经典教材：30分钟学会ICP-MS.pdf

FirstA Little HistoryICP-MS(Inductively CoupledPlasma Mass Spectrometry)is not anew technique at all.In fact,it isover 20 years old!The first paperson ICP-MS were published in the1980s and PerkinElmer SCIEXintroduced the first commerciallyavailable ICP-MS instrument in1983.Since that time,manyimprovements and refinementshave been made to each generationof ICP-MS instrumentation.Today,PerkinElmer SCIEX is on its fifthgeneration ICP-MS and thetechnique is used daily by manylaboratories as a routine analyticaltool.ICP-MS offers many benefits tolaboratories performing trace metaldeterminations.IICP-MS offers detection limitsequal to or better than thoseattainable using GraphiteFurnace Atomic Absorption(GFAA)with much higherproductivity.IICP-MS can easily handle bothsimple and complex samplematrixes.IICP-MS has detection limitcapabilities that are superior to those obtained in InductivelyCoupled Plasma OpticalEmission Spectrometry(ICP-OES).What Can Be MeasuredWith an ICP-MS?For laboratories using flame orfurnace AA or ICP-OES,ICP-MSoffers the opportunity to achievehigher productivity and obtainlower detection limits.Lets look atwhat an ICP-MS instrument can doand how it does it.The ICP-MS instrumentmeasures most of the elements inthe periodic table.The elementsshown in color in Figure 1 can beanalyzed by ICP-MS with detectionlimitsaat or below the part pertrillion(ppt)brange.Elements thatare in white are either not measur-able by ICP-MS(the upper righthand side)or do not havenaturally occurring isotopes.TheDetection Limit RangesFigure 1.Elements determined by ICP-MS and approximate detection capability.aThe detection limits are based on a 98%confidence level(3 standard deviations).bIdentifying a single part per trillion of an element in a solution is analogous to locating a single white raisin in a house(2,700 sq.ft)full of regular raisins.The 30-Minute Guide to ICP-MSTECHNICAL NOTEI C P M a s s S p e c t r o m e t r yLife SciencesOptoelectronicsInstrumentsFluid Scienceslines shown for each element inFigure 1 depict the number andrelative abundance of the naturalisotopes for that element this issometimes referred to as theisotopic fingerprint of the element.The naturally occurring isotopes ofeach element all have the sameatomic number(number of protonsin the nucleus),but differ by theatomic mass.This is the result ofthe different number of neutronspresent in the nucleus of eachisotope.ICP-MS shares some compo-nents,including the quadrupolemass spectrometer,vacuum system,and detector,with other types ofmass spectrometers such as LC/MSand GC/MS.However,the plasmaion source used in ICP-MS is muchdifferent than the lower-energy ionsources used in LC/MS and GC/MSinstruments.The extreme hightemperature of the plasma ionsource completely breaks apart themolecules present in a sample.As aresult,the ICP-MS detects onlyelemental ions.This makes ICP-MSmuch more comparable to otherinorganic techniques,such as AASand ICP-OES,for elemental analy-sis.In addition,the standard andsample preparation techniques,sample introduction systems,andpotential interferences are all verysimilar to those in AAS and ICP-OES.One advantage of ICP-MS overthese other inorganic techniques forelemental determination is that ICP-MS can determine the individualisotopes of each element.Thisallows ICP-MS to perform isotoperatio and isotope dilution measure-ments.A Quick OverviewAn ICP-MS consists of the followingcomponents(see Figure 2):ISample introduction system consists of the peristaltic pump,nebulizer,and spray chamberand provides the means ofgetting samples into the instru-mentIICP torch generates the plasmawhich serves as the ion source ofthe ICP-MS,converting theanalyte atoms to ionsIInterface links the atmosphericpressure ICP ion source and thehigh vacuum mass spectrometerIVacuum system provides highvacuum for ion optics,quadru-pole,and detector ILens focuses ions into a beamfor transmission into the quadru-poleIQuadrupole acts as a massfilter to sort ions by their mass-to-charge ratio(m/z)IDetector counts individual ionspassing through the quadrupoleIData handling and systemcontroller controls all aspectsof instrument control and datahandling to obtain final concen-tration resultsMost samples analyzed by ICP-MS are liquids.However,solidsamples can be analyzed usinglasers or heated cells to vaporize the sample.Gas samples can bemeasured by direct introductioninto the instrument.The mostcommon sample introductionsystem used on an ICP-MS consistsof a nebulizer and spray chamber.The nebulizer converts the liquidsamples into very small droplets.These droplets are carried throughthe spray chamber and into the tubeor injector that is the center channelof the torch and then into theplasma.The plasma ionizes theelements present in the droplets.These ions then pass through theinterface and the ion lens.Afterbeing focused by the ion lens,theions are separated by their mass-to-charge ratio in the mass spectrome-ter and measured by the detector.Once the detector measures theions,the computerized data systemis used to convert the measuredsignal intensities into concentra-tions of each element and generate a report of the results.The dottedblue arrow in Figure 2 shows thedirection of travel of the sampleFigure 2.Components of an ICP-MS system.through the ICP-MS.The next fewpages briefly describe the functionof each part of the ICP-MS in moredetail.Sample IntroductionThe sample introduction system onan ICP-MS is very similar to thatused on a flame AA or ICP-OES.When measuring metal content in a liquid sample,the liquid must bein a form acceptable to the instru-ments plasma.This is achieved byintroducing the sample to a nebuliz-er such as a cross-flow type shownin Figure 3.A cross-flow nebulizerworks on the same principle as theold hand-pumped insecticidesprayer:1.A flow of gas is passed at rightangles over the end of a tubecontaining a liquid.2.The flow of gas shears the liquidinto very small droplets formingan aerosol.3.The droplets then pass through aspray chamber that eliminates alldroplets except those that are theright sizecand velocity forintroduction into the plasma.Other types of nebulizers are alsouseful for introducing the liquidsample to the ICP-MS instrument.The specific type of nebulizer usedcan be selected based on the sampletype and volume.The ICP Torch-Making IonsThe plasma generated in the ICPtorch creates a very hot zone thatserves a variety of functions.At atemperature of approximately6000C,the plasma is about 10times hotter than a pizza oven,three times hotter than a weldingtorch,and equal to the temperatureat the surface of the sun.Theplasma is generated by passingargon through a series of concentricquartz tubes(the ICP torch)that arewrapped at one end by a radiofrequency(RF)coil.Energy sup-plied to the coil by the RF generatorcouples with the argon to producethe plasma.During their voyage into theplasma,shown in Figure 4,theliquid droplets containing thesample matrix and the elements tobe determined are dried to a solid,and then heated to a gas.As theatoms travel through the plasma,they absorb more energy from theplasma and eventually release oneelectron to form a singly chargedion.The singly charged ions exitthe plasma and enter the interfaceregion.Similar processes occur in theflame or plasma of AA and ICPinstruments,respectively.Thedifference is that ICP-MS measuresthe ions themselves and the othertechniques measure light instead.The Interface Sampling IonsPlacing a plasma,operating at6000oC,near an ion focusingdevice,operating near roomtemperature,is like placing theEarth about a half-mile away fromthe sun.In addition to a largetemperature difference,the plasmaoperates at a pressure that is muchhigher than the vacuum required bythe ion lens and mass spectrometerportions of the instrument.The interface allows the plasmaand the ion lens system to coexistand the ions generated by theFigure 3.Cross-flow nebulizer.Figure 4.Form of sample in ICP-MS.ArgonNebulizerFlowLiquid SampleAerosol(fine droplets)cOne micron diameter(1 X 10-6 meters)is considered an ideal diameter for introduction into the plasma.plasma to pass into the ion lensregiond.The interface(shown inFigure 5)consists of two invertedfunnel-like devices called cones.The sampler cone is located next tothe plasma and the skimmer cone islocated several millimeters behindthe sampler cone.Each cone has anopening of approximately one-millimeter in diameter at the apexthat permits the ions to passthrough.The cones are typicallymade of nickel or platinum and aremounted into a water-cooled metalhousing to prevent damage from theheat of the plasma.The regionbetween the two cones is evacuatedto a pressure of a few Torreby amechanical roughing pump.Theholes in the cones or orifices mustbe large enough to prevent un-vaporized materials from cloggingthem.The orifices must also besmall enough to maintain a consis-tent vacuum on the other side of theinterface.As samples pass throughan ICP-MS instrument,materialswill eventually deposit on thesecones and will need to be removedby occasional cleaning.How oftenthe cones require cleaning is animportant maintenance item thatmust be considered when selectingan ICP-MS.The Vacuum System Provides Correct OperatingPressureThe distance from the interface tothe detector of an ICP-MS istypically one meter or less.If an ionis to travel that distance,it cannotcollide with any gas molecules.This requires removal of nearly allof the gas molecules in the spacebetween the interface and thedetector to create a vacuum.Thistask is accomplished using acombination of turbomolecularpumps and mechanical roughingpumps called the vacuum system.The turbomolecular pumps worklike jet turbines and are capable ofrapidly pumping a chamber to apressure of 1 x 10-5Torr.Theroughing pump connected to theinterface area(where the cones arelocated)removes most of the excesssample matrix ions and gasses.Routine maintenance on thevacuum system generally consistsof changing the oil in the roughingpumps every 2 to 3 months.The Lens System-FocusingIonsThe ion lens is positioned immedi-ately behind the interface.It isresponsible for focusing ions intothe quadrupole region.Since theions generated in the plasma arenearly all positively charged,theyhave a natural tendency to repeleach other.In order to get as manyof the ions as possible into thequadrupole for mass separation andeventual measurement,it is neces-sary to keep the ion beam fromdiverging.This is achieved bypassing the ions through a chargedmetallic cylinder that acts as afocusing lens.Since the charge onthe lens is the same as the charge onthe ions,the ions are repelled(likepushing the positive poles of twomagnets together)back toward eachother to form a focused ion beam.The small disk located between theskimmer cone and the cylinder lensis the shadow stop.This devicekeeps the photons and unionizedmaterials emitted from the plasmafrom moving downstream wherethey could have an adverse effecton the performance of the ICP-MS.Again,focusing ions in a massspectrometer is like focusing lightin an optical spectrometer.Howev-er,the optical spectrometer uses anoptical lens to bend the light beam,while the mass spectrometer usesan electrical field to focus the ionbeam.The Quadrupole-Separating IonsThe mass spectrometer separatesthe singly charged ions from eachother by mass,serving as a massfilter.Three main types of massspectrometers are used in commer-cial ICP-MS systems:quadrupole,time-of-flight,and magnetic sector.The quadrupole is the type mostcommonly used in routine analyti-cal instrumentation.A quadrupoleconsists of 4 rods approximately 20 cm in length and 1 cm indiameter arranged as shown inFigure 6.The quadrupole mass spectrome-ter works by allowing only onemass to pass through to the detectorFigure 5.The ICP-MS interface.dAn Improved Interface for Inductively Coupled Plasma-Mass Spectrometry(ICP-MS)”,D.J.Douglas and J.B.French,Spectrochimica Acta,Vol.41B,No 3,pp.197-204,1986.eThe Torr is a unit of pressure equal to 1-760th of an atmosphere.The Torr is named after the physicist Evangeslista Torricelli(1608-1647)who is credited with inventing the barometer.at any given time.The quadrupoleactually sorts on the mass-to-charge(often referred to as m/z)ratio of theions.The quadrupole does this bysetting up the correct combinationof voltages and radio frequencies toguide the ions with the selected m/zbetween the four rods of thequadrupole.Ions that do not havethe selected m/z pass out throughthe spaces between the rods and areejected from the quadrupole.Themass spectrometer can move to anym/z needed to measure the ele-ments of interest in the sampleanalyzed.For example,to measuresodium,which has a single isotopeat mass 23,the mass spectrometercan be set to allow ions with m/z=23/1 to pass.For copper,which hasan isotope at mass 63,the massspectrometer can be set to pass ionswith m/z=63/1.If a doublycharged ion were formed,forexample Ba+2,the mass spectrome-ter would need to be set for a m/z of69fto allow this ion to go through.Even though the quadrupolemass spectrometer only allows onem/z to pass through the rods at anygiven time,the voltage settings onthe rods can be changed rapidly.The quadrupole on the ELANSeries ICP-MS from PerkinElmerSCIEX can see from m/z=1 to m/z=240 in less than 0.1 seconds.Thisis called the scan speed of thequadrupole.This is the reason ICP-MS can determine so many differ-ent elements quickly even thoughonly one mass goes through thequadrupole at a time.Another way to think of this is to compare it to AA or ICP-OES.Instead of using an optical gratingin a monochrometer to separate outthe wavelength of light for theelement being determined,the massspectrometer separates the ions onthe basis of their mass and charge.The Detector-Counting Ions The ions exiting the mass spectrom-eter strike the active surface of thedetector and generate a measurableelectronic signal.The active surfaceof the detector,known as a dynode,releases an electron each time anion strikes it.In Figure 7,the ionexiting the quadrupole strikes thefirst dynode which releases elec-trons and starts the amplificationprocess.The electrons releasedfrom the first dynode strike asecond dynode where more elec-trons are released.This cascading of electrons continues until ameasurable pulse is created.Bycounting the pulses generated bythe detector,the system counts theions that hit the first dynode.Compared with AA and ICP-OES,the detectors used in ICP-MS arenot much different than the photo-multiplier tubes used as thedetectors in these optical instru-ments.Instead of detecting the lightemitted by an analyte,the actualanalyte ion itself is being detected.Data Handling and SystemControllerAll ICP instruments require com-puters and sophisticated software to control the plasma and massspectrometer as well as performcalculations on the data collected.The following is an overview of themain software features provided bythe ELAN software on allPerkinElmer SCIEX ICP-MSi