印第安纳大学课件GCMS介绍.PDF

609Chapter 31Gas Chromatography Mass SpectrometryRonald A.HitesIndiana UniversitySchool of Public and Environmental Affairsand Department of ChemistrySummaryGeneral Uses Identification and quantitation of volatile and semivolatile organic compounds in complex mix-tures Determination of molecular weights and(sometimes)elemental compositions of unknown or-ganic compounds in complex mixtures Structural determination of unknown organic compounds in complex mixtures both by match-ing their spectra with reference spectra and by a priori spectral interpretationCommon Applications Quantitation of pollutants in drinking and wastewater using official U.S.Environmental Protec-tion Agency(EPA)methods Quantitation of drugs and their metabolites in blood and urine for both pharmacological and fo-rensic applications610 Handbook of Instrumental Techniques for Analytical Chemistry Identification of unknown organic compounds in hazardous waste dumps Identification of reaction products by synthetic organic chemists Analysis of industrial products for quality controlSamplesStateOrganic compounds must be in solution for injection into the gas chromatograph.The solvent must bevolatile and organic(for example,hexane or dichloromethane).AmountDepending on the ionization method,analytical sensitivities of 1 to 100 pg per component are routine.PreparationSample preparation can range from simply dissolving some of the sample in a suitable solvent to ex-tensive cleanup procedures using various forms of liquid chromatography.Analysis TimeIn addition to sample preparation time,the instrumental analysis time usually is fixed by the durationof the gas chromatographic run,typically between 20 and 100 min.Data analysis can take another 1 to20 hr(or more)depending on the level of detail necessary.LimitationsGeneralOnly compounds with vapor pressures exceeding about 1010 torr can be analyzed by gas chromatog-raphy mass spectrometry(GC-MS).Many compounds with lower pressures can be analyzed if they arechemically derivatized(for example,as trimethylsilyl ethers).Determining positional substitution onaromatic rings is often difficult.Certain isomeric compounds cannot be distinguished by mass spec-trometry(for example,naphthalene versus azulene),but they can often be separated chromatographi-cally.AccuracyQualitative accuracy is restricted by the general limitations cited above.Quantitative accuracy is con-trolled by the overall analytical method calibration.Using isotopic internal standards,accuracy of20%relative standard deviation is typical.Gas Chromatography Mass Spectrometry611Sensitivity and Detection LimitsDepending on the dilution factor and ionization method,an extract with 0.1 to 100 ng of each compo-nent may be needed in order to inject a sufficient amount.Complementary or Related Techniques Infrared(IR)spectrometry can provide information on aromatic positional isomers that is not available with GC-MS;however,IR is usually 2 to 4 orders of magnitude less sensitive.Nuclear magnetic resonance(NMR)spectrometry can provide detailed information on the ex-act molecular conformation;however,NMR is usually 2 to 4 orders of magnitude less sensi-tive.IntroductionLike a good marriage,both gas chromatography(GC;see Chapter 8)and mass spectrometry(MS;seeChapter 30)bring something to their union.GC can separate volatile and semivolatile compounds withgreat resolution,but it cannot identify them.MS can provide detailed structural information on mostcompounds such that they can be exactly identified,but it cannot readily separate them.Therefore,itwas not surprising that the combination of the two techniques was suggested shortly after the develop-ment of GC in the mid-1950s.Gas chromatography and mass spectrometry are,in many ways,highly compatible techniques.Inboth techniques,the sample is in the vapor phase,and both techniques deal with about the same amountof sample(typically less than 1 ng).Unfortunately,there is a major incompatibility between the twotechniques:The compound exiting the gas chromatograph is a trace component in the GCs carrier gasat a pressure of about 760 torr,but the mass spectrometer operates at a vacuum of about 106 to l05 torr.This is a difference in pressure of 8 to 9 orders of magnitude,a considerable problem.How It WorksThe InterfaceThe pressure incompatibility problem between GC and MS was solved in several ways.The earliest ap-proach,dating from the late 1950s,simply split a small fraction of the gas chromatographic effluent intothe mass spectrometer(1).Depending on the pumping speed of the mass spectrometer,about 1 to 5%of the GC effluent was split off into the mass spectrometer,venting the remaining 95 to 99%of the an-alytes into the atmosphere.It was soon recognized that this was not the best way to maintain the highsensitivity of the two techniques,and improved GC-MS interfaces were designed(2).These interfacesreduced the pressure of the GC effluent from about 760 torr to l06 to 105 torr,but at the same time,they612 Handbook of Instrumental Techniques for Analytical Chemistrypassed all(or most)of the analyte molecules from the GC into the mass spectrometer.These interfaceswere no longer just GC carrier gas splitters,but carrier gas separators;that is,they separated the carriergas from the organic analytes and actually increased the concentration of the organic compounds in thecarrier gas stream.The most important commercial GC carrier gas separator is called the jet separator;see Fig.31.1(3).This device takes advantage of the differences in diffusibility between the carrier gas and the or-ganic compound.The carrier gas is almost always a small molecule such as helium or hydrogen with ahigh diffusion coefficient,whereas the organic molecules have much lower diffusion coefficients.Inoperation,the GC effluent(the carrier gas with the organic analytes)is sprayed through a small nozzle,indicated as d1 in Fig.31.1,into a partially evacuated chamber(about 102 torr).Because of its highdiffusion coefficient,the helium is sprayed over a wide solid angle,whereas the heavier organic mole-cules are sprayed over a much narrower angle and tend to go straight across the vacuum region.By col-lecting the middle section of this solid angle with a skimmer(marked d3 in Fig.31.1)and passing it tothe mass spectrometer,the higher-molecular-weight organic compounds are separated from the carriergas,which is removed by the vacuum pump.Most jet separators are made from glass by drawing downa glass capillary,sealing it into a vacuum envelope,and cutting out the middle spacing(marked d2 inFig.31.1).It is important that the spray orifice and the skimmer be perfectly aligned.These jet separators work well at the higher carrier gas flow rates used for packed GC columns(10to 40 mL/min);however,there are certain disadvantages.Packed GC columns are an almost infinitesource of small particles upstream of the jet separator.If one of those particles escapes from the column,it can become lodged in the spray orifice and stop(or at least severely reduce)the gas flow out of theGC column and into the mass spectrometer.Part of this problem can be eliminated with a filter betweenthe GC column and the jet separator,but eventually a particle will plug up the orifice.In fact,sometimesit is not a particle at all,but rather tar(mostly pyrolyzed GC stationary phase)that has accumulated inthe spray orifice over time.Clearly,these devices require maintenance.Currently,the most common strategy,which is ideally suited for capillary GC columns,is to passall of the carrier gas flow into the ion source of the mass spectrometer(4).This works only if the GCgas flow is sufficiently small and the pumping speed of the mass spectrometers vacuum system is suf-Figure 31.1 The jet separator,a device for interfacing a packed column GC with an MS.The three distances are typically d1,100 m;d2,300 m;and d3,240 m.Gas Chromatography Mass Spectrometry613ficiently high to handle the gas flow.For most capillary GC columns,the gas flow is 1 to 2 mL/min,and for most modern mass spectrometers,the pumping speed is at least 300 L/sec.The development offlexible,fused silica capillary columns has made this approach routine.In fact,the only time a jet sep-arator is now used is for a few applications that require packed or thick stationary phase GC columns(for example,for permanent gas analysis).In practice,most GC-MS interfacing is now done by simply inserting the capillary column direct-ly into the ion source.Fig.31.2 is a diagram of one such system.The fused silica column runs througha 1/16-in.-diameter tube directly into the ion source.Other gases,such as methane for chemical ion-ization,are brought into the ion source by a T joint around the capillary column.One of the other twolines into the ion source is used for a thermocouple vacuum gauge tube so that the pressure in the ionsource can be roughly measured.The remaining line into the ion source is for the delivery of the massspectrometer calibration standard,perfluorotributylamine.Most joints are welded together to avoidleaks when this inlet system is thermally cycled or vented.The only removable(Swagelok)fitting isat the junction of the GC column and the far end of the inlet tube(marked with an asterisk in Fig.31.2).This fitting uses Vespel ferrules.Once the ferrules are on the GC column and it is in the ion source,itis desirable to cut off a few centimeters of the column,if possible.This eliminates the possibility offine particles partially occluding the end of the column.If the end of the column cannot be placed directly in the ion source,the material in the GC-MSinterface becomes important.The interface is held at 250 to 280 C;thus,it should not include a reactivemetal(such as copper).In some interfaces,glass-lined stainless steel tubing has been used,even thoughthis tubing is difficult to bend properly.Figure 31.2 A typical GC-MS interface for fused silica capillary GC columns.The end of the GC column enters the ion source of the mass spectrometer.614 Handbook of Instrumental Techniques for Analytical ChemistryIn summary,for capillary GC-MS,the best interface is no interface at all;run the flexible,fusedsilica GC column directly into the ion source.Using a column that is 25 to 30 m long by 220 to 250 minner diameter gives an ion source pressure of 106 to 105 torr,a more than acceptable pressure at whichto obtain electron impact spectra.This gives a helium or hydrogen GC carrier gas velocity of 25 to 35cm/sec or a flow of about 1 to 2 mL/min.The GC columns most widely used for GC-MS are those inwhich the stationary phase has been chemically bonded to the fused silica;DB-5 is a common tradename.Occasionally,there have been problems with the plastic cladding on the outside of the GC col-umn.This cladding is usually hot(typically 250 C)and under vacuum.Thus,it may decompose,givingbackground ions in the mass spectrum or weakening the fused silica itself.The Data SystemThe amount of data that can be produced during one GC-MS experiment is overwhelming.In a typ-ical GC-MS experiment,the mass spectrometer might be scanned every 2 sec during a 90-min GCrun,whether GC peaks are entering the mass spectrometer or not.Assuming that each mass spectrumhas an average of 100 mass/intensity measurements,one such GC-MS experiment will give 270,000mass/intensity pairs.Because these data have several significant figures and because other ancillarydata are also obtained,the data output from a typical GC-MS experiment is about 1 megabyte.Tomanage this high data flow,computers are required;thus,it is virtually impossible to purchase a GC-MS system without a powerful(but small)computer acting as a data system.How do data systems work?Two things are going on at the same time(5).There are two differ-ent rates within the system.There is a slow rate that times the start and stop of the mass spectrometerscan.This is usually set such that 10 to 15 mass spectra are obtained across a typical GC peak.Be-cause these peaks are usually on the order of 20 to 30 sec wide,the mass spectrometer scan speed isusually set at 2 to 3 sec per spectrum.While this scan is going on,the computer must read the outputof the electron multiplier at a rate fast enough to define the mass peak profile.In most commercialGC-MS data systems,the voltage output from the preamplifier on the electron multiplier is convertedfrom an analog signal to a digital value(using an analog-to-digital converter)at a rate of 10,000 to100,000 times per sec.This process generates large amounts of data:If the analog-to-digital convert-er worked at 10,000 conversions/second,each minute of the GC-MS experiment would generate600,000 numbers.This would quickly fill most bulk storage devices;thus,to avoid saving all of thesedata,most data systems find the mass peaks in real time and convert them into mass intensity pairs,which are then stored on the computers hard disk.Once the most recent mass spectral scan is stored,this cycle is repeated until the end of the gas chromatogram is reached.Each of the spectra stored onthe hard disk has a retention time associated with it,which can be related directly to the gas chro-matogram itself.The latter is usually reconstructed by the GC-MS data system by integrating themass spectrometer output.All modern GC-MS data systems are capable of displaying the mass spec-trum on the computer screen as a bar plot of normalized ion abundance versus mass-to-change(m/z)ratio(often called mass).Like the other parts of the GC-MS instrument,the data system must be cal-ibrated.Typically this is done by running a standard compound,such as perfluoro-tributylamine.What It DoesGas chromatographic mass spectrometry is the single most important tool for the identification andGas Chromatography Mass Spectrometry615quantitation of volatile and semivolatile organic compounds in complex mixtures.As such,it is veryuseful for the determination of molecular weights and(sometimes)the elemental compositions of un-known organic compounds in complex mixtures.Among other applications,GC-MS is widely used forthe quantitation of pollutants in drinking and wastewater.It is the basis of official EPA methods.It isalso used for the quantitation of drugs and their metabolites in blood and urine.Both pharmacologicaland forensic applications are significant.GC-MS can be used for the identification of unknown organiccompounds both by matching spectra with reference spectra and by a priori spectral interpretation.Theidentification of reaction products by synthetic organic chemists is another routine application,as isthe analysis of industrial products for control of their quality.To use GC-MS,the organic compounds must be in solution for injection into the gas chromato-graph.The solvent must be volatile and organic(for example,hexane or dichloromethane).Dependingon the ionization method,analytical sensitivities of 1 to 100 pg per component are routine.Samplepreparation can range from simply dissolving some of the sample in a suitab