mass_spectrometer Pfeiffer обзор (1248468), страница 5
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The preferred applicationfield for this rugged ion source is residualgas analysis. The filament is made ofrhenium or tungsten.Grid Ion Source (Fig.12)The special selection of materials in conjunction with the open construction giveFundamentalsestablish best matched conditions of massspectrometry for the various measuringtasks. The following illustrations show thechief types of ion sources for the massspectrometers described in this catalog.Fig. 12:Grid ion source forextreme ultra highvacuum measurements.are bombarded with electrons, a numberof species such as H+, O+, F+, Cl+ are desorbed directly, often with high yield.
EID ionsoriginate from adsorbed layers whose cause is often found in the history of the ultrahigh vacuum apparatus or of the ion source, and they usually have an initial energyof a few eV. This property can be exploitedfor distinction with respect to ions fromthe gas phase (Fig. 13). Based thereon,Redhead has proposed the ”extractor“tube for total pressure measurement,giving good discrimination with respect toions with initial energy [7]. The quadrupole mass spectrometer can be combinedwith a grid ion source with a similar functional principle. The mass numbers 1, 16,19, 35, 37, which are typical for EID ions, inmost cases have only very limited significance for residual gas analysis.
All gaseswhich contribute to these peaks can alsobe detected on other mass numbers.Fig. 11: Axial ion source.the grid ion source very low desorptionrates. This ion source also has the important advantage of easy degassing byelectron bombardment. Possible applications are therefore residual gas analysisand partial pressure determination in theultra high vacuum field (Fig.
13), as well asdesorption measurements. Two filamentsare available in the form of the ringcathode with centre tap. By virtue of itslow vapor pressure, tungsten is thepreferred filament material for this ionsource.During measurements in the pressure range < 10-10 mbar the ”EID ions“ mentionedabove can be observed [6]. When surfacesFig. 13:UHV spectrum recorded with the QMA 125grid ion source and90° off axis SEVand QME 125-1,(1-100 AMU)Ion Current [A]E–09ptot = 5 · 10-11 mbar5E–105E–115E–1205101520253035404550Mass [amu]151 Fundamentals of mass spectrometryIntensityH2+Field axis voltage 14 V10-08 AO+35Cl+F+37Cl+CO+10-09 A+HCO2+H2O+C+Field axis voltage 1 V10-08 A-09Fig. 14:The different behaviour of ions from thegas phase (red) andEID ions (blue).10O+AF+35Cl+37Cl+H+ +HCross-Beam Ion Source (Fig. 15)The open construction of the cross-beamion source (Fig.
15) permits operation witha directed gas jet largely without wall interaction. The particle beam (gas jet), theelectron beam and ion extraction aremutually at right angles. The special „thinsheet metal construction“ with low heatloss lets the source interior heat-up to200°C, so that condensation of vapors islargely prevented. The cross-beam ionsource is equipped with two filamentsoutside the formation space of the ions,so that reactions of the gases to be analysed on the hot surfaces of the filamentsare suppressed, giving long service lifeFig. 15:Cross-beam ionsource.16for this construction too.
Tungsten as wellas rhenium and yttrium oxide coated iridium are used as filament materials. In conjunction with a gas feed pipe it is possibleto obtain inlet conditions similar to molecular beam operation for improved signal/background ratio. The applicationfields for the cross-beam ion source consequently range from general residualgas analysis via analysis of corrosive orcondensable gas mixtures to molecularbeam measurements and the determination of isotope ratios. As a further constructional variant, the cross-beam ionsource is also used in conjunction with aspecial aperture construction for monitoring material flows and for source controlin thermal coating processes.By virtue of its suitable geometry, this ionsource can also be used for detectinglaser-induced ions (with switched-offfilament) as simple ion transfer optics.With the help of a magnet system theelectron density can be increased in thepart of the ion source from which the ionscan be effectively focussed into the rodsystem.
This can considerably increasethe sensitivity of the ion source. The electron-guiding magnet also has the effectthat the majority of the electrons impingeon locations of the formation space whichare less critical in terms of electron optics.A special version of this ion source is shown in Fig. 16. This version is particularlysuitable for analysing gases under highpressure which are fed directly into theionization space via orifices or gasdosing valves. Utilizing a “gas tight“ ionsource achieves low gas consumption(and therewith slight loading of the pumping system), low background pressure inthe rest of the analysis vacuum chamberand a very short time constant of the ionsource.
Typical application fields are consequently trace analysis of ultra puregases, respiration analysis in humanmedicine and isotope analysis. However,it must be borne in mind that the dangerof possible coating of the inner surfacesof the ion source increases. This is due tothe lower effective pumping capacity ofthe vacuum system in the formationPrisma Ion SourceFig. 16 shows the ion source of thePrismaTM family in open and in gas tightform. Both versions have a dual filamentconfiguration, so that two cathodes areavailable. Tungsten, or yttrium oxidecoated iridium in the open version, isused as filament material.
The configuration of the electrodes and the potentialcharacteristic are similar to those of thegrid ion source.For the gas tight version of the PrismaTMion source it must be borne in mind thatgas input is possible only in the axialdirection, i.e. in the direction of ion extraction. The conductance value is about 1 l/sfor nitrogen.Fundamentalsspace of the ion source. The conductancevalue of a gas tight cross-beam ion sourceis about 1 l/s for nitrogen. Therefore thisconstructional form of the cross-beam ionsource is only conditionally suitable foranalysing strongly condensing gas mixtures. A gas tight ion source is also notrecommended for classical residual gasanalysis at pressures < 1 · 10-6 mbar,because of the closed construction.1.2.2.
Mass separationIons can be separated according to theirmass/charge ratio in a high frequencyquadrupole field produced in the idealcase with four hyperbolic rod electrodeswith an apex separation of r0. Cylindricalelectrodes are used in most cases forcommercial applications because of thetechnical difficulties involved in makinghyperbolic electrodes with the necessaryquality. Relatively good approximation ofthe ideal quadrupole field is achieved bymaking the rod radius equal to 1.144times the field radius r0. The equations ofmotion for ions injected into a quadrupolefield are described by the Mathieu differential equations. Only a brief phenomenological description of the functionalprinciples is given here.
See [3, 8, 9] for adetailed description.Fig. 16:PrismaTM ion sourceLeft: open versionRight: gas tightversion.171 Fundamentals of mass spectrometrytions with increasing amplitude and are neutralised.yz plane: With increasing amplitude Vthe positive ions execute stable oscillations with decreasing amplitude and can reachthe detector.1.
Only a direct voltage U is applied to therod electrodes U:xz plane: The positive ions experiencea repelling potential of theelectrodes and can reach thedetector.yz plane: The positive ions are attracted by the nearest electrodeand become neutralised sothat they cannot reach thedetector.3. For a fixed mass/charge ratio M wehave:xz plane: For V < V1 the ions can reachthe detector.For V > V1 transmission is suppressed.yz plane: For V < V1 transmission is suppressed.For V < V1 the ions can reachthe detector.2. A high frequency voltage of amplitudeV is superimposed on the direct voltageelectrode potential:xz plane: With increasing V the positiveions execute unstable oscilla-xzU + V · cos · tyxzyz+Utransmission:full1–Utransmission:none+ U + V · cos · ttransmission:low-pass2– U – V · cos · ttransmission:high-passi+i+3V1VV1i+Vi+4M1MM1M5superimposition of xz and yzi+yzxzIFig.
17:The principle ofseparating the ionsaccording to theirm/e ratio.18(U/V fixed)IIIIIMU/V resolutionsensitivityE–055Fundamentalsresolutiona/d setting2551005030152Ion Current [A]E–065E–075E–085E–0916171819[amu]4. For a fixed U/V ratio we have:xz plane: For ions with M < M1 transmission is suppressed.Ions with M > M1 can reachthe detector.yz plane: Ions with M < M1 can reachthe detector.For ions with M < M1 transmission is suppressed.5. For the combination of both planes theresulting ion current i+ with fixed U/Vratio is:Region I: The ions cannot pass throughthe rod system (xz plane).Region III: The ions cannot pass throughthe rod system (yz plane).Region II: The transmission factor for amass number M is determined by the ratio U/V.
A compromise must always bemade between the desirefor high sensitivity and thedesire for high resolution(Fig. 17).Fig. 18:Variation of the resolution taking the H2Ogroup as an example(measurements withPrismaTM M2, fieldaxis = 3.74 V).The oscillation amplitudes of the ions inregion II remain finite and smaller than r0.All other ions are rejected. For “stable“ions we have:V = 14.438 · M · ƒ2 · r02ƒ = HF frequency, when U/V is held justbelow 0.1678 .Thus the resolution can easily be variedelectrically, by changing the U/V ratio, foradapting to various tasks.If the direct voltage U is set to zero, thequadrupole operates as high-pass massfilter. With low HF amplitudes the ions ofnearly all masses initially move on stablepaths and reach the detector (Fig.
18, resolution ”255“). This is exploited only formaking total pressure measurements. Withincreasing HF amplitude, commencingwith light masses ions with increasingmass number M become unstable and aretherefore rejected. Mass number scan is191 Fundamentals of mass spectrometryIon Current [A]field axis15,0 V6,0 V3,0 V1,5 V0,5 VE–055E–065E–075E–085Fig.
19:Modification ofthe injectionconditions byvarying the fieldaxis voltage(measurementswith PrismaTM M2,resolution = ”25“).E–09161719[amu]achieved by varying V, giving a linearmass number scale.The ratio U/V can be controlled as a function of the mass number such that not theactual resolution m/⌬m but instead theline width ⌬m remains constant. Thismeans increase of resolution proportionalto the mass number. In spite of the increasing resolution proportional to the massnumber, the reduction of transmissionwith increasing mass number (massnumber discrimination) can be avoidedby making the quadrupole rod systemsufficiently exact and ensuring that the ionsource fulfils the correct conditions for2018injecting the ions into the rod system.
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