mass_spectrometer Pfeiffer обзор (1248468), страница 4
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This permits smaller power rating ofthe HF module for the same performancequality of the mass filter, and thus a spacesaving and cost effective constructionaldesign of the entire electronics. However,there are limits to the possible utilisationof such a design if high ambient temperatures (> 40°C) or increased radiationstress, for example in direct attachment toelementary particle accelerators, can arise.Utilization of the PrismaTM devices downto the region of very small pressures(p < 1 · 10-10 mbar) is possible without problems by virtue of the degassing function ofthe ion source, the use of suitable materials for the components under vacuumand the maximum bakeout temperature of300 °C for the analyzer when the electronics is detached.Fig. 5:Mass spectrometer PrismaTM M1.Ionization is the part of the procedure foranalysing neutral particles which has thegreatest effect upon the sample gas [4, 5].A small fraction of the atoms or moleculespresent in the gas phase are convertedinto an ionized state by bombarding themwith low energy electrons.
This producessingly and multiply charged positive ions.The energy of the collision electrons has astrong effect on the number and on thetype of ions which are produced (Fig. 6). Theionization process of the neutral particlescommences at a minimum energy (the”appearance potential“) of the electrons.The number of ions produced increasesrapidly with increasing electron energy,reaching a maximum at 50 – 150 eV depending on the type of gas, then falling slowlyagain as the energy is increased further.The yield of ions – and therewith the sensitivity – should be as great as possible,therefore electron energies in the range70–100 eV are used in most cases. Theionic current i+k of a gas component k canbe calculated according to the followingformula:i+k = i- · l · s · pk [A]wherei- = Electron (emission) current[A]l = Mean free path of the electrons [cm]s = Differential ionization of k1[cm · mbar]pk = Partial pressure of k[mbar]Fundamentals1.2.1 The ionization processFor the ionization of molecules, the number of possible kinds of ions increasesrapidly with increasing complexity of themolecules.
Fragment ions appear in addition to singly and multiply charged molecular ions.ABC + e- ➞ ABC +++ABC+AB ++BC ++A++C++B++2e+ 3eC + 2eA + 2eBC + 2eAB + 2eA + C + 2e-Rearrangement ions, such as AC+, canappear in addition to these species. Theappearance and relative abundances ofthe individual species of ions are characteristic for a certain kind of molecule andserve as important clues for identifying themolecule and thus for qualitative gasFig. 6:Ionization producedby electron impact,as a function of theelectron energy.111 Fundamentals of mass spectrometryCO2100%512 1612 16 +C O16 +10%5O12 +C12 16++13 16C O21%5+C O2Rel. Intensity+C O212 16 18 +C O O13 16 +13 +C OC1000 ppm5100 ppm10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48Mass [amu]Fig.
7:The fragmentdistribution of CO2.analysis. Fig. 7 below shows the distribution of fragments (fragment distribution orcracking pattern) of the simple moleculeCO2 recorded with 70 eV electron energy.The spectral library provided in the QuadStarTM software contains further fragmentdistributions for some gases and compounds which are frequently of interest.These and other distributions taken fromspectral libraries can only serve for guidance, because the actual distributiondepends on various parameters such asionization energy, temperature and thetransmission characteristics of the massanalyzer.As shown in Fig. 8, the production of multiply charged ions can be strongly suppressed by using smaller electron energies,here < 43 eV.
This effect is exploited, forexample, to analyse Ar/Ne mixtures, inorder to minimise the contribution to themass number 20 produced by 40Ar++ andthus to achieve a lower detection thresholdfor 20Ne on mass number 20.8+Arions/cm · mbar642+++Ar++ArFig. 8:Ionization by electronimpact as a functionof the electron energyfor argon.12(· 10)004315,710088200300electron energy [eV]rent intensity at mass number 28 the contributions of N2 (determined at massnumber 14 when the fragment ion ratioN+/N2+ is known) and of CO2 (determinedon the mass number 44 when the fragment ion ratio CO2+/CO+ is known).Depending on the composition andconcentration ratios of the gas mixturewhich is to be analysed, it is thus possibleto devise suitable algorithms and calibrating procedures for a particular measuringtask.
Before carrying out a quantitative gasanalysis, the respective calibration factorsfor each individual gas component mustbe determined by feeding suitable calibration gas mixtures with respective nonoverlapping components. Thereafter theconcentration and partial pressure of thesegases can be determined within the scopeof a matrix calculation.The QuadStarTM software supports suchmatrix calculations and the required gasspecific calibration routines.FundamentalsFor all mass spectrometers described inthis catalog (except for the QMG 422,QME 125 variants) the energy of the collision electrons can be varied continuouslyover the range 10 – 150 eV.The problem of overlapping ion currentsof different origin on certain mass numbers is frequently encountered when analysing mixtures of several gas components.Fig.
9 shows that in this example there aremass numbers whose intensity is determined exclusively by a single gas component (e.g. argon at mass number 40,oxygen at mass number 32, carbon dioxide at mass number 44, water at massnumber 18).For other mass numbers the total intensityof the detected ion current is determinedby superimposition of the contributionsmade by various fragment ions originatingfrom different gas components.
In thisexample the ion current intensity on massnumber 16 is determined by fragment ionsfrom oxygen, water, carbon monoxide andcarbon dioxide. Therefore this mass number is less suitable for quantitative determination of the oxygen content or oxygenpartial pressure. In this example onewould use instead the intensity measuredat mass number 32. In this example itwould be particularly difficult to determinethe CO content, which could be calculatedonly by subtracting from the total ion cur-CO+O+O+H2+H2+20O+H++13 +C+H20++Ne5HydrogenNitrogen1016O2+CO++Ne142213Ar+CO2+Ne++NC+0Ar++H2O+OH+OH2OCONe++C16O18O+36Ar+13 12C CO2+14N15N+1520OxygenWater25303540Carbon monoxideNeon4550ArgonCarbon dioxideFig.
9:Mass spectrum of agas mixture, recordedwith 90 eV ionizationenergy.13Rod systemInjection apertureFocusBaseplateExtractionWehneltFilamentIonization space1 Fundamentals of mass spectrometryElectron beamIonsElectrode systemV4Neutral particles0VV5Potential characteristicV1V2V3100 VFig. 10:Electrode configuration and potentialcharacteristic of across-beam ion source taken as example.–100 VOperation of Ion Source (Fig. 10)The neutral particles arriving in the ionizing space (formation space) are ionizedby the electrons which are emitted by thefilament and accelerated in the formationspace.
From the potential characteristics itis evident that a repelling potential alwaysresults for the electrons relative to theenvironment (mass frame potential), sothat no electrons are emitted into the environment, and electrons are acceleratedonly towards the formation space. Theproduced positive ions are rapidly accelerated out of the formation space and thendecelerated by the applied electric fielddown to the energies corresponding to thefield axis potential (V4).
This achievesshorter dwell times of the ions in the formation space:reduces undisired ion-neutrals-reactionsfaster penetration of transition fieldsExcept for the QMG 422, with QME 125mass spectrometers, all voltages on theelectrode configurations and the emissioncurrent can be varied continuously via the14QuadStarTM software, so that optimisingthe ion source for a particular measuringtask is a very simple procedure.Tungsten (W), rhenium (Re) and yttriumoxide coated iridium are used as coatediridium filament material. Tungsten filaments are preferred in the ultra-high vacuum range, or where the vapor pressure ofRe could produce disturbances.
It is necessary, however, to bear in mind the brittleness of tungsten filaments produced bythe tungsten-carbon-oxygen cycle, i.e.through the formation of W2C. Yttrium oxide coated iridium is used increasinglyinstead of the former pure metal filaments.The advantages of these filaments are theconsiderably lower operating temperatureand the relative insensitivity to air in-rush.Consequently the preferred applicationfields for these filaments are analysis ofthermally sensitive substances (such asmetal-organic compounds) or the analysisof impurities in gas mixtures which contain a large oxygen fraction.Different constructional designs of the ionsources have been developed in order toAxial Ion Source (Fig.11)Electron beam orientation and ion extraction in the axial direction ensure high sensitivity and good injection conditions forthe ions into the downstream quadrupoleseparating field.
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