Fundamentals of Vacuum Technology (1248463), страница 43
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Between 1 á 10-6 mbar and 1 á 10-4 mbarthere are minor deviations from linear characteristics. Above 1 á 10-4 mbarthese deviations grow until, ultimately, in a range above 10-2 mbar the ionsfor the dense gas atmosphere will no longer be able to reach the ion trap.The emergency shut-down for the cathode (at excessive pressure) isalmost always set for 5 á 10-4 mbar. Depending on the information required,there will be differing upper limits for use.In analytical applications, 1 á 10-6 mbar should not be exceeded if at allpossible. The range from 1 á 10-6 mbar to 1 á 10-4 mbar is still suitable forclear depictions of the gas composition and partial pressure regulation (seeFig.
4.12).4.5.7 Information on surfaces andamenability to bake-outAdditional information required to evaluate a sensor includes specificationson the bake-out temperature (during measurement or with the cathode orSEMP switched off), materials used and surface areas of the metal, glassand ceramic components and the material and dimensions for the cathode;data is also needed on the electron impact energy at the ion source (and onwhether it is adjustable). These values are critical to uninterrupted operationand to any influence on the gas composition by the sensor itself.log iEvaluating spectraContinuous change in the voltages applied to the electrodes in theseparation syssem (ÒscanningÓ) gives rise to a relationship between the ionflow I+ and the Òatomic numberÓ which is proportional to the m/e ratio andexpressed as:M=Mrne(4.2)(Mr = relative molar mass, ne = number of elementary charges e)This is the so-called mass spectrum, i+ = i+(M).
The spectrum thus showsthe peaks i+ as ordinates, plotted against the atomic number M along theabscissa. One of the difficulties in interpreting a mass spectrum such asthis is due to the fact that one and the same mass as per the equation (4.2)may be associated with various ions. Typical examples, among many+++others, are: The atomic number M = 16 corresponds to CH4 and O2 ;+M = 28 for CO+, N2 and C2H+! Particular attention must therefore be paidto the following points when evaluating spectra:1) In the case of isotopes we are dealing with differing positron counts inthe nucleus (mass) of the ion at identical nuclear charge numbers (gastype). Some values for relative isotope frequency are compiled in Table 4.2.ElementOrdinalnumberAtomicnumberRelativefrequencyH11299.9850.015He2340.00013Å 100.0B5101119.7880.22C6121398.8921.108N7141599.630.37O816171899.7590.03740.2039F919100.0Ne102090.92210.257228.8223100.0+RegulationExact measurementrangeAutomatic shut-down:≈ 5 · 10 –4–81010–7Fig.
4.12 Qualitative linearity curve10–6–510–41010–3log PNa11Table 4.2 Relative frequency of isotopes102HomeMass spectrometryOrdinalnumberAtomicnumberRelativefrequency12Al1327100.010Si1428293092.274.683.05P1531100.0S163233343695.060.744.180.016Ions formed per cm · mbarElementAr+86420100200300400500Electron energy (eV)ClArKrXe17183654353775.424.63638400.3370.06399.607880828384860.3542.2711.5611.5556.9017.371241261281291301311321341360.0960.0901.91926.444.0821.1826.8910.448.87Thresholdenergyfor argon ionsAr+ 15,7 eVAr++ 43,5 eVAr3+ 85,0 eVAr4+ 200 eVFig.
4.13 Number of the various Ar ions produced, as a factor of electron energy level4) Finally, gas molecules are often broken down into fragments byionization. The fragment distribution patterns thus created are the socalled characteristic spectra (fingerprint, cracking pattern). Important: Inthe tables the individual fragments specified are standardized either againstthe maximum peak (in % or ä of the highest peak) or against the total of allpeaks (see the examples in Table 4.4).Both the nature of the fragments created and the possibility for multipleionization will depend on the geometry (differing ion number, depending onthe length of the ionization path) and on the energy of the impactingelectrons (threshold energy for certain types of ions).
Table values arealways referenced to a certain ion source with a certain electron energylevel. This is why it is difficult to compare the results obtained using devicesmade by different manufacturers.Often the probable partial pressure for one of the masses involved will beestimated through critical analysis of the spectrum. Thus the presence of airin the vacuum vessel (which may indicate a leak) is manifested by thedetection of a quantity of O2+ (with mass of 32) which is about one-quarter2) Depending on the energy of the impacting electrons (equalling thepotential differential, cathode Ð anode), ions may be either singly or multiplyionized. For example, one finds Ar+ at mass of 40, Ar++ at mass of 20 andAr+++ at mass of 13.3. At mass of 20 one will, however, also find neon, Ne+.There are threshold energy levels for the impacting electrons for allionization states for every type of gas, i.e., each type of ion can be formedonly above the associated energy threshold.
This is shown for Ar in Fig.4.13.3) Specific ionization of the various gases Sgas, this being the number ofions formed, per cm and mbar, by collisions with electrons; this will varyfrom one type of gas to the next. For most gases the ion yield is greatest atan electron energy level between about 80 and 110 eV; see Fig. 4.14.In practice the differing ionization rates for the individual gases will be takeninto account by standardization against nitrogen; relative ionizationprobabilities (RIP) in relationship to nitrogen will be indicated (Table 4.3).Ions formed per cm á mbarTable 4.2 Relative frequency of isotopesElectron energy (eV)Fig.
4.14 Specific ionization S for various gases by electrons exhibiting energy level E103HomeMass spectrometryType of gasAcetone (Propanone)AirAmmoniaArgonBenzeneBenzoic acidBromineButaneCarbon dioxideCarbon disulfideCarbon monoxideCarbon tetrachlorideChlorobenzeneChloroethaneChloroformChlormethaneCyclohexeneDeuteriumDichlorodifluoromethaneDichloromethaneDinitrobenzeneEthaneEthanolEthylene oxideHeliumHexaneHydrogenSymbol(CH3)2CONH3ArC6H6C6H5COOHBrC4H10CO2CS2COCCl4C6H4ClC2H3ClCHCl3CH3ClC6H12D2CCl2F2CH2Cl2C6H4(NO2)2C2H6C2H5OH(CH2)2OHeC6H14H2RIP3.61.01.31.25.95.53.84.91.44.81.056.07.04.04.83.16.40.352.77.87.82.63.62.50.146.60.44Type of gasHydrogen chlorideHydrogen fluorideHydrogen iodideHydrogen sulfideIodineKryptonLithiumMethaneMethanolNeonNitrogenNitrogen oxideNitrogen dioxideOxygenn-pentanePhenolPhosphinePropaneSilver perchlorateTin iodideSulfur dioxideSulfur hexafluorideTolueneTrinitrobenzeneWater vaporXenonXylolsSymbolHClHFHIH 2SI2KrLiCH4CH3OHNeN2NON 2OO2C5H17C6H5OHPH3C3H8AgClO4Snl4SO2SF6C6H5CH3C6H3(NO2)3H2OXeC6H4(CH3)2RIP1.61.43.12.21.71.91.61.80.231.01.21.71.06.06.22.63.73.66.72.12.36.89.011.03.07.8Table 4.3 Relative ionization probabilities (RIP) vis ˆ vis nitrogen, electron energy 102 eVElectron energy :GasArgonSymbolArCarbon dioxideCO2Carbon monoxideCONeonNeOxygenO2NitrogenN2Water vaporH2OMass40203645442816122928161412222010343216292814191817162175 eV (PGA 100)Σ = 100 %Greatest peak = 100 %74.910024.733.10.9572.78.311.76.151.8991.31.11.73.59.289.60.840.4584.215.00.786.312.81.46016.11.95.015.51.310011.516.18.42.01001.21.93.810.21000.930.5310017.80.8100152.3100273.28.420102 eV (Transpector)Σ = 100 %Greatest peak = 100 %90.91009.1100.30.81841009.2117.69560.9192.61001.920.84.650.91190.11009490.19.90.992.66.51001111001274.118.51.51.54.410025226Table 4.4 Fragment distribution for certain gases at 75 eV and 102 eV104HomeMass spectrometryNo1234567891011121314151617181920212223242526272829303132333435363738394041424344GasAcetoneAirAmmoniaArgonBenzeneCarbon dioxideCarbon monoxideCarbon tetrachlorideCarbon tetrafluorideDiff.
pump oil, DC 705Diff. pump oil, FomblinDiff. pump oil, PPEEthanolHalocarbon 11Halocarbon 12Halocarbon 13Halocarbon 14Halocarbon 23Halocarbon 113HeliumHeptaneHexaneHydrogenHydrogen sulfideIsopropyl alcoholKryptonMethaneMehtyl alcoholMethyl ethyl ketoneMechanical pump oilNeonNitrogenOxygenPerfluorokerosenePerfluor-tributylamineSilaneSilicon tetrafluorideTolueneTrichloroethaneTrichloroethyleneTrifluoromethaneTurbomolecular pump oilWater vaporXenonSymbol(CH3)2CONH3ArC6H6CO2COCCl4CF4CH3CH2OHCCl3FCCl2F2CClF3CF4CHF3C2C13F3HeC7H16C6H14H2H2SC3H8OKrCH4CH3OHC4H8ONeN2O2C12F27NSiH4SiF4C6H5CH3C2H3Cl3C2HCl3CHF3H2OXe1 = 10043/10028/10017/10040/10078/10044/10028/100117/10069/10078/10069/10050/10031/100101/10085/10069/10069/10051/100101/1004/10043/10041/1002/10034/10045/10084/10016/10031/10043/10043/10020/10028/10032/10069/10069/10030/10085/10091/10097/10095/10069/10043/10018/100132/100215/4232/2716/8020/1077/2228/1112/5119/9150/1276/8320/2877/8945/34103/6087/3285/1512/731/58103/6241/6243/921/532/4443/1686/3115/8529/7429/2541/9122/1014/716/11119/17131/1831/8087/1292/6261/87130/9051/9157/8817/25129/98358/2014/615/851/1816/916/247/5131/539/7316/1663/2927/2435/1650/1650/1419/669/4085/5529/4957/8533/4227/1683/2014/1632/6772/1657/7310/129/151/1231/629/3128/1239/1299/61132/8531/4941/761/6131/79414/1016/314/250/1712/629/182/4219/443/5931/962/2729/2366/1535/1231/931/550/1931/5027/4029/8436/429/1082/2013/815/5027/1655/64131/1151/528/2833/1065/626/4397/6450/4255/7316/2134/39527/1940/152/1545/135/3991/3297/864/2146/1747/1235/750/852/1151/4157/3427/6535/241/780/41/428/1657/671/20100/550/332/886/545.5/427/3160/5712/471/522/2136/33642/839/1022/1121/2947/838/726/831/1087/521/1153/2571/2856/5039/612/22/1642/539/1931/4114/233/247/551/463/2735/3169/49130/15Table 4.5 Spectrum library of the 6 highest peaks for the TRANSPECTORof the share of N2+ with its mass of 28.
If, on the other hand, no oxygen isdetected in the spectrum, then the peak at atomic number 28 wouldindicate carbon monoxide. In so far as the peak at atomic number 28reflects the CO+ fragment of CO2 (atomic number 44), this share is 11 % ofthe value measured for atomic number 44 (Table 4.5). On the other hand, inall cases where nitrogen is present, atomic number 14 (N++2 ) will always befound in the spectrum in addition to the atomic number 28 (N2+); in the caseof carbon monoxide, on the other hand, there will always appear Ð inaddition to CO+ Ð the fragmentary masses of 12 (C+) and 16 (O++2 )).Figure 4.15 uses a simplified example of a Òmodel spectrumÓ withsuperimpositions of hydrogen, nitrogen, oxygen, water vapor, carbonmonoxide, carbon dioxide, neon and argon to demonstrate the difficultiesinvolved in evaluating spectra.105HomeMass spectrometryCO+O+O+H2+Ar++CO+H 2 O+H2+O2 +Ar+CO2+O+N+H+Ne+OH+C+H3O+13C++Ne++ CH+N2+O+13CO+22Ne +16O18O+13C16O +236Ar+14N15N+051015HydrogenNitrogen202530OxygenWater3540Carbon dioxideNeon4550ArgonCarbon monoxideEvaluation problems: The peak at atomic number 16 may, for example, be due to oxygen fragments resulting from O2, H2O, CO2 and CO; the peak at atomic number 28 from contributionsby N2 as well as by CO and CO as a fragment of CO2; the peak at atomic number 20 could result from singly ionized Ne and double-ionized Ar.Fig.
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