mass_spectrometer Pfeiffer обзор (1248468), страница 7
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bakeout temperature300°C400°Cat 2.5 kV251 Fundamentals of mass spectrometryratio. In this manner, current amplifications of up to 108 can be reached. Thanks tothe high intensity, the subsequent electrometer preamplifier can operate in a highermeasurement range and is thus very fast.The SEM 217 can be operated both in theelectrometer mode (for positive ions) aswell as in a “single ion counter” mode (inthis case for positive and negative ions).Figure 27 shows the various operatingmodes and the corresponding electricalcircuitry.However, the use of a secondary electronmultiplier as a detector in a mass spectrometer, whether it is a continuous or aexample.
In order to avoid this effect, theSEM 218 is equipped with an additionalgalvanically separated conversion dynode.The voltage applied to the conversion dynode is a constant – 6.3 kV and the ion energy is correspondingly higher (Figure 27, b).A further advantage of this variant is related to the dual high-voltage supply HV421: the amplification of the SEM can beselected independently of the conversionrate.Furthermore, the state of the surfaces andthus the yield of secondary electrons canchange during operation.
If more accuratequantitative measurements are to be car-b) SEV 218 / EP 422a) SEV 217 / EP 422EP422EP422EPEPHV–0 . . . –3.5 kVHV–HV+– HV420 +Fig. 27:CDHV421. . . –6.3 kV0 . . . –3.5 kVHV+– HV421 +CDa) Detection ofpositive ions in theelectrometer mode(current amplifier)b) Detection ofpositive ions in theelectrometer modewith a separateconversion dynodec) Detection ofpositive ions in thecounting moded) Detection ofnegative ions in thecounting moded) SEV 217 / CP 400c) SEV 217 / CP 400EPEPHV–0 .
. . –3.5 kVHV+– HV420 +HV421discrete dynode SEM, does have somedisadvantages. In quantitative analyses, italso brings uncertainties and sources oferror. The number of electrons ejected perion impacting the conversion dynode isdependent on both the mass of the ion,the type of ion, as well as on the energy ofthe ion. In a C-SEM and in the SEM 217,the energy of the impacting ion approximately corresponds to the operating voltage of the SEM, i.e. max. 3.5 kV. For this ionenergy, the conversion rate of ions in themass range between 100 AMU and 500AMU may be reduced by a factor of 3, for26CP422CP422HV–+3.1 kV0 . .
. –3.5 kVHV+– HV421 +ried out, the amplification must bechecked at regular intervals and the unitmust be recalibrated if necessary. Comparative measurements with the Faradaycup as the detector are suitable for thispurpose. The SEM can be essentially freed from these sources of error if it is notoperated as a “current amplifier” (whichalways averages over a certain period oftime) but as an “ion counter” (Figure 27, cand d).
The ions produce short pulseswhich can easily be detected individually.In this case, the detection limit is improved : less than 1 ion per 10 seconds canA mass spectrum recorded before theadmittance of air showed water to be themajor component. The intensity of the ioncurrent for the mass number 28 was 3.5 ·10-8 A or 2.1 · 10-12 A, as measured by theC-SEM and Faraday detector respectively;it is neglected in the following considerations. After the introduction of air, the totalpressure ptot increased to 3.0 · 10-6 mbarand the spectrum shown in Figure 28 wasrecorded.Channeltron1100 VFaraday-CupIon Current [E–05A]Ion Current [E–09A]1.00.9Fundamentalsbe measured.
The pulses triggered by theindividual ions are counted directly or aresummed after a corresponding standardization. In order to keep the countingloss as low as possible, both the ion energy as well as the amplification (> 106)should be sufficiently high. The accuracyof single-ion counting is not influenced somuch by differences in the ion/electronconversion and the SEM amplificationthan in the case of current measurement.However, for large counting rates (> 106counts/s), account must be taken of deviations in non-linearity caused by the timeoverlapping of pulses [14].
The doublepulse resolution of the SEM 217, the ioncounter preamplifier CP 400 or the counter unit IC 421 amount to a total ofapprox. 20 ns.The determination of the sensitivity andthe amplification of a secondary electronmultiplier is explained using a simpleexample. Air is admitted into a vacuumchamber flanged onto a mass spectrometer(PrismaTM M2). The total pressure beforethe air was admitted was 8.2 · 10-8 mbar.1.0I = 9,6 · 10–06 A0.90.80.80.70.70.60.60.50.50.40.40.30.30.20.2140.11414+N N15N15N+,N14N+I = 5,7 · 10–10 A0.1140.0N14N+0.0262728293031Mass [amu]262728293031Mass [amu]Fig. 28:Spectrum measuredwith a C-SEM andwith a Faraday detector with otherwise thesame measuring conditions.271 Fundamentals of mass spectrometry0.500000.45000Scan # 1Ion Current [E–12A]scan speed: 0.1 s/AMU0.400000.350000.300000.250000.200000.150000.100000.050001220.500000.45000124126Scan # 1Ion Current [E–12A]128130132134136138140134136138140scan speed: 1 s/AMU0.400000.350000.300000.250000.20000Figure 29:Measurement of Xeisotopes in air atvarious scan speeds(QMA 400 with crossbeam ion source andcollimation magnet,inlet pressure5·10-6 mbar).0.150000.100000.05000122124126128130The proportion of nitrogen cN2 in air is78.1 vol.-% (see Appendix), so that thepartial pressure pN2 of nitrogen in thevacuum chamber ispN2 = ptot · cN2pN2 = 3.0 · 10-6 mbar · 0.781 ≈ 2.3 · 10-6 mbar.Because of the dominance of the nitrogenisotope 14N in the natural distribution with99.6 % compared to the nitrogen isotope15N with 0.36 % (see Appendix), only themass number 28 is evaluated.The sensitivity of the mass spectrometersunder the measuring conditions and settings of the ion source amounts to:132EN2 = I (28) / pN2EN2 = 9.6 · 10-6 A / 2.3 · 10-6 mbar ≈4.2 A/mbar with the C-SEM detector(1100 V)orEN2 = 5.7 · 10-10 A / 2.3 · 10-6 mbar ≈2.5 · 10-4 A/mbar with the Faradaydetector.The amplification of the C-SEM at a voltage of 1100 V can be determined quitesimply from the intensity ratios.V = 9.6 · 10-6 A / 5.7 · 10-10 A ≈ 1.7 · 104The detection of ions is a statistical process: ions hit the detector at random intervals.
If a particular accuracy is required,then a certain number of events must be28suitable averaging over many scan cycles(1000 scans with each 1 s/AMU), the Xeisotopes 124 and 126 (with a concentrationof approx. 90 ppt in air) can also be detected [13].For rapid measurements of the gas composition, and in order to obtain a highmeasurement speed, an operating mode isgenerally used which is limited to theintensity maximum of the mass numbersof interest and which averages over a defined time interval (Multi Ion Detection).1.2.4 Control and signal evaluationThese modules are individual units that areinterconnected via an internal data andcontrol bus.
This modular constructionallows a technically optimized, cost-effective design of the control unit for each task.Communication with a computer (PC) andthe QuadStarTM software installed on it iscarried out either via a RS-232-C or anoptical LAN-ArcNet interface. The requirements of the computer are relatively low,they are almost exclusively determined bythe operating system Microsoft Windows(WIN 95,98 or NT 4.0) that is required forthe QuadStarTM operating software.
At thestart of a measurement, the control parameters are first sent from the computer tothe control unit. This set of parameters isstored internally in the control unit and thecurrent measurement is carried out. Thedata of the measurement signal are intermediately stored in the internal storageunit of the control unit and continuouslysent back to the computer.
The computeris then used to visualise and store theresults for their subsequent processing. Incase of an interruption in the communication between the computer and the controlunit, the measuring job sent last is continued and the equipment status remainsunchanged.The control of the components of the quadrupole analyzer described in the previoussections necessitates various electronicmodules that are installed in the controlunits (QMS 422, QMI 422 or QME 200). Themost important functional modules are:the quadrupole controller (QC) to control the mass scan and to process themeasured signalinternal data and parameter storageand a computer-interface for the transfer of data and parametersthe voltage supply for the ion source(IS)the voltage supply for a secondaryelectron multiplieroptional modules for the digital andanalog output of the measured signals(DO, AO)optional modules for the digital andanalog input of external signals (DI, AI)Fundamentalsmeasured. For N events (i.e.
registeredions), the statistical error is ~ 1/√ N. If, forexample, 100 counts/s are measured (thiscorresponds to a partial pressure of 10-13 –10-14 mbar ), then for a measurement timeof 1 s, the relative statistical error is 10 %.For a given sensitivity of the mass spectrometer, this error can only be reduced byan increase in the measurement time(Fig. 29).A scanning rate of 64 data points perAMU was used in the analog scan showin Fig. 29.It should be mentioned that a furtherincrease in the measurement time andoptional local operating console(CS 422)291 Fundamentals of mass spectrometryIon sourceMass filterIon detectionFaradayEP 422/1SEMEP 422/2QMH400/410IS 420QC 422OptionsHV420/421QMS 422 with system busFig.
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