mass_spectrometer Pfeiffer обзор (1248468), страница 10
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Incertain applications it is also possible touse oil-sealed rotary pumps equipped withappropriate devices for reducing oil backflow (vacuum side) and oil mist (outputside).A very cost-effective solution is to use aSplitFlowTM Turbo backed by a diaphragmpump (Fig. 47). This turbopump has amain pumping port for the high vacuumand an additional lower pumping speedport designed to pump the inlet.
However,this requires a very good match betweenthe dimensioning of the gas throughput inthe capillary and the downstream pressure stages of the vacuum chamber. Foranalyzing gas mixtures containing a component with high temporal fluctuations(e. g. 10 ppm – 100 %) of hydrogen or helium, such coupling is unsuitable becauseof the backflow entailed in the SplitFlowTMTurbo.The choice of pumping system for thevacuum chamber depends on the requireddetection limits and on the selection of theanalyzer. The selection criteria alreadypointed out in Section 1.2 apply here.40A stainless steel capillary (internal diameter 0.15 mm, length 1 m) and an interchangeable aperture for pressure reduction areutilized in the gas inlet system GES 010(Fig.
43). The all-metal valve, GEV 010, serves for opening and closing the gas inletin the analyzer unit. The sample gas flowof about 2–3 sccm is not interrupted evenwhen the valve is closed. The gas inlet canbe axial or at right angles to the rodsystem axis in conjunction with a crossbeam ion source. It is suitable for generalgas analysis in the range from 100% toabout 10 ppm.Fundamentalscontinuous pressure reduction in capillary1000 mbar1 mbarGEV 010 aperturepressure in ion sourceopen: p < 5 · 10-6 mbar,pressure in vacuum chamberp < 5 · 10-6 mbarGES 010inlet pressure heated SS capillaryinlet pressure900–1200 mbarGEV 010 with aperture(open/close)PrismaTMopen ion sourceC-SEMthrottleTMH 071MVP 015-2The inlet system shown in Fig. 44 employsa quartz capillary (internal diameter 0.15mm, length 1 m) and an all-metal gasdosing valve for pressure reduction.
Withthe valve combination UDV 040 and EVB016 in the intake line, the gas throughputsand the pressure ratios can easily bevaried, so that the setup can be optimizedfor various applications (different inletpressures, sample gas consumption, analysis of corrosive gas mixtures). The flowinto the vacuum chamber as well as theentire sample gas flow can be adjusted orturned off with these valves.A molecular beam inlet into an opencross-beam ion source is preferred particularly for analyzing gas mixtures containing corrosive or other components thatmay condense on the analyzer. Thisresults in better stability of the measurements and extends the length of time between analyzer maintenance.With the help of separate heating circuitsfor the capillary and the valve, the respective temperatures can be matched optimally to the pressure conditions and to the gasto be analyzed. Fig.
44 shows the cold trapTMU 071MVP 015-2Fig. 43:Design of ananalysis system withthe PrismaTM massspectrometer.The heated region ofthe gas inlet ismarked red.which reduces the water vapor residualpartial pressure in the vacuum chamber. Acryo-baffle which is thermally connected tothe cold trap surrounds the cross-beam ionsource. The gas transfer pipe passes throughthe cryo-baffle (without thermal contact)directly into the formation chamber of theion source.
With this configuration it is alsopossible, for example, to achieve detectionlimits in the sub-ppm and ppb ranges formethane and other hydrocarbons.An inlet system optimized for trace analysis of ultra pure gases is shown in Fig. 45.Pressure reduction is here effected with aquartz capillary and a fixed gold aperturedirectly upstream of the ion source.
Thissystem has been optimized with regard tominimizing the internal surface areas andthe volume, so it is constructed withoutany valves in the gas flow. The gas inletinto the vacuum chamber is permanentlyopen. Except for the gold aperture, thesample gas has no contact with metal surfaces before entering the ion source, therefore with this system it is also possible toreliably detect reactive gases such as oxygen in concentrations down to < 10 ppm.411 Fundamentals of mass spectrometrycontinuous pressure reduction in capillary1000 mbar1 mbarUDV 040pressure in ion sourceopen: p < 5 · 10-6 mbar, molecular beampressure in vacuum chamberp < 5 · 10-6 mbarQMA 400 withCross-Beam ion sourceLN2 cryo trapEP 422QMS 422heated Quarz capillaryUDV 040 withgas transfer pipeSEV 217inlet pressure100–1200 mbarFig 44:Gas analysis systemwith a QMG 422 andLN2 cold trap.The gas inlet is heredepicted in simplifiedform, turnedthrough 90°.The heated regionof the gas inlet is depicted red.QMH400-5TMU 071TMH 071MVP 015-2Very short reaction times can be achievedin conjunction with a gastight ion source.If valves are required upstream of thecapillary for isolating the entire analyzerunit or for gas routing, special valves designed for ultra-pure gas technology mustbe used.
The demand for increasing detection limits of course also entails stricterrequirements for the vacuum system ofthe analyzer unit. With the pumpingsystem shown in Fig. 45, base pressures inthe lower 10-10 mbar range can be achieved without gas input to the vacuumchamber.Apart from these dedicated solutions forparticular measuring tasks, compact massspectrometer systems are used for manydifferent applications.
Figs. 46 and 47 showthe very compact internal construction andgas inlet of such benchtop instruments.The design of the gas inlet of the ThermoStarTM is similar to that of the GES 020. Byvirtue of its construction without valvesand its small internal surface area, thisdevice is also suitable for detecting reactive gases. The principal application field42MVP 015-4for these instruments is their couplingwith thermal balance systems. For example, the temperature and weight loss ofthe sample can be read directly into thedata record of the mass spectrometer viaits two analog inputs, and processed alongwith the mass spec data. The sample gasinlet is permanently open, so high puritycarrier gases, so-called “zero gases”, areused for exact determination of the massspectrometric background.Fundamentalscontinuous pressure reduction in capillary1000 mbar1 mbaraperturepressure in ion sourcegas tight: p < 5 · 10-4 mbaropen:p < 5 · 10-6 mbar, molecular beampressure in vacuum chamberp < 5 · 10-6 mbarQMA 410 withCross-Beam ion sourceEP 422QMS 422inlet pressure900–1200 mbarGES 020heated quarz capillary gas transfer pipewith gold apertureSEV 217TMU 200 MTMH 071MVP 015-2For the OmniStarTM, the gas inlet in thevacuum chamber as well as the entiresample gas flow can be turned on or offwith the valve combination in the inletsection.
This is useful, for example, forconnecting several sample gas lines to ananalyzer system and to reduce the consumption of sample gas. This isolates theanalyzer equipment from the actual process. For analyzing gases containingexplosive or corrosive components, automatic shut off in the case of a problem isan important safety requirement. A specialvariant of the OmniStarTM (C version) hasbeen developed especially for such applications.
In this version the injection of purge gas (Ar, N2) ensures that the gas mixture at the analyzer system output is nevercapable of combustion and the pump bearings are protected from corrosion. Thedilution factor for the sample gas is about500.Optimum matching to inlet pressures inthe range 1200–100 mbar is possible overlarger capillary cross sections and greateraperture diameters. However, a pressureQMH400-1TPD 011MVP 055-3Fig. 45:Gas analysis systemusing a QMG 422 withQMA 410 analyzer.The heated regionof the gas inlet ismarked red.controlled gas inlet system is required ifthe input pressure for the analyzer unitchanges by more than one decade duringthe measurement.
Two variants of theOmniStarTM are available. Here the signalof the total pressure gauge in the vacuumchamber is used as control variable. In amass spectrometer with a pressure controlled gas inlet, only the concentrationvalues have real significance. The absoluteFig. 46:Picture of anOmniStarTM instrumentwith cover removed.431 Fundamentals of mass spectrometry1000 mbaraperturep < 1–4 mbargas-tight ion sourcep < 8 · 10-5 mbarp < 5 · 10-6 mbarQuadStar™heatedapertureheated silica glas capillaryPrismaTM withgas-tightion sourceC-SEM1/16”ThermoStarTMRS-232-C2 x relay output2 x analog output2 x analog input1 x relay inputTMU 071-3SplitFlowTM Turboexhaustheated stainless steel capillaryheated valve andaperturePrismaTM withgas-tightion sourceC-SEM1/16”OmniStarTMRS-232-C1 x relay output2 x analog output2 x analog input1 x relay inputTMU 071-3SplitFlowTM Turboexhaustheated stainless steel capillaryFig.
47:Schematic of theThermoStarTM andOmniStarTM benchtopinstruments.The temperaturecontrolled heatedregion of the gas inletis marked red.heated valve andaperture1/16”OmniStarTMCorrosive-VersionRS-232-C1 x relay output2 x analog output2 x analog input1 x relay inputTMU 071-3SplitFlowTM Turbopurge gas2–7 bar, 1000 sccmSwagelock 1/8”1/4 inch exhaustvalues of the ion currents and the partialpressures of the mass numbers of interest,do not relate directly the process gassesbeing analyzed. Only their mutual ratios,i.e.
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