mass_spectrometer Pfeiffer обзор (1248468), страница 6
Текст из файла (страница 6)
Anion which is only slightly unstable must beallowed sufficient time before it leaves thefield, i.e. its velocity must not be too great.The following further requirements existfor the injection conditions in order to enable the ions to pass through the limitedquadrupole field:The following general conclusions forselection can be drawn for the quality of aquadrupole mass filter from the relationships pointed out above:The quality of a quadrupole mass filterincreases with increasing rod diameter (r)and increasing rod length (L).
(Fig. 20, 21)Furthermore, with greater rod radius it iseasier to fulfil the conditions for geometricaccuracy (production tolerances andassignments for operating the QMS).The quality of a quadrupole mass filterincreases with increasing frequency f ofthe high frequency field (Fig. 22)These considerations are offset by the larger space required for the analyzer and thelower required operating pressure andespecially the necessary increased powerrating of the HF generator with the corresponding effort needed.Ion Current [A]E–05Fig. 20:Rod systems withdiameters of 6 mm,8 mm and 16 mmfor the analyzersQMA 200 (QMA 125),QMA 400 (QMA 430),QMA 410.The required power rating N of the HFgenerator isN = Constant · C · Mmax · r4 · ƒ5where C is the capacitance of the rodsystem including the connecting leads andMmax is the maximum mass number.
Nincreases proportional to high powers ofthe frequency and of the rod radius.Fig. 20 shows the rod systems of the massspectrometers described in this catalog.E–06E–06E–07E–07E–08E–08E–09E–09E–10E–10E–11E–11E–12E–12E–13E–134He5 ppmE–1405Ion Current [A]E–054He5 ppmE–1410 15 20 25 30 35 40 45Mass [amu]QMA 400,rod diameter (d): 8 mmrod length (L): 200 mmfrequency (f): 2.25 MHz0510 15 20 25 30 35 40 45Mass [amu]QMA 200,rod diameter (d): 6 mmrod length (L): 100 mmfrequency (f): 2.0 MHzFig.
21:Air spectra; the inputpressure was about5 ·10-6 mbar in eachcase.21FundamentalsIons parallel to the z axis must be injected within an injection aperture havingdiameter D = 1/2 · r0 · (m/⌬m).The maximum injection angle must satisfy the conditiontan < 11.85 · r02 / L2.1 Fundamentals of mass spectrometryIon Current [A] PrismaTMFrequency (f): 1.70 MHzMass Range: 1–300 amuE–07E–08E–09E–10E–11E–12E–13E–140510Ion Current [A]E–07E–0815202530354030354045504550PrismaTMFrequency (f): 2.46 MHzMass Range: 1–100 amuE–09E–10E–114He5 ppmE–12E–13Fig. 22:Air spectra, recordedwith the QMA 200with variousHF generators.E–1405101520Mass [amu]An other important criteria for the size ofthe rod system is the mass discrimination.The greater the diameter and the lenghtIon Current [E–09A]QMA 400, QMH 400-5(f= 2,25 MHz)I ( 285) : I (50) = 1.840.160000.140000.120000.10000Fig.
23:Spectrum of TFMT (2,4, 6-Tris(trifluormethyl)-s-triazine) takenwith QMA 400, (left)respectively QMA 200(right) with 70 eVionization energy.The inlet pressure was1 · 10–6 mbar.22250.080000.060000.040000.020000.0000004080 120 160 200 240 280Mass [amu]of the rod system, the lower is the massdiscrimination. (Fig. 23)Ion Current [E–10A]0.52000QMA 200, RF 2030.48000(f= 1.7 MHz)0.44000I ( 285) : I (50) = 0.530.400000.360000.320000.280000.240000.200000.160000.120000.080000.040000.000000 40 80 120 160 200 240 280Mass [amu]PrismaTMQMG 422Rod diameter6 mm6 mm6 mm8 mm8 mm8 mm16 mm16 mm16 mmRod length100 mm100 mm100 mm200 mm200 mm200 mm300 mm300 mm300 mmMaterialstainless- stainless- stainlesssteelsteelsteelMoMoMoMoMoMoAnalyzerQMA200QMA200QMA200QMA400QMA400QMA400QMA410QMA410QMA410Mass number range 1-1001-2001-3001-5121-10241-20481-1281-3401-16HF generatorRF201RF202RF203QMH400-5QMH410-1QMH410-2QMH400-1QMH410-3QMH402Frequency2.46MHz2.0MHz1.7MHz2.25MHz1.7MHz1.3MHz2.05MHz1.4MHz2.05MHz8 kVA8 kVAPower0.08 kVA0.1 kVA0.1 kVA9 kVA7 kVA8 kVA7 kVAContributionto adjacentmass number4He/ 540Ar/ 4110ppm1020ppm20100ppm5010ppb10<1ppb<110ppb10He/D2resol.possibleTransmissionfor Xe–10 %35 %–50 %–Sensitivity for Arwith Faradaydetector in A/mbar> 5·10-4> 3·10-4> 1·10-3> 5·10-4–> 1·10-4The combination QMH 410 with QMH 402(a modification of the QMH 400-1 for working in the so-called second stability region) is an exception.
This combination has> 5·10-4> 1·10-4been specially developed for analysinggas mixtures such as 4He/D2, 3He/HD,H3/3He, CH3/15N and CH4/16O.Ion Current [E–09A]0.900000.800000.700000.600000.500000.400000.300000.20000> 2·10-4Ion Current [E–08A]0.120000.100000.080000.060000.040000.020000.000001 % D2 in 99 % He3,5 % He in 96,5 % D2Ion Current [A]Ion Current [A]E–075E–075E–085E–085E–095E–095E–10E–104He : 4,003 AMU D2 : 4,028 AMU4He : 4,003 AMU D2 : 4,028 AMUFig. 24:Spectra of 4He/D2gas mixtures in linear(top) and logarithmic(bottom) plot.23FundamentalsOverview of the possible combinations of rod systemsand HF generators1 Fundamentals of mass spectrometry1.2.3 Ion detectionThe ions, which have been separatedaccording to their mass/charge ratio in therod system, can be electrically detected byvarious types of detectors:– Faraday cup– continuous dynode secondary electronmultiplier (C-SEM) and– discrete dynode secondary electronmultiplier (SEM).The choice of detector is primarily basedon the required detection sensitivity andthe detection speed.
It is also determinedby other application-specific requirements,such as the required stability, the thermaland chemical stability and the amount ofspace available.In the simplest case, but also that withthe least systematic errors, the ions hit aFaraday collector (Faraday cup) whereis therefore integrated into all analyzersdescribed in this catalog. In combinationwith an electrometer preamplifier, theFaraday cup can only be used to detectpositive ions.Figure 25 shows a detector set-up with aFaraday cup and a C-SEM detector asused, e.g. in QMA 200.
This C-SEM is acontinuous dynode secondary electronmultiplier in which the active layer has thetask of multiplying the electrons as well asapplied gain voltage distribution. This twinfunction limits the maximum current flowand the thermal stability of the continuousdynode detector. The utilizable amplification ratio of approx. 106 (at 2.5 kV) isessentially limited by the dark current inthe active layer.The C-SEM is positioned somewhat off thecentral axis of the rod system. The positiveions are deflected by applying a negativeFaraday-CupQMA 200HV (–)QME 200HV200C-SEMEP 200Fig. 25:Arrangement of theFaraday cup and C-SEMin QMA 200.they give up their charge.
A sensitivecurrent/voltage converter (electrometerpre-amplifier EP 422, EP 200) convertsthe resulting current into a voltage signalthat is proportional to the ion current.The detection limit lies between 1 · 10-16 –1 · 10-14 A, depending on the time constant (from a couple of seconds to 100ms). The Faraday signal is not affected bydegradation or mass-discriminationeffects at the detector. In addition to thesimple and robust design, a Faradaydetector also has long-term stability andhigh thermal resistance.
The Faraday cup24high voltage to the mouth of the C-SEM.The advantage of this arrangement is thatit is very simple and quick to change between the two detectors. This change-overcan also be carried out automaticallydepending on the ion current. In combination with the common electrometer preamplifier, the C-SEM is used to detectpositive ions (from the originally neutralspecies) of small ion currents and rapidprocesses.If there are very low ion currents and avery high measuring speed is required, theinfluence from photons must be reduced.mass filterFaraday cupFundamentalsmounting flangemounting flangedeflection unitFaraday cuphousingQMA 400 with a Faraday cupSEM 217/218QMA 400 with a Faraday cup and SEMIn this case, a discrete dynode secondaryelectron multiplier (SEM) is used as thedetector in a 90° off-axis arrangement, asshown in Figure 26.The ions leaving the rod system are reaccelerated to several KeV.
They are thendeflected so that they hit the first dynode(conversion dynode) of the SEM. Herethey eject a number of electrons which arethen multiplied in a series of further dynodes (16 in SEM 217, 17 in SEM 218). TheFig. 26:Arrangement of theFaraday cup andSEM 217 in QMA 400.voltage for the individual dynodes is distributed via a separate resistance network.
Inorder to minimize the contribution ofimpacting photons, soft x-rays or fast neutral particles (which are also coming fromthe direction of the rod system), the SEMis offset by 90° from the axis of the rodsystem. The ions are deflected by an electrostatic field which does not have anyeffect on uncharged particles. This produces the highest possible signal-to-noiseC-SEM (continuous)SEV 217 (discrete dynode)voltage dividercontinuous1 M/Dynodeamplification106108 at 3.5 kVarrangementoff-axis90° off-axismax. permissible current10-6 A10-5 Amax. nominal temperature range120°C150°C (at 1kV)max.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
