Performance and application of a fourfold monopole mass spectrometer

Infernafior& 0

Ekevier

Journal Scientific

of Mass

Spe&om&y

Physics.

and Ion

Publishing Company,

Am&erdam

J-k

RI&ARDS

349

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OF A FQURF~L&!IONOPOii _-. ..

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Department of Electrical Qld_ 481 I (Australia)

R.M.

APPLICkTON *

349-357.

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PERFQRMAN CE AND MASS !SPECTROWE~R

27 (1978).

7 Printed in -The Netherh+s

Engineering,

James

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Cook

University

of North

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Townsv~lle.

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S&hog1 of.Ek&ical 2033 (AustraZin)

(Received

Engineer&~

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Kensingion,

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3 November

1977)

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Some preliminary tests with an experimental fourfold monopole msss spectrometer are described, illustrating that the device performs acceptably (at the low resolutions used) despite the fact that-the fkld:forming surfaces of the driven electrodes are OXI&one qua@rant of a cylinder_ Coupling between adjacent channels is shown-not to be a problem so that applications re-quirin g simultaneous measurements using two or more of the rnoi nopole chanbels can be entertained_ Owing to its parallel structure the instrument is suggested as being suited partic=uIarly.to isotope ratio measurem ents with precisiork which could be significantly better tbanwould be possible with a quadrupole device. _~

INTRODUC+ION

When the ideaI hyperboloidal electrode ofationo@ole&ss spectrotieter is replaced,& ~eusualmanner;byacircularcylindridalr6de2~ti~deauseful modific+on~ of the inktnkenk +com& apparetit;-By eXtendixig the angle electrode inti a [email protected]&G squa%cylind~r,~the rod:-e!ectrdde%.shar&& cknmon by the four“angl~electr~des"so fo~ed,thuScrtiatin~~geonietry throtigh which four~ion beams cak be &ass-a@ys&d simulti&tiusly. Ifthe circular cylindri&l elect&de is'th&i~sepak&d intb..quadrants, ~&&~tially foti hidependentmakskp&z~me$eti~-_are formed withi&a~&inmoti &&h&d ~ousin~akillu&atedin Fig;-l;Massaq~~~-can_nbwbe-~ed-q~tind~~en_ dently-on fdkSion~be&% kiptilied to th&~device;~Irithe foll&Sng~desi~ con&derati&%~df sti&' a_ fourfold_ tidnop‘bk m&&spectrometer ~~kkYtion@c-tid~som6 prelh@nary iridicatiqns-ofpe~ormanck-are-~ven~~~t.-

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*Present address: Kensington, N.S.W.,

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School of Australia.

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January ‘25-_28.1977, Editor-1

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)sssection of a fourfold monopoIe mass spectrometer electrode configuration.

iabihty of the instrument_ Applications to which it appears :e also discussed, particularly with regard to what appears ty to the quadrupole mass jilter in certain circumstances_

uniquely to be its

%ND CONSTRUCTION lost critical mechanical design parameter in a quadrupole or _monos spectrometer is the radius of the rod electrode relative to the disthe rod from the apex of the angle electrode (the so-called “field hEnor errors in rod radius can have severe effects on peak shapes ;itivity_ Before constructing the fourfold monopole device, to be by using the finite differi below, this param eter was determined :hnique to generate the field distribution in the inter-electrode The rod radius was varied until the comparison between that disfzind the ideal linear field was as close as possible (determined on the an r-m-s_ error measure taken over a substantial portion of the anagion)_ The radius which gave the best comparison was chosen as opnd used in the construction of an experimental fourfold monopolerice was designed so that the hollow square cylinciricaI electrode between a pair of 2-75~in. (70 mm) OD vacuum flanges, In this manelectrode forms a substantial portion of the device’s vacuum conhe ion source mounts on one flange and the detectors on the other, >y balls to align and insulate the various components. In the source, Aron streams are produced by a common filament which, in turn, our ion beams in a standard electron impact ion source arrangement four sets of apertures, etc. The complete ion source was intentiontnted a small distance away from the entrance to the analysing structllow ions to experience some fringing field, since computer modelmmends this to be beneficial in obtaining high transmission [I] *_ rical connections to the instrument were made via a rear mounted electrical feedthrough, Connections to the ion source, in particular, nowledgements

351

“middle" ofthefourquadrant-shapedanalysingelectrodes, passthroughthe which were madeannularforthatpurpose. PERFORMANCE

Figure 2 shows peaks obtained with one channel ofthe fourfoldmonopole mass spectrometer; the half-peak height resolution is about 70_Arectangular waveform oflMHz was used to drive the device and, forthelarge peak (28 am-u.), the waveform had a peak-to-peak amplitude of about 73 V. Notwithstanding the small field-forming surfaces employed in the instrument, it can be seen thatthepeaksareverywell formedatthemoderately low resolutions tested. (Computer modelling has recommended that well-shaped peaks should be obtainable, athigh transmission, to resolutions ofatleast300.) Owingto the geometry of the devicetherearetwomechanismsbywhich analysis in one channel can be affected by neighbouring channels (i.e.cross talk). These are the capacitive coupling between electrodes and the chance that rejected ions from one channel could possibly traverse the inter-electrode space into anadjacentchannel. To assessthelikelydegree ofthiscross talk a spectrum was recorded on onechannelwithandwithoutanrf. signal beingfedtoaneighbouringelectrode.Theneighbouringchannelwas~so fed with ions so that anycrosstalkmeasuredwoufdbeares~ultboth ofcoupling and of rejected ions crossing the inter-electrode space. The signalfedtothe adjacentchannelhadthesamerepetitionrateandpeak-to-peakm~itudeas

Fig_ 2. Mass peaks at 28 and 29 a-m-u. mass spectrometer.

recorded

on one channel of a fourfold

monopole

352

that applied to the “main” channel, with both normal and inverted polarities_ (In practice whilst the amplitudes may be different the repetition rates would generally be identical.) The results of these tests are shown in Fig_ 3 wherein only minimal shifts in calibration will be notedFor the invertedpolarity, adjacent-electrode signal the shift is bigger owing to a slight change introduced into the waveshape by the inverting process used- There is no noticable increase in background noise in the spectrum, implying that cross talk resulting from rejected ions also appears not to be a problem. Certainly for the moderately low resolutions used at present there seems to be no need to provide means to reduce coupling -say, by providing inter-electrode shielding - although at very high resolutions such precautions may perhaps be necessary_ A further test of cross talk was carried out by supplying the adjacent electrode with a 25-V p-p500~kHz square wave, with a phase free-running with respect to the l-MHz analysing potential_ This is a severe test since the waveform is subharmonic to the driving potential - a condition known to be indidetrimental to peak shapes. As shown in Fig. 4 the effect is substantial, cating that in practice different frequencies could not be used in different monopole channek unless some form of shielding was used. Finally Fig_ 5 shows the effect of earthing one adjacent electrode_ Such a procedure will lead to distortion of the monopole field in the channel of

Fig_ 3_ An indication of the effect of capacitive coupling between adjacent electrodes in a fourfold monopoleThe middle trace is with all adjacent electrodes unexcitedThe lower trace is with one adjacent electrode driven with the same waveform as the channel of interest whereas the top trace is with the adjacent electrode driven with the inverse of the azxalysing potential-

353

Fig_ 4. The trace)_ The

effect of driving top mace is when

an adjacent

electrode

with

interest, near the electrode which has been the operation of -the device is not affected, region of analysis of a monopole instrument

Fig_ 5. The effect having ali adjacent

a subharmonic

waveform

(bottom

the adjacent electrode is unexcited.

of earthing an adjacent eIectrode electrodes floating (top trace)_

earthed. However, as observed, suggesting that the principal is well towards the centre of

(bottom

trace)

by

comparison

to

354

the field (in the near-region of the y-axis), which affected by such changes in “boundary conditions”-

is not

significantly

APPLICATIONS

Owing to its ability to perform four simultaneous, independent mass analyses the fourfold monopole mass spectrometer would appear to be particularly suited to a number of interesting applications_ These include the monitoring of relatively fast chemical reaction kinetics, respiratory gas analysis and the precision measurement of stable isotope ratios. In each of these the mass spectrum would not be swept; instead each channel of the instrument would be set to monitor one particular mass number, leading to a speed of response limited only by the time constants of the detecting and recording apparatus used _ For the remainder of this paper the particular application of peak height (isotope) ratio measurements will be discussed. If a mass spectrometer of any type is used in a normal continuous spectrum-scanning mode to determine the concentration ratios OF a number of species then the precision of the result will be affected both by random and long-term variations in ion source and analyser conditions_ Much the same applies for instruments operated in stepping or so-called peak switching modes, in v*-hich operating conditions are s@xzhed periodically between the species of interest. As a result multiple (dual) collector techniques have evolved in which measurement of the intensity of each peak is made simultaneously at separate collectors (usually Faraday cups, although secondary electron multipliers are also used when -only moderate precisions are required). In this manner ion source and analyser conditions appear as “common mode parameters” and a change, for esample, in total ion current wi.2 not affect the measured peak height ratio, even though the individual intensities will vary_ Using this technique precisions to about O_OI% in isotope ratios have been recorded with dual collector magnetic mass spectrometers [Z] _ When combined with a dual inlet system as well, in which both the sample and a standard are presented to the machine periodically, the difference in isotope ratios has been measured with precisions of the order of O_OOl% [ 2]Tne quadrupole mass filter unfortunately cannot operate in a dual collector mode for precise peak-height ratio studies since it does not use focussing of the ion beam to achieve mass analysis_ Instead it relies upon path stability. There is no means therefore by which multiple collector-like operation can be implementedAs a result random jitter in the voltages and frequency and particularly in the so-called d-c_ to r-f. ratio - will affect the precision of any ratio measurement whether spectral sweeping or peak switching is employedit can be shown that the d-c_ to r-f_ ratio needs to be stabilized to about 10 ppm if peak height stability is to be better than 1% Peak-height ratio measurements with a quadrupole mass filter therefore will be limited to

355

a precision of about 1% since 10 ppm can be regarded almost as a technological limit in the ratio of two voltages, one being- the amplitude of a timevarying function_ There have however been some reports of precisions slightly better than 1% (0.15%+)_9%) using a computer-controlled quadrupole mass filter with total peak intensity integration times of about 5 mm [3]_ Moreover a recent investigation using a quadropole mass fiber with a dual inlet system was able to achieve a measurement precision of about 0.1% (four standard devitations) in isotope-ratio differences [4]_ This however is an order of magnitude or two poorer than precisions achievable with magnetic mass spectrometers in the same configuration_ Recent simulations have shown that the monopole mass spectrometer may be considerably more suited to precision analysis than the quadrupole largely because it is less dependent upon critical path stability for its mass-filtering action [5]_ The outlet end of the device is constrained to an exit aperture and this imposes a degree of space-focussing on the operation of the instrument, as a result of which sensitivity of transmission to d-c; to r-f_ ratio variations is considerably reduced_ For example, a 10 ppm change in d-c, to r-f_ ratio will lead to something of the order of O_l% transmission change - an order of magnitude better stability than is possible with a quadrupole, By a careful choice of operating parameters it may be possible to reduce transmission sensitivity to d-c_ to r-f_ ratio variations even further with the monopole mass spectrometer. Whether this can be exploited for precise peak-height ratio measurements however will depend upon jitter in ion source conditions, since the monopole configuration, as with the quadrupole, offers no common mode advantages_ Owing to its structure the fourfold monopole mass spectrometer does permit multiple collector-like measurements on peak-height ratios and, in addition, possesses the insensitivity of the ordinary monopole mass spectrometer to d-c. to r-f_ ratio variations_ Since peak intensities can be measured simultaneously both long- and short-term variations in ion source conditions wiR have little effect on the ratios of the peak heights provided the four ion beams are derived from a common source or else provided ‘-tracking” ion sources are used. To take full advantage of the fourfold monopole instnmxent for precise peak-height ratio studies it is important that the transmission-determin~mg properties of each monopole channel (i-e_ the d-c_ to r-f_ ratios) be not mutually varying_ Indeed the tolerance on mutual variations for a given peakheight ratio precision will be of the same order as that for the same peakheight stability in a single-channel instrument, viz_ 0.1% peakheight ratio stability for 10 ppm mutual stability of d-c_ to r-f. ratios. Unfortunately it is difficult enough producing one excitation potential (r-f. plus d-c_) of that tolerance let alone four which are within that tolerance relatively, given that they are at different levels corresponding to the different mass components being measured and thus given that they have passed through independent output stages in the -electronic control unit. ThuS even though d-c_ to r-f_ ratio may be stabilized at a prim&y waveform generator, voltage level flue-

356

tuations, as a result of random processes such as noise in the subsequent circuitry,are 'Likelyto lead to transmission variations which are differentin each channel bymore than 0.1%. It is speculated that this problem can be readily overcome by operating the fourfold monopole device with a single asymmetric rectangular waveform ratherthanthe combined d.c_and sinusoidal r.f.potentials normally used_ Suchafunctionhasanintrinsicd.c.level which automatically tracks the peak-to-peak magnitude of the waveform provided the positive porch length and the period are well stabilized (to about 5ppm for O_l% transmission stabilityin a monopole mass spectrometer), It is ofcourse relativelyeasy to achieve ahighdegreeofstabilityin tmnngp ammeters and figures as good asO_Olppmforperiodcanbereadily obtained using a simple oven-controlled crystal oscillator_In the fourfold monopole mass spectrometer the primary rectangukir waveform,ofasuitabIe mark-to-space ratio, would be generated by a clock circuit and then modifiedappropriatelyinmagnitudebeforeapplicationto '+hefouranalysing electrodes Any independent, or common, randomvoltage-level fluctuations will have no effectonthe relativetransmissionsofthechannelssincetiming ~Ptherectangularwaveform will not be affected_Rather,only asmall calibration shift will occur(atitdoesalso ford_c.plussinusoidal r-3.excitation) whichwihbeneghgibleforallpracticalmassranges. CONCLUDING

REMARKS

In view oftheabovecommentsitissuggestedthatthefourfoldmonopole mass spectrometer,driven by a rectangular r-f.potential, should be capabIe ofpeak-heightratio me asurementswithoneorperhapstwoordersofmagnitude better precision than is possible with currently available quadrupole mass spectrometers_ E'urthermore the device can be readily changed from measuring one set of peak-height ratios to another,simply by alteringthe magnitudes oftheexcitationpotenti&s_Bycompar2son,somemagneticmass spectrometersrequiremechanicaladjustmenttodothis,owingtotheuseof space focussing-Moreoverdualco&ctormagneticmassspectrometersgenerallyrequire theionsto becloseinmass,whereas withthe fourfold monopole technique(orwithpeak-switchedquadrupoIes)this isnotthecase, Notwithstanding the proximity of the four analysing channels in the experimental device described above,capacitive coupling between channels has been seen not to be aprobIem,atleast for the Low resolutions tested. This is theresult ofdrivingtheelectrodes with waveform generators which havelow output impedances by compariso n to thecoz?pIingcapacitivereactances, ACKNOWLEDGEBSlNTS

Mr_J_L_Wint.ersandMr_R_A_M the determination of optimum

usumeciweresubstantiallyresponsiblefor &z&roderadius forthe fourfoldmonopole

357

instrument and for the computer modelling which led to the determination of other design p arameters, It is expected that this work will be published in the near future, The work described is supported by the Australian Research Grants Committee and the Australian Institute of Nuclear Science and EngineeringREFERENCES 1 P-H_ Dawson, J. Vat_ Sci. Technol., 8, No. 1,263. 2 L-F. Henog, New techniques for measuring isotope ratios by maSS spectrometry, 26th Annual Pitt&burg Conference on Analytical Chemistry and Applied Spectroscopy, March 3-6.1975. 3 P.M. Caprioli, W.F. Fies and MS. Story. Anal_ Chem_, 46 (1974) 453k 4 T-W_ Schmidt and J.A. Favre, Int. J. Mass Spectrom. Ion Phys., 20 (1976) 379. 5 R.A. Mmumeci and J-A Richards, J. Vat- Sci. Technol., 14 (1977) 1180,