Chemical Instrumentation

Topics in..

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Chemical Instrumentation Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079

These articles are intended to serve the readers O ~ T H JOURNAL E by calling attention to new developmafs i n the theory, design, or availability of chemical laboratory instrumentation, a by presenting useful insights and ezplanations of topics that are of practical importance lo those who use, or leach the use of, modern instrumentation and instrumental techniques.. The edilor invites correspondence from prospectiue contributors.

XLIII. Mass Spectrometers - Part Two GALEN W. EWING Part 2.-Mass-Analyzer Detectors

Systems;

Tho disporsian in mass speetromctcrs is achieved by combinations of cleetrieal and magnetic fields. The resultant spertrum is ordered according to m/e, the mass-to-charge ratio, for the accelerated ions. Analyzcr systems can be classed as static, where ions of a particular m / e valuc are brought to 1~ focus by steady (as opposed to alternating) ficlds, and dynamic, in whieh AC ficlds play a kcy part.

whieh shows that ions of a given kinetic energy will follow the same path, regardloss of their initial kinetic energy spread. If the ions enter the radial ficd as a slightly divergent beam, as shown, "+riatian in the distanec covered within t h e field will result in directional focusing on a n exit slit. This radial electric field alone cannot produce dispersion in terms of nyss (or mle) but only in kinetic energy. Passage of a beam of ions through a magnetic field also rcsults in a circular trajectory (Fig. 10). The radius of tho

able with sectors of 60, 90, and 180".A restriction on design is that the slit from the ion source, the apex of the magnetio sector, and the exit slit must be colinear in order for the ions to be focused on the exit slit. The exactness of this restriction is dependent upon ions entering and leaving the magnetic field normal to its boundary. This is not completely achievable in practice due t o edge effects of the magnetic field; hence the focusing of the ions is somewhat less than perfect. Usual practice is to allow a mechanical adjustment of the position of the magnet; exact alignment is determined by optimum response of the detector for ions of a given m/e. One way to avoid edge effects in the magnetic field is to employ a 180' sector with the source of ions immersed in the magnetic field. This will cause some curvature of the trajectory of the ions before leaving tho ion source itself, but this can be allowid for by careful design. Figure 11 shows a design of this type. Ions of a particular m/e value are brought to a n exeellcnt focus a t the exit slit 8 . Ions of other mle values will come t o a focus a t

STATIC METHODS As pointed out in Part 1, all ions emcrging from thc ion gun have bccn given the same amount of energy, eV (Eqn. 1 ) . However, in general the ions will have possessed varying initial kinetic cncrgics, so that thc beam is ncvcr ontircly homogcncous a t thin point. Thc cffoct of this sprcad of cncrgics can bc ncarly climinatcd by passing thc bcam through a radial electric ficld, as in Figuvc 9. If wc desig

Figwe 10.

Magnetic rector analyzer.

path is given by

Figure 9.

Radial electrostatic rector.

nate the strcngth of this ficld as E (volts per eentimctsr), a c can write thc equality,

eEr = nw2

(2)

nhcrc v is thc radius of curvature of thc circular trajcebory of ions with mass-toclrargo ratio m/e moving with a vcloeity u. IIcnec, r = m?/eE

(3)

where B is the magnetic ficld strength. In this easo the radius is awn to be proportional to momentum rathw than mcrgy. Magnetic Sector Speetrometcrs. Probably tho nmst familiar typc of mass spcctromcter is the magnetic scetor wricty. Thc pht,h of the ions is dct.cl.mincd by a combination of the cleebrieal aecclcrat,ing potential, as in Eqn. l, with thc cffcct of the magnctic field as in Eqn. 4. Combining thcse two equations, with thc dimination of u, rcsults in the relation

Tho sector spectromctcr can bo dcsigncd with any angle of magnctic scetor up to 180". Commercial instrumont,~am avail-

Figure 11. 180' magnetic onolyrer. The source and the exit slit S ore immersed in the magnetic fleld. S' indicates the region where other m/e ions may focus.

other points, such as S'. Thme can be swept across the exit slit by vnriation of cit,her thc aceelcrating voltagc or thc mxgnetic ficld. Figure 12 shon-s that farus achieved by this method rannot br prrfrrl. Trlljcetories dcscrihed in a nmgnotic fidd (Continued on page A160)

Volume 46, Number 3, March 1969

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Chemical Instrumentation

described by a point on a eirclc which is rolling along a stmight linc. Thcrc arc thrcc types of cycluids, thc common ryaloid in which t,hr: moving point is on tho oircumfweneo of thc tdling circle, tla: prulole cyeloid in which the point lies l,ct,wcan thc

Figure 12. Trojedorier of identical ions diverging from a ,lit in a magnetic Reid. The lower half would not be realized in an octuoi rpectrometer.

in t h e absence of a n electric field must be circular, but if they are, they cannot come together perfectly a t a n exit slit. This aberration is inherent in the method, and limits the permissible angle of divergencc of the ion beam as i t emerges from the source. The resolution is, however, considerably greater than with a simplc sector spcetromctcr. C&oirlal Speelro,rrelers. I n this stylc the nmenotie and clcetric ficlds arc suocrimposed, and the resultant trajcetory fallows a eycloidd path, as in Figurc 13. I t may bc rceallcd t h a t a cycloid is a cuwc

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Figure 13. Cycloidol trqjectorie. in cmrred electric and magnetic fleldr. A, curtote, 8, common, C, prolate cycloidr.

late cycloid in which thc point lics a n it radius extcndcd lwyond Lhc cireumfcrcnee of tho eirclc. I t can bc shown mathcmat,ically t h a t ians of it givcn mass-to-cha1.g~ ratio lcaving thc sourcc slit s t s n y rtngle

will bc bt.ought to x conunun focus by tho thrcc types of cycloirld paths. Tlrc eut.tatc geulnct1.y gim:s bcst results, and is alnwst, invariably chosen. This is slmwn a t A in the figurc. The difficulty with thc common and p r d a t c eyeloids is that the ions slow dam in the vicinity of tllc cusp a t H ,and this p~~uducos u considc~.aldcslmcc charge which tcnds t,o dcfoeus thc tmsm of ions. For prceisc focusing in a cycloidal spcctromctcr, both firlrls must he quitc unifarm. I t is customary to incruasc thc uoiformity of thc electric ficld by mcans of u stack of guard rings, which arc shown in scetion in thc figurc. Thesc are hcld at sucecssive potenbials by means of z voltage dividcr. 1)oulrlr-Focas Speetromelers; MallauchHerzoy Geomelr!,. I n this typc of spcctromctcr thc ions are pesscd through a radial clcctric field, like that of Figure 9, followcd by a '30" magnetic soctor. The ave~.allgcomebry is shown in Figurc 14. The anglc s"btended by thc elcctrie iicld is 31' 50', which is equal t o ~ ( 4 4 radians. ) I t can be shown that for this particular snglc, ions of a given mass-to-chargc ratio arc collimated, 01. t o put i t another way, ians leaving the clcctrie seet,or are dispcrscd actmding t,o cncrgy (see Eqn. 3), and foeuscd atinfinity. Tlicsc ions are now brought t o a focuv simultaneously by the magnctie licld, d o n g a plmc surface. This n m k ~ si t passible t o rccord all ians a t onc time on a. plmtogmphic platc laid d o n g this planc. Alternatively thc spectrum can be seitnncd across a n exit slit as in mod& prcviausly considet.ed. Hcnce this is both it spcotrom(Conlini~edon page A15Z)

Chemical Instrumentation ctor and a spectrograph. It has much grcater resolution than any of tho singlefocus systems. Nzer-Johnson Geometry. This is another version of a double-focus mass spcctromctcr. The ions are led through a radial clcctric sector, usually 90D,fallowcd by a magnetic sector which may be 60 or 80'. This differs from thc Msttaueh-Hcreog design in that thc ion bssms as they lcavc thc electric scetor aro convergent, so that an intcrmcdiatc slit can be mounted between thc olectric and magnetic sectors. Ions of only one value of m l e are sharply in focus a t any given combination of ficld strcngths, hence this gcomotry is not suitahlc for a spectrograph. The resolution is comparable to that of the MattauchHcrzog instrument. Details arc shown in Figure 15.

DYNAMIC METHODS The static mass analyzers discussed in the previous pages all employ focused ion beams, and hence requiro s slit source. The first two dynamic instruments to be described-the linear rediefrequency and time-of-flight designs-utilize a wide beam of ions, and direction focusing is not involved. Hence their ion guns differ in detail from the corresponding parts of static mass spectrometers, although using the samc principles, usually electron bamhardmcnt. Slits may be replacod by screcns or grids as olcmcnts for establishing clectric field gradimts. Radio-Pmpeney Spectrometers. An assembly of grids arrangod as in Figure 16 will permit selection of ions according to their velocities. This will be tantamount to m l e sclcction if the entcring ions have uniform energy. Alternate grids arc canncctcd togcthcr to a steady potential; the other set of alternate grids arc conncetcd

Figure 15. Nier-Johnson spectrometer.

Figure 14.

Maltauch-Herrog double-focus mars spectrometer-spectrograph

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double-focus

maa

to a radiefrequency sourcc of the order of 10 to 100 MHz. The ions which enter are either accelerated or retarded by the alternating ficld in accordance with the relation hetwoon the ion volocity and the f r p (Continued on page A156)


.

quency. IIcncc a t n given frcqucncy only ion, .i particular velocity (and thercfol.~

-- 1, , , , , ;;;;;,IIIIII,I

Ion beam

I , I

I

I

I

Detector

I I

Accelerator

\ /

D r i f t spaces

Figure 16. Linear radio-frequency m a s spectrometer. General Electric Company.

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I

Retarder

(ol ,chemotic,

ibl

photograph, courtesy

mass) can gct through. Thc fivld-fwc drift spaccs permit further rlisorimination irctwccn thc iworcrl ions and thosc drn!a~.l,p vcloeitics which may lravc suecccdrd in gct,ting past. tllc first scl of grids. Subx q u c n t modulatw stxgcs can r,zcl.t s grcatcr sclcrt.ion bctwcon tlrcsc ~.csulting bunches ui ions, and a t thv samr timc rcject ions with harmonically rclatcd vclorities. Tha ch:rtrodc lahrlcd "l.etardw" is given a positiva voltagtr whir11 p(wni1.s passage only of thosc ions which hxvc gained more than a spceified fraction of the total available oncmv. Tho ranm! of oscillator frcqucncy. Time-&Flight Spertro,,rrtrw F i g u ~ c17 shows thc dcsign of a n inritrumcnt in which ions arc formed only dwing n very short interval of time, h:ss than n mirrosccond tvnicallv. durine which the clwxccclcrating grid is givcn a pulsc of ncgative charge which cl~.swsout thc ions from thc ionizing rcgion and accelcratcs t h o m Sinco all ions havc essentially the samc cncrgy a t this point, theil. velocities will 1,c inversely prapot.tiona1 t o thc square-roots of their masses. The ions arc now alloactl t o move down t,hc lcngth of a fiald-frec space with whatcvcr vcloait,, tllcy nmv lravc aequirccl. llcnec thc ions 1,cromo scparlrtcd into bunchcs according to thcir masses. Thc ions of low mass arrive a t thc dctcctor carlier than thosc of higher mass. (Thc dctcctol. will bc described latw.) The pulse ~.cpctitionratc in thc timc-of-flight, (Contimierl on page A1,58)

slnyle pulse i m n a laset..

-I= -I=

-

170 CM

ELECTRONS

ELECTROMETER

ION

SOURCE

DRIFT PATH

MULTIPLIER

Figure 17. Time-of-Flight mars spectrometer, courtesy Bendix Corporation.

s p r e t ~ ~ a m c tr a n~ ~hc: of t h r ordw oi 10 k l l a , which mrans t,hat t h r romplr%r mass spcetrum of a sampla can llo ~ . c p m t c d 10,000 times in onc sccond. I n view of this spectl, thc only practieshlc rcsd-out dcvirc 1s a cathode-my osrillosropc at thr: output, of t h : dctcetw. Tlta 1inn:-swoop of tlr: oscilloscope must bv synchmniacrl with thc pulsc rcpctition ratc of t h c spr,ctromr~ta~.. T h c high spcrd maim this spwttmmater particularly sppropriatc I'or t h r st,udy of

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Quadrzpole Specfro,r~rtrvs.T h c q u a r l n pole mass spcetromcter in an instrument in which clectromagnctir fivlds intcrart in such a rvxy t,hnt only ions a i one particular m / ? ratio o:tn yet. tI~~.oue(h to thc dctcrtor. Ilenre t,hey arc oltau r d w n d to as nrrrss fillers r:rtIw tl,nn mass sl,crtt.mc!trt.r.T h c ;tnalyow rrnmists of fr~ut. ~ x t r a l l d rylinin crass srrrlt.ical rods : L I T : L ~ns~ sllou.n P~ tion in I'ignvr 18. I)ingc,nnlly o p p a a d rods ((!ouli,z twd o,,, pa!,^ .A 1611


Figvre 19. Quadrupole corporation

Figure 18. Quadrupole mars filter, diagrommatic crorr section.

A , / , imd n,C s7.c connrrlril t o g ( ~ t l ~vIv(.w tl.irnlly. lloth I)(! and AC potontids (fwqucnry of tllo o~clrt.of 500 lklls) aw applhal I,rt,wmw tlrr 1.wo p~i1.s~ L S5\10\\.11 :md n b c n n ~of ions is : d l o a d to 1mss ~1ownlhc ~:anl.t~;~I i~xis. l2igul.r I!) shows n typical drsign of quntltupola m:ws npwt,romotcv. T h e ion gun a t 1111,loft is vssrntially sirnilst. to t h o x Iwwiously ct,nsirlwrd

Fig. 17). Sceondary clcetrons an: eonstraincd by a magnctic ficld t o follow circular paths causing thcm t o hit the samc elcctt.ode from xhich they acre cmittcd, but a t a different point. Sueccssivcly emitted electl.ons then move along by a series of scmici~.cularjumps. Most of thc path lies b c t w e n a pair of glass platcs coatcd n i t h a high rcsistancc ~nctallie film. An clcetric gmdicnt is imp~.esscd along the lengths of these plates to producc tlw n e d d aecelm.atian. Thc magnctic ficld is p r a d w r d b,v a numbcl. of small Ixnnancat magnt!ts.

Literature Cited (4) Dawson, P. H., and Whetten, S . R., Rescarch/l)ri~elopmmt, 19, N o . 2, 46 (1!168).

Part three, eonelusion. q f "Mass Speetvometers", will appear i n the April issue. Figure 23. 16-Stage electron multiplier detector, courtesy Consolidated Electrodynomicr Corporation.

Ucndix has dcvclopcd a magnetic clcetron multiplier especially for use n i t h their

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The late Velmer 8.Fish's article on "Autom a t i a of Organic Elemental Analysis" will follow i n the May issue.

Chemical Instrumentation

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