Banerjee, A

Dutta, N C

Ghoshal, S N

Mitra, N N

English

IACS Institutional Repository

Construction and operation of a 90" Nier type single focusing mass spectrometerS N  CuO SlIAL, N  ( \  J)ITTTA, (JVllR ) A  lUNKK.MCr, ANH N  N , MlTKAMa<ss Hpcdmmchn Lahoiatorj/, DepavImeHl oj Pht/sirs, Prcsnicnvy (UdUrjf,(hlvntia-t2[nernvvd 24 March 1972).'V 90‘" Nier type mass-sped loinetor a nomma] radius ol‘ ciirvaiuro of J 5 em Ibr ioii 1 rajedoiy has been set up. An eleoiromap;ii(d. capable of «j;en(‘iiiting imigndu held u]) io GOOD oersUds is used in the-instTumenl Tins is eiiei gi/A',d by a stabilized magnet eummt supply oiicuit, in whieli 1 he euru'ut can be varied eontinuously from 10 to 115 mA (on aeecderat- jug voltage is ,su])plied from a highly stabiliziul 2000 volts supply together with a potential dividing ciieuit I'hlament emission is also controlled Ironi a suitable stabilized circuit 1’he samjile handling system and vacuum system are described in the ])aper.In tr od u c tionA 00 ' sector field single focusing mass spectrometer with a nominal radius of enr- vatnre of 15 cm for the ion trajectory has been construoled and snccessfully ope­rated in tins laboratory (Nier 1940). The instrument has been built tor sindies of ionization and fragmmif-ation of inolociiles by electron impact.The principle of the first-order single-roensing, sector-field mass spectrometer is well known iSince in these instrumcjils only direetional focusing is used, the magiietic field actually gives a momentum sjiectrum. This neeessitates the use ol an ion source {eg., of ihc e-leetron hombardment type) Avhicli produces nearly mono-energetic lolis The usual mass spectrometer equation then becomesyli/c -  4 .7 9 x ]0 "V i/2 /f 'w'lun'c M IS the mass of tlic ion in a in u , e is the charge of the ion m niiii/S of electronic charge, r is the radius of the ion trajectory in the magnetic lieild, II is the magnetic field in oersted, and V is the ion-accclerating potential in volts Jn the present instrument r has a nominal value of 15.24 cm (6 inches) and so for singly charged ions,M V  -  1.11x10 2//2.V can be varied from 0 to 2000 volts and H from about 600 to 6000 oersteds. The usable mas.s range is, therefore, from 4 to 400 a.m.u , although the resolution at higher masses will necessarily be poor.Jnflian J Phyt! 4 7 , 5 3 -5 ! )  (19711)6354 Ghoshal, D iitta , (Mrs.) B anerjee and M itraAnalyser TubeThe analyfior tube, made of copper, was obtained as a gift from Prof. H, G. Thorlo of McMaster University, Hamilton, Canada, and iias been described olse- wliore (Graham et al J947, Thodo at al 1949). It is made from a 3/4" copper tubing of I /32" wa]J thickness, bent through 90'’ on a 6" radius The sector portion is flattened to 3/8" to fit in the poJe gap of the electromagnet/ 9’he original id3Tox glass enclosures at the two ends for housing the ion-chamber and collector assembly have been dispensed with. Instead, two 5" copper flanges bave been silver soldered to the analyser tube just beyond the two ends of the sector, the analyser tube extending about 4" beyond these flanges on both sides The ion source along with the focusing half plates and the ion entrance slit forms one composite assembly, the ])roiruding tubular end of which fits snugl}'^  into the analyser tube at one end. Sitnilarly, the exit slit with secondary electron suppressor plate forming a eomjiosite as'S(‘nibl,y \citli a ])i‘otruding tubular end fits snugly into the analyser tube at the other end. The entrance' and t*xit shts he in jflanes defined bj^  the two free ends of the analyser tube.Tile spacings and the sizes of the various electrodes and slit sizes on the source sid(^  and in the collector assembly ana the same as in the original instrument of 1’hodc. The filament is a 6-mil diameter tungsten wire in the form of a helix, spot-welded to the leads r>f a two-wore press seal at the end of a pjo’ex glass stem.Stainless steel tubes with copper flanges can be fitted on to the flanges fixed to the analyser tube to make vacuum-tight enclosures on the source and collator sides with the help of 0-rings, the other ends of the stainless steel tubes being /closed by copper plates silver-soldered to them. Electrical leads are brought out by means of glass to metal seals soldoied to the copper flanges attached to the analyser tube, so that the stainless steel tubes enclosing the ion chamber and the collector assembly at the opposite ends can he romoA’^ cd, wdiencvcr needed, AAith- ont disturbing the electrical connections to the various electrodes.Tu'o Avindows arc proAudod on the source side enclosure hy drilling t\^ o^ 0 4" Jiolcs on the back plate of the stainless steel tube forming the casing. The purpose of this is to provide a means for seeing the ion source assembly which is necessary for proper positioning of tiic sample inti oducing tube. The openings arc covered Avith small glass plates each of which is placed hotween two small copper flanges AAuih 0-rings in between. The Avhole arrangement is bolted on to the back plate over the holes and made vacuum tight by moans of O-rings.Some modifications have been later made in the collector assembly of the instrument to adapt it to the study of the measurements of kinetic energy of the ions produced hy electron bombardment on some organic samples, In the new arrangements, we propose to maintain the analyser tube at high voltage, whereas, the source and the collector assembly vill be almost at ground potential Th  ^details of these modifications will bo communicated iu due course,Construction and operation of a etc. 55[oua passing through the exit slit are colleeted on a collociiiig electrode and the ion current measured by nieaiLS of an EKGO Vibrating Reed Electrometer.V acu um  yysTKMA Metrovac 03B diffusion pump (of 30 litres per second unbaified pumping speed) is used for pumping the vacuum chamber The ])umping manifold in­cludes a flexible coi)per bclloAvs for providing small inovements to the analyser tidjc whicli might be required for maximising tlic lou current. The fore-]mmp used is a rotary vacuum pump of Cudian make having a nominal pumping speed of about 100 liters per sec capacityThe gauge manifold consists of a copper tube of IV' internal diameter litted Nvith flanges on both sides On one side, it is connected to ihe side arm of the stainless steel easing on tlie source side ol the spectrometer tube, while on the othci side it is eonnoeted to the pumping manifold. All vacuum light connections are made with the help of 0-rings. Vacuum measuring gauges are comieotod through copper-glass seals to the three small tubes attached to the imiiiilold. The pressure of the vacuum system is measured with the help ol a i;ommereial combined 1 iiani and loiii/atioii Gauge circuit. The ultimate vacuum attained without the use of a cold trap is of the order ot 2 X torr or less.An all-metal liquid air trap designed in this laboratory has been incorpo­rated in the vacuum system The volume of the liquid air container is about 700 c c. and once fully charged it can worlv for about ciglit to ten hours, rressiiius of the order of 5x10“® mm ol Hg have been obtained when the Iraj) is charg(M.l with Hiiuid air.filLlSCTJiOMAGNETTlie electrornaguot is of a double-yoke type with the pole laces machined to the proper sector shape so that the ions can describe a circular arc-shaped trajec- lory of 15 cm nominal radius within the magnetic held, the deflection of the ions in going through the field being 90°. The magnet is made of mild carbon steel with less than .08% of carbon and has been cast for us by a local firm. The pole pieces arc held ou to the yokes through tapered holes by means of a single bolt and a clamp on each side. The gap between the pole pieces is adjustable and is now set at 0.5 inch. The magnet is energized by passing current through four- coils, two on each pole piece. Each coil has about 22,000 turns of 29 swg super-onamellod copper' wire which are protected on two sides with 1/8" smdanyo discs and the lop and bottom being protected with glass-mica-shcots. Each layer, having exactly the Same number of turns of wire, is varnished by biuehing with Dow Oomiug silicone varnish. The total resistance of tlj. f.mr coils is about 10,000 ohms.56 G hoshal, G u tta , (M rs.) Banerjee and M itraJ3L1SCTR1CAL C'moXTlTSTlio (ilootrical circuila iiaed for the operation and control of the mass spectro­meter are of conventional design (Graham el al 1947).The coils of the electroinagiiet arc energized hy means of a highly stabilized higli-A'oltage low-current power supply system. The current is continuously variable betwotui 10 and 115 mA and j)rovid(‘s a maximum magnetic field of about 0000 oeiKtods, The long-time stability of the magnetic field is better than 1 part in 6000, 'riiis js inferred from the reappearance of the ion peaks at a given magnetic fi(4d setting Avhen the ion acceleration voltage is brought back to the valu(‘- at which the peaks hud been originally observed.The ion acc.eleration voltage is supplied from a 2000 volts highly stabilized Jiigh v’oJtagi' unit Ih-ecision wire wound resistances arc used aud tlui measured stabilization ratio is ol the order of 1 part in 10‘^  Tlie acceloratiuii voltagi'is varied with tlie help of ii potential dividing system, using a set ol wire wound resistors of total resisi-ance 0.5Mt2 which ad s as a cousLaiii- loail to the high voltage supply Kelvin-Varley priiicijile is ajijihial to select a constant pai’t of the*, volt­age. This again is appli<*d to a high resistance; hank so that an almost linear variation ol' the voltage with rcNSislance is obtained.Tlui current for heating the filament is sujiplied fioni an e,mission control unit, wliich utilizes the pmicijiJe of the ion gauge' eoifij-ol cii'cuit dcvisc'.d by Kida- mmr A Lanipsoii (1937). Tim filament emission ciiiTcnt ju'udnn'S a voltage droj) across a riisistancc winch is ap])lied to the grifl of an amplifier tube Any fluctuation in this voltagi'. drop resulting from lliietnalions in the filament emission burreutis amplified and helps to contiol f hi' plat(‘ ciirreut in a pair of tubes acting as a variable load on tJio secondary ol the' Iraiisfouiier supplying the filami'iit ciirreiii. Tlu' corresponding vaiiation in fh(i prijnaiy cunent keeps the filament cniTent under contrr)!. A hue voltage variation of 10 volts produces negligible (jhangos in the emission curr(mt. This circuit also supplies the necessary voltage (or electron acceleration, AvJiich is vaiiahle from 5 to 100 volts.As mentioned earlier, the loii (airrent is measured with an EKCO Vibrating Keed h]lectr(»meter. Th(‘ smallest curj’eiit which can be detected by it is ol tlu' order of 6 x  10 '■'•amp. Zero shift of tlie inatriimeiit is a fretpient trouble, which can however, he minimized by blowing liot air over the input components and oounectiiig leads.l^ iSBFORMANCJfl TiiSTliJOIn oide.r lu U*st tlu' jiciformaiice of the iustrument we liave studied llu' fol­lowing gases : argon, carbon dioxide and chluriiu' The mass peaks ]ia\e been scanned l»y keeiiing the magnetie field eoiistant at a suitable value so that the ionConstruction and operation of a etc. 57acoelerating potentials are as high as possible. Ionization efJicionoy curves for the different mass peaks have also been determined. The corresponding appear­ance potentials arc in satisfactory agreement with accepted values. Although the lowest detectable ion current by our electrometer Avas about 5 X 10~^ ‘*amp we have not studied in details peaks Avhoso heights were below 1X lO^^ a^mp. The peak heights of the residual gases and of fractionated hydrocarbons Averc considerably less than lO^^ a^mp.The peaks of the two chlorine isotopes CP** and 01“’ are easily recognizable. Ill addition, jicaks of and HKjP’ aa'c t o  t>bservod, since HOI gas Avas presensas contamination in the chlorine used The heights of these peaks at the mas-4 numbers 3G and 38 wei e however much reduced when care Avas taken to remove all traces of tlio HCl gas from the raAV chlorine The I’atio of the peak heights of OP'* and CP’ is in fair agreement Avith the standard value.The calculated value of the resolving power of the instrument, considering the slit Avidths and the radius of curvature would be i/130 I’relimiiiary measure- meiits AA'ith oui* [lolential dividing system showed Jair agreement with the cal- eulated value and Ihi; peaks of consecutive mass numbers weie found to be v^ ell sepalated (figimi 1) The measured values of the resolving pOAA'er were hoAS'evm-Figure 1 Mass [Ksaks at MJa -  .‘Ifl, 30, 37, 3S, using oUloniu) guHnot very accurate, since the assumption of lineox* variation of the voltage with resistance in the potential dividing system used in these measurements was not justified. A modified potential dividing system which did away Avith this858 Ghoshal, D u tta , (M rs.) B auer joe and M itra(liMculty wfts latt'r iiicorporaled ami nsod for Hludying tho variation of reriolving power will) prob'suK'H (Choshal et al 1971).1. ConipariHoii ol' tlio porooiitagci yield ol’ fragments for n-propyl alcoholViiliU'S ohl.iiuu'd with oiii Mist 1 uiiicntValm*.'-. ohtuuuMl hv (uirlioi* AVorloM J^’i'ioclel et a l ] 9156JU l.() 1 0927 is.:i IK. 92'l 10 1 17.0:iu 2.(i 2 5.7:u 1(10 100a:i 1 0(i 1 :i9>11 (1, KS 0 90i'2 (1 71 9.:is111 ■{ 17 :i >17l.T 2 :i:i 1 815S 0 0 240'fh(i iiislrumejit was also use.d for preliminary studies of the cracikmg ])aUcrn of some* organic comianindsKigun^  2. Mii«s H|Kiflriiin ot n-pi-opyl alcciliolR(!Su I1h of a typical case (ii-propyl a loohol) 'are shown in figure 2, and are com­pared ith earlier work (Fj'icdel H al 1950) in table 1 Except at M je  — 19, tl.l and 45 the agrecuiient l)ctwe('.n our measurements and earlier work is (piite good.(Construction and operation of a etc.\lim m m m sFnndol K. A., iSoKultz .1, L & Sharkey A. G lOfiG A n a l .  C h m .  28, 02(1,Gi'fthiim R. L., ITnrkness A T & Thoclr M. (h 1 9 -1 7 S n  24. 119(Uioshnl S N,. J)n(in.i\ ( ',  Milr/i N. V Ar Ihinn-joc^  A 1971 I m l im i  J .  44, 201^Ninr A 0 19(0 Uvr, SV/ Imt 11. 212.RidaiUMir L N, L.inipstm AV 19;i7 U a  Svi /as/, 8, Ki2 TlindeH (( , Mocniumira d I't ('nllniH T. R V U \)  Can J  Phj fs B27. :l(ll.ni)

Construction and Operation of a 90° Nier Type Single Focusing Mass Spectrometer

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Construction and Operation of a 90° Nier Type Single Focusing Mass Spectrometer

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