Topics in..
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Chemical instrumentation
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Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079 signed to minimize variations in this initial energy. Figure 1 shows a representative ion source. Ions can also be formed thermally or electrically. The requisite energy is supplied by local heating as with a laser, or by means of a, radio-frequency spark or gas discharge. These methods make possible mass spectrometry of many kinds of solids, in spite of low vapor pressure. Combination of thermal and electronbombardment excitation is often appropriate. An example is the Knudsen cell (Fig. 21, which consists of a crucible containing the sample: upon heating in vacuum molecules of the sample stream out through the narrow opening directly into the space traversed by the electron beam.
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hew a r t i ~ ~OTP e s intendtd to s m r Ihe r ~ a h r . qO ~ T IJOURKAL I ~ ~ 6u rallinu all~nlionto new d ~ t ' ~ 1 o ~ m einn tthe s theoru. ". &&m. " , or nvoilnbilitu " ol . c h e m i c a ~ l a b ~ r instrumentakm, at~ or by presenting useful insights and ezplanations of topics that are of practical importance to those who use, m teach the use oj, modern instrumentation and instrumental techniques. The editor invites correspondence from prospective contributors.
XLIII. Mass Spectrometers-Part
One
GALEN W. EWING INTRODUCTION There has been considerable activity in the desien of new instruments in this field in the five years since the review by Wiberley and Aikens (1). Several improved models of magnetic deflection spectrometers have appeared, and in addition, incressed emphasis is placed on quadrupole and other dynamic designs. Hence, it s e e m desirable to outline briefly the working principles of the major classes of instrument, then to describe specifically the major mass spectrometers and spectrographs now available.
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Part 1. Ion Sources, Sample Handling, Vacuum Systems These systems and components are common to all laboratory mass spectrometers, and so will be considered first. All mass spectrometers act on gas-phase ions. The ions are produced in a variety of ways depending on the nature of the sample. The most widely applicable is electron bombardment. A beam of electrons emitted from a. hot-wire filament (usually tungsten or rhenium) is passed through an evacuated space into which a gaseous sample is allowed to diffuse. Molecules of the gas, upon collision with sufficiently energetic electrons, become ionized by loss of electrons and often by more deep-seated fragmentation. A greater number of positive ions than negative ions are formed, and the bulk of mass spectrometr~c measurements are made on positive ions. A rather weak electric field transverse to the electron beam extracts the ions from the source space through one or more small openings. A higher voltage V then accelerates the ions, giving them kinetic energy
cV =
rn"'
(1)
where e is the charge on the ion, expressed in multiples of the electronic charge, m is the mass of the ion in a.m.u., and v is its velocity. This relation indicates that
-
Figure 1. A precision ion source, the CEC Irotron. la) Schematic; the battery symbols reprerent seporotely odjuttoble voltages from a regulated rectifier; RI and R2 energize the pair of repeller plotel, F, and F,, the two focus plates, the voltage source marked "scan" is continuourly variable from 250 to 3500 volts. [bl Exploded view of the CEC lrotron; the major ports, left to right, ore: the mounting bose, the body of the ionizing chamber with the two remi-circulor repeller plates on either ride, the flrst slit, the two focus plater (which ore coplanar when ormmbledl, and the cover plate which contoins the third slit [not visiblel; the small spheres are sapphire insulating spacers. [cl The CEC Irotron, fully assembled; the filament and heater orwnbly, ,how" separately, mounts on the Rat side of the lkotron and i s covered b y the shield shown at lower right. The goseour sample is introduced through the oxial port visible at the left. The length d the lhotron is about 3 cm. (Pholor rourtca,, Consolidated Rleclrod~,namica Coiporolion, Pnriidenn)
all ions will attain the same kinetic enerav (for a given value of e ) , but this is ad; true if they start with negligible kinetic energy. The ionisation chamber and associated circuitry must be carefully de-
SYSTEMS If the sample is a gas or an porized liquid or solid, it be introduced (Continwd on page A70)
Volume 46, Number 2, February 7 969 Circle No. 123 01 Readen' Sewice Card
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o t h e ionization chamber through a. valvChemical lnstr'~ItIent~ti0nting system ,,,,h those ,how,, in Figures 3, 4, and 3. The Bendix assembly of Figure 3 is representative of the prod-
Figure 2. The Bendii M o d e l 1 0 3 0 Knvdsen cell source. The crucible is heoted electrically to as high 0 s 2,500°C, ond emits a rnoleculor beom vertically upward into the ioniring chamber. The crucible temperature can b e monitored with o thermocouple or, through the viewing window with on opticol pyrometer. (Courtcry Seicnlific Inalrumonlr Diuirion of l i e Bendir Corporation. Cincinnnti)
ucts of several manufacturers. T h e sample is introduced through either a n ambient-temperature port outside the oven or s, heated one inside. The first is provided with a 10/30 standard taper joint for atbachment of interchangeable gas flasks. B y appropriate manipulation of valves, t h e sample is admitted t o the section of tube marked "sample volume" in the diagram, a t a pressure measured by the external mercury manometer. This segment is then closed a t the lower end and opened a t the upper in such a way t h a t the gas expands into the 5-1 "expnnsion volume," a t a pressure lower by a known factor, such as 0.002, depending on t h e ratio of volumes. Next a valve is opened admitting the sample t,o the mass spectrometer through a "moleculnr leak." The molecrtlar leak most commonly consists of one or more tiny needle holes in a thin gold membrane. T h e holes must be small compared t o the mean free path of t h e gas molecules s t expected pressures; a diameter of 0.01 mm is ahout right. This ensures conformity t o Graham's law of effusion: The components of n mixture of gasw will pass through t h e orifice inversely as the square-roots of their. molecular weights. Since most of the molecules (those which escape ionization) follow the same law in leaving the ionizing chamber, the relative partial pressures will be the same in the chamber as in the expansion volume l.e..ervair, a n eisential condition for quantitative analysis. Figure 4 shows t h e all-glass inlet system provided by Consolidated Eleetrodynam-
(Continued on page A7%)
Chemical Instrumentation
., ,,,
ic.. Corporation (CEC) for use with several models The [o,, black cylinders are encapsulated iron slugs which are manipulated by ring-shaped
Figure 4. The CEC Model 21-084A glmr inlet system. This ir o four-valve system, somewhot simpler than that shown in Figure 3. This unit must be attached by welded glass to the molec"lor leak and to sampling volver. ( C o m l c a ~Conaolidoled Rlccbodynnmice Coipora1.on. P"s"dcna)
Figure 3. The Bendii Model 1071 heated inlet system. See text for detmiled description. The three molecular leokr hove slightly different diameters. (Couilcs" scientific Inrl7ummla Division o/ lhc Bcndi. Co?poralion, Cincinnali)
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permanent magnets (not shown) to operate the valves. A rather different inlet system available from Micro-Tek is diagrammed in Figure 5. The sample is held in a small dipper or capsule a t the end of a long stainless steel rod. With the ball valve 1 closed, the rod is inserted through the Teflon seal b, and the space surrounding it evacuated through a pump corrneeted a t 4. The ball valve is then opened and
Chemical Instrumentation (.he rod poahed throngh i t and the second Teflon seal 3, until i t seats against. the constrictiorl a t 5, with t h e small sample container projecting into the side arm of the evacuated expansio~i chamber 7. This vesxel is maintained at a n elevated temperatme in an oven, 6, so that the sample immediately vaporizes. T h e tnbe 8 leads t o the molecular leak and spertmmeter. The valves a t .9 and I 1 permit evacuation of the system via diffusion and mechanical pumps connected a t 13. The fitting a t 12 is intended for a pressure gage. A similar probe can be inserted directly into the ionization chamber in many mass spectrometers, and is especially useful for solids and viscous liquids of low vapor pressure. Figure 6 shows such a probe arranged for laser evaporation of a refraetory solid sample. The sample is manipulated until i t is sharply in the field of t,he viewing microscope. The sliding diagonal mirror is then withdrawn which leaves t h e ssmple precisely aligned with the laser beam. The laser energy can be expected to fragment molecr~lesof the vaporized ssmple, producing ions without the need of electron bombardment,.
GAS CHROMATOGRAPHY Although fractions can be collected for later examination, continuons sampling is highly desirable if t h e mass spectrometer is t,o monitor t h e emuent from a gas chromrttograph (GC). A special eonnideration which becomes important in this service is the effect of the UC carrier gas. In order t,o obtain a sufficient pressure of an eluted sample in the m a s spectromet,er, relatively large quantities of carrier must be handled. If i t is admitted t o t h e spectrometer, i t is apt t o degrade the vacuum too greatly for proper action. Hence, i t is preferable to remove t,he bulk of carrier prior to entry into the instrument. A separator for this purpose has been described by Watson and Riemann (2), as shown in Figure 7. The entire efRuent from t,he GC is passed t h r o ~ ~ gahporous glass tube, 20 em long by 4 mm id, R mrn ud, mounted in a. jacket which is eontinually pumped. T h e helium carrier preferentially diRuses through the porom walls, allowing the sample (with residual heliom) t o pass t o the mass spectrometer. The entrance and exit const,rietions are intended t o maintain the internal pressrlre low enough t h a t effusion through the pores will be governed by Graham's l m . Enrichment up t o about 50% is reported. Several manufacturers of mass spectrometric equipment offer versions of this separator ar its equivalent. Another feature t o consider in t h e GCmass combination is t h e relation of t h e mass-scanning time t o the G C elution times. I t is desired to obtain the mass spectrum just as a particulw component of the sample is entering the spectrometer, and this requires observat,ion of some non-selective G C detector. One way t o accomplish this is t o split the GC effluent stream, one part t o enter the mass spec(Continued on page A761
Chemical Instrumentation
Figure 5.
The Micro-Tek MS-7500 universal inlet ryrtem. See text for dexription. ( C o u r h y Mlrvo-Tek D i ~ i r i o nof Tiocor, Ine., Bnton Kouor)
trometer, while the other goes l o a conventiunal GC detector. Another way is t o arrange an auxiliary electrode i n t h e . mass spectrometer itself l o intercept a fixed fraction of t,he ion heam before its dispersion. Figure 8 shows these relat i . The curve represeuts n GC peak
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seen either hy thc ( X detectw or the ,,,-be,, interseptor ,here acting as n GC detector). If the seauning time of the RS
mass spectrometer is .;hurt curnpawd 111 the dillation of the peak, then it can be triggered (time span 1) 11, ruiwidc with the peak maxim~un. It may also Ijo passihle t o run mass pec1l.a at timer 2 and 3
(Conlim~edon page .I%')
Chemical Instrumentation
Figure 6. The Bendir laser-heated direct insertion system. The sample can be or large as =bout 9 mm rquore, ond the vaporized spot only 0.01 mm diameter. tcmmw scianlific Indiumed: IXui.+o,! of lhr Bendiz co7porotzon. C,nc,nnn1il
Figure 7. Helivm separator, after Wotron ond nicmann (21.
Figure 8. GC-Detector response verrur time. The numbered intervals moy b e of interest for moss spectrometric rompling.
VACUUM REQUIREMENTS The analyzer of a. mass spectrometer operates under dynamic conditions. Sample gas is constantly entering and heing (Conlinued on page A80)
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t,he condensed ail 10 the point whe1.e it will not flaw back to the hoiler fast enough. If t,his is likely to be a dilliculty, the water shordd be prewa~.med tn roam tempern~.emoved. Hence, speed as well as ultiture. mate v a c n u n mmt be rotwidered in ne A cold trap is nearly always required lecting a pumping sy.item. The ultimate between the diff~isio~l pump and the specvaeutm seldom need go below 1W7 turr. ttometer. Liquid ~iitl.ogenis the c o o l a ~ ~ t The speed of pumping depcnda largely uf choice, though a slurry of solid CO* 011 the geometry of t h e entire system, as with chloroform ur other suitable liqnid well as on the kind of pump and lemperawill serve in a pinch. A ~trmbel.ui devices ture of the trap. S w h cmsideratio~mare a1.e available which will automnte filling too involved t o detail he1.c. t h e trap with liquid nitrogen from n stanThe conventional pompittg system fur a dard storage dewar. I n i h e anthon's exmass spectt.ameter consists u l s dilTnsim 11el.ience with a CEC 21-104 maw specpump backed with at oil-sealcd mcehnntrometer, thc trap holds enungh nitrogen ical roughiug pump. The dillusion pump t o last 24 hours, though the antomntic may use eit,lier m e w w y o r oil. h1el.eul.y filler adds smxllorincrements more often. has the advantage tdlat. any of its vapor A 4000 cu. it. storage dewar lasts about which finds its way into the ioniaaliolr 12 days. region of rhe spectrumetel. will only a m The SnrgenbWelch Scientific Co. mantribute easily recognized peaks rurrccpoudufactures a. compressed-air operated cryoing t o t h e rneucwy isotopes; a h it is less genic refrigerator which can replace the sobjeat t o chemical attack ii air is inliquid-nitrogen trap, boltirrg auto the same advertelttly admibled to ormlact will1 tile Hwga in m y standard vscoum syslem. hat vspol.. However, many organic oils This operates by expansion-cooling of air, provide h i g h a pumping speeds atid lower It has and is stated t o r e ~ c h-140°C. ult,imnte pressnres. A compal.ative disbarn reported t o be sntisfnctory 101. mass cussiov of fifleen pumping fluids hay been speet,rr,metl.y, and is certnidy mwe c o w presented by Rondeau (3). I t s h m l d be venicnt. noted that the rilieone oils are 1101 s d i r Imr pumps, Lhough mare expensive, am fnct,ory far mass spect,romet,er ap~>li~:ation, sometimes srtbstituted i m difiusion pllmps. RB they pl.oduce a slow build-op of a They dc, no1 requi1.e either forepump or siliceous deposit on i,he irm sowce slits, cold trap, except during starling pmwhich is next t,o impmsible to remove. ccdwes. Iuu pllmps are uat ordinndy A paint which may not be aenerally s ~ t i ~ f acry c t for pnmpine helinm and other known is t,hat (,he cooling water fed to tho ,noble Rases, and su slanrld not he relied condenser of an oil-diRorion purnij ran be (Cmlinaed on page 489) too mld. I t will inrronsc Lhe viscosity of
Chemical lnstrumentation
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Chemical Instrumentation upon for operation iu conneetinu with GC unless the manufacturer specifically approves sueh use. Many mass spectrometers require more than one pumping system. I t may he desirable to divorce the mass analyzer chamber from the pumps which remove the un-ionized sample from the ionization chamber; both will require high performance diffusion or ion pumps. Sample handling systems sueh as those of Figures 3 and 5 must be evacuated prior to introduction of a, sample, and a separate difusion pump is generally employed here without a cold trap. Any probe sampleintmdnction device (as in Figs. 5 and 6) must include a vacuum lock to prevent entry of air into the spectrometer; in some models a separate diffusion pump is provided far this purpose while in others a mechanical pump is sufficient. An efficient system of interlocking pressure-operated switches is essential, to prevent heating the diffusion pumps when the forepressure is too high or when cooling water is not flowing. Vacuum Gages. The high pressure side of the inlet system is commonly supplied with a closed-tube mercury manometer, abaot I50 torr full-scale (Fig. 3). The pressure in the expansion chamber may be measured precisely with a diaphragm micromanometer (sensor indicsted in Fig. 3); this is a rather expensive secesso1.y which is only needed for high precision quantitative analysis. Rugged gage, typically thermocouple or Pirani types, are needed to monitor the various mechanical forepomps. An ionization gage is appropriat,e for the main analyzer vacuum. I t should have good sensitivity and a wide range (e.g., lo-' to torr), but need not he highly precise. I t is this gage that is used for a continual check on the proper operation of the overall vacurim and ga5-handling system.
General References Reed, R. I., "Modern Aspects of Mass Spectrometry," Plenum Press, N. Y., 14RR .. -...
Mcl)owell, C. A,, "Mass Spectrometry," McGraw-Hill, N. Y., 1963.
Literritute Cited (1) Wiherley, S. E., and Aikens, D. A,, J. CHI:M. EDUC., 41, A75, A153 llOR4)
(2) Watson, J. T., and Biemann, K., Anal. Chem., 37, 844 (1965). (3) Rondeau, R. E., J. CHEM.EDUC., 42, A445, A551 (1965).
I n fallowing installments, mass analyzer systems and detectors will he considered, snd complete spectrometers described.
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