An integrating multiscanning field ionization mass spectrometer

139 InrmnncimaI Journal of Mass Sprcrronmr_~ and fan P&s& Q Ekvicr Scicntilic Publishing Company, Amsterdam -

AN INTEGRATING SPECTROMETER

IONIZATlON

MASS

Sran,cOrd Research lruricure. :Wzss Spccmcwncfry Rcxcarch Insrirurc. Mcnlir Park.

California

M

E. SCOLNICK,

MULTISCANNING

17 (I 975) 139-146 The Netherkxds

Printed in

W_ H_ ABERTH

AKD

FIELD

hl_ ANBAR 9m25

(U.S.A.) (First

rcccid

S

July

1974; in final form

3 Deazmbcr

1974)

A mass spectrometer comprising a field ionization source and a modified Wien fitter is described_ This instrument has been developed for analytical applications in the mass range 100 5 Al _I 400 a_m.u., which is scanned repeatedly by varying the E field of the velocity filter synchronousiy with an integrating multichannel analyzer_ The magnetic field is held constant_ A linear relationship between mass number and channel number is achieved electronically. Ion optics and electronic circuitry are described_ Performance is exemplified by unfragmented moIecuIar ion spectra of mineral oils.

INTRODUC.TlON

A mass spectrometer [l] comprising a field ionization source and a Colutron (Colutron Corporation, BouIder, Colorado 80302) velocity filter has been constructed for routine analytical applications. The Colutron 123 is an E x B velocity filter with tapered magnet pole faces and an array of electrostatic plates and guard rings for reducing the astigmatic focusing that is inherent in the design of the Wien filter- The characteristics of this ion mass separator have been described by Seliger [?] and Wahlin [Z]. Until now it has been used exclusively in applications requiring efficient ion transmission over a narrow mass range, such 8s io9 implanting and isotope separation systems The instrument described in this report has been developed for low resolution identification of complex mixtures, such as crude or refined mineral oils and plasma or urine fluids, over mass ranges spanning several hundred a.m.u.

The measurement of molecular wei_ght profiles ofsas mixwrc; and vWoriZ~-d liquids and solids is fitcilitatcd by the use of nonfngmcnting ionization sourcesIn particular, the advantages of the field ionizaticn source have been described by Beckey [4] and Chait [SJ_ However. the partial pressures of the vaporized con&tents of a mixtureof Eiquids or solids at any given tem_perature will no?. in general, represent their relative concentrations in the mixture. The spectrum resulting from a sin$e mass scan of field ionized molecules represents the molecular weight profile of thevolatilized sampEe pent in the ionizer during the scan. A spectrum Lhat is rcprexntativc of the composition of the total sample can tx obtained by operating the spectrometer in a multiscanning mode with an integating multichannel scaler for data xtxunulation. ln this mode of operation the misturc is evaporated. at P rate that is slow in comprison to the m;Lss scan rate. by gradually increasing the temperature until the sample is exhausted- The combinrrtion of IklJ ionization aw.l multiscanning. which is especially useful in complex mixture analyses. h&s not been reported heretofore_

The FI source consists of an array of ;r 1000 coppcr points spxxxi 25 jlrn apart on a 2 mm’ copper grid and a gold extraction grid placed 50 /fin above the points_ A potential diftirence of 2000 V bctwxxn points and cstrxtion grid is suflicient for field ionization of most organic molecules_ The multipoint FI source bar been described in detail by Anbar and Aberth [6] Samples are introduced into the vacuum system and delivered IO Lhc ionizer by a direct insertion quartz probe_ Figure I s1.a~~ schematic cross seaions of the ionizer and sample probe. The ionizer is maintairwd at ;Lte.mperature that is higher than the maximum probe temperature LOkeep the alumina insulator clean and LO prevent sample memory effects from occurring_ An image of the points away is focused onto ;L 0.2-mm-wide vertical slit by the electrostatic fens shown schematically in Fig_ 2_ The horizontal and vertical

defkctors shown in Fig 2 arc used for minor corrections

of beam alignment- The are electrically insulated from

fens elements, deflector plates, and ionizer assembly each other and mounted on a vacuum flange_ The Lange is held against z.xv3cuunl

seal O-ring in the end phte of the spectrometer by four bolts that pass through oversized holes in the flange_ Four translating screws are provided for aligning the ionizer-iens assembly by moving the flange_ A schematic diagam of the total ion op:ia system is shown in Fig- 3. The einzel lens focuses an image of the 0.2~mm slit spa-w-c onto the ion detector plancThe fens is equipped with a variable diameter circularflstop for removing nonpzra.xial ions_ The beam cross section at rheflstop is z 2 mm in diameterAfter passing through the velocity filter, the ion beam can be directed by the

141

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MITER

AL-a

OVARTZ - - PROBE

-

Fig_ I_ .S&cm;ltic

r.

cross sations

I

of multipoint

sow?=

HEATER

\

S.=.UPLE

THE-LE

ionizer

and solid s;lmpIc probe_

cv

P *

I

Ll

II

7

. L

__.__. -- - -_.

___.

Fig_ 1_ Details of ionizer lens and defiecting phtc ktyout. (a) hemispherktl inlet port; (b) ion@r smbly: (c) ionizer heater. L, tirst lens element. L2 second lens clement, L, third lens element rvith fixed slit aperture. and horizontal (H) and Vertical (V,) beam deflectors.

142

Fig_ 3_ Major compo~cs of ion optics end lcuum sysnxnsr (al source lens and icnizcr assembly with horizontal and vertical steering phtcs; (2) cinzcl &ns with wuiablcfl stop; Q) Ex B \-elocity fikc; (4) wrt.icd dcfkctin~ plates; and (5) detector and imss intcnsificr assemblies.

vertical deflectors VI (set Fig 3) onto either sn electron multiplier ion detector or an image intensifier v] which has a phosphor screen that can be viewed through a LucitemS window in the vacuum chamber_ The image intensifier enables the spectrometer operator to adjust beam focusing and aIignment parameters until a well-defined image of the “illuminated” slit aperture appears on the phosphor screenThe mass to charge ratio of ions transmitted through an E x B velocity fitter is proportional to l/E” [2] _ A linear dependence between mass/charge and time cm be achieved by pro-mmming the cIcctric field according to the equation E= I/@ + br)*, where Q :md b constants and t is time. The E field of the velocity

filter is progammcd in the followins way_ When the multichannel sc4er receives a start puke, it sweeps through its 4096 channeis at a constant time rate_ A digitalto-analog converter in the muItiscaIer provides a linear ramp voltage for use at the horizontal sweep input of a display oscilloscope- This signal is amplilTed Iinearly by a circuit whose output is Y, = II-li, where the slope k, can be varied by

adjusting a potentiometer_ The voltage V, is supplied to an inverse square root circuit that has an input bias control for adding a constant kz to r, and that provides an output Y2 = I/(X-, f ktt)*_ This signal is then supplied to a high-voltage linear amplifying system that has a gain G_ The final voltage applied to the plates cf the velocity filter is thus given by Y = + G/(k, + k ,t)*. where the f sign refers to the plus and minus pIates, respectively. The E fieid. therefore? is equal to 2 V/D, where D is the distance between the plates. For a beam of sir&y chara~ monoenergetic ions, the mass of ions transmitted through a constant B field veIocity filter is proportional to (kJG’) )[I + (k&)t]_ At f = 0, M = MO, the lower lkmit of the mass r,?ge_ At I = z, the the muItichanne1 sweep period, AZ = I%&_. the upper limit of tke mass range. Hence,

where MO oc kJG* and k,/kz = (M_-AI,)/IM,r_

The controls for the gain (G)

143

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tcI.ll

=-_=-_= ==I=-_,

Fig_ 4

Schematic

of field ionization

iingcrprint

=s=-

_--szi

-=--_nnn

b

1

apparatus.

and slope (k,) enable the lower limit and the relative range of the mass scan to be varied independentlyA block diagram of themass scanning and data acquisition circuitryis shown in Fig. 4_ A trigger circuit detects the collapse of the linear ramp voltage at the end of each swee~pperiod and gcnenres a pulse that initiates the next cycle. The countote meter (Cmberra Model 14Sl!!_, Canberra Industries, Meriden, Conn. 06450.) monitors the rate at which ions arrive at the electron multiplier ion detector averaged over a selectable ?ime interva!. The sample probe heater control circuit is coupled electrically to the count&ate meter in ;L fcedback configuration which programs the sample temperature for a preset ion countrate. A similar circuit has been described by Franzen et al. [Sl. Thus, as the more volatile constituents of the sample mixture are depleted, the sample temperature is kreased automatically, maintaining a virtually constant total pressure in the ionizer unti1 the sample is exhausted_ By using a ‘%onstant feed” sample heating program, the time required for analyzing a complex mixture in minimized and high-voltage arcing in the field ionization source, which is due to too high a pressure, is avoided. A schematic diagram of the sample heater temperature control circuit is shown in Fig

5 With the switch SI in the “rate meter” position, the power ampli-

fier control circuit uses the chart recorder output of the countrate meter in the spectrometer detector circuit to maintain the ion countrate at a value that is determined by the position of the control potentiometer_ The period over which the countnte is averaged can be varied by using the time constant control of the countrate meter or by changing the value of the feedback capacitor in the sample heater control circuit- With Sl in the grounded position, *thecontrol potentiometer can be used for manual controi of the sample temperature.

Fis 5_

Stmplcprobe hcxtcr tempcrz~turccontroi circuit.

PERFORSf22NCE

The mokcular ion mass distributions shown in Fis 6 are the result of analyzing 2:No_ 6 fuel oil and illustrate the spectrometer’s capability to reproduce data accurately_ The dates on which these two spectra were obtained were separated by three months?during which time the mass range was changed several times and the field ionizers w-we mpfaced and reaIigned_ Each spectrum represents the integration of z 1500 mass scans over a period of 20 min_ The linearity of the mass scare is due to velocity filter E fieId programming The dezadation in resolution with increasing mass is inherent in EX B dispersing elements_ The features that appear at the low mass limits of both spectra are artifacts caused by the collapse of the E field at the end of each cyck The muitiscan spectrum at the bottom of Fig_ 7 shows the molecular weight profile of a crude oil distiIlate_ The absence of ions below mass 190, the cutoff point of this distillate, is a characteristic of field ionization spectra (viz, the absence of mokcular fmgmentation). Part of this spectrum is shown in an expanded mass scale to demonstrate the resolution of the velocity filter in an analysis that spans the mass range 75 < rti c 440 am-u_

145

220

210

22o

239

240

ISO

260

27oEkn

290

ax,

310

32o

330

310

35o

38)

370

6_ Two tingerprints of a No_ 6 fuel oil obtained 3 months apart- The difference in mass range bctwen the two spectra is due to spcctromctcr tuning.

Eg_

Fig_ 7_ Wide mass spectra of Statesbury. Missouri Crude Oil- Upper trace shows spectrum of moiecular fragment ions produced by susfaincd ekctric discharge in source_

During a duplication of this analysis, a stable electrical discharge developed in the ionizer. A comparison of the two spectra shown in Fig. 7 illustrates two characteristics of the field ionization-multiscxtnning velocity filter spectrometer. First, the spectrum at the top of Fig. 7. which resulted from a combination of field ionization at the beginning of the analysis and electron impact ionization produced by the electrical discharge later, shows peaks at M c 190 amu. which are due to molecular fmgment ions. The absexwe of these peaks in the spectrum at the bottom

of Fig. 7 iilustrates the nonfngmenting characteristic normally associated with field ionization. Second, taken together, the two spectra demonstrate the spectrometer’s analytically useful resolution over a significantly wider mass range than has been reported heretofore for instruments that utilize the Wien filter or its modification, the Colutron, for ion mass dispersion.

ACLN0\VLEoGEbiENT

This work ~3s done with partial support from the U. S. Coast Guard under Contract No. DOT-CC-X?. 996-A and from the National Institutes of Health, National Cancer Institute under Grant No. CA-13312

REFERENCES I W. H. Abcrth. C. A. Spindt, M. E. Scolnick. R. R. Spcrty and hi. An&u. Proc. 6th Intcmational Mass Spattometry Con& Edingburgh. Scuthnd. in A. R. West (Ed.), Acfrunws in dfuss Sjwtircmc~~. Vol. 6, Ekevies Applied Science. Ike... 1974. p. 437. 2 L Wahlin. Nffcf. fawwx k~fetlk&r.27 (1964) 55. 3 R. L. S&i-. llrli Sjmpvs%tn on E7ccfr-o~.fan. andLaser Bkuzn Tccfxnohg~-, thrrlrkr, Colomb. !hn Fran&co Ptess, Calit, 1971. 4 H. D. Becky. Anger. Cknr. Int- Ed. fig_. 9 (1969) 623. 5 E M. C&sit. Amzi. Ckn.., 44 (1972) 77A. 6 W. Anbar and W. Abcr& Anal. Chcnr.. 46 (1974) 59A. 7 W. Aberth and R R Sperry, Rec. Sci. Zns~rnnx_~ 45 (1974) 128. 8 3. Fr~nen. H. Kupcr and W. i?icpq 1tzr.J. Afar Spccrmm Ion P&s-, IO tiPEj73) 353.