Improved interfacing of a field ionization source with a quadrupole mass analyzer

309 International

@ Ekcvicr

Journal of Mius Specltomctry Scientific Publishing Company,

and Ion Physics, 22 (1976) 309-3 14 Amsterdam - Printed in The Netherlands

IMPROVED INTERFACING OF A FLELD IONIZATION A QUADRUPOLE MASS ANALYZER *

SOURCE

WITH

M. ANBAR AND ERIC ICEhIPSTER Mas (First

Spccfrometr_~- k?csearch Cenrcr. sranford reazived

11 March

1976;

in final form

Research Insxiture, Metdo 30 April

Park. CA 9u)-S

(USA.)

1976)

ABSTRACT

Significant improvement of interfacing field ionization (FI) sources with quadrupole mass analyzers (QMA) has been achieved by placing a ferrite insert at the detector end of the QMA and by installing a repeller elecirode opposite the electron multiplier (EM)_ By installing a multipoint FL source ca_ 1 cm from the entrance to a QMA, operated at a resolution of 250, a molar counting elliciency of ca. 3 x IO’ 7 counts mol” has been achieved_ This is comparable to that obtained with a sector magnet mass analyzer at the same resolution.

The interfacing of field ionization (FI) sources with quadrupole mass analyzers (QMA) is constrained by two factors First, there exists interference by highly-excited fast neutral species formed by glancing collisions of fieldionized ions with the counter electrode fl]_ These neutrals are then field-ionized in the RF field inside the quadrupole and collected with very poor mass resolution by the detector system. Second, there is high divergence of tht ion beam emerging from the FI source [2]_ The first constraint can be removed completely by setting the quadrupoie off-axis [3, 43 with or without double deflection plates. This approach results, however, in a substantial loss in transmittance of the mass analyzing system because of the ion beam divergence cited above_ The loss

can hardly be tolerated in view of the low ion transmittance

achieved,

even with

on-axis arrangements_ Attempts to increase the ion transmittance by acceleration and subsequent exponential deceleration does not lead to a significant improvement in the transmittance, in comparison to mounting the FL source as closely as pracrical to the QMA 121. By lowering the voltage between the field ionizer points and the counterelectrode grid, it has been possible to improve the performance of the FI-QMA

system_ This voltage lowering C;LI~be achieved by using as sharp field ionizing surfaces as possible and by minimizing the distance between t hcse and the counterelectrode- Using esIxciaI!y sharp multipoint sources of the Stanford Research Institute type IS] and point-to-grid distances in the ran_= 25-40 pm, it was possible to operate these field ionization sources at 5OO-IO00 V grid-to-point voltageThe lower voltage ions tend to produce nruclr Iess excited neutral sRccies [I 3, and aIso the divergence of the ion beam is minimized under these conditions [3]_ Consequently, we have been operating Fl-QMA systems at minima1 pointto-grid voltages and at minimal distance between the FI source and the entrance to the QMA- Under optimal conditions, an overall efficiency of 6 x 1016 ion mol- ’ at a resolution of 250 at mass 125 was achieved in the FI-QMA system comprising Extranuclear 324/9 I9-mm rods. This eficiency is bttter by a factor of ca_ IO over that achievable with a 16-mm set of QMA rods and by a factor of ca. 100 over that attainable with a 95mm set under the same Ff-QMA geometry and resolution conditions_ Ln this paper- a series of experiments will be described in which the geometrical arrangement of the QMA system has been changed, resulting in an additional five-fold improvement in the overall sensitivity of the FI-QMA systemThe geometrical parameters changed in our experiments included the removal of the ferrite insert of the Extranuclear QMA from the front end of the poles, installing it at the back end, changing the distance between the anode of the FL source and the entrance to the QMAz and changing the position of the electron multiplier (EM) detector, together with the installation of a repeller electrode opposite to the entrance dynode of the EM_ The rationale for these modifications has been as follows: since the ion beam from the FI source is highly divergent, it is possible that the front end of the quadrupole will act as a converging lens if the ferrite insert, which also acts as a mechanical f-stop, is removed, The insert narrows the real acceptance an@e of the QMA_ Further, it was assumed that since most of the ions enter the QMA at angles signiIiuntly different from O’, a substantial frrction of mass-analyzed ions may leave the analyzer at a divergent angle_ These ions, which would normally escape detection, can be counted if appropriate electrostatic fields steer them into the EM. The experimental results presented in Table I corroborate these assump tions_ The experimental apparatus comprised a 4096 multichannel analyzer synchronized with the sweep of the QMA, This arrangement allows accumulation of the ion count in a pre-selected mass range while evaporating a known amount of sample into the FI source [6]_ From the known number of molecules (M) crossing the porous grid of the multipoint source and the number of parent ions detected (P) overa given mass range (lia_m_u_), the overall efficiency (E) can readily be cakulated: E = (P/M) x A, The molar counting efficiency. i.e. ions detected per mok sampk, is then simply E- N, where N is Avogadro’s number- The molar

311 -T-ABLE DEIlECTlON

Ex-p. h-o-

1 EFFICIESCIO

OF THE

Disramxgrid

FI-QXA

SlSYE31

USDER

11.0

3 4

1s 16 4s S-0 11.0

z i

11-O

COXDIllOSS=

Fen-irepatiliorf EAP

dfolur counling e$Ticienc~~ (x IO’” count+lmoJ)

F N N B B B B

0.55

ro pofes C7znl)b I 1

DIFFER-T

P P P P P P R

1.00 0.75 IA0 IA0 l-70 1so

a Mcasurcd by counting the integrated number of ions at 126 amu produced whiIe scanning owr the mass range 1~0-130; resolution kept constant at AI/:IAf = 250. a Operating condition: points KOgrid voltage 750 V, grid to QMA voltage 17.3 V. c F - from mounting (regular); B = back mounting: N = totally removed. d’pzz normal as used bcforc modification. R = modified position u-ith repeller installed. C Molar counting cffkicncy = number of ions x number of amu per scan x Avogadro‘s numkr~numbcr of molccuIa in sample.

efficiency alIows an easy estimate of the minimum amount of sampIe measurable in a given mass spectrometric systemIt can be seen from Table I that removal of the ferrite increased the eRiciency of detection at constant resolution by a factor of about 2_ Bringing the source closer to the analyzer, which is possible when the ferrite is removed, did diminish the efficiency at constant resolution, probably because of interference of the fringe fields of the QMA. Insertion of the ferrite at the back end of the analyzer however, increased the efficiency of detection by a factor of 2_ Inserlion tif a grounded shield with a 12-mm hole in front of the dnalyzerentranc-e while moving the source back to a distance of 4.5 mm lowered the efficiency from I-4 x IO” to ca_ 1 x 10” counts mol-‘_ The efficiency with the ferrite at the detector end reached a maximum of u. 1.7 x lOI counts mol-‘_ This efficiency increased, however, to 2-S x 10” counts mol- ’ when a repeller electrode was mounted opposite to the EM entrance dynode (see Fig_ l)_ The effectiveness of the repeller became constant at a positive voltage of 1800 V_ Briefly, by removing the ferrite from the front end of the QMA., installing it at the exit end, and by installing a repelling electrode opposite to the multiplier, a factor of 5 was gained in counting efficiency, with no loss in resolutionBringing the FI-QMA system to a counting efficiency of ca_ 3 x 10” counts mol- ’ makes this system comparable in performance in the low mass range with a 90” sector magnet mass analyzer [7]_ It shoJld be remembered, however, that this efficiency has been achieved with a Qkwith exceptionally large rods (I9 mm) which have a limited mass range (up to 350a.m.u_)_ In addition, counting

312

MULTIPOINT

QUADRUPOLE lwA.SS FILTER WITH FERRITE ELFS REPELLER

+ HV

4096 CHANNEL MULTISCALER SPECTRUM INTEGRATOR STORAGE

Fis

I_ Quadmpok

integrating multisunning

field iotization mass ~pcctrocnc~cr.

QMAs have intrinsiczdiy higher background counts than magnetic anaiyzers. Consequently, in spite of the comparabIy high efficiency- the smallest amount

of thymine measurzbPe with a QMA would require the accumulation of 5OW counts at the mass cf the parent ion in a iO-a.m_u_ scan, i-e_, IS x 10‘ l3 mol or CL 20 pg- With a sector magnet, owing to its signifi~tiy lower background, 1 pg of material could be assayed with a precision -z &- IO%_ Notwithstanding this limitation, the QMA is undoubtedly the instrument of choice for multicomponent analysis, where the sample amount is generally not critical [6]_ Frgures 2 and 3 illustrate the acceptabie quality of the spectra obtained (with 16mm rods) by a modified FI-QMA system_ The efficiency of this system was u. 2 x lOI counts mol- r. Although we were not sample-limited here, one would like to use even in this GWZ as little sample as possible, if only to avoid frequent maintenance shutdowns of the mass spectrometerThese experimental results also suggest that the construction of a QMA with huger rods and with sticient length to allow a greater mass range at reasonable RF may resuIt in even higher detection efficiencies, making the QMA a superior mass analyzer for FI systems- Moreover, since no extensive attempts were made to optimize the ion collection system at the detector end of the QMA, there might be room for additonal gain in detection efficiency following this lineIn any case, the substantial gap between the performance of FI sector magnet

313

ADENINE

THYMINE

CYTOSINE

I1

Fig 2 Intcgt-atcdtield iotition E Coli.

1

I

I

mcl Fig.

I

I 220

mass spectrum of an HF hydrolysatc of DNA

I

! 240

I

I

I

260

I 280

I

exuaacd

I 300

I

from

I 320

3_ Integrated field ionization mass spcnrum of No. 6 fuel oil.

systems and FI-QMA referred to previously 183 seems to have been eliminated by the more recent developments.

314

The authors wish to acknowledge the assistance of Mr_ Arthur C_ Scott in the generation of some of the experimental data. The work was carried out with partial support from the National Institutes of HeaIth, N3tional Cancer lustitute under Grant No_ CA-I:33!2_

REFERENCES 1 H_ L- Brown md M- hbar_ fnr.I_ Radiuf- Ph~x Chem_. 7 (1975) 281_ Z H. t Brown and R H- Crass, -ngs of 23rcfAnnrml Conf~ on M&s Specfrontetr~ d AUid Topics. 1975, p. 87, 3 H_ J_ Hcinen, ch Hotzel and H. D- Becky, IIU. 1. MTSS S’pecttvm_ Ion Ph_n.. 13 (1974) 55. 4 H_ L BKND. W- H- Abtrth ad M- AI&S, Pmmsurgr of 21~1 Amud Conjkrtnre on Mass S’mezry turd A&Ii&lTopics. 1973. p- 456_ 5 (L k Spindt. .L A&Phys-. 39 (1968) 350% 6 M. Scolnick. W- H- Atih and M- Anbar, IIZL J- Afuss Spcwvm- Ion Phlx., 17 (1975) 139_ 7 M- Anhar amcl G- k St John. AidCXmn, 48 (1976) 198, S Mm Anbar, in ZWmaf Approaohtr KOGas Chroma~og~ph_~bfa.ss

Mdi&,

DHEW

Publication

MH

75-762.

March

1975

Spmmmerr_r in Laborator__e-