134

Section III. Spectrometers, energy and mass analyzers

Recent development of ion-optical studies for mass spectrometerand mass spectrograph designTakekiyo Matsuo and Toru SakuraiInstitute of Physics, College of General Education, Osaka University, Toyonaka 560, Japan

Morio IshiharaAnalytical Instruments Division, JEOL Ltd., Aktshtma, Tokyo 196, Japan

Ion-optical studies in the field of sector-type mass analyzers (mass spectrometer and mass spectrograph) investigated in our groupare reviewed . First ion-optical predictions were made, and ion-optical calculations were checked by experiments . Good agreementbetween both results suggests that ion-optical calculation is reliable and indispensable for designing mass analyzers .

1. Introduction

The report is divided into the following three sec-tions :

(a) High-performance mass spectrometer . Best fo-cusing condition (resolution) can be satisfied only at acertain fixed deflecting radius . Full mass spectra has tobe obtained by scanning either the magnetic field or theaccelerating potential and electric field. While ion inten-sity is lost during scanning, this type is, at the moment,most commonly used .

(b) Normal-performance mass spectrograph . Thefirst mass analyzer was a mass spectrograph with photo-plate detection. Since the invention of an electron multi-plier, mass spectrometers have been more widely em-ployed because of easy and fast data processing. Theappearance of simultaneous ion detection systems (mi-crochannel plate, fluorescent plate, photodiode arraysystem) stimulates the development of a mass spectro-graph again.

(c) Tandem mass spectrometer system. Many combi-nations of a mass spectrometer with another analyzerhave been developed. A most typical example is thecombination with a gas chromatograph as a pre-separat-ing apparatus (GC/MS). GC/MS has become a com-plete technique for analysis of gas mixture samples.Recently, liquid chromatography also has been almostcompleted and now is effectively combined with massspectrometry as LC/MS. As a pre-separation instru-ment, a mass spectrometer can also be used : interestingion beams having certain mass are selected by the firstmass spectrometer (MS1) and this selected beams areanalyzed by the second mass spectrometer or massspectrograph (MS2) after collisional dissociation withinert gas. This tandem arrangement of two mass

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analyzers is called MS/MS technology and is going tobe very important for practical analysis of mixturesamples.We are going to review mass spectrometer and mass

spectrograph design from the above classification . Thereare two methods of ion trajectory calculation, the" transfer matrix method" and the "ray-tracing method".In some cases, ion beams are calculated by the ray-trac-ing method and the result can be expressed and furthercalculation executed using the transfer matrix . Thishybrid calculating method is useful . Recent progress incomputers (from personal computer to supercomputer)makes it possible to express the ion trajectories in visualform very easily. Such a method helps us a lot tounderstand the ion-optical characteristics of systemsand to design new systems.

2. High-performance mass spectrometer

We would like to define the term "high-performancemass spectrometer" as follows: A mass spectrometer thathas a mass resolution, ion transmission and maximumdetectable mass as high as possible under the restrictionsof a certain magnet sue . Firstly, there is the well knownmass spectrometer equation,

where

M = the

ion

mass,

B = the

magnetic

fieldstrength, p = the trajectory radius in the magnet, V= theaccelerating potential, and q = the ion charge. Eq . (1)defines the upper limit of detectable mass under thegiven p and the maximum field strength Bmax . Themagnetic field strength is limited because of the satura-tion of magnet material . To detect heavier ions, the

T. Matsuo et al. / Ion-optical studies for mass analyzer design

135

Fig. 2. Photograph of the large-scale mass spectrometer .

1m

2m

3m

4m

5m

6m

Fig. 1 . Ion-optical block diagram of the large-scale mass spectrometer at Osaka University (from ref. [3]) .

trajectory radius in the magnet has to be larger, and the

Secondly, the equation which determines the theoret-magnetic deflection angle has to be smaller accordingly

ical mass resolution is :in order to fit in the fixed magnet size .

R=A.,/(A x, + d +,A),

(2)

III . SPECTROMETERS/ANALYZERS

136

where A Y = the mass dispersion coefficient, Ax = theimage magnification, d = the width of the detector slit,d represents higher-order aberrations, and xo = thesource slit width.

To realize higher mass resolution, larger AY andsmaller Ax , d and A are preferable . Mass dispersion AYis proportional to the radius p . In order to make theimage magnification Ax smaller, the ion beam enteringthe magnet should be dispersed widely. In other words,the area of the envelope of the ion beam in the magnetshould be large from the Q-value concept [1] .We conclude that a magnet with larger trajectory

radius p, wider pole piece and smaller deflection angle0. is required for high-performance MS. One of thedisadvantages of a smaller deflection angle is the in-crease of second- and third-order image aberrations.For the purpose of compensating higher-order aberra-tions, multipole fields such as quadrupole, hexapole andoctupole lenses have to be introduced .

Some examples of high-performance MS of reversegeometry with quadrupole lenses were proposed byMatsuda [2] . A large-scale instrument which employedone of these geometries was constructed at Osaka Uni-versity [3] . Its ion-optical characteristics compared withother instruments were discussed in ref. [4] . An opticalblock diagram is shown in fig . 1 . The specifications areas follows :Total path length = 7.6 m,Magnet radius = 1.25 m,Maximum magnetic field = 1 .8 T,Mass range = 12 500 at 20 kV and 25000 at 10 kV .Fig. 2 illustrates the whole instrument . To see clearlythe progress of ion optics in mass spectrometry, we havedevised schematically on paper four instruments withthe same magnet radius (1 m), simulated peak shapesand estimated the theoretical mass resolution by usingthe computer code BEIS [5,6] under the assumption ofthe same source slit width (100 Wm) and detector slitwidth (19 gm) (see fig. 3) . Mass dispersion, whichmeans the distance between the peaks of a mass dou-blet, is of nearly similar magnitude because we assumethe same magnet radius . On the other hand, imagemagnification and higher-order aberrations have im-proved considerably, contributing greatly to improvedmass resolution and transmission .

In fig. 4, we see the beam envelopes of four massspectrometers both in the horizontal and vertical planes .In Mattauch-Herzog and Nier-Johnson type instru-ments, only cylindrical electrodes and homogeneousmagnets were used, which lack focusing action in thevertical direction. Satisfactory beam transmission can-not be expected by using only these combinations,especially on a large-scale instrument . Introducing anelectric quadrupole lens greatly improves the verticaltransmission, as shown in the figure . The function ofthis type of lens is not only to focus the ion beam in the

T Matsuo et al. / Ion-optical studies for mass analyzer design

Mattauch-Herzog

Nier-Johnson

CQH (Matsuda)

GEMMY

Mattauch-Herzog

Nier-Johnson

CQH (Matsuda)

GEMMY

0

Fig. 3 . Theoretical image shapes of four mass analyzers underthe same slit condition (from ref. [4]) .

vertical direction, but also to diverge the beam in thehorizontal plane . This is particularly important becausethe ion beam entering the magnet is dispersed morewidely, thus causing it to be focused on the detector at a

Horizontal Vertical

Mass resolution

3600

5200

9800

25000

Fig. 4 . Beam envelopes of four mass analyzers (from ref. [4]) .

70 80 90 100 110

much sharper angle . The image magnification is thensmall, and a higher mass resolution can be expected .

After completing its construction, ion-optical specifi-cations (mass range, mass resolution) were examined byexperiment :(1) Mass range : A spectrum of CsI clusters up to m/z

= 48 000 at an acceleration voltage of 5 kV wasobtained as shown in fig . 5 .

(2) Mass resolution : The mass resolution was checkedby taking a normal EI spectrum of phosphazinecompounds at m/z = 3628. An experimental massresolution of 24300 was obtained with 100 [Lmsource slit width, whereas the theoretical mass reso-lution was expected to be 28000 . This agreement issatisfactory .This large-scale mass spectrometer is now routinely

used mainly for peptide analysis . The results obtainedwere reported in ref. [7] .

3. Normal-performance mass spectrograph

Fig. 5 . Mass spectrum of Csl clusters ; acceleration voltage of 5 kV (from ref . [3]).

One of the most effective features of mass analyzerscompared with other analytical instruments is their

Electrostatic analyzer

T. Matsuo et al. / Ion-optical studies for mass analyzer design

Quadrupole lens

120 130 140 150 160 170 180(n)

137

higher "quantitative sensitivity" . For increasing sensitiv-ity, we have investigated ion optics of normal-perfor-mance mass spectrographs in which higher sensitivity isexpected because there is no beam scanning. "Normal"means here that the theoretical mass resolution factorAy/Ax is not so high and second- and third-order imageaberrations are not so small as compared with high-per-formance instruments discussed in the previous section .In mass spectrography, ions with various mass numberscan be detected simultaneously on the focal plane . TheMattauch-Herzog instrument is a famous mass spectro-graph with straight focal plane . We have investigatedmass spectrographs of normal combination arrange-ment having straight focal planes. One example of suchsystems is shown in fig . 6 . A detailed discussion will begiven elsewhere .

4. Ion-optical techniques related to tandem mass spec-trometers

By connecting two double-focusing instruments,MS/MS study becomes possible . Ion beams with cer-tain mass number are selected by the first instrument

Homogeneous magnet

Fig . 6 . An example of a normal combination mass spectrograph. Beam envelopes are drawn by the ray-tracing method .

111 . SPECTROMETERS/ANALYZERS

138

(MSI) and are led into a collision chamber to undergocollisional dissociation . Fragment ions produced arethen analyzed by the second mass analyzer (MS2),which is a mass spectrometer or mass spectrograph .This is the standard MS/MS strategy which is effectivefor structure investigation and analysis of compounds .We do not intend to review ion optics of the tandemmass spectrometer system itself here, but propose threeion-optical techniques related to tandem MS.

One of the most important techniques in MS/MS isthe scanning of the second MS in a proper way or theuse of the mass-spectrographic detection in order toanalyze all fragment ions with wide mass distributionhaving different energies . These techniques have beenalmost completed using microprocessors [8] or simulta-neous position-sensitive detectors . Other importanttechniques are the transmission of ion beams selectedby MS1 into a collision chamber and into the MS2.

4.1 . A quadrupole doublet interface [9]

In order to carry out MS/MS experiments, ionsselected by MS1 have to be led effectively into a colli-sion cell and further to MS2 for analysis of producedfragments ions . A quadrupole triplet was employed forthis purpose. The first two quadrupole lenses guide theion beam through the collision cell . The point-to-pointfocusing from D1 (the detector slit of MS1) to S2 (thesource slit of MS2) is achieved in the horizontal direc-tion by choosing suitable values for Ql and Q2 . Opti-mum beam transmission is obtained in the verticaldirection too, as shown in fig. 7b . Q3 is off in this mode .

A mode

enhanced resolution0,

mode

B mode

MS/MS mode

T Matsuo et al. / Ion-optical studies for mass analyzer design

01

0.01

horizontal direction

Q1 Q2

Q3

0 521 1

0.01

Q1 Q2 Q3

The routine MS/MS measurements are executed in thismode and the typical results are given in ref . [10] .

4.2 . Enhanced mass resolution to a four-sector mass spec-trometer [11]

A quadrupole triplet can also be used for transmit-ting an ion beam efficiently from MSl into MS2 afterremoving a collision cell for the purpose of obtainingbetter-resolved mass spectra at the detector of MS2,using two double-focusing MS as a four-sector singleMS . It should be noted that the ion beam intensitythrough MSl and MS2 could remain almost the same asfor MSl only if one carefully chooses the ion trajecto-ries through the entire system . The key feature of maxi-mizing ion beam transmission is the proper use of theelectric quadrupole lenses . The overall transfer matrixcan be simply obtained by the matrix multiplication of[MS2][QT][MSi]. Here, [MSl] and [MS2] represent theoverall transfer matrices of a single double-focusing MSand [QT] refers to the transfer matrix of the interfaceregion . The expected beam envelopes through MS1 andMS2 in the enhanced mass resolution mode are shownin fig . 8 . The beam envelopes at the interface area areshown expanded in fig. 7a . The point-to-parallel focus-ing is achieved in the horizontal direction by choosingsuitable values for QI and Q2 . Q3 is adjusted to focusthe virtual image to S2 . Focusing in the vertical direc-tion is required in order to optimize beam transmission .We have checked this principle of enhanced mass

resolution experimentally by using a tandem mass spec-

IIIIIIIIIIIIIIIIIIIIIIII I i 1

I

vertical direction

Ql Q2

Q3

.. unru. "e.un,

QI Q2 Q3

Fig. 7. Beam envelopes in the interface region (from ref . [ll]) .

horizontal direction

XMAX = .00005AMAX _ .002CMAX = .000DMAX = .002

T. Matsuo et al. / Ion-optical studies for mass analyzer design

vertical direction

Q E Q

M

QQ Q E Q

M

trometer (JEOL HX110/HX110) . A xenon and PFKmass doublet at m/z = 131 was obtained in the EIionization. The mass spectra obtained are shown in fig .9. An improvement in mass resolution of a factor 2.37was obtained, whereas the theoretical improvement fac-tor is 2.49, calculated on the basis of transfer matrices .Theoretical and experimental values agree quite well .This confirms that the ion-optical prediction is reliable.As a practical example, the FAB mass spectrum ofmelittin was then recorded by MS1 and MS1 + MS2,respectively . The results are shown in fig. 10 . Massresolution is clearly improved, while a transmission

w

ZZQZZmQw

5w

29

MS1 only

47

1 1 11- 1 1111 1

400 420 440 460 480

SAMPLE POINTS

R1 = (47/29)*(131/0 .087) = 2440

6 .387

S

YMAX = .00075BMAX = .002

6.387

139

Fig. 8 . Beam envelopes through MS1 and MS2 in enhanced mass resolution mode ; horizontal and vertical envelopes (from ref. [11]) .

efficiency of 90% was experimentally checked using aCsI cluster .

4.3. Quadrupole doublet plus octupole lens system forvarying mass dispersion [12]

An array detector placed at the focal plane canincrease the sensitivity of a mass spectrometer by afactor of 10 to 100. Because the size of a present arraydetector is limited to one or two inches at the moment,it is necessary to develop a mass spectrometer in whichone can change the mass range, in other words, vary themass dispersion. One of the authors (M.I .) has devel-

MS1+MS2

400 420 440 460 480

SAMPLE POINTS

R2 = (73/19)*(131/0 .087) = 5790

Fig. 9. El mass spectra of xenon andPFK mass doublet at m/z =131 ; (a) MS1 only, (b) MS1+MS2(from ref. [11]).

111 . SPECTROMETERS/ANALYZERS

140 T. Matsuo et al. / Ion-optical studiesfor mass analyzer design

MS1+MS2MS1 only

� I �� I ,-

m/z

m/z

Fig . 10 . The (M+H)+ region of the FAB mass spectrum of melittin ; (a) MSl only, (b) MS1+MS2 (from ref. [ll]).

oped a new method for this purpose . In this method, aquadrupole doublet (two electrostatic quadrupole lenses)and an electrostatic octupole lens are placed between amagnet and a detector . Schematic drawing of the geom-etry is shown in fig . 11 . The key points of this arrange-ment are as follows :(1) The lens strengths of two quadrupole lenses (QD1

and QD2) are adjusted so as to focus the image toP1 . There are many combinations of QD1 and QD2to satisfy this condition .

(2) Mass dispersion depends on the values of QDl andQD2.

(3) Inclination angle of the focal plane B is also depen-dent on QDl and QD2.

(4) Double-focusing condition is always satisfied at PIfor any QDl and QD2.-

5 . Summary

Original Double Focusing Point (P -

Fig. 12 shows the simulated ion trajectories calcu-lated up to a third-order approximation under typicalconditions . The focal plane is usually curved because ofthe third-order aberrations of the system. It is, however,possible to correct the curvature by using a third-orderfocusing element, for example an octupole lens . Fig. 13shows the simulated ion trajectories with and withoutthe octupole lens . It is clearly shown that the octupolelens can improve the curvature of the focal plane to alarge extent.

Mass spectrometry consists of (1) ion creation, (2)mass separation and (3) ion detection . Recent develop-ments in ionization and detection techniques are

Fig . 11 . Schematic drawing of quadrupole doublet, octupole system .

surprising. In order to support such techniques, the

mass analyzer part itself must advance accordingly. We

have reviewed some ion-optical techniques developed

by our group.

Acknowledgements

We would like to express our thanks to Prof. H.

Matsuda for his support on construction of a large-scale

mass spectrometer . We thank Prof . I. Katakuse for his

cooperative work using a large-scale mass spectrometer .

(C) 6= 16'

Fig. 12. Simulated ion trajectories under typical conditions.The inclination angle 0. shown in this figure is not real,because the vertical and horizontal scales are not identical. (a)0= 38' and R=0.24, (b) B~ = 27 ° and R =0.12, (c) 0. =16 °and R =0.04. R is the simultaneously detectable mass range,defined as R =(m2- MO/M01 where mo is the mass of theion detected at the centre of the detector, and mr and m2 are,respectively, the minimum and maximum masses of the ions

simultaneously detected (from ref. [121) .

T. Matsuo et al. / ]on-opticalstudiesfor mass analyzer design

~b)

Fig. 13 . Simulated ion trajectories at the detector region with(a) and without (b) octupole lens (from ref. [12]) .

References

[41[51

50mm

H. Wollnik, in : Optics of Charged Particles (Academic,New York and London, 1968) p. 259.H. Matsuda, Int. J. Mass Spectrom. Ion Processes 66(1985) 209.H. Matsuda, T. Matsuo, Y. Fulita, T. Sakurai and I .Katakuse, Int. J. Mass Spectrom . and Ion Processes 91(1989) 1.T. Matsuo, Mass Spectrom . Rev. 9 (1989) 203.T. Matsuo, H. Matsuda, Y. Fulita and H. Wollndc, MassSpectrosc. (Japan) 24 (1976) 19 .

[61 T. Matsuo, T. Sakurai, H. Matsuda and H. Nakabushi,Mass Spectrosc. (Japan) 33 (1985) 261 .Y. Wada, T. Matsuo and T. Sakurai, Mass Spectrom Rev.8 (1989) 379.

[8] K. Sato, T. Asada, M. Ishihara, F.Kunihiro, Y. Kammei,E. Kubota, C.E . Costello, S.A . Martin, H.A . Scoble andK. Biemann, Anal. Chem. 59 (1987) 1652.M. Ishihara, F. Kunihiro and Y. Kammei, Proc. 2ndJapan-China Joint Symposium on Mass Spectrometry,eds. T. Matsuda and Liang Xiao-Tian (1987) p. 137.

[101 K. Biemann and S.A. Martin, Mass Spectrom . Rev. 6(1987) 1 .

[111 T. Matsuo, M. Ishihara, S.A. Martin and K. Biemann, Int.J. Mass Spectrom . and Ion Processes 86 (1988) 83 .

[12] M. Ishihara and Y. Kammei, Rapid Commun . in MassSpectrom. 12 (1989) 420.

III. SPECTROMETERS/ANALYZERS