Instruments and Experimental Techniques, Vol. 43, No. 5, 2000, pp. 635--639. Translatedfrom Pribory i Teldmika Eksperimenta. No. 5, 2000, pp. 59-63. Original Russian Text Copyright 9 2000 by Mamontov, lvlev.

GENERAL EXPERIMENTAL TECHNIQUES

A Hyperboloid Mass Spectrometer with a Monopolar Ion Trap E. V. Mamontov and D. A. Ivlev Ryazan State Academy of Radio Engineering, ul. Gagarina 59/1, Ryazan, 390000 Russia e-maih nich @rricnit, ryazan.su Received September 14, 1999; in final form, February 2, 2000 Abstract A hyperboloid mass spectrometer with an analyzer utilizing a three-dimensional ion trap bounded by the plane z = 0 is described. Potential distribution in a monopolar trap is numerically simulated and the influence of distortion function AU(z, r) on the trajectories of charged particles is investigated. Mass peaks for various operating modes of the analyzer are calculated. An experimental mass spectrometer with a monopolar ion trap has been investigated. Spectra of the residual atmosphere and of that with tetrachloromethane CCI 4 puffed into the vacuum chamber have been measured. The resolution obtained is R0.5 = 1.2 x 103. INTRODUCTION

A MONOPOLAR ION TRAP

The electric field in quadrupole mass analyzers proposed by Paul in 1953 was linear, which ensured the independence of charged-particle motion relative to all three coordinates [1]. Mass selectivity of linear analyzers depends on the accuracy of the manufacturing and assemblage of hyperboloid electrode systems. In order to obtain the resolution R > 103, the accuracy of the electrode system must be better than 10-4. With improvement in the electrode-system accuracy, the performance characteristics of mass specrtometric equipment deteriorate and its cost rises. Quadrupole mass spectrometers with simplified electrode profiles [2] have better performance characteristics. Development of efficient methods for ion sorting in nonlinear fields made it possible to considerably soften the accuracy requirements to hyperboloid analyzers.

The electrode system of an analyzer with a bounded ion trap is presented schematically in Fig. 1. The monopolar analyzer consists of end 1 and annular 2 axially symmetric hyperboioid electrodes, whose minimal distances from the coordinates origin are Zol and r01, respectively. The electrode system is situated in the hemisphere z -> 0 and constitutes a half of the electrode system of a three-dimensional ion trap. Shielding electrode 3 is located along the boundaries of the hyperboioid electrodes and, behind the annular-electrode opening, semitransparent correcting electrode 4 is placed. In the central part of electrode 2, in the plane z = 0, an aperture with diameter 2r01 is formed, through which a charged particle can be injected into the analyzer and

The idea of a nonlinear analyzer is embodied in a three-dimensional ion trap with displaced electrodes [3]. In [4], methods of ion sorting are considered in monopolar space z >- 0, with quadratic potential distribution formed by two hyperboloid electrodes. Another version of a nonlinear analyzer with monopolar ion sorting is a three-dimensional ion trap with an annular electrode bounded by the plane z = 0 [5]. A simple design of the monopolar ion trap makes it possible to improve its production and operating characteristics, to find an efficient solution to the problem of injection and extraction of ions, and to slow down the formation of dielectric films on the field-generating electrodes. The analytical capabilities of a monopolar-ion-trap mass spectrometer were evaluated through numerical simulation of ion sorting in a monopolar nonlinear analyzer and in the course of experimental investigations. Below, the simulation and experimental results are given.

,..,

z

D

F

Electrons

ID

I

ro, I

.

.

.

.

.

Ions Fig. 1. The electrode system of a monopolar hyperboloid analyzer. (I) end electrode, (2) annular electrode, (3) shielding electrode, and (4) correcting electrode.

0020-4412/00/4305-0635525.00 9 2000 MAIK "Nauka/Interperiodica"

-2

MAMONTOV, IVLEV

636

AUIUt (a)

0.08 ~

Uc = -0.0176U1

0.04 ""~*'~ r = 0

LOIX

"'~

',

..... r

- 0 04L

=

0.2

i

i

i

AUIUI

Uc = 0.0176U1

"o~ .........

.-~

". i

/

It

-0.004

i

(b)

0

-0.002

I

. . . _ ~ ~ :~.~. g..o..=_...r ~ ~: = = = . . . . . . . . ....

--- . . . . . .

~

t

:t

ao ~

I

3r= 0.2 I

0.06

I

1

0.18

z/z01

1

I

0.3

Fig. 2. The distortion function of a monopolar trap with rodzol = 0.18 and U. = 0.5Ui.

Z

extracted from it. When the annular electrode is bounded by the plane z = 0, nonlinear field distortions occur in the analyzer working zone z -< 2r01. In order to localize these distortions within an area z < 0.2z0t and reduce their level, the hyperboloid-electrode parameters z01 and rot are limited by the ratio r0t/z01 < 0.2. The pattern and degree of deviation of the field from the linear one depend on correcting-electrode potential Uc. The sorting space of the monopolar trap corresponds to the positive coordinates z; therefore, the RFinduced oscillations of the analyzed particles can be associated only with z(t) > 0. With perfectly quadratic potential distribution in the analyzer, such oscillations occur when the working points for the ions with analyzed mass mo are at the boundary of the stability diagram [4]. In a bounded trap, the potential distribution differs from the perfect one by AU(z, r). In this case, the properties of the trap as a mass analyzer depend to a great extent on the form of distortion function AU(z, r). Function AU(z, r) is determined through numerical simulation of the potential distribution in the monopolar trap with parameters Zot/rol = 5.333 and D/zol = 2.5 at various correcting-electrode potentials U c, where D is the outer diameter of the annular electrode. Depending on AU, the sorting space is divided into a r e g i o n with slightly nonlinear distortions and a region of a "perfect" field. In the perfect-field region, 0.2 _
AV=~'~I(z-a)6-1--'~ ( 1"z - Zot a )t_ 4 r 2 U (l)

+ 4-~( z - a )'r'r4 - ~6 r6] . For rol/Zo~ = 0.18, the constants are A 6 = 0.3586, a = 0.255, the maximal potential deviation is AU(0, O)/UI = 8 • 10-3; in the perfect-field region, the distribution accuracy is better than 3 x 10.'*. MASS-SELECTIVE SORTING OF IONS IN THE MONOPOLAR TRAP Fig. 3. The trajectories of ions for m = 0.99m0, k = 0.33, and the number of sorting periods n = 20: (1) in the perfect field; (2, 3) in the field with nonlineardistortionsA 6 = --0.3586 and 0.3586, respectively.

In order to determine the analytical properties of the monopolar analyzer, we carried out a numerical simulation of charged-particle sorting, taking into account the distortion function AU(z, r). Ion trajectories were

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A H Y P E R B O L O I D MASS SPECTROMETER

investigated, and mass peaks were plotted for the analyzer fed with a pulse RF voltage of the form

U(t) = UI-

637

//t 0.6

U2 = U + V~/(f.ot),

where U and V are the dc component and RF amplitude of the supply voltage, ~(tot) is the normalized periodic impulse function with period To = 2nRo. Charged particles were injected into the analyzer before initiating the sorting under the no-field space conditions for U1 = U2 = Us = Ur = 0. The initial phase of RF oscillations tp0 was matched with the ions' initial coordinates and velocities [5]. Charged-particle trajectories in the z and r directions were calculated by numerically solving a system of two nonlinear differential equations of the second order with periodic coefficients [6]

0.4

0.2

d2 + 2q~(tot)](z + kOAU~ O, d t-~z2+ [a Oz J = d2r I2+q~(c~ dt 2

0.98

(2)

0.99

1.00

m/mo

Or J = 0,

where a and q are the coefficients depending on z01,

Fig. 4. Mass peaks of the analyzer with a truncated trap. The number of sorting periods n = 20, ~6 = -0.3586.

parameters U, V, to, and RF-voltage phase tp0; k = (z021 + 2

r01 )/2(Ul - U 2 ) . The slope of the working line of the analyzer stability diagram is governed by the parameter ~. = a/2q. The motion of light-mass ions (m < m0) is affected most by the deviation of the potential from the perfect distribution AU(z, r), because, in the course of sorting, their trajectories in z, z(t) - 0, and, finally, the ions reach the region of the weakly nonlinear field. Characteristic trajectories of light-mass ions are shown in Fig. 3. Curve I corresponds to the motion of ions with m = 0.999m 0 along the z-axis in the ideal ion trap. Curves 2 and 3 are calculated for a bounded trap with potential deviations of form (1) at various parameters of the nonlinear field. At AU(z, r) < 0, the nonlinear field has a straightening effect on the charged-particle trajectories when oscillations z(t) occur in the region z > 0. At the initial stage of sorting, light ions are in the perfect-field region; in the course of time, they move along the z-axis towards the origin and into the region of nonlinear distortions, where the field intensity increases as compared with the linear one. Under the influence of this field, light ions return to the perfectfield region without crossing the plane z = 0. The process becomes an oscillatory one with a periodic envelope. The envelope period is half the period of ion-trajectory envelopes in a linear analyzer. The effect of trajectory straightening is also observed for a limited mass range (m l < m < m0). The boundary mass ml depends on the value of the coefficient A 6 in (1). In the mode of trajectory straightening in the nonlinear analyzer, lasting confinement of particles by the RF field is possible. In this case, the massINSTRUMENTS AND EXPERIMENTAL TECHNIQUES

End

Ioos

Ul

I

Fig. 5. The experimental mass spectrometer with a bounded trap: (1) electron source, (2) RF generator, (3) scan processor, (4) secondary-emission multiplier, (5) registration amplifier, and (6) personal computer.

selectivity properties of the analyzer depend on the range of the confined masses m r m o. In the mode of one-dimensional sorting, when the r-coordinate of the working point of the analyzed ions is deep inside the stabilit), zone, the range mrmo, depending on parameter A., is (0.01-0.04)m 0, and the resolution is low (R = 25-100). In order to increase R, one should move the working point towards the stability-diagram vertex and sort the light ions in the r-coordinate. In this case, the operation mode of the monopolar-analyzer is similar to the massselective mode of confinement of charged particles in a Vol. 43

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638

MAMONTOV, IVLEV sensitivity [5]. In the monopolar trap, by varying the voltage U c at the correcting electrode, one can change the sign of the distortion function AU(z, r) and control the modes of ion injection, sorting, and extraction. AN EXPERIMENTAL MASS SPECTROMETER WITH A MONOPOLAR TRAP

/

I.

18

28

m, ainu

FigT,6. Fragment of the residual-atmospherespectrum. P =

lff-~ Tort, n = 27, Uc = 4V, and R0.5 = 840.

119

121

123

m, amu

Fig. 7. Speetrul/aobserved on puffing CCI 4 into the chamber. Pffi~x 10-~ n = 19, Us= 170V, Uc= 6V, andR0.5= 1.2 x 10".

conventional three-dimensional trap, with the only difference that ions oscillate with respect to the z-axis at z(0 > O. Figure 4 shows mass peaks of the monopolar analyzer in the mode of ion sorting with respect to coordinates z and r. The peaks are obtained by calculating the trajectories of 5 • 103 particles with initial coordinates in a range Zo = (0.05--0.95)Zol, r0 = -rot ... +rot and initial thermal velocities. With the mass-scan line approaching the stability-diagram vertex ( ~ = 0.3455), the resolution of the monopolar analyzer increases. For g = 0.344, R = 850 at the relative mass-peak intensity !11o= 0.2. As seen from Fig. 2, at Uc > 0, the potential distribution in the vicinity of the annular-electrode opening varies so that the field decreases. This field variation contributes to a speedy withdrawal of light-mass ions from the analyzer (curve 3 in Fig. 3), and mass-selective ion sorting thus becomes possible with respect to the z-coordinate. With respect to the r-coordinate, the working points of the ions under analysis are moved deep into the stability diagram (g < k0), where the oscillation amplitudes are limited (r m < rot). The advantage of one-dimensional sorting is that it provides greater instrumental resolution without a significant drop in its

The actual potentialities of a monopolar trap were estimated in the process of an experimental investigation of a mass spectrometer with two hyperboloid electrodes with parameters z01 = 32 m m and r01 = 6 m m (Fig. l). The electrode diameter D = 80 m m is chosen such that a 2-mm-thick beam of ionizing electrons passes between the field-forming electrodes. The experimental facility is represented schematically in Fig. 5. The monopolar-analyzer was powered by a pulse RF generator forming two pulse voltages with amplitudes Usl = 210+ 20V and Us2 = 210V. The voltages Ui and /./2 applied to the end and annular electrodes are offset relative to each other by a half period To of the RF signal. The value of the sorting parameter ~. was set by varying the amplitude Usl at the end electrode. The correcting-electrode voltage, Uc, was formed by changing the annular-electrode voltage U 2 by a constant value AU = • V. In the course of sorting, the shielding-electrode voltage was maintained constant, U s = 100-200 V. By adjusting U s, one varied the field distribution in the analyzer working zone z > 0.2Zot, so that optimal values of R and !/I o could be attained. Ions were produced near the end electrode under the action of the electron beam current !e = 5-15 I.tA. The ionization time was Ti = 2T0 in every analysis cycle. Sorting started at the RF-field phase q~0t = 0. Mass scan was performed by a discrete exponential variation of RF-oscillation frequency in a range of 0.08-4).64 MHz, which corresponds to the mass range 15-1000 amu. Exponential scan ensured the same number of measurements per mass peak (at R = 103, ten measurements per peak were made). Ion sorting took 7 to 63 periods and was completed at the RF-signal phase tpB = r,,/4, which ensured extraction of the sorted-out ions through the annular-electrode opening. The ion current was amplified by a B 3 Y 2 amplifier and by the current-to-voltage converter with a gain of 10-6 V/C. Then, the voltage was converted into a 10-bit code. A personal computer was used as a storage and display device. In order to increase the mass-analysis sensitivity, a mode of data accumulation over 16 measurement cycles was used. The spectra of the residual atmosphere and of that with tetrachloromethane puffed into the vacuum chamber at pressures of 10 -6 to 3 x I0 -s Torr (the evacuation equipment is the same as in [6]) were investigated. The experimental results are presented in Figs. 6 and 7. Figure 6 shows a fragment of the residual-atmosphere spectrum measured in a scanning time of 0.3 s for a pressure of 10-5 Torr, number of sorting periods n = 27, and U c = 4 V. The half-mass-peak resolution is

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A HYPERBOLOID MASS SPECTROMETER R0.5 = 840. Figure 7 shows the spectrum measured when CC14 was let into the chamber at a pressure P = 3 x 10-5 Torr, n = 19, Us= 170V, and Uc = 6V. The resolution is R0.5 = 1.2 x 103.

639

olution R > 103. With the experimental mass spectrometer using a monopolar ion trap, a resolution R0.5 = 1.2 x 103 was obtained. The mass-spectrometer parameters can be improved through optimization of the fields and the sorting modes of the analyzer, as well as by injecting the ions through the opening in the annular electrode.

CONCLUSION The application of monopolar methods of massselective sorting of ions in nonlinear RF fields allows one to achieve high resolution of hyperboloid analyzers, simplify the design of their electrode system, and to efficiently solve the problem of injection and extraction of charged particles. The working zone of an analyzer with a bounded ion trap consists of a region zm >z ->2rm of a perfect field (AUIU ! < 2 x 10-4) and a region 0 <_z -< 2rm of a weakly nonlinear field (10 -2 _
INSTRUMENTS AND EXPERIMENTALTECHNIQUES

1. 2. 3. 4. 5. 6.

REFERENCES Paul, W. and Steinwedel, H., Z. Naturforschung, 1953, vol. 8, p. 448. March Raymond, E. and Hughes Richard, J., Quadrupole Storage Mass Spectrometry, New York: Wiley, 1989. Franzen, J., Int. J. Mass Spectrometry and Ion Processes, 1993, no. 125, p. 165. Mamontov, E.V., lzv. Ross. Akad. Nauk, Ser. Fiz., 1998, vol. 62, no. 10, p. 2039. Mamontov, E.V. and Ivlev, D.A., Pis'ma Zh. Tekh. Fiz., 1999, no. 10, p. 51. Mamontov, E.V., Prib. Tekh. Eksp., 1999, no. 1, p. 83.

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