A time of flight mass spectrometer with field free interaction region for low energycharged particle-molecule collision studiesK. C. Rao, V. S. Prabhudesai, and S. V. K. Kumar Citation: Review of Scientific Instruments 82, 113101 (2011); doi: 10.1063/1.3653393 View online: http://dx.doi.org/10.1063/1.3653393 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/82/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Low energy (e, 2e) study from the 1t 2 orbital of CH4 J. Chem. Phys. 137, 024301 (2012); 10.1063/1.4732539 Shock tube/time-of-flight mass spectrometer for high temperature kinetic studies Rev. Sci. Instrum. 78, 034101 (2007); 10.1063/1.2437150 A laser ablation electron impact ionization time-of-flight mass spectrometer for analysis of condensed materials Rev. Sci. Instrum. 73, 3003 (2002); 10.1063/1.1490419 On the combination of a linear field free trap with a time-of-flight mass spectrometer Rev. Sci. Instrum. 72, 2900 (2001); 10.1063/1.1373666 A mass spectrometry study of n-octane: Electron impact ionization and ion-molecule reactions J. Chem. Phys. 114, 2166 (2001); 10.1063/1.1334898

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REVIEW OF SCIENTIFIC INSTRUMENTS 82, 113101 (2011)

A time of flight mass spectrometer with field free interaction regionfor low energy charged particle-molecule collision studies

K. C. Rao, V. S. Prabhudesai, and S. V. K. Kumara)

Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400 005, India

(Received 4 May 2011; accepted 27 September 2011; published online 3 November 2011)

A new design of a linear time of flight mass spectrometer (ToFMS) is implemented that gives nearlyfield-free interaction region without compromising on the mass resolution. The design addressesproblems that would arise in a conventional Wiley-McLaren type of ToFMS: (i) field leakages intothe charged particle-molecule interaction region from various components of the mass spectrometer,including that through the high transparency mesh used to obtain evenly distributed electric fields;(ii) complete collection and transportation of the ions produced in the interaction region to the detec-tor, which is essential for high sensitivity and cross section measurements. This ToFMS works overa wide range of masses from H+ to a few hundred Daltons and would be the most suitable for lowenergy charged particle-molecule interaction studies. Performance of the ToFMS has been tested bymeasuring the partial ionization cross sections for electron impact on CF4. © 2011 American Instituteof Physics. [doi:10.1063/1.3653393]

I. INTRODUCTION

Today, mass spectrometers that work on various princi-ples and types are available which are used in a variety offields. Out of them, time of flight mass spectrometer (ToFMS)is relatively simple to design and fabricate and it is also eco-nomical. Since the invention of the two-stage ToFMS byWiley and McLaren,1 its use in various areas of physics,chemistry, and biology has taken a giant leap. In particular, thearea of photon, electron, and ion molecule collision studieshas benefited and reliable ionization and fragmentation mea-surements have been carried out. One of the major goals inthese studies is to measure the cross section of the processunder study where it is required to collect and transport allthe ions that are generated during the interaction, to the de-tector irrespective of their initial kinetic energy and angulardistribution. Another important requirement of the charge par-ticle interaction studies, especially at low projectile energies,is that they must be carried out in a field-free environmentavoiding the influence of any external stray fields to obtainthe correct information on the interaction process. For exam-ple, in the case of electron impact ionization, the main chal-lenge is to produce the electron beam of precise energy and itstransportation to the point of interaction without any changein its energy and direction profile. Any external stray fieldcan distort the incident ionizing electron beam. To avoid leak-ing fields from different regions of the ToFMS, pulsed electricfields are used to extract and accelerate the ions to the requiredvelocities. One possible source of stray field can be from thefield leakage through the fine wire mesh used to obtain a uni-form field in the two zones of a Wiley-McLaren ToFMS andthe other possibility being from the sides of the flight tube dueto the finite size of the extraction electrode. It is essential tosolve this problem especially in the case of low (<25 eV) and

a)Author to whom correspondence should be addressed. Electronic mail:svkk@tifr.res.in.

very low (<10 eV) energy electron molecule collision studies,where one intends to measure ionization threshold energies,partial ionization cross sections, resonance reactions, disso-ciative electron attachment reactions from ground and excitedmolecules and radicals, etc. In addition, to carry out completeand reliable studies of low energy charge particle collision ex-periments one more issue that needs to be addressed is com-plete collection of ions that are produced during the ionizationprocess. It is also important that the design should be such thatthe mass resolution of the ToFMS should not be sacrificed toachieve the above goals, as this affects the capability of theinstrument.

Wiley-McLaren designed their two-stage ToFMS for thepurpose of mass analysis of residual gases using electron im-pact ionization. However, one often has to change the con-figuration or geometry of their system when one intends todo different kinds of experiments, even when their basic con-cept is retained. For example, de Heer and Milani2 developeda ToFMS for atomic clusters beams with a large ionizationvolume of the order of 200 cm3 keeping the two-stage accel-eration principle of Wiley-McLaren. Chandezon et al.3 pre-sented a modified version of the mass spectrometer wherethey achieved the mass resolution higher than 2000 with largeion volumes, of the order of 0.2 cm3 attributing the highermass resolution to the second-order compensation of the ini-tial position effect. Nagesha et al.4 have implemented a seg-mented ToFMS to achieve the complete ion transportationfrom the interaction region to the detector by compromisingthe mass resolution. The lowering of the mass resolution isdue to the segmented nature of the flight tube which guidesthe ions to the detector.

In this article, we demonstrate the implementation of alinear ToFMS based on the basic Wiley-McLaren principle,which has been modified to provide a field-free interactionregion, and provides complete collection and transportationof the generated ions in the interaction region without com-promising the mass resolution.

0034-6748/2011/82(11)/113101/7/$30.00 © 2011 American Institute of Physics82, 113101-1

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113101-2 Rao, Prabhudesai, and Kumar Rev. Sci. Instrum. 82, 113101 (2011)

II. DESIGN OF THE TIME OF FLIGHTMASS SPECTROMETER

There have been many articles5–7 which clearly describethe Wiley-McLaren principle, and in general the technicaland physical aspects of a ToFMS. The present ToFMS hasbeen designed based on the basic Wiley-McLaren principlewhere the ions are accelerated in two stages to achieve thebest mass resolution. The purpose of the present design isto make a short medium resolution ToFMS where chargedparticle/photon-molecule interaction occurs in a nearly field-free condition and all the high kinetic energy ionic fragmentsproduced by this interaction are collected. H+ and H− ionsare one of the fragments that are often formed from most or-ganic molecules, and being the lightest, they are the most dif-ficult ions to collect as they carry most of the excess energyof the process. If due care is not taken they tend to fly faraway from the formation point during the extraction pulse de-lay. The achievement of a good design for total ion collectionneeds to be judged by the performance of the ToFMS by thecomplete collection and transportation for H+/− ions.

The schematic of the present linear ToFMS is shownin Figure 1. The main feature of the spectrometer is a dou-ble mesh arrangement (puller) that separates the interac-tion region from the acceleration region. The region betweenthese two grids is named dead zone. The grids made of thehigh transparency (88%) electroformed nickel mesh (Preci-sion Eforming LLC) are separated by a distance of 2 mm.Another important feature of our design is a cap electrodethat holds these two meshes, and is grounded. This makes theelectric field gradient in the dead zone very small. This cap ismounted with a proper vacuum compatible insulating materialon the 160 mm long flight tube. The length of the interactionand the acceleration regions are 10 and 8 mm, respectively.

The main factors that limit the performance of a ToFMSin terms of its ability to separate the masses of the ions are dueto their initial spatial spread and kinetic energy with whichthey are born. Correspondingly, the space and energy resolu-tions were defined for a given ToFMS by Wiley and McLaren.

FIG. 1. (Color online) Schematic of the present time-of-flight Mass Spec-trometer. Z-1–Interaction zone cum the first acceleration zone of a conven-tional Wiley-McLaren ToFMS; Z-2–dead zone that has been introduced in thepresent design; Z-3–main acceleration zone which is also the second accel-eration zone of a conventional Wiley-McLaren ToFMS; Z-4–field free flighttube; and Z-5–ion acceleration needed to match the detector requirement. E1to E5 and T1 to T5 are the electric fields and time of flights of the ions cor-responding to the zones Z-1 to Z-5, respectively. s is the distance from thefirst grid G1 to the centre point of the ion cloud and ds is the diameter of thiscloud. w, d, L, and D are the lengths of the zones Z-2 to Z-5, respectively.

Space resolution (ms) is defined as the ability of a spectrom-eter to resolve the masses from the dispersion of the initialspace distribution of the target particles. Whereas energy res-olution (me) is defined as the ability of spectrometer to resolvethe masses due to the time spread introduced by the initialkinetic energy distribution of the ions. It is known that theions closer to grid will gain less energy compared to thosecloser to pusher. Therefore, the ions which gain less energywill be overtaken by the ions which gain higher energy by thetime they reach the detector. This spatial distribution of ionsaffects the mass resolution of the spectrometer, hence, to re-duce the time distribution of the arrival of ions of the samemass to charge ratio (m/q), the electric field E1 need to bekept small. This consideration works for zero initial kineticenergy of ions, but the ions, when they are formed, alwayspossess initial kinetic energy due to the gas temperature andthe dynamics of the fragmentation process, which introducesa time spread in their arrival time at the detector, affecting theresolution of the mass spectrometer. One can reduce this timespread by applying a high accelerating field (E1) such thatthe initial kinetic energy is only a very small quantity com-pared to the energy gained in this region. While increasing E1

helps me, it works against ms making whole exercise counter-productive. Wiley and McLaren1 split the applied accelerationfield into two regions, e.g., E1 and E3 and optimized the valuesto obtain the best overall mass resolution m. Translating theWiley-McLaren criterion to the present case for positive ions,taking into account that in our case the interaction zone is tobe kept field-free during the electron/ion/photon-molecule in-teraction, no electric field (voltage) is applied to zone 1 (Z-1,see Figure 1) while a negative field is applied to zone 3 (Z-3)and distance of zone 2 (Z-2) is zero. The electric field ap-plied to zone 3 (Z-3) is usually much larger than that for zone1 (Z-1). This high field applied to zone 3 leaks into the in-teraction region (Z-1) as shown in Fig. 2, which would affect

FIG. 2. (Color online) SIMION simulation of the interaction region (first ac-celeration region) Z-1, the second acceleration region Z-3, and part of theflight tube for a conventional Wiley-McLaren-type of ToFMS. The pusherelectrode and the puller grid in this simulation are kept at ground potential,and −860 V is applied to the flight tube. The electrode separation is givenin the text. The equipotentials due to the infringing field from the sides aremarked by their respective values.

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113101-3 Rao, Prabhudesai, and Kumar Rev. Sci. Instrum. 82, 113101 (2011)

FIG. 3. (Color online) SIMION simulation of the interaction region showing the equipotential lines while the pusher and the puller (cap electrode) are heldat ground potential and the flight tube at −860 V for (a) single grid, (c) 1.5 mm, (e) 2 mm, and (g) 3 mm width of dead zone, respectively. (b), (d), (f), and(h) corresponding ion trajectories from the interaction region to the detector for H+ ions born with a kinetic energy of 2.8 eV and pushed into the zone Z-2by a +250 V pulse of 1 μs width. Legend: (1) Pusher, (2) puller cum cap-electrode, (3) flight tube, (4) earthed detector housing, (5) ion detector, and (X)electron-atom/molecule overlap region.

the electron/ion/photon-molecule interaction process. Simu-lations of the electric field lines carried out using SIMION 8.0clearly show that the field leakage of the order of 2.5 V (seeFig. 2) at the projectile-target overlap region. The field in-fringing into the interaction regions due the finite size of thepuller electrode was addressed by replacing the flat electrodewith a cap electrode that holds the high transparency mesh.The length of the cap was again determined based on the

SIMION simulations, which showed that increasing the lengthbeyond 26 mm would not provide any additional benefit. Thecurrent cap electrode can hold back the infringing fields evenwhen −3 kV is applied to the flight tube. However, the resid-ual field observed in the interaction region is mainly due tothe electric field leaking through the high transparency mesh(G1) from the voltage applied (−860 V) to the flight tube (seeFig. 3(a)).

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113101-4 Rao, Prabhudesai, and Kumar Rev. Sci. Instrum. 82, 113101 (2011)

To solve the problem of field penetration through hightransparency mesh (G1), a second high transparency mesh(G2) separated by a small distance from G1 and held at thesame potential as that of G1, viz. ground potential, was intro-duced and the simulations were carried out (see Fig. 3(c)). It isalso important to determine the optimum separation (w) of thetwo grids so that the leaking field is minimized and its effecton the ion transportation to the detector and on the mass res-olution is examined. The advantage of the double grid setupis shown in Figures 3(c), 3(e), and 3(g) where it can be seenthat the field penetration in the interaction region is reducedto near zero volts. For example, the leaking field is reducedfrom 2.5 V to ∼25 mV for a separation length w = 3 mm.The ion trajectory simulations shown in Figures 3(b), 3(d),3(f), and 3(h) are carried out for grid separation of 0, 1.5, 2and 3 mm, respectively, and for H+ ions with initial kineticenergy of 2.8 eV with 250 V pulsed extraction voltage arriv-ing 100 ns after the ion generation. The voltages to be appliedto the pusher and the flight tube were first determined, to afirst approximation, by carrying out trajectory timing calcu-lations for the best mass resolution assuming uniform fielddistributions and no leaking fields using homemade softwareTOF3.8 This software can optimize the voltages to be appliedto various electrodes to obtain the best mass resolution at agiven m/q taking into account the interaction volume and thekinetic energy of the ions produced. This program can alsowork for a ToFMS with reflectron geometry. The simulationsshow the extraction, focusing, and transportation of the ions tothe detector in all the cases. The simulations clearly show thatthere is a complete collection and transportation of the ionsas well as the suppression of the leaking field in the interac-tion region. However, to determine the optimum separationw between the grid G1 and grid G2 to achieve the best pos-sible mass resolution, analytical treatment to determine themass resolution of the entire system was carried out, whichincludes the initial kinetic energies of the ions.

The conceptual diagram of the present ToFMS along withall the nomenclatures is shown in Fig 1. As can be seen fromfigure, there are five regions in the time of flight mass spec-trometer Z-1 to Z-5. Let T1, T2, T3, T4, and T5 be the timeof flight of the ions in each of regions namely interaction(Z-1), dead zone (Z-2), acceleration (Z-3), flight tube (field

free) (Z-4) and detector (Z-5), respectively,

T1 = 1.02(2m)1/2

q E1[(U0 + qs E1)1/2 ± (U0)1/2], (1)

T2 = 1.02 (2m)1/2 w

2(U0 + qs E1)1/2, (2)

T3 = 1.02 (2m)1/2 1

q E3[(U0 + qs E1 + qd E3)1/2

−(U0 + qs E1)1/2], (3)

T4 = 1.02 (2m)1/2 L

(U0 + qs E1 + qd E3)1/2, (4)

T5 = 1.02 (2m)1/2 1

q E5[(U0 + qs E1 + q DE5)1/2

−(U0 + qs E1 + qd E3)1/2]. (5)

Total time of flight of the ions is given by

T = T1 + T2 + T3 + T4 + T5. (6)

Here m, q, and U0 are the mass, charge, and initial kineticenergy of the ions, respectively. s is the distance between thecentre of the electron beam-molecular beam overlap regionfrom the grid G1 in the interaction region. d and D representthe lengths of acceleration region and flight tube-detector re-gion, respectively. E1, E3, and E5 are the electric fields in Z-1,Z-3, and flight tube-detector region Z-5, respectively. w is theseparation between G1 and G2, and L is the length of flighttube. Defining

k0 = (U0 + qs E1 + qd E3)

qs E1, (7)

and

k1 = (U0 + qs E1 + q DE5)

qs E1(8)

with the space focusing condition as explained in Ref. 1, d T/d s

is set to 0. Then the expression for the optimum value of theflight tube length becomes,

L = 2(q E1)1/2(k0s)3/2

⎡⎢⎢⎣

(U0 + qs E1)−1/2 − w

2(q E1)(U0 + qs E1)−3/2 + E1

E3[(k0qs E1)−1/2 − (U0 + qs E1)−1/2]

+ E1

E5[(k1qs E1)−1/2 − (k0qs E1)−1/2]

⎤⎥⎥⎦ . (9)

The change in the flight time �Ts with respect to smallchange �s in the value of s can be expressed as seriesexpansion of T(v, s) about point s. If the initial velocityv = 0, i.e., initial kinetic energy U0 of ions to be zero, then

we have

�Ts =∞∑

n=1

1

n!

(dnT (0, s)

d sn

)(� s)n . (10)

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113101-5 Rao, Prabhudesai, and Kumar Rev. Sci. Instrum. 82, 113101 (2011)

Applying Wiley-McLaren space focusing condition and ne-glecting higher order terms,

�Ts = 1

2

(d2T (0, s)

d s2

)(� s)2 . (11)

Equating it to the separation time of adjacent masses, we have

T

2ms= 1

2

(d2T (0, s)

d s2

)(� s)2 . (12)

Expressing it in terms of mass resolution, the space resolutionwould be

ms = T(d2T (0,s)

d s2

)(� s)2

. (13)

To understand the effects of initial kinetic energy of the ions,consider two ions having the same (m/q) but with oppositevelocities be present at the same point s, i.e., one moving to-wards the detector and one away from the detector along theaxis of the spectrometer. The ion, which is moving opposite tothe detector, i.e., towards the pusher will be slowed down dueto E1 and will stop eventually, makes a turn around and thengets accelerated in the direction of detector. Now, this ion willgain its original speed to reach its point of start, i.e., s. Nowboth ions have identical velocities, but the ion, which initiallymoved away from the detector, will lag by this turn-aroundtime, which is twice the value of deceleration time that can beexpressed as

�Te = 1.022vm

q E1= 1.02

2 (2mU0)1/2

q E1. (14)

Therefore, considering only the effects of initial energy, themaximum resolvable mass, termed as the energy resolution,would be

me = T

2�Te. (15)

Finally, combining expressions (13) and (15) for total massresolution would be

m = msme

ms + me. (16)

Using the above analytical treatment for the present geometryand for the applied electric fields, viz. +250 V pusher volt-age and −860 V flight tube voltage, for H+ ions possessing2.8 eV initial kinetic energy, the total mass resolution of thesystem is calculated as a function of the grid separation lengthw. It can be seen from Fig. 4 that for w value of 1.98 mm, thebest mass resolution is obtained. Calculations were also car-ried out for various ds values from zero to 10 mm and it canbe seen from Fig. 5 that the best total mass resolution is ob-tained for w value of 1.98 mm. The peak in the total massresolution is due to the large contribution coming from thespace resolution component. The peak value for space resolu-tion can be attributed to the compensation of the spread in theflight time due to spatial spread of the ions, (ion cloud vol-ume) during the ion flight time though the dead zone (Z-2),which is similar to the single field ToFMS but with an appro-priate flight tube length to match the requirements.1 However,it has a marginal negative effect on the energy resolution, as

FIG. 4. (Color online) Calculated mass resolution of the present ToFMS,optimized at mass 40 Da with initial kinetic energy of 0.1 eV, as a functionof the width of zone 2 for a spatial spread ds [ion cloud volume] of 2 mm.

the energy resolution falls continuously from the beginningof Wiley-McLaren point (single grid condition, i.e., w = 0)as the dead zone width increases. This is due to the conflict-ing requirements for improving space and energy focusing.Wiley-McLaren have introduced the second higher field ac-celeration region (Z-3) for the above purpose. As this schemeis disturbed by the introduction of zone Z-2 in the presentdesign some amount of negative effect on the total mass res-olution is expected. It was observed during the calculationsthat increasing the electric field of zone Z-3 in order to com-pensate the loss of the mass resolution beyond a certain valuedoes not seem to have much effect. Besides, the overall reduc-tion in the mass resolution is found to be very small comparedto the overall benefit of achieving a near field-free interactionregion (Z-1).

FIG. 5. (Color online) Calculated mass resolution of the present ToFMS,optimized at mass 40 Da with initial kinetic energy of 0.1 eV, as a functionof the width of zone 2 and for various spatial spread ds [ion cloud volume]from 1 to 4 mm.

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113101-6 Rao, Prabhudesai, and Kumar Rev. Sci. Instrum. 82, 113101 (2011)

FIG. 6. (Color online) (a) Artist’s sectional view of the ToFMS fabricatedbased on the design criteria described in the text. (1) Housing of the elec-tron gun, (2) electron beam, (3) pusher electrode, (4) Faraday Cup, (5) Capelectrode housing the two high transparency meshes, 6 – insulator separat-ing the cap electrode and the flight tube, (7) flight tube, (8) channel electronmultiplier, and (9) electron multiplier housing and shield. (b) Detailed viewshowing the interaction region and the three zones Z-1 to Z-3 and the threeGrids G1 to G3.

III. RESULTS AND DISCUSSIONS

A ToFMS was fabricated as per the above design and thecomputed voltages based on the TOF3 (Ref. 8) and SIMION

simulations. In order to evaluate the performance of the massspectrometer, electron impact ionization cross section of CF4

was measured. The experimental arrangement is shown inFig. 6 in which the electron beam crosses the molecular beamorthogonally. The electron beam is generated by a Pierce-typeof electron gun working in the energy range of 0–500 eV withan energy resolution of 0.5 eV full width at half maximum(FWHM). A Faraday cup placed opposite to the electron guncollects the unscattered electrons. A weak magnetic field of5 mT is used to guide the electrons through the electron gun,the interaction region, and into the Faraday cup. The electrongun is mounted orthogonal to the axis of the ToFMS. The gasunder study is introduced in the interaction zone through along tube of 1.0 mm diameter embedded at the centre of thepusher plate. The electron beam is pulsed at 5 kHz with the ontime being 100 ns. The ion extraction pulse of +250 V last-ing for 1 μs is switched on after a delay of 100 ns so that theunreacted electrons fly away from the interaction region. Thedata is collected using a home built data acquisition system.9

The experimental mass spectrum taken with the above-designed ToFMS for a combination of target molecules CF4

(carbon tetrafluoride) and argon (see Fig. 7) is compared withthat from SIMION 8.0 simulations. Argon is used as a calibrat-ing gas. The mass spectrum is obtained using 50 eV electronbeam. The simulated spectrum has been generated taking intoaccount the rise, fall, and on times of the pusher pulse. The ex-perimental data is acquired for a flight tube voltage of −860V dc and pusher voltage +250 V of width 1 μs. The measuredmass resolution t / (2�t), (where t is the flight time of the ionand �t is the full width at half maximum of the mass peak un-der consideration) of the ToFMS is 28, 32, and 29 at mass 12,40 and 69 Da, respectively, while the SIMION 8.0 simulationsfor this system carried out with an initial ion kinetic energiesof 0.04, 0.03, and 0.06 eV, respectively. These energies are thesum of thermal and the additional energy acquired during thedissociation process, where applicable.

FIG. 7. (Color online) Simulated (full line) and measured (dashed-dottedline) mass spectrum of a mixture of CF4 and Ar at an electron impact energyof 50 eV.

It is observed that the experimental mass resolution islower by ∼11% compared to the simulated one over entiremass range of the spectrum. This deviation between the exper-imental and the simulated values can be attributed to variousfactors such as possibility of broader electron-molecule over-lap region than that assumed, imperfect electron beam over-lap with respect to the centre of the interaction region, someamount of pusher pulse ringing due to impedance mismatchresulting variable acceleration field experienced by the ions,and possible distortion in the flatness of the mounting of thehigh transparency grids, etc.

It can be noticed that both the measured and the SIMION

simulated mass resolutions are found to be lower as comparedto that derived using the analytical formulae. The simulationswere carried out by calculating the ion flight times with a flatdetector that is perpendicular to the axis of spectrometer. Thecomputations using the analytical formulae predict unit massseparation at 58 Da with the FWHM being 0.0141 μs (for 0.1eV initial kinetic energy), while a more realistic SIMION sim-ulation for the same initial kinetic energy predicts unit massseparation at 43 Da with a larger FWHM of 0.0207 μs. Thisdifference can be expected as the analytical formulae assumeideal conditions. We use a channel electron multiplier (CEM),for the sake of sturdiness, as the detector which has non-negligible (20 mm) depth as compared to the flight tube length(160 mm). As a result, the simulated spectrum with the ionsplatting onto the conical detector with the uncertainty of thesplatting point equivalent to the depth results in a larger timespread (0.06 μs). The mass spectrum generated by SIMION

simulations carried for ToFMS with a CEM detector and ini-tial kinetic energies as given above compares very well withthe one obtained experimentally (see Figure 7), supporting thedesign methodology.

Consequently, one can improve the mass resolution withthe use of a detector that is flat, e.g., a microchannel plate in-stead of a CEM, especially for short flight tube lengths. Onecan reduce this temporal spread occurring inside the detector

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113101-7 Rao, Prabhudesai, and Kumar Rev. Sci. Instrum. 82, 113101 (2011)

TABLE I. Comparison of measured cross sections with some selected val-ues reported in the literature for electron impaction ionization of CF4.

Partial ionization crosssection (10−20 m2)

Electron energy C+ F+ CF3+

50 eV 0.15 0.10 4.13 Present0.12 0.10 2.70 Ref. 100.08 0.06 2.52 Ref. 110.08 0.06 3.42 Ref. 12

80 eV 0.28 0.42 4.40 Present0.24 0.36 3.17 Ref. 100.20 0.31 3.14 Ref. 110.15 0.16 4.10 Ref. 12

100 eV 0.33 0.58 4.40 Present0.28 0.47 3.22 Ref. 100.24 0.47 3.26 Ref. 11

160 eV 0.31 0.66 3.87 Present0.30 0.58 3.11 Ref. 100.29 0.71 3.19 Ref. 11

by increasing the length of the flight tube, which results in thedecrease in the ratio of this time spread to the total time offlight of ion. But in any case, when the detector is having a fi-nite depth, this temporal spread will be an unavoidable sourceof spread and decreases the overall mass resolution.

To check the overall performance of the mass spectrom-eter, we have tested it by measuring the absolute cross sectionof electron impact dissociative ionization of CF4. As can beseen from Figure 7 that all the mass peaks are well separatedand our ToFMS can be used to measure absolute partial crosssections by electron impact for the dissociative ionization pro-cesses. The measured dissociative ionization cross section forCF4 is compared with the available data in the literature10–12

(see Table I), as this would be an authentic check to evalu-ate whether all the ions which are formed in the interactionzone are fully extracted and transported to the detector. Thecross sections measured are put on an absolute scale using therelative pressure technique13, 14 with the cross section valuesfor argon15 used as the standard reference. In this technique,the ionic currents of various species that are formed fromthe target molecule under test are compared with those of thestandard gas whose absolute cross section is known. Both themeasurements are carried out with the stringent condition thatthe molecular flow pattern of test gas as well as reference gasmust be similar at electron beam molecular beam overlap re-gion. This condition is ensured by keeping the Knudsen num-ber Kn ≥1 where Kn = λ/d with λ as the mean free path andd as the diameter of the capillary. This is achieved by main-taining low enough pressure behind the capillary. Under suchconditions the flow pattern depends only on the geometry ofthe capillary, independent of the gas.13 Hence for such a modeof operation, the cross sections obtained can be put on the ab-solute scale using the expression,

σCF+3(E) = ICF+

3PAr

IAr+ PCF4

σAr+ (E),

where PCF4 , PAr are respective pressures of the test gas andreference gas behind the capillary. The pressure behind the

capillary is measured using a capacitance manometer (MKSBaratron) in our experiment. ICF+

3, IAr+ , σCF+

3(E), and σAr+(E)

are ionic currents and absolute partial ionization cross sec-tions of the particular specie (e.g., CF3

+ from CF4) from thetest gas and reference gas (e.g., Ar), respectively, at a givenenergy E of the ionizing electrons. The same methodology isused for other ionic fragments.

It is observed from the table that the present values of themeasured cross sections are close to or higher than the litera-ture values,10–12 and this difference is even more noticeable inthe case of the lighter ions such as F+. This can be attributedto the fact that F+ is formed with large kinetic energies of upto 8 eV (Ref. 12) and earlier measurements might have missedout on the complete collection of the light mass ions formedwith high kinetic energies.

IV. CONCLUSIONS

The purpose of the present design of the ToFMS, whichis to have a nearly field-free interaction region and achievecomplete ion collection and transportation to the detectorwith minimum loss to the mass resolution has been real-ized. The introduction of the cap-shaped puller electrode ina small-sized mass spectrometer has resulted in preventingfield leaking into the interaction region from the sides, whilethe introduction of a dead zone flanked by high transparencymeshes between the interaction zone and the main acceler-ation zone has stopped the electric field leakage thorough asingle high transparency mesh. Analytical study of the iontrajectories followed by SIMION simulations has helped inrealizing total ion collection and transportation of the ions,making the present design of the ToFMS useful for measur-ing reliable charged particle/photon-molecule absolute ion-ization/attachment cross sections.

ACKNOWLEDGMENTS

The authors thank Mr. Satej Tare and Mr. YogeshUpalekar for technical help.

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