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Influence of trapping parameters on ion injection and dissociation efficiencies in a quadrupole mass filterion trap tandem instrument

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JOURNAL OF MASS SPECTROMETRY
J. Mass Spectrom. 34, 511È520 (1999)
InÑuence of Trapping Parameters on Ion Injection
and Dissociation Efficiencies in a Quadrupole Mass
Filter/Ion Trap Tandem Instrument
V. Steiner, C. Beaugrand, P. Liere and J.-C. Tabet*
Laboratoire de Chimie Structurale Organique et Biologique, Universite Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris
cedex 05, France
Investigations on ion injection into a laboratory-built quadrupole/ion trap tandem instrument (Nermag R1010/ITMS Finnigan) with a 90Ä geometry are described. The quadrupole dual electron impact/chemical ionization
(EI/CI) source was used as an external source to prepare ions under EI (or CI) conditions. They were selected by
the quadrupole analyzer and injected into the ion trap by using an electrostatic quadrupolar deÑector. The e†ects of
major trapping parameters (i.e. ion injection r.f. level and helium pressure) were considered. The injection efficiency
of the stable CF ‘ (m/z 69) fragment ion from FC43 and also those of the m/z 40 and 84 atomic cations of argon
3
and krypton depend upon the injection r.f. level conditions. As expected, e†ects of non-linear resonances were
observed during these experiments. Under certain injection and trapping conditions, ion motion can be strongly
a†ected, yielding ion ejection, because non-linear Ðelds (resulting from device imperfections) are present in addition
to the quadrupolar potential required for ion trapping. The inÑuences of non-linear e†ects according to their
strength were investigated to show the best and worst trapping conditions. The non-linear resonances are characterized by the expected b values. In order to determine the average internal energy deposited on the selected polyatoz
mic ions during injection, butylbenzene molecular ions were chosen as a model for internal energy measurement.
The e†ects of the r.f. injection level and helium pressure on the injected ion fragmentation were studied. The M‘~
ions were only observed through a narrow injection q window. At low q values, only low critical energy m/z 92
z
z
ions were detected, while the higher critical energy m/z 91 ions were observed at higher q values. Copyright (
z
1999 John Wiley & Sons, Ltd.
KEYWORDS : ion trap mass spectrometry ; ion injection ; non-linear resonances
INTRODUCTION
The efficacy of ion trap mass spectrometry for the
analysis of volatile organic compounds has been amply
demonstrated over the past 15 years or so. During this
period, there have been a number of signiÐcant
improvements, in particular extension of the mass
range1,2 and enhanced sensitivity and mass
resolution.3h5 Furthermore, an attractive feature of the
ion trap is the ability to perform multiple massselective, (MS)n, experiments6h8 with high sensitivity in
a single small space. These improvements in performance have led to a large number of publications
and reviews9 on fundamental and analytical studies on
gas-phase ion reactivity and ion structure.
* Correspondence to : J. C. Tabet, Laboratoire de Chimie Structurale Organique et Biologique, Universite Pierre et Marie Curie, 4 Place
Jussieu, 75252 Paris Cedex 05, France. E-mail : tabet=ccr.jussieu.fr
Contract/grant sponsor : CEB
Contract/grant sponsor : DGA.
CCC 1076È5174/99/050511È10 $17.50
Copyright ( 1999 John Wiley & Sons, Ltd.
During the 1980s, the commercially available quadrupole ion trap instruments were restricted to analytical
investigations of covalent compounds of relatively low
molecular mass. With such instruments, volatile compounds were ionized by in situ electron impact (EI) and
by chemical ionization (CI). However, conditions within
the ion trap were not conducive to the formation of
negative ions. Indeed, several attempts were made to
detect negative ions prepared in situ by electron capture
processes but negative ions were formed and trapped
together with positive ions obtained by electron
impact.10,11 Unfortunately, the use of the space charge
conditions was required and these are not the best conditions for analytical purposes.12,13
As negative ion analysis and the range of compounds
compatible with in situ ionization are limited, there was
great interest in developing an external source/ion trap
combination.14,15 Consequently, the performance of an
EI/CI external source combined with the ion trap was
optimized.16,17 With such a source, negative ions could
be produced under high-pressure conditions18 within
the dual source and subsequently injected into the ion
trap. A major advantage of an external source is that it
Received 6 May 1998
Accepted 27 January 1999
512
V. STEINER ET AL .
largely eliminates self-ionization processes which arise
from the interactions of nascent ions with parent molecules throughout the ion residence period. Note that
alternatively for the preparation of the speciÐc negative
ions (e.g. by charge exchange or by proton transfer), the
injection of externally generated ions into the ion trap
brings about selective ionÈmolecule reactions between
the selected reagent and the analyte directly
introduced19 through a gas chromatographic interface.
Such a device limits the competitive ionization mode
occurring in a high-pressure source.20 The development
of desorption/ionization techniques17,21h23 brought
non-volatile compounds within the aegis of the ion trap.
Coupling of electrospray ionization (ESI) with the ion
trap has also been developed22 and has now become
available on commercial instruments.24,25
An alternative approach that is fairly e†ective is to
interpose a conventional ion beam analyzer between the
external source and the ion trap to form a tandem
instrument. Initially, double sector analyzers, such as
E/B26 and B/E,27 were used to inject externally formed
ions into an ion trap. In order to ensure that the ions
injected were of low kinetic energy, a collision cell was
introduced immediately before the ion trap28 so as to
decelerate the mass-selected ion beam. Even a quadrupole ion store (quistor) was used as a collision cell in a
BE/quistor/quadrupole system.29 The major difficulty
with this latter apparatus was the necessity to Ñoat the
ion trap at high potential. Other quadrupole tandem
instruments have been constructed such as a
quadrupole/ion trap instrument30 and a quadrupole/ion
trap/quadrupole device ;31 the latter system permitted
the study of ionÈmolecule reactions and the measurement of reaction rate constants.
In order to investigate selected ionÈmolecule reactions, an o†-axis tandem mass spectrometer consisting
of a quadrupole mass Ðlter and a quadrupole ion trap,
linked by a 90¡ quadrupole deÑector, has been constructed in our laboratory. Mass-selected ions transmitted by the mass Ðlter are deÑected through 90¡ and
directed into the ion trap analyzer. This system has permitted the study of ionÈmolecule reactions in which the
ions are essentially thermalized through collisions in the
external ion source at relatively high pressure so that
they are of low vibrational energy ; furthermore, the
reactant ions may be either negatively or positively
charged. An instrument having similar o†-axis geometry
has been used already by Pedder and Yost,32 who
found that this device prevented the occurrence of secondary reactions that would have impaired the reliability of their kinetic and equilibrium measurements.
The improvement with our tandem system was the
additional di†erential pumping between the device
vacuum manifold containing the mass Ðlter and the ion
trap. Since chemical noise is reduced in the tandem
arrangement and the o†-axis injection prevents fast neutrals and photons originating from the external source
from reaching the ion trap detector, the signal-to-noise
ratio is enhanced in this type of instrument.
The roles played by the various trapping parameters
in a†ecting ion injection have been scrutinized.16,33 It
was found that the helium bath gas pressure and the
amplitude of the r.f. potential are the two most inÑuential factors a†ecting both the efficiency with which the
Copyright ( 1999 John Wiley & Sons, Ltd.
injected ions are detected and the mass resolution of
these signals upon detection. The conÐnement of
charged species injected into an ion trap is enhanced by
the presence of helium bu†er gas and the optimum mass
resolution with which these ions may be detected is
achieved at a pressure of helium of the order of 10~3
Torr (1 Torr \ 133.3 Pa). The conÐnement of charged
species having initial kinetic energies in the region of 10
eV15 requires a Ðnite r.f. amplitude. However, as the r.f.
amplitude is increased, the injected ions become accelerated by the resulting Ðeld and can acquire much higher
kinetic energies.16 When polyatomic ions are injected
under conditions of relatively high helium pressure and
r.f. amplitude, the charged species detected may be
largely fragment ions formed by collision-induced dissociation (CID) ; under such conditions, few polyatomic
parent ions may survive the injection and trapping
process. In most of the commercial ion trap instruments, the separation of the end-cap electrodes is
stretched in order to improve the trapping efficiency,
but owing to this change in the ion trap geometry,
imperfections are induced in the trapping Ðeld ;34 these
imperfections are responsible for the appearance of nonlinear resonances that can severely perturb the ion
injection process. However, these resonances have been
found to be very useful since resonant ejection at such
particular non-linear resonances has already been used
to extend the m/z ratio range of the ion trap.35
In this paper, the inÑuence of the trapping parameters
on ion injection experiments in our o†-axis quadrupole/
ion trap tandem instrument is reported. At this stage,
sensitivity is a limiting factor since the overall transmission efficiency of the tandem system does not exceed 1%
of the selected ion beam. Numerous simulation studies
have been made in order to understand and improve
the injection process.36 Di†erent methods have recently
been used for enhancing the injection efficiency, such as
the use of a d.c. pulse during the injection step.36a On
the other hand, in order to enhance the trapping efficiency, the trapping Ðeld can be increased.37
Mordehai38 suggested the use of an additional trapping
electrode at the back of the end-cap close to the detector device and a further modiÐcation of our tandem
system in that respect is in progress. In the present
study, the e†ects of injection r.f. level and helium pressure were investigated thoroughly. The observed variations in ion injection efficiency are explained in terms of
non-linear resonances, CID and the stability of ion
motion within the quadrupole ion trap. With respect to
the determination of the optimum ion trapping conditions for maximum sensitivity, monoatomic Ar`~ and
Kr`~ ions were employed since such species do not
undergo CID ; CF ` was also employed since it does
not undergo CID 3during normal ion trap operation.
With respect of the incidence of collisional excitation
leading to CID during ion injection, a well known thermometer ion, the molecular ion of butylbenzene,39h41
was employed in the determination of the average internal energy deposited ; its M`~ molecular ion gives rise
to the competitive formation of two fragment ion
species, at m/z 91 and 92, for which the critical energies
and thermochemistry have been determined previously.39h43 In spite of the poor efficiency of the injection system, the results reported in this paper are
J. Mass Spectrom. 34, 511È520 (1999)
ION INJECTION AND DISSOCIATION EFFICIENCIES
consistent with those observed elsewhere on ion injection.
EXPERIMENTAL
Experiments were performed with a laboratory-built
tandem quadrupole mass Ðlter/ion trap mass spectrometer having an orthogonal geometry such that the ion
beam emerging from the mass Ðlter is directed through
90¡ and towards an ion trap. The instrument, as shown
in Fig. 1, is composed of a Nermag mass Ðlter (R10È10)
and a Finnigan ion trap mass spectrometer (ITMS). The
ion beam deÑection system consists of a quadrupolar
electrostatic deÑector44,45 with associated lenses. The
SIMION46 program was used to maximize the transmission of ions through the instrument.
The Nermag dual EI/CI source was used as an external source for the quadrupole mass Ðlter of m/z range
10-1000 Th. The main ion beam was directed axially
into the ion trap, of m/z range 15È450 Th, through the
d.c. quadrupole deÑector. In order to optimize the ion
beam efficiency, supplementary holes in the entrance
end cap electrode were made. The electron gate electrode incorporated in the leading end-cap electrode was
modiÐed to optimize the ion beam transmission efficiency. However, the overall transmission efficiency for
a selected ion has been estimated to be lower than 1%.
This value was achieved (i) by considering that D30%
of the initial selected ion beam was transmitted through
the quadrupole Ðlter (for a Ðxed d.c./r.f. ratio) and (ii) by
measuring the total ionic current lost at the di†erent
parts of the 90¡ quadrupole deÑector, at each ion trap
element (end-cap electrodes) and at the conversion
dynode compared with the initial ion beam current.
The channeltron-type electron multiplier used for ion
detection was equipped with a conversion dynode held
at a potential of ^4000 V depending upon the polarity
of the ions under study. The key element of this
513
laboratory-made tandem instrument is the 90¡ ion deviation system connecting the two analyzers ; this element
permits a high degree of experimental versatility that is
achieved by variation of the potentials applied to the
component electrodes. Three operating modes are as
followed : (i) a direct in-line arrangement where the ion
beam crosses the quadrupole mass Ðlter, passes through
the deÑector without deviation and impacts upon a conversion dynode, whereupon the ions are detected by the
quadrupole detector, as shown Fig. 1 ; (ii) a direct in-line
arrangement [orthogonal to mode (i)] where electrons
emitted by the ion trap Ðlament shown in Fig. 1 pass
through the deÑector without deviation and enter the
ion trap so as to bring about in situ EI ionization ; and
(iii) an orthogonal arrangement where the mass-selected
ion beam issuing from the mass Ðlter is deviated by the
deÑector and directed through the injection lens and
into the ion trap. The electron energy was 90 eV and
the extractor and source potentials were adjusted so as
to maximize ion beam transmission efficiency. Normally, the source potential V
and the extractor
potential V
were held at source
16 and [39 V, respecextractor
tively, for the
injection of positive ions. A triple-element
Einzel lens is mounted just after the quadrupole deÑector ; the potential applied to the central electrode, called
the “electron gate,Ï was held at ^150 V so as to control
the polarity of ions injected into the ion trap [mode
(iii)] and the injection of electrons [mode (ii)]. For
maximum ion injection efficiency, the quadrupole
deÑector electrodes were held at potentials of [45 and
[10 V and the gate electrode was held at [150 V
during ion injection.
The scan function used in our studies is shown in Fig.
2. The duration of the ion injection period was Ðxed
typically at 50 ms. Throughout the ion trapping investigation described here, where the r.f. drive potential
amplitude was varied over as wide a range as possible,
the magnitude of the r.f. amplitude is expressed in terms
of low-mass cut-o† (LMCO). The LMCO value corresponds to the ion species of lowest m/z that can remain
Figure 1. Schematic diagram of the quadrupole/ion trap tandem mass spectrometer.
Copyright ( 1999 John Wiley & Sons, Ltd.
J. Mass Spectrom. 34, 511È520 (1999)
V. STEINER ET AL .
514
Figure 2. Description of the scan function used in positive ion injection studies.
stable in the ion trap since its m/z corresponds to a q
value of D0.904 (\0.908 due to axial modulation),z
where
InÑuence of the LMCO on the injection of Ar‘~, Kr‘~
and CF ‘
3
\ (LMCO ] 0.908)/(m /z)
(1)
z, i
i
When the mass-selective instability scan47 was activated, the r.f. amplitude was ramped at a rate of 5555 Th
s~1 and was combined with an axial modulation48 (1.5
V
and a frequency \550 kHz) so as to improve the
php
sensitivity
and m/z resolution. Mass spectra were
obtained as the average of 15 microscans. The experiments performed in mode (iii) concerned either the
inÑuence of the r.f. amplitude on the trapping efficiency
of Ar`~, Kr`~ and CF ` at a Ðxed helium pressure of
3 inÑuence of helium pressure,
1.5 ] 10~4 Torr or the
varied over the range 1.5 ] 10~4È1.5 ] 10~3 Torr, on
ion dissociation. The reported pressures were corrected
using the gauge manufacturerÏs factor for helium. Argon
and krypton, having a purity of [99%, were purchased
from Air Liquide and butylbenzene, also having a
purity [99%, was obtained from Aldrich. Butylbenzene
was introduced into the ion source without further puriÐcation via a leak valve, as was FC43 for the preparation of CF `~. Ions were formed in the EI source at
a pressure of 3D10~5 Torr for argon or krypton and
10~6 Torr for butylbenzene and FC43. The vacuum in
the quadrupole manifold was maintained by two
primary pumps and two di†usion pumps ; the ion trap
has its own vacuum chamber that is pumped independently by a turbomolecular pump to obtain a background pressure of 10~7 Torr as measured in the
absence of helium by a Penning gauge attached to the
vacuum chamber.
E†ect of non-linear resonances on ion injection. An important factor in the trapping of injected ions is the role
played by non-linear resonances. This well known
phenomenon34 is caused by the superimposition of
higher order Ðelds upon the main quadrupole Ðeld ; the
existence of such non-linear resonances can perturb the
motion of the injected ions49 by causing them to move
away from the center of the ion trap. These perturbations, Ðrst observed in tandem mass spectrometric
(MS/MS) experiments50 performed in a conventional
ion trap and later during ion injection experiments,51
cause newly injected ions, or fragments thereof, to be
ejected readily when they encounter non-linear resonances at relatively large excursions from the center of
the ion trap. The further an ion is from the ion trap
center, the more it can be a†ected by non-linear resonances since the inÑuence of such non-linear resonances
increases with the distance from the ion trap center.
Yost and co-workers,51 in a study of the causes of ion
cloud excursions in the vicinity of the end-cap electrodes, found that space-charge, the absence of helium,
the use of resonant excitation and ion injection could
perturb ion trajectories, yielding substantial ion losses.
Ar`~ (m/z 40), CF ` (m/z 69) and Kr`~ (m/z 84)
3 were injected into the ion trap
formed in the EI source
by mode (iii) while the LMCO was varied from 0 Th to
the m/z value of the ion selected for injection ; the subsequent variations of the observed ion signal intensities
are shown in Fig. 3.
The LMCO threshold value for the appearance of
each ion species is in the region of 15 Th. The striking
feature of each of Fig. 3(a)È(c) is the appearance of deep
troughs or “black holesÏ where the ion signal intensity
drops dramatically at particular LMCO values. In
order to Ðnd the best trapping conditions, the inÑuence
of helium pressure was scrutinized, as shown in Fig. 4.
Once the pressure exceeds 1.5 ] 10~4 Torr, the trapping efficiency is not signiÐcantly improved. The m/z 69
ion is very stable towards dissociation, so at helium
pressure of 1.5 ] 10~4 Torr the cooling is sufficient for
efficient ion trapping.
q
RESULTS AND DISCUSSION
The e†ects of the LMCO during the ion injection
period, the helium pressure and the injection ion kinetic
energy on the efficiency of ion trapping are considered
Ðrst, followed by an examination of the LMCO and
helium pressure inÑuence on the dissociations of
injected ions.
Copyright ( 1999 John Wiley & Sons, Ltd.
J. Mass Spectrom. 34, 511È520 (1999)
ION INJECTION AND DISSOCIATION EFFICIENCIES
515
Table 1. Major holes observed during injection of Ar‘~ (m/z
40), Kr‘~ (m/z 84) and fragment CF ‘ ion (m/z 69),
3 the non-linear
related to the ion axial frequency and
Ðeld involved in the resonance
Ion
Ar½~(m /z 40)
CF ½(m /z 69)
3
Kr½~(m /z 84)
Figure 3. Variation of the ionic current of the selected and
injected ions as a function of the injection low mass cut-off and
the q stability parameter of (a) argon (m /z 40) ion, (b) FC43 (m /z
z
69) fragment
ion and (c) krypton (m /z 84) ion. The b values of
z b ¼0.52
the main black holes are : (i) b ¼ 0.32 (a), b ¼ 0.43 (b),
z
z
z (c),
(c), b ¼ 0.68 (d) ; (ii) b ¼0.34 (a), b ¼ 0.42 (b), b ¼ 0.52
z (d) ; (iii) b ¼0.34
z (a), b ¼ 0.42
z (b), b ¼ 0.51
z (c), b ¼
b ¼0.67
z (d).
z
z
z
z
0.66
Upon inspection of Fig. 3(a)È(c) and with reference to
the abscissa labeled q [Eqn (1)], it is seen that the prinz same q values for each species ;
cipal holes occur at the
z 1. It should be noted
these holes are identiÐed in Table
that b is obtained from a continued fraction expressed
z
as a function
of the well known trapping parameters a
and q ,9 ) is the r.f. drive frequency in radi s~1 and uz
and uz are the radial and axial secular frequencies,r
z
respectively
:
u \ b )/2
(2)
r, z
r, z
Similar troughs in the ion signal intensity have been
described by Guidugli and Traldi.50 Cooks and coworkers,16,52 who emphasized that the LMCO is an
important parameter in determining ion trapping efficiency, detected a black hole at q \ 0.635 and ascribed
z
its origin to the inÑuence of a non-linear
resonance at
)/4, that is, with b \ 1/2. All of the major holes in Fig.
3(a)È(c) have beenz matched to axial frequencies, as
shown in Table 1, and, for each species, hexapole, octapole, decapole and dodecapole non-linear Ðelds were
identiÐed.
In a quadrupole ion trap, the quadrupolar trapping
Copyright ( 1999 John Wiley & Sons, Ltd.
Observed holes
b
q
z
z
0.43
0.57
0.66
0.79
0.46
0.55
0.66
0.78
0.46
0.55
0.65
0.78
0.32
0.43
0.52
0.68
0.34
0.42
0.52
0.67
0.34
0.42
0.51
0.66
Frequency o
z
(o ¼ b O/2)
z
z
b ¼ 1/3,
z
b ¼ 2/5,
z
b ¼ 1/2,
z
b ¼ 2/3,
z
b ¼ 1/3,
z
b ¼ 2/5,
z
b ¼ 1/2,
z
b ¼ 2/3,
z
b ¼ 1/3,
z
b ¼ 2/5,
z
b ¼ 1/2,
z
b ¼ 2/3,
z
o ¼ O/6
z
o ¼ O/5
z
o ¼ O/4
z
o ¼ O/3
z
o ¼ O/6
z
o ¼ O/5
z
o ¼ O/4
z
o ¼ O/3
z
o ¼ O/6
z
o ¼ O/5
z
o ¼ O/4
z
o ¼ O/3
z
Non-linear field
Dodecapole
Decapole
Octapole
Hexapole
Dodecapole
Decapole
Octapole
Hexapole
Dodecapole
Decapole
Octapole
Hexapole
Ðeld can deviate from the ideal linear Ðeld53 owing to
perturbations introduced by electrode truncations, deviations from hyperbolic geometry, misalignment of electrodes and perforations in one or more of the three
electrodes. All of these physical changes introduce to
the trapping Ðeld, higher order multipoles, that are
manifested as non-linear resonances. For example, in
the commercial ion trap, the stretching of the distance
between the two end-electrodes so as to enhance mass
assignments has introduced non-linear resonances. In a
pure linear Ðeld, ion trajectories are characterized by
the secular frequencies u and u and their sideband
r i is az positive integer and
frequencies i) ^ u , where
u
u \ r, z. In the case of the non-linear Ðelds, additional
higher harmonic frequencies ku (where k [ 1) and sideu order Ðeld can be
band frequencies of the higher
observed. Non-linear resonances appear when higher
harmonics match sideband frequencies.34 When the
secular frequency of an ion corresponds to a non-linear
resonance, the ion oscillates at a subharmonic of the r.f.
frequency and the amplitude of the ion trajectory
increases, whereupon the ion gains kinetic energy from
acceleration by the r.f. driving potential ; under such
conditions, the ion can be ejected from the ion trap or
collide with the electrodes. However, ion loss is limited
since, as the trajectory amplitude increases, the secular
frequency of the ion changes so that the ion is no longer
in resonance and its trajectory amplitude decreases.
In the ion trap, two opposing forces come into
play :49 relaxation of the ion kinetic energy or
cooling54,55 under the inÑuence of collisions with
helium bath gas opposes the increase in ion kinetic
energy brought about by a non-linear resonance that
leads to increased ion axial excursions from the ion trap
center. In the case of in situ EI ionization, the cooling
process e†ects a focusing of the ion cloud near the ion
trap center where non-linear resonance e†ects are experienced weakly, if at all. However, during MS/MS
experiments when the application of a tickle voltage can
lead to fragmentation or, in the limit, to ion ejection,
kinetic energy thermalization by collisions with helium
cannot compensate sufficiently rapidly for the increased
ion motion manifested by higher kinetic energy and
greater excursions from the ion trap center. Hence ions
J. Mass Spectrom. 34, 511È520 (1999)
V. STEINER ET AL .
516
Figure 4. Variation of the ionic current of the FC43 (m /z 69) fragment ion as function of the LMCO and helium pressure (Torr).
excited in this manner are subjected to perturbations
due to non-linear resonances and can be ejected.
The role played by the ion kinetic energy upon the
injection step was also examined. At an helium pressure
of 1.5 ] 10~5 Torr, the ion energy was varied from 10
to 15 eV (Fig. 5). No inÑuence on the non-linear resonance e†ects was noticed, but the trapping efficiency
seems to depend upon the injected ion kinetic energy.
Indeed, to enter the ion trap, ions need to have enough
kinetic energy to cross the potential barrier created by
the r.f. drive potential and collisions with helium.
It has been pointed out by Traldi and co-workers56
and by Eades and Yost57 that non-linear resonances do
not exist only on the q axis but can stretch across the
entire stability diagram z; when they are prolonged in the
a direction, the non-linear resonance lines have been
z
termed
“black canyons.Ï Recently, Alheit and coworkers58,59 were able to map non-linear resonances
Figure 5. Influence of the injected ion energy on the variation of the ionic current of the FC43 (m /z 69) fragment ion as function of the
LMCO.
Copyright ( 1999 John Wiley & Sons, Ltd.
J. Mass Spectrom. 34, 511È520 (1999)
ION INJECTION AND DISSOCIATION EFFICIENCIES
over the entire stability diagram. The most important
ones are identiÐed as associated with b \ 2/3 as a
z
hexapolar Ðeld contribution, b \ 1/2 as an octapolar
z
Ðeld contribution and b \ 1/3 as a dodecapolar Ðeld
z
contribution.
E†ect of the m/z values of the selected ion on the r.f. level
threshold. By comparison of the dependence of the three
injected ions upon the r.f. level values (Fig. 3), the ion
appearance threshold can be determined according to
the selected ion (Table 2). The r.f. level threshold
increases as the m/z ratio of the selected ion, i.e. the q
z
trapping parameter decreases as the injected ions are
larger. This behavior is in agreement with that found by
Cooks and co-workers ;17 by studying the di†erent fragment ions of perÑuorotributylamine (FC43), it appears
that an increase in the r.f. voltage threshold is required
for trapping larger size selected ions, whether or not q
decreases. This trapping phenomenon has been ration-z
alized using the Dehmelt model.49 The trapping efficiency depends upon (i) the initial kinetic energy of the
selected ions and (ii) the depth of the pseudo-potential
well (denoted D on the z-axis). D depends on the m/z
z ions and the r.f.z voltage amplitude V
ratio of the studied
according to the following equation within the adiabatic
approximation (i.e. for q \ 0.4) :
z
4zez 2 V 2
0
D \
(3)
z m(r 2 ] 2z 2)2)2
0
0
where ze is the charge, m the ion mass, r and z the
0
0 and
radial and axial dimensions of the trap, respectively
) the angular frequency of the r.f. drive potential Ðeld.
Hence, for a given injection LMCO, when the m/z of the
selected ion is increased, the D value decreases and the
ions are no longer efficientlyz trapped and are much
more sensitive to weak trajectory perturbation. Therefore, to detect high m/z ions, higher LMCO values are
required. However, in the case of polyatomic ions, CID
processes occur more efficiently in the ion trap as the
injection r.f. level was increased,33 as it will be seen
later.
InÑuence of trapping conditions on the stability of
injected polyatomic ions
A number of experiments were carried out using butylbenzene as the reagent substrate in order to investigate
the inÑuence of ion trapping parameters on the kinetic
and vibrational energies of mass-selected ions injected
into an ion trap. The butylbenzene molecular ion is
considered as an “energetic thermometerÏ in that the rate
constant ratio for two competitive pathways can be
found directly from measurement of the ion signal
517
intensity ratio of m/z 91 and 92. The favored process
from molecular ions, characterized by the higher internal energy, is the formation of the [C H ]` (m/z 91)
7 7
ions according to a simple cleavage mechanism having
an activation energy of 1.7 eV.40,43 The competitive
decomposition leading to the formation of [C H ]`
7 8
(m/z 92) ions occurs from molecular ions having a lower
internal energy of 1.1 eV.40 Thus, the ion signal intensity ratio of m/z 91 and 92 provides an estimate of the
internal energy carried by molecular ions under a
variety of experimental conditions. Let us consider now,
in turn, the e†ects of variations in helium pressure and
LMCO.
E†ects of helium pressure on the injection of
butylbenzene molecular ions
Enhanced ion trapping and the associated trade-o†s.
While collisions with helium bu†er gas result in focusing of the ion cloud to the ion trap center, this process
does not necessarily preclude the accumulation of internal energy in the charged species. Let us examine these
opposing e†ects with reference to injected butylbenzene
molecular ions.
As shown in Fig. 6, the total ionic current (TIC), consisting of molecular and fragment ions, is approximately
doubled by a 10-fold increase in helium pressure across
most of the range of LMCO investigated.33 Also, the
LMCO threshold is lowered from 18 to 12 Th over the
same 10-fold increase in helium pressure. However, the
trade-o†s are that the increase in TIC is accompanied
by a degradation of the mass resolution and a diminution in TIC at high LMCO. The mass resolution of the
molecular ion is degraded in that the peak shape is
asymmetrically broadened towards lower m/z. This
resolution degradation is due to fragmentation of the
m/z 134 ions as the working point of this ion species
approaches the b \ 1 stability diagram boundary. The
z
m/z 134 ions, having
become vibrationally excited in
many collisions with helium immediately following
injection, are accelerated during the analytical scan by
the increasing r.f. amplitude and, after further energizing
collisions with helium, dissociate. The nascent fragment
ions, having unstable trajectories in the ion trap, are
ejected, giving rise to broadening of the m/z 134 peak
shape towards lower m/z.
McLuckey et al.33 have shown that sensitivity can be
enhanced when H is used in place of helium as a bath
2
gas ; however, when
argon60 or air61 is used as a
damping gas, the trapping efficiency is markedly
Table 2. Injection r.f. level for the
appearance of the selected ion
Selected ion
Ar½~ (m /z 40)
CF ½ (m /z 69)
3
Kr½~ (m /z 84)
(Threshold)
exp
q
Th
13
16
18
Copyright ( 1999 John Wiley & Sons, Ltd.
z
0.32
0.21
0.18
Figure 6. Evolution of the total ionic current of the butylbenzene
ions with the LMCO and the helium pressure.
J. Mass Spectrom. 34, 511È520 (1999)
V. STEINER ET AL .
518
Figure 7. Evolution of the m /z 91/92 ratio as a function of the
LMCO and the helium pressure.
lowered. The higher sensitivity achieved with light bath
gases is due to the small scattering angle of the heavier
collision partner in binary collisions that results in
partial loss of kinetic energy without dispersion. The
lowering of the LMCO threshold with increased helium
pressure illustrates clearly the requirement for low
kinetic energy for e†ective trapping of ions in the relatively shallow potential well formed at low q values. At
z
high LMCO the reduction in the TIC at high
helium
pressure, as can be discerned in Fig. 6, is ascribed to
dissociative processes and the subsequent loss of fragment ions having q values in excess of 0.908.
z
Internal energy considerations. In order to investigate
the inÑuence of collision number on the internal energy
of the butylbenzene molecular ion, the ion signal intensity ratio for m/z 91 and 92 was monitored as a function
of helium pressure and LMCO value. The variation of
the ion signal intensity ratio, expressed as m/z 91/m/z
92, as a function of these two parameters is shown in
Fig. 7. Since the fragment ion m/z ratios di†er by only
one unit, their working points are similar so that these
fragment ions experience essentially the same trapping
Ðeld with or without non-linear resonances. An advantage of the present experimental arrangement is that the
remote ion source ensures that most of the collisions
su†ered by ions in the ion trap are with helium. From
Fig. 7, it is clear that the ion signal intensity ratio is
largely independent of the 10-fold increase in helium
pressure within the LMCO range 10È50 Th. The
absence of signiÐcant e†ects may indicate that the cir-
cumstances of the observed fragment ions are similar in
that dissociation has occurred in the central region of
the ion trap. Concomitantly, when dissociation occurs
in the proximity of the end-cap electrodes, the fragment
ions are ejected from the ion trap and the ratio of the
ejected fragment ion intensities cannot be determined.
Similar studies carried out with negative ions by
McLuckey et al.33 showed that CID processes were
enhanced in that more fragmentations were observed
when the helium bu†er gas pressure was increased up to
7 ] 10~3 Torr.
E†ects of the LMCO on ion trapping and ion dissociation. In
Fig. 6, it is seen that the TIC is slightly modulated as
the LMCO is changed, presumably owing to the presence of non-linear resonances. The e†ects of black holes
are discerned more clearly when the individual ion
abundances that contribute to TIC are plotted as a
function of LMCO value at 7 ] 10~4 Torr helium, as
shown in Fig. 8. Here the abundance of the molecular
ion at m/z 134 and the fragment ions at m/z 92, 91 and
65 are displayed as a function of LMCO.
T rapping of m/z 134 ion. Generally, the trapping efficiency of the m/z 134 molecular ion appears to be poor
in that such ions are detected with relatively low abundances and only for low LMCO values in the range
18È36 Th. Furthermore, the injection of molecular ions
is always accompanied by fragmentation so that fragment ions may be observed even when no molecular
ions are detected. For example, for an LMCO of 15 Th,
the q value of the molecular ion is D0.1 whereas the q
z for the m/z 91 and 92 ions are close to 0.15 andz
values
the fragment ions are trapped with greater efficiency
than the molecular ion. Hence the injection of molecular ions with high kinetic energy leads to CID processes,
as was observed earlier by Cooks and co-workers16 in a
study of perÑuorotributylamine ions injected from an
external source. They found that the abundance of the
fragment ion of m/z 219 was very low whereas that of
the fragment ion of m/z 131 was anomalously large
compared with its abundance in an EI mass spectrum
obtained with a mass Ðlter.
T rapping of fragment ions. Fragment ions were detected
over the entire range of LMCO investigated, that is,
Figure 8. Variation of the ionic current as a function of the LMCO. x, m /z 134 ; +, m /z 92 ; K, m /z 91 ; L, m /z 65.
Copyright ( 1999 John Wiley & Sons, Ltd.
J. Mass Spectrom. 34, 511È520 (1999)
ION INJECTION AND DISSOCIATION EFFICIENCIES
from 15 to 80 Th, whereas molecular ions were
observed only in the LMCO range 18 ThÈ36 Th.
Clearly, the absence of molecular ions indicates their
complete dissociation or ejection from the ion trap.
However, the relative constancy of the TIC over the
major portion of the LMCO range employed (except for
the troughs caused by non-linear resonances) shows
that molecular ions are not ejected any more at high
LMCO than they are at low LMCO values. At low
LMCO values, the m/z 91/m/z 92 ratio is less than unity
and increases with increasing LMCO until the ratio is
unity at an LMCO of 35 Th. At this point, the molecular ion has a q value of 0.237 whereas the m/z 92 and
z
91 fragment ions have q values of 0.345 and 0.349,
z
respectively. Both fragment and molecular ions should
be trapped under these conditions, yet only a trace of
the molecular ion is detected. The constancy of the TIC
together with the steady increase in the m/z 91/92 ratio
throughout the LMCO range from 15 to 50 Th suggest
that molecular ions are trapped throughout this range
but become progressively more internally excited prior
to dissociation. Even in the LMCO range from 35 to 50
Th, where no molecular ions are detected, the fragment
ion ratio changes smoothly so that their precursors
must have remained in the ion trap sufficiently long to
sample the ambient r.f. Ðeld.62 The absence of the oddelectron m/z 92 ions means that the competition related
to frequency factors at high internal energy favors the
formation of the even-electron m/z 91 ions that is characterized by a higher frequency factor.39h43 The proposed increase in molecular ion internal energy with
LMCO value is supported by the appearance of the m/z
65 ion due to consecutive decomposition of m/z 91. The
large drop in ion abundance at LMCO \ 64 Th is due
to the loss of m/z 91 (subjected to the b \ 1/2 nonz
linear resonance) which constitutes the entire
TIC at
that point.
CONCLUSION
The use of laboratory built quadrupole/ion trap tandem
mass spectrometer, Ðtted with a 90¡ selected ion beam
transfer device, prevents injection of the primary neutral
from the external ion source to the ion trap. Hence it is
possible to avoid undesirable ionÈmolecule reactions.
Several trapping parameters, including the helium pressure and the driving potential r.f. level for ion injection,
were investigated. It was shown that trapping efficiency
of the selected ion injection depends on many “black
519
holes.Ï In the EI mode, the trapping yield of the monoatomic Ar`~ and Kr`~ ions, and also stable CF ` frag3
ment ions (prepared from the FC43 used for
calibration), decreases strongly when the injection step
takes place at particularly b values such as 1/2, 2/3 and
z of several less important
1/3. Moreover, the presence
“black holesÏ has been found. Such e†ects are related to
ion trap device imperfections. The trap stretching and
the supplementary holes for ion injection and detection
lead to the production of non-linear Ðelds. The linear
quadrupolar Ðeld and the surperimposed ones contribute to the trapping of the selected ions. Then, for the
previous b values, non-linear resonances related to the
z
octapole, hexapole and dodecapole Ðelds contributions
appear and lead to unstable ion motions. Furthermore,
an optimum r.f. level is required to trap the injected
ions. This appearance r.f. threshold seems to depend
strongly on the mass/charge ratio characterizing the
ions. This phenomenon has already been explained by
using the Dehmelt pseudo-potential well depth.
To explore the e†ect of helium pressure and LMCO
on CID processes, and to estimate the average internal
energy, an “energy thermometerÏ was used. The relative
abundance of both the competitive m/z 91 and 92
decompositions occurring from the molecular ion of
butylbenzene were measured. These decompositions are
characterized by well known thermochemistry and
kinetic properties. The internal energy carried by the
selected ions during the injection step was demonstrated
to be related to the injection q value. At low q values,
z
z
trapping is not efficient but low-energy
dissociation
(i.e.
rearrangement cleavage, m/z 92) can occur. It is difficult
to observe the parent ions. Alternatively, at higher q
values, the trapping is better and the ions are then sub-z
mitted to higher collision energies. The molecular ions
carry more internal energy, enhancing the processes
favored at high collision energy (i.e. simple cleavage
yielding m/z 91). Consequently, in the ion trap, the polyatomic parent ions can survive only through a weak
injection q window and then be detected. Helium pressure plays zan important role in the ion trapping step by
increasing the efficiency of ion trapping. The results suggests that large ion motion amplitudes on injection are
strongly decreased by kinetic energy cooling after some
r.f. cycles.
Acknowledgements
The authors thank the CEB and DGA for Ðnancial support.
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