- Email: [email protected]
Time-of-flight detection of ions ejected from a radiofrequency quadrupole trap: experimental determination of their fz secular frequency
Volume 174, number 6
CHEMICAL PHYSICS LETTERS
23 November 1990
Time-of-flight detection of ions ejected from a radiofrequency quadrupole trap: experimental determination of their fi secular frequency G. Brincourt, R. Catella, Y. Zerega and J. And& Laboratoire des Interactions Ioniques et Mokwiaires (Unite de Recherche AssociPeau CNRS No. 773). Universitkde Provence, Centre de Saint JtWme, Case A61, 13397 Marseille Ceakx 13, France Received 11 May 1990; in final form 6 September 1990
We have studied the temporal behaviour of a very small number of ions injected into a quadrupole trap and detected by ejection. The sampling of the movement of the ions in the trap, associated with a time-of-flight detection, leads to an ion s@nal which gives thef, secular frequency.
1. Introduction Withdrawal of ions from a radiofrequency quadrupole trap has become an increasingly wide subject of study due to the possibility of mass separation and detection with or without an external mass analyser *‘. Further, it has been shown [2] that the motion of ions during ejection from the trap is complex when applying a pulse to the end-cap electrodes through which the ions are ejected. Observations were made on the dependence of the efficiency of ion ejection with the amplitude [ 31 and the width [ 41 of the pulse, with the phase of the rf field at which the pulse is applied to the caps [ 5 1, and with the end-cap mode ejection [6]. On the other hand, many investigations have been carried out on the injection of ions in a trap; and the efficiency of confinement of such injected ions have been discussed in terms of various ion-injection situations (ring, cap, or asymptotic injection) [ 7-141. Concerning the ejection of the ions from the trap, improvements in electronic devices like rf signal gating and opposite ejection pulses applied to the end-cap electrodes have reduced the complexity of the ejected ion motion. However, to our knowledge, very few investigations have been reli’ For a general account of the theory and applications of rf quadrupole devices, see ref. [ 11.
626
ported into the experimental determination of the spatio-temporal position of ions in a trap and the sampling volume [ I 5 1. In the following, we report on an experimental study on ejected SF, molecular ions from a quadrupolar ion trap, based on the time-of-flight (TOF) analysis of the ions after the ejection pulse has been turned on for different confinement times. The SF; molecular ions are created outside the trap by electron attachment in collisions between SF, molecules and xenon Rydberg atoms Xe** according to the reaction SF6 +Xe**+SF,- +Xe+ . The choice of these ions results from parallel studies on their lifetime [ 16,171 but, evidently, many other ions may be used provided, as here, that they are few in number, and initially located in a very small volume in the trap.
2. Theory We briefly report below the main theoretical considerations we need. In our experiment, the radiofrequency electric field employed by the quadrupole trap to confine the ions is generated by applying a rf voltage V, cos S2tto both end caps made with a wire
0009-2614/90/$03.50 0 1990 - Ekvier Science Publishers B.V. (North-Holland)
Volume 174. number 6
CHEMICAL PHYSICS LETTERS
mesh, and a continuous voltage V, to the ring as shown in fig. 1. In the scheme, the voltages applied to the caps come from separated circuits for potential modification required for ion injection in the trap through the lower end cap (LEC), and ion ejection from the trap through the upper one (UEC). Such a biasing of the electrodes was decided in earlier experiments in order to keep the ring electrode to the low voltage V0 (a few volts) to prevent field ionization of the Xe** atoms when entering the trap through this electrode. The potential in the trap is then:
that is the phase of the alternating field when the ion first experiences its influence. As we are mainly interested in the ion stability in the z direction and have assumed a singly charged positive ion of mass m, it follows that a, and qz are defined in our case by a,=-
-
4eUo
rnziQ2
(1)
where z and I (r2=x2+y2) are the coordinates, referenced to the center of the trap, of a confined ion, z being on the symmetry axis of the trap. The relative position of the ring and end-cap electrodes is such so that ri =2zg (fig. 1). When the initial angular velocity of the ion around the z symmetry axis is zero, the equation of ion motion takes the Mathieu form [ 18-201:
(2) where U=X, y, or z and c= 4Qt. & is the initial phase, AMP. COUNTING
MULTIPLIER
UPPER
If I
/
CONTROL GRID v d.c.>O
and
qZ=--
2eVo mz@2
’
Connected relations are obtained for a, and qr. The secular frequency in the z direction is given by the fundamental frequency of the z motion: fi = BZQ/47r,
tu,+V,cosm],
23 November 1990
(4)
where BZis a complex function of a, and qZ. When q&0.4, an approximation for fiZis [ 2 1 ] /?Z=(aZ+;q:)“’
(5)
and then (6)
Thus, the projection of the trajectory of the trapped ion in a plane containing the z axis is generally a Lissajous curve composed of the two corresponding frequencies and with a superimposed micromotion of the rf ripple at the frequency G/2x. The a, and qu parameters must he inside the stability diagram for the ion to be indefinitely trapped, that is with finite and stable trajectories in both the r and z directions inside the trap. The boundaries of the diagram in a, and g. coordinates are known, so one deduces from (3) the values of V. and V. to be used for trapping, with given values of m, z. and L?
-_____/
3. Experimental U.
+ EJECTION
dcc. VOLTAGE
PULSE
GRID INJECTION (IP)
a\ PLATE k
h
Fig. 1. Schematic arrangement of the biasing and command voltages.
INJECTION
PULSE
trap with its associated
Fig. 1 shows the trap arrangement. A SF6 molecular beam and a pulsed Xe** atomic beam interact between the injection plate (IP) and the LEC of the trap. With the gas beams, the pressure is of the order of 8 x lo-’ Torr in the trap, while the density is about 2 x 10” particles crnm3 in the crossed-beam region. The Xe** atoms are created by pulsed electron excitation of a xenon beam in a second vacuum chamher, and when the density of Xe** atoms is maximum under the trap, the SF; ions created are axially 627
injected into the trap by a negative voltage pulse (NVP) of circa - 5 V applied to the IP electrode, the rf voltage applied to the end caps being off, and that applied to the R electrode having its normal value (0 to - 1.5 V ). Fig. 2 shows the different voltage-timing sequences. After a time delay r, following the leading edge of the NVP applied to IP, a positive voltage pulse of circa 5 V is applied to the LEC electrode for a time Tz to decelerate the ions once in the trap (cooling). The rf voltage of frequency 160 kHz is then turned on for confinement, with a given amplitude in the range of 25 to 80 V. It is obvious that the length and the amplitude of the previous voltage pulses are of great importance for suitable confinement of the ions. After a defined confinement time, the ions are ejected by two opposite voltage pulses of circa 100 V applied to the LEC and UEC electrodes and superimposed on the rf voltage with a given relative ejection phase. An adjustable positive voltage is applied to the control grid to have the best ion-collection efficiency. The ejected ions are detected by an electron multiplier followed by an
L-d
beam pulse
+iOV
electro?
amplifier and a single-channel analyser associated with a temporal window the width and position of which select the ions by TOF for counting. Due to the very small number of created ions, many cycles of creation-injection-confinement-ejection (CICE ) are performed for each channel and the results added. For pulse-timing sequence and data acquisition, a PDP 1 l-03 minicomputer was used. All the signals are strictly phase synchronised and adjusted in length and amplitude. For each CICE cycle, the number of the channel concerned in the ejection is incremented, the channel width T, being 12.5 or 25 us. Since these values are compatible with the range of the secular frequencyf, of the SF; ions in the trap, it is possible to count the ions only when they are in a well-defined place on the secular trajectory by adjusting the time delay T3 and the width Tw of the counting window. Thus, we have a sampling of the z(t) coordinate of the ions with the frequencyf,= l/T,, and we always operate with fs/fi= k? E with k an integral number >2 and ec 1.
4. Results and discussion
one CICE cycle
electron
and nepive
ions
scavenger positive vottage
Fig. 3a shows a typical curve of confinement with the sampling effect. Since the ratioS,/f, is not an integer, stroboscopic effects are visible. Here we have k=4, and so we have four lines of sinusoidal amplitude modulation with the same frequencyf,; then fr =f,lk+fm
Counting! window
1
Fig. 2. Pulse timing sequences for operation ofthe trap. For practical reasons, the signals are not to the same temporal scale. T, is the time of deceleration of the injected ions.
628
23 November 1990
CHEMICAL PHYSICS LETTERS
Volume 174. number 6
.
(7)
SinceS, is connected with /I=,(4)) the sign must be considered before f, is deduced from the evolution offm with U0 or V,. In the case of fig. 3a, we have f5= 80 kHz and the curve gives directly f, x 676 Hz; we deduce fi = 19.8 kHz while the calculations from (6 ) and ( 3 ) and Fourier analysis give, respectively, f,=20.2 and 19.5 kHz, that is, an error of less than 4% between the results. Fig. 3b shows a curve obtained under the same experimental conditions as the previous one except for fs which has a value half the previous one, i.e. 40 kHz; here k=2 and from the determination off, we deduce_& 19.7 kHz in very good agreement with the values above. Due to the narrowness (0.4 &s) and the position of the counting window, we count only
Volume 174, number 6
CHEMICAL PHYSICS LETTERS
23 November 1990
1
COUNTS
I.
CHANNELS
Id
CICE cycles per channel
128
Fig. 3. Total number of SFz ions counted in each of the I28 channels for 1000 CICE cycles and with Ue= -0.5 V and Vo= 30 V. (a) Channel width T, is 12.5 us: the dashed line is one of the four amplitude modulation lines (k=4); (b) T,=25 us (k=2).
circa 0.5 ion per CICE cycle; hence, for good visibility, each channel is concerned, here, with lo3 CICE cycles. One can note the high degree of reproducibility, sensitivity and noise immunity of the experiment. As expected, no significant influence of the initial phase &,has been noticed, and the number of ejected ions was weakly dependent upon the ejection phase, the best value of which was zero as indicated in fig. 60,,
1
2. When shifting the counting window to set it at one boundary of the statistical ion packet, it is possible completely to avoid ion counting for some channels by TOF selection as shown on fig. 4, where, furthermore, we have adjusted V, and V, for the ratio f,/S, to be an integer (heref,= -$). We can see that the channels numbered 4+3n with n=O to 12 are practically empty, the background noise being, moreover, undetectable. We note that for n> 12, the
COUNTS
per
channel
Fig. 4. Total number of SF; ions counted with T,= 12.5 us, UO=- 1 V, Vi,= 39 V and for 500 CICE cycles. The counting window is shifted towards a boundary of the ion packet. For some channels, no ion falls inside the counting window.
629
Volume 174, number 6
CHEMICAL PHYSICS LETTERS
channels are tilling up indicating a gathering of the ions towards the center of the trap during the confinement. This is attributed to a cooling of the ions by collisions with residual vacuum particles. The curve of fig. 3b shows clearly this behaviour too, the decrease in amplitude of the tops of the upper envelope of the modulation being compensated by the increase in amplitude of the troughs of the lower one for a given mean value of the amplitude (amplitude of the nodes of the modulation). In conclusion, by appropriate electronic devices and timing sequences leading to a very good temporal and phase reproducibility of the CICE cycles, it is possible to determine the secular frequency S,of confined ions by TOF of ejected ions. And, as _Lis related to the mass of the ions, mass analysis of stored ions may be achieved by this TOF method. When ions of different masses are simultaneously confined, it is obvious that the method using the relation (7) becomes hazardous compared to the Fourier analysis of the curves. In addition, in our experimental conditions, the confinement and the ejection of ions practically did not show a significant influence of the initial phase &,,or of the ejection phase.
References [ 1 ] P.H. Dawson, ed., Quadrupole mass spectrometty and its application (Elsevier, Amsterdam, 1976); R.E. March and R.J. Hughes, Quadrupole storage mass spectrometry (Wiley-Interscience, New York, 1989). [ 21 P.H. Dawson and C. Lambert, Intern. J. Mass Spectrom. Ion Phys. I4 ( 1974) 339. [3] R.M. Waldren and J.F.J. Todd, Intern. J. Mass Spectrom. JonPhys. 31 (1979) 15.
630
23 November 1990
[4] R.M. Waldrcn and J.F.J. Todd, Intern. J. Mass Spectrom. Ion Phys. 29 (1979) 315. [ 51 J.F.J. Todd and R.M. Waldren, Intern. J. Mass Spectrom. Ion Phys. 29 (1979) 301. [ 61 J.E. Fulford and R.E. March, Intern. J. Mass Spectrom. Ion Phys. 30 (1979) 39. [ 7 ] S.-S. 0 and H.A. Schuessler, Intern. J. Mass Spectrom. Ion Phys.40 (1981) 53. [ 81 M. Nand Kishorc and P.R. Ghosh, Intern. J. Mass Spectrom. Ion Phys. 29 (1979) 345. [9] PK. Ghosh, AS. Arora and L. Narayan, Intern. J. Mass Spectrom. Ion Phys. 23 ( 1977) 237. [IO] J.F.J. Todd, D.A. Freer and R.M. Waldren, Intern. J. Mass Spectrom. Ian Phys. 36 ( 1980) 37 1. [ 111 C-S. 0 and H.A. Schuessler, J. Appl.Phys. 52 (198 1) 1157. [ 121J.E. Curtis, A. Ramar, R.E. March and U.P. Schlunegger, Proceedings of the 35th Annual Conference of the American Society of Mass Spectrometry, Denver, CO (24-29 May 1987) pp. 237,238. [ 131J.N. Louris, J.W. Amy, T.Y. Ridley and R.G. Cooks, Intern. J. Mass Spectrom. Ion Phys. 88 ( 1989) 97. [ 141 H.A. Schuessler, C.-S. 0 and H.S. Lakaraju, Physica Scripta T3 (1983) 27. [ 151 E.R. Mosburg Jr., M. Vedel, Y.Zerega, F. Vedel and J. Andre, Intern. J. Mass Spectrom. Ion Phys. 77 ( 1987) I. [ 161M. Vedel, J. Andre, G. Brincourt, Y. Zerega, G. Werth and J.P. Schermann, Appl. Phys. B 34 ( 1984) 229. [ 171 G. Brincourt, S. Rajab Pacha, R. Catella, Y. Zcrega and 1. Andre, Chem. Phys. Letters 156 ( 1989) 573. [ 181 N.W. McLachlan, Theory and applications of Mathieu functions (Clarendon, Oxford, 1947). [ 191 R. Campbell, Thtorie g&i&ale de l’tquation de Mathieu (Masson, Paris, 1955). [ 201R.F.Bonner, Intern. J. Mass Spectrom. Ion Phys. 23 ( 1977) 249. [ 2 I] R.F. Wuerker, H. Shelton and R.V. Langmuir, J. Appl. Phys. 30 (1969) 342.
CHEMICAL PHYSICS LETTERS
23 November 1990
Time-of-flight detection of ions ejected from a radiofrequency quadrupole trap: experimental determination of their fi secular frequency G. Brincourt, R. Catella, Y. Zerega and J. And& Laboratoire des Interactions Ioniques et Mokwiaires (Unite de Recherche AssociPeau CNRS No. 773). Universitkde Provence, Centre de Saint JtWme, Case A61, 13397 Marseille Ceakx 13, France Received 11 May 1990; in final form 6 September 1990
We have studied the temporal behaviour of a very small number of ions injected into a quadrupole trap and detected by ejection. The sampling of the movement of the ions in the trap, associated with a time-of-flight detection, leads to an ion s@nal which gives thef, secular frequency.
1. Introduction Withdrawal of ions from a radiofrequency quadrupole trap has become an increasingly wide subject of study due to the possibility of mass separation and detection with or without an external mass analyser *‘. Further, it has been shown [2] that the motion of ions during ejection from the trap is complex when applying a pulse to the end-cap electrodes through which the ions are ejected. Observations were made on the dependence of the efficiency of ion ejection with the amplitude [ 31 and the width [ 41 of the pulse, with the phase of the rf field at which the pulse is applied to the caps [ 5 1, and with the end-cap mode ejection [6]. On the other hand, many investigations have been carried out on the injection of ions in a trap; and the efficiency of confinement of such injected ions have been discussed in terms of various ion-injection situations (ring, cap, or asymptotic injection) [ 7-141. Concerning the ejection of the ions from the trap, improvements in electronic devices like rf signal gating and opposite ejection pulses applied to the end-cap electrodes have reduced the complexity of the ejected ion motion. However, to our knowledge, very few investigations have been reli’ For a general account of the theory and applications of rf quadrupole devices, see ref. [ 11.
626
ported into the experimental determination of the spatio-temporal position of ions in a trap and the sampling volume [ I 5 1. In the following, we report on an experimental study on ejected SF, molecular ions from a quadrupolar ion trap, based on the time-of-flight (TOF) analysis of the ions after the ejection pulse has been turned on for different confinement times. The SF; molecular ions are created outside the trap by electron attachment in collisions between SF, molecules and xenon Rydberg atoms Xe** according to the reaction SF6 +Xe**+SF,- +Xe+ . The choice of these ions results from parallel studies on their lifetime [ 16,171 but, evidently, many other ions may be used provided, as here, that they are few in number, and initially located in a very small volume in the trap.
2. Theory We briefly report below the main theoretical considerations we need. In our experiment, the radiofrequency electric field employed by the quadrupole trap to confine the ions is generated by applying a rf voltage V, cos S2tto both end caps made with a wire
0009-2614/90/$03.50 0 1990 - Ekvier Science Publishers B.V. (North-Holland)
Volume 174. number 6
CHEMICAL PHYSICS LETTERS
mesh, and a continuous voltage V, to the ring as shown in fig. 1. In the scheme, the voltages applied to the caps come from separated circuits for potential modification required for ion injection in the trap through the lower end cap (LEC), and ion ejection from the trap through the upper one (UEC). Such a biasing of the electrodes was decided in earlier experiments in order to keep the ring electrode to the low voltage V0 (a few volts) to prevent field ionization of the Xe** atoms when entering the trap through this electrode. The potential in the trap is then:
that is the phase of the alternating field when the ion first experiences its influence. As we are mainly interested in the ion stability in the z direction and have assumed a singly charged positive ion of mass m, it follows that a, and qz are defined in our case by a,=-
-
4eUo
rnziQ2
(1)
where z and I (r2=x2+y2) are the coordinates, referenced to the center of the trap, of a confined ion, z being on the symmetry axis of the trap. The relative position of the ring and end-cap electrodes is such so that ri =2zg (fig. 1). When the initial angular velocity of the ion around the z symmetry axis is zero, the equation of ion motion takes the Mathieu form [ 18-201:
(2) where U=X, y, or z and c= 4Qt. & is the initial phase, AMP. COUNTING
MULTIPLIER
UPPER
If I
/
CONTROL GRID v d.c.>O
and
qZ=--
2eVo mz@2
’
Connected relations are obtained for a, and qr. The secular frequency in the z direction is given by the fundamental frequency of the z motion: fi = BZQ/47r,
tu,+V,cosm],
23 November 1990
(4)
where BZis a complex function of a, and qZ. When q&0.4, an approximation for fiZis [ 2 1 ] /?Z=(aZ+;q:)“’
(5)
and then (6)
Thus, the projection of the trajectory of the trapped ion in a plane containing the z axis is generally a Lissajous curve composed of the two corresponding frequencies and with a superimposed micromotion of the rf ripple at the frequency G/2x. The a, and qu parameters must he inside the stability diagram for the ion to be indefinitely trapped, that is with finite and stable trajectories in both the r and z directions inside the trap. The boundaries of the diagram in a, and g. coordinates are known, so one deduces from (3) the values of V. and V. to be used for trapping, with given values of m, z. and L?
-_____/
3. Experimental U.
+ EJECTION
dcc. VOLTAGE
PULSE
GRID INJECTION (IP)
a\ PLATE k
h
Fig. 1. Schematic arrangement of the biasing and command voltages.
INJECTION
PULSE
trap with its associated
Fig. 1 shows the trap arrangement. A SF6 molecular beam and a pulsed Xe** atomic beam interact between the injection plate (IP) and the LEC of the trap. With the gas beams, the pressure is of the order of 8 x lo-’ Torr in the trap, while the density is about 2 x 10” particles crnm3 in the crossed-beam region. The Xe** atoms are created by pulsed electron excitation of a xenon beam in a second vacuum chamher, and when the density of Xe** atoms is maximum under the trap, the SF; ions created are axially 627
injected into the trap by a negative voltage pulse (NVP) of circa - 5 V applied to the IP electrode, the rf voltage applied to the end caps being off, and that applied to the R electrode having its normal value (0 to - 1.5 V ). Fig. 2 shows the different voltage-timing sequences. After a time delay r, following the leading edge of the NVP applied to IP, a positive voltage pulse of circa 5 V is applied to the LEC electrode for a time Tz to decelerate the ions once in the trap (cooling). The rf voltage of frequency 160 kHz is then turned on for confinement, with a given amplitude in the range of 25 to 80 V. It is obvious that the length and the amplitude of the previous voltage pulses are of great importance for suitable confinement of the ions. After a defined confinement time, the ions are ejected by two opposite voltage pulses of circa 100 V applied to the LEC and UEC electrodes and superimposed on the rf voltage with a given relative ejection phase. An adjustable positive voltage is applied to the control grid to have the best ion-collection efficiency. The ejected ions are detected by an electron multiplier followed by an
L-d
beam pulse
+iOV
electro?
amplifier and a single-channel analyser associated with a temporal window the width and position of which select the ions by TOF for counting. Due to the very small number of created ions, many cycles of creation-injection-confinement-ejection (CICE ) are performed for each channel and the results added. For pulse-timing sequence and data acquisition, a PDP 1 l-03 minicomputer was used. All the signals are strictly phase synchronised and adjusted in length and amplitude. For each CICE cycle, the number of the channel concerned in the ejection is incremented, the channel width T, being 12.5 or 25 us. Since these values are compatible with the range of the secular frequencyf, of the SF; ions in the trap, it is possible to count the ions only when they are in a well-defined place on the secular trajectory by adjusting the time delay T3 and the width Tw of the counting window. Thus, we have a sampling of the z(t) coordinate of the ions with the frequencyf,= l/T,, and we always operate with fs/fi= k? E with k an integral number >2 and ec 1.
4. Results and discussion
one CICE cycle
electron
and nepive
ions
scavenger positive vottage
Fig. 3a shows a typical curve of confinement with the sampling effect. Since the ratioS,/f, is not an integer, stroboscopic effects are visible. Here we have k=4, and so we have four lines of sinusoidal amplitude modulation with the same frequencyf,; then fr =f,lk+fm
Counting! window
1
Fig. 2. Pulse timing sequences for operation ofthe trap. For practical reasons, the signals are not to the same temporal scale. T, is the time of deceleration of the injected ions.
628
23 November 1990
CHEMICAL PHYSICS LETTERS
Volume 174. number 6
.
(7)
SinceS, is connected with /I=,(4)) the sign must be considered before f, is deduced from the evolution offm with U0 or V,. In the case of fig. 3a, we have f5= 80 kHz and the curve gives directly f, x 676 Hz; we deduce fi = 19.8 kHz while the calculations from (6 ) and ( 3 ) and Fourier analysis give, respectively, f,=20.2 and 19.5 kHz, that is, an error of less than 4% between the results. Fig. 3b shows a curve obtained under the same experimental conditions as the previous one except for fs which has a value half the previous one, i.e. 40 kHz; here k=2 and from the determination off, we deduce_& 19.7 kHz in very good agreement with the values above. Due to the narrowness (0.4 &s) and the position of the counting window, we count only
Volume 174, number 6
CHEMICAL PHYSICS LETTERS
23 November 1990
1
COUNTS
I.
CHANNELS
Id
CICE cycles per channel
128
Fig. 3. Total number of SFz ions counted in each of the I28 channels for 1000 CICE cycles and with Ue= -0.5 V and Vo= 30 V. (a) Channel width T, is 12.5 us: the dashed line is one of the four amplitude modulation lines (k=4); (b) T,=25 us (k=2).
circa 0.5 ion per CICE cycle; hence, for good visibility, each channel is concerned, here, with lo3 CICE cycles. One can note the high degree of reproducibility, sensitivity and noise immunity of the experiment. As expected, no significant influence of the initial phase &,has been noticed, and the number of ejected ions was weakly dependent upon the ejection phase, the best value of which was zero as indicated in fig. 60,,
1
2. When shifting the counting window to set it at one boundary of the statistical ion packet, it is possible completely to avoid ion counting for some channels by TOF selection as shown on fig. 4, where, furthermore, we have adjusted V, and V, for the ratio f,/S, to be an integer (heref,= -$). We can see that the channels numbered 4+3n with n=O to 12 are practically empty, the background noise being, moreover, undetectable. We note that for n> 12, the
COUNTS
per
channel
Fig. 4. Total number of SF; ions counted with T,= 12.5 us, UO=- 1 V, Vi,= 39 V and for 500 CICE cycles. The counting window is shifted towards a boundary of the ion packet. For some channels, no ion falls inside the counting window.
629
Volume 174, number 6
CHEMICAL PHYSICS LETTERS
channels are tilling up indicating a gathering of the ions towards the center of the trap during the confinement. This is attributed to a cooling of the ions by collisions with residual vacuum particles. The curve of fig. 3b shows clearly this behaviour too, the decrease in amplitude of the tops of the upper envelope of the modulation being compensated by the increase in amplitude of the troughs of the lower one for a given mean value of the amplitude (amplitude of the nodes of the modulation). In conclusion, by appropriate electronic devices and timing sequences leading to a very good temporal and phase reproducibility of the CICE cycles, it is possible to determine the secular frequency S,of confined ions by TOF of ejected ions. And, as _Lis related to the mass of the ions, mass analysis of stored ions may be achieved by this TOF method. When ions of different masses are simultaneously confined, it is obvious that the method using the relation (7) becomes hazardous compared to the Fourier analysis of the curves. In addition, in our experimental conditions, the confinement and the ejection of ions practically did not show a significant influence of the initial phase &,,or of the ejection phase.
References [ 1 ] P.H. Dawson, ed., Quadrupole mass spectrometty and its application (Elsevier, Amsterdam, 1976); R.E. March and R.J. Hughes, Quadrupole storage mass spectrometry (Wiley-Interscience, New York, 1989). [ 21 P.H. Dawson and C. Lambert, Intern. J. Mass Spectrom. Ion Phys. I4 ( 1974) 339. [3] R.M. Waldren and J.F.J. Todd, Intern. J. Mass Spectrom. JonPhys. 31 (1979) 15.
630
23 November 1990
[4] R.M. Waldrcn and J.F.J. Todd, Intern. J. Mass Spectrom. Ion Phys. 29 (1979) 315. [ 51 J.F.J. Todd and R.M. Waldren, Intern. J. Mass Spectrom. Ion Phys. 29 (1979) 301. [ 61 J.E. Fulford and R.E. March, Intern. J. Mass Spectrom. Ion Phys. 30 (1979) 39. [ 7 ] S.-S. 0 and H.A. Schuessler, Intern. J. Mass Spectrom. Ion Phys.40 (1981) 53. [ 81 M. Nand Kishorc and P.R. Ghosh, Intern. J. Mass Spectrom. Ion Phys. 29 (1979) 345. [9] PK. Ghosh, AS. Arora and L. Narayan, Intern. J. Mass Spectrom. Ion Phys. 23 ( 1977) 237. [IO] J.F.J. Todd, D.A. Freer and R.M. Waldren, Intern. J. Mass Spectrom. Ian Phys. 36 ( 1980) 37 1. [ 111 C-S. 0 and H.A. Schuessler, J. Appl.Phys. 52 (198 1) 1157. [ 121J.E. Curtis, A. Ramar, R.E. March and U.P. Schlunegger, Proceedings of the 35th Annual Conference of the American Society of Mass Spectrometry, Denver, CO (24-29 May 1987) pp. 237,238. [ 131J.N. Louris, J.W. Amy, T.Y. Ridley and R.G. Cooks, Intern. J. Mass Spectrom. Ion Phys. 88 ( 1989) 97. [ 141 H.A. Schuessler, C.-S. 0 and H.S. Lakaraju, Physica Scripta T3 (1983) 27. [ 151 E.R. Mosburg Jr., M. Vedel, Y.Zerega, F. Vedel and J. Andre, Intern. J. Mass Spectrom. Ion Phys. 77 ( 1987) I. [ 161M. Vedel, J. Andre, G. Brincourt, Y. Zerega, G. Werth and J.P. Schermann, Appl. Phys. B 34 ( 1984) 229. [ 171 G. Brincourt, S. Rajab Pacha, R. Catella, Y. Zcrega and 1. Andre, Chem. Phys. Letters 156 ( 1989) 573. [ 181 N.W. McLachlan, Theory and applications of Mathieu functions (Clarendon, Oxford, 1947). [ 191 R. Campbell, Thtorie g&i&ale de l’tquation de Mathieu (Masson, Paris, 1955). [ 201R.F.Bonner, Intern. J. Mass Spectrom. Ion Phys. 23 ( 1977) 249. [ 2 I] R.F. Wuerker, H. Shelton and R.V. Langmuir, J. Appl. Phys. 30 (1969) 342.