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External trapped ion source for ion cyclotron resonance spectrometry
International Journal of Mass Spectrometry and Zon Processes, 87 (1989) 237-247 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
237
EXTERNAL TRAPPED ION SOURCE FOR ION CYCLOTRON RESONANCE SPECTROMETRY
P. KOFEL
*, M. ALLEMANN
and HP. KELLERHALS
Spectrospin AG., Fiillanden (Switzerland) K.P. WANCZEK Inorganic Chemistry, (Received
University of Bremen, Bremen (Federal Republic of Germany)
16 July 1987)
ABSTRACT An external trapped ion source employs the stray magnetic field of the superconducting magnet of an ion cyclotron resonance (ICR) spectrometer to accumulate and to trap ions for several hundred milliseconds. A pulsed transfer scheme is used to transfer the ions as a spatially confined cloud to the ICR cell. The trapping and the transfer of the ions are studied. The ions can oscillate between the ion source and the ICR cell because the grounded vacuum system also shows trapping properties. By an accumulation of the ion signals, the signal-tonoise ratio and the sensitivity of the method can be increased strongly. The theoretical signal-to-noise values for an ICR spectrometer coupled to a gas chromatograph are compared for four configurations: single cell, dual cell, external ion source and external trapped ion source.
INTRODUCTION
The applications of ion cyclotron resonance spectrometry have been greatly extended in recent years [l]. In addition to the very successful application of the method to the study of ion/molecule reactions and light-induced phenomena of trapped ions, the first analytical applications are now also being seen. This has been brought about mainly by three instrumental developments: the introduction of pulse spectrometry with a trapped ion ICR cell by McIver [2], of the Fourier transform technique by Comisarow and Marshall [3], and of high field superconducting magnets by Allemann et al. [4]. However, the use of the ICR method is complicated by
* Present address: Department of Chemistry and Guelph-Waterloo Research Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ont. N2L 3G1, Canada.
016%1176/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
238
the very low pressure, below lo-’ mbar, necessary in the analyser cell to obtain very high resolution, one of the major advantages of the technique. Therefore it is difficult, if not impossible, to incorporate techniques such as chromatography which operate with heavy gas loads. The ICR cell is only difficult to access because it is located in the room temperature bore of the superconducting magnet. Three different approaches are described in the literature to overcome one or both of these difficulties. The first is the dual cell introduced by Ghaderi and Littlejohn [5]. One ICR cell is the source cell where the ions are generated, while the second is the analyser cell. With the dual cell, differential pumping is possible and the analyser cell can be kept at low pressure. With this instrument the ions can be trapped in both the source and the analyser cells. However, both cells are difficult to access. The second approach is the tandem quadrupole ICR spectrometer introduced by McIver et al. [6]. The ions are generated outside the magnet and guided by a quadrupole mass spectrometer through the fringing fields into the magnet to the ICR cell. A second quadrupole located directly behind the ion source can be utilized for mass analysis. All three stages of the instrument are differentially pumped. This tandem spectrometer is very versatile but also very complicated. Kofel et al. [7] introduced an external ion source without any guiding structures and showed that this instrument has a high transmission efficiency in spite of the simple construction. In this instrument, with an external source without trapping properties, only a portion of the ions generated in the ion source can be trapped in the ICR cell, even at 100% transfer efficiency. The duty cycle can be characterized by the ratio of two times: the flight time of the ions through the ICR cell and the ionization time applied to the external source. In this work an external trapped ion source is presented which employs the residual magnetic field of the magnet to trap the ions. Therefore accumulation of ions in the external ion source is possible before the ions are injected into the ICR cell. This greatly improves the sensitivity and the dynamic range of the technique. EXPERIMENTAL
The basic ICR spectrometer has been described in detail in the literature [4,9]. Therefore only a brief description will be given here. The vacuum system consists of two chambers which are pumped differentially by two turbomolecular pumps [lo]. The two compartments are connected by an orifice of typically 1 mm in diameter and a pressure differential of a factor of 1000 can be maintained. The base pressure is below 10P9 mbar. The part of the vacuum chamber which contains a cylindrical ICR cell of radius 6 cm
239
A
FXi--
I I G
B
I
4 /IFF _______________--!__________!--_--./-_ I” El i rl 1
z
S
Q
II R
Fig. 1. Schematic diagram of the ICR spectrometer with the external trapping ion source: (A) external trap; (B) ICR cell. The broken line represents the magnet corner. For other details see text.
and length 6.1 cm fits into the room temperature bore of diameter 150 mm of a 4.7 T superconducting magnet. The spectrometer is controlled by an Aspect 3000 minicomputer with a 24-bit word length equipped with an array processor for fast Fourier transform and a fast A/D converter with a sampling frequency of 20 MHz and a buffer memory of 124 K. The external trapped ion source has the same dimensions as the ICR cell. The experimental arrangement is shown in Fig. 1. The trapping ion source was located approximately 10 cm outside the room temperature bore of the magnet and the distance to the ICR cell was 52 cm. The residual magnetic field at the site of the external trap was 0.15 T. The external trap was equipped with a filament F which was operated at a filament current of 3.5 A. The electron energy was 70 eV. At the anode K an electron current of 2 PA was measured. The electron beam could be pulsed with the grid G and for the experiments a pulse of 20 ms duration was employed. Before an experiment all the ions outside the trapped ion source were quenched. All side electrodes (parallel to the direction of the magnetic field) were at the same potential. The two trapping electrodes were at the trapping potential,
Fig. 2. Pulse sequence employed for the external trap. EJ is the ion ejection pulse of the external trap. QE, TR and DE are the quench, trap open and detection pulses of the ICR cell, respectively. At is the difference in duration between the pulses TR and EJ.
240
and had holes 4 mm in diameter (E) and 8 mm in diameter (T). The ions were extracted by pulsing the trapping electrode E. The ICR cell was equipped with a grounded shielding electrode S, a Faraday cup K (anode) and a ramp electrode R. For the injection of the ions the front trapping plate T of the ICR cell was “opened” and “closed” by pulsing it to ground and to the trapping potential, respectively. The duration of this pulse is TR in the pulse sequence and is shown in Fig. 2. The pulse for the ejection of the ions from the external trap EJ starts at the same time as TR but is Aat shorter. By scanning At, time-of-flight ICR can be obtained. This method has been described in detail elsewhere [8]. RESULTS AND DISCUSSION
On their way to the collector K the electrons generate ions in both the external trapping ion source and the ICR cell and in the vacuum system between the two cells. For the study of the external trap, first all the ions in the vacuum system and the ICR cell have to be quenched by pulsing the trapping plates of the ICR cell. To evaluate the optimum conditions of ion transfer, the dependence of the ICR signal intensity on the potential of the external trap and on the transfer voltage, and the influence of ion oscillations have been studied. The molecular ion of benzene, m/z = 78, was used. For the study of the dependence of the relative signal intensities as functions of the external trap voltage and of the transfer voltage, the extraction potential of the external trap and the delay TR between ejecting the ions out of the external trap and closing the ICR cell were optimized for each measurement. Figure 3 shows the relative intensities of m/z = 78 as a function of the potential of the external trap. In the potential range from 0 to 8 V a pronounced optimum is found at 1 V. All the other potentials of the external trap give lower transfer yields. The variation in the potentials of the external trap, V,, and of the ICR cell, Yic., keeping the difference constant at 0.3 V, gives the ion yield as a function of the transfer voltage, which is approximately proportional to the transfer energy. The results are shown in Fig. 4. The transfer yield is increased by using a transfer voltage of a few volts and has a broad maximum. For the determination of the absolute transfer yields an ICR spectrum is recorded with the same experimental settings. The ions are generated in the ICR cell alone, By comparison with the maximum signal in Fig. 4 a yield of 12% was calculated. This indicates that the ions are trapped very effectively in the external trap and are transferred with great efficiency. Figures 5 and 6 show the time dependence of the transfer process. Spectra of the benzene molecular ion, m/z 78, have been recorded at different
241
r
I 0
1
2
Fig. 3. Relative abundances external ion trap.
3
4
of the ion C,Hl
5
, m/z
6
7
8
= 78, as a function
CW
of the potential
of the
values of At, the difference in duration between the ICR cell open and external trap ejection pulses. In Fig. 5 a transfer voltage of 0.3 V and in Fig. 6 a transfer voltage of 8 V has been applied. It is seen from the figures that
0
Fig. 4. Relative
1
abundances
2
3
4
5
6
7
of ion C,Hz , m/z = 78, as a function
8
CVI
of the transfer
voltage.
242
0.3
0.1
05
0.6
018
07
091 25
tCmsl
Fig. 5. Relative abundances of the benzene molecular ion, m/z = 78, as a function of TR. The transfer voltage is 0.3 V.
the ions oscillate between the external trap and the ICR cell if At is large enough. Owing to diffusion, reflection by the magnetic field, the long distance ‘which the ions travel between the two cells and the low magnetic
0. I
08
1.2
1.6
2.0
2.1
2.8
3520
tCmsI
Fig. 6. Relative abundances of the benzene molecular ion, m/z = 78, as a function of TR. The transfer voltage is 8 V.
243
field at the site of the external trap, many ions are lost in the oscillations. The ion packet which is originally concentrated in the external trap diffuses also in the direction parallel to the magnetic field, because of this energy distribution. The first arrival of the ion packet in the ICR cell yields the largest ion intensities at both transfer energies. The duration of maximal signal intensity depends, as expected, on the transfer voltage. For a transfer voltage of E,, = 0.3 V it is 0.3 ms, for E,, = 8.3 V it is 0.1 ms. With time-of-flight measurements the residence time of ions with m/z = 78 in the ICR cell has been determined to be approximately 0.1 ms [8]. Therefore the duration of the maximum in Fig. 6 at the high transfer voltage is determined by the residence time of the ions in the open ICR cell. This is not the case for the low transfer voltage in Fig. 5. Here the ion cloud already has suffered diffusion in its flight direction, because the energy distribution of the ions is more important at low transfer energies. Oscillation of the ion cloud also has been observed for the dual cell by Giancaspro et al. [ll]. In this case the reduction in intensity during the oscillations is much smaller because both cells are located close together in the region of the approximately homogeneous, strong magnetic field. THEORETICAL COMPARISON OF THE SENSITIVITY OF THE EXTERNAL WITH OTHER ICR METHODS IN GC-ICR EXPERIMENTS
TRAP
The coupling of ICR spectrometry with gas chromatography (GC) [12] is difficult because a large amount of carrier gas contains only a small amount of sample, and the heavy gas load increases the pressure in the ICR cell. Therefore trapping time and resolution are degraded. The width of a gas chromatographic peak is of the order of 1 s. Therefore several ICR spectra can be recorded for each gas chromatographic peak. To reduce the heavy gas load, several methods have been published in the literature. One procedure is to split the flow of effluent or to use separation techniques [12]. Only a portion of the gas is allowed to enter the ICR cell; the rest is lost. With a pulsed valve [13], gas is allowed to enter the ICR cell only during the ionization period. Again a large portion of the sample is lost. Use of a pulsed valve is complicated with superconducting magnets. Furthermore, a dual cell configuration [5,14] and external ionization [16] can be employed. In the dual cell the two cells are differentially pumped. They are connected by a common trapping electrode with a small hole. A pressure differential can thus be maintained. The ions are generated and trapped at high pressure in the source cell and they are transferred through the hole from the ion source cell with the high pressure to the analyser cell with the low pressure. By this method the applicability of the GC-ICR instrument is greatly enhanced. The size of the hole, however, limits this method. In order
244
to have a high efficiency in the transfer of the ions from the source cell to the analyser cell the hole has to be large because the ions diffuse from the centre of the cell after generation. To obtain a large pressure gradient between the source and the analyser cell, however, the hole must be as small as possible. For the spectrometer employed in this study the optimum diameter of the hole has been calculated to be approximately 4 mm [16]. With the external source the pressure problem can be solved by adding further differential pumping stages. However, the sensitivity is limited by the short residence time of the ions in the open ICR cell. In an external ion trap the pressure problem can be solved in the same way as with an external source. The sensitivity is greatly enhanced because the ions can be accumulated and transferred to the ICR cell after accumulation. The signal-to-noise ratios in the frequency domain, S/N,, will now be compared theoretically for four configurations: GC capillary with split, introduced’directly into a single ICR cell; dual cell with split; external ion source with continuous extraction; and external trap with discontinuous extraction. The S/N, value for a compound which passes out of the GC and gives upon ionization only a single kind of ion (e.g., the molecular ion) is given by S/N, = E4 (TD)“’ Np,W where TD is the effective transient duration, N is the number of ions generated per millibar, ps is the pressure in millibar in the ion source and is given by
Pq=qdL IV is the sample-to-carrier
w=-
gas ratio given by
N,RT qpvTT
where n is the split ratio, qPv is the carrier gas flow through the capillary, L is the conductance of the ion source, NT is the total number of molecules injected into the GC, TT is the total duration of the GC peak under consideration, R is the gas constant and T the absolute temperature. E, is the sensitivity factor which is constant for each instrument. It is given by E = 9
K’,(N=1, 4, Pop,) VN
245
wherepop1 is
the optimal radius of ion orbit, determined experimentally and V, is the noise voltage (n V(Hz)-‘I*) of the preamplifier. The signal voltage V, for a capacitively coupled preamplifier is
where N is the number of ions of the same mass, orbiting with radius p -=scr with charge q (rotating monopole), and r is the cell radius. The capacity C is the sum of the cell capacity and the capacities of the feedthroughs and leads between preamplifier and ICR cell. SL, is a geometry factor. (For the nomenclature, see ref. 17.) The theoretical results for the four arrangements are listed in Table 1. A total cycle time of 100 ms was chosen. Because the ionization time for the external source is negligible, two scans can be performed in 100 ms. The effective detection time TD is 50 ms. This corresponds to unit resolution at m/z = 1900. Therefore the pressure in the detection ICR cell has to be lower than approximately 2 X lo-’ mbar. The electron current was 1 mA in all the calculations. The number of ions N produced by electron impact can be calculated according to
where e is the elementary charge, Tr is the ionization duration, 1, is the electron current, I, is the length of the trapped ion source (the path length of the ionizing electrons), p is the partial pressure of the neutral gas and S,
TABLE 1 Comparison of the theoretical signal-to-noise ratios for different ICR spectrometric configurations. (For details see text) Parameter
Single cell
Dual cell
External source
External trap
Pressure differential
-
Differential pumping
Differential pumping
T, (s) I, (cm) pq (mW L (1 s-l)
0.05 6 2x10-7
Hole 4 mm in diameter, 100 1 s-i 0.05 6 1.4x10-5 100 1: 3.6 70 10000
3x10-4 0.6 5x10-3 1 1:l 15 2100
0.05 6 5x10-4 10 1:l 2500 350000
n
N* S/N,
30 1: 830 1 140
246
is the ionization yield. Ions formed from the carrier gas are not considered. They can be ejected after closing of the ICR cell, or trapping can be avoided using the time-of-flight method [8]. In Table 1 the ratios of the total numbers of ions which are generated by electron impact during the ionization period are given. The signal-to-noise ratios, S/N,, also shown in Table 1 are proportional to the total numbers of ions. The other fixed parameters were Eq = 0.027 ion- ’ s -‘I2 (broad band mode [16]), q = 5 X 10e3 mbar 1 s-1, N = 3.33 x lo-l2 mol (1 ng sample with molecular mass of 300), T = 300 K, TT = 2 s and therefore W = 8.3 x 10-6. The pressure for external ionization has been chosen so that thermal ions can leave the ion source without collision. The values in Table 1 are theoretical values. They are the maximum values which can be reached without any ion losses through imperfect trapping or during transfer and with neglect of space charge effects. For high resolution the split factor has to be reduced and the acquisition time has to be increased. The table shows that the sensitivity increases drastically in the order single cell, external source, dual cell, external trap. The external source with continuous extraction is placed between the single cell and double cell because the path of the ionizing electrons is short and there is no possibility of ion accumulation. GC measurements are possible with all four arrangements, as has been shown experimentally for the first three; the fourth configuration with an external trap should be much more sensitive than the other three. However, this has to be proved experimentally. Like the external ion source, the external trapped ion source can be operated at higher electron currents compared with an ICR cell with an electron gun located inside the magnet at high magnetic field.
CONCLUSIONS
The ions stored in the external ion trap can be transferred as a spatially confined cloud to the ICR cell. The external ion trap described in this paper employs only the stray field of the superconducting magnet to trap the ions and has therefore to be located very close to the magnet. If the ion trap is moved away from the magnet to allow improved differential pumping, the trapping time in the external trapped ion source is rapidly reduced. In this case either a second magnet or another storing device such as a radio frequency ion trap has to be used. The external trapped ion source combines the advantages of the external ion source and the dual ICR cell. The theoretical limit of the detectable partial pressure is increased by at least one order of magnitude compared with all other ICR cell designs.
247 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
K.P. Wanczek, Int. J. Mass Spectrom. Ion Processes, in press. R.T. McIver, Rev. Sci. Instrum., 41 (1970) 555. M.B. Comisarow and A.G. Marshall, Chem. Phys. Lett., 25 (1974) 282. M. Allemann, Hp. Kellerhals and K.P. Wanczek, Chem. Phys. Lett., 75 (1980) 328. S. Ghaderi and D.P. Littlejohn, Proc. 33rd Annu. Conf. Mass Spectrom. Allied Top., San Diego, CA, 1985, p. 727. R.T. McIver, R.L. Hunter and W.D. Bowers, Int. J. Mass Spectrom. Ion Processes, 64 (1985) 67. P. Kofel, M. Allemann, Hp. Kellerhals and K.P. Wanczek, Int. J. Mass Spectrom. Ion Processes, 65 (1985) 97. P. Kofel, M. Allemann, Hp. Kellerhals and K.P. Wanczek, Int. J. Mass Spectrom. Ion Processes, 72 (1986) 53. M. Allemann, Hp. Kellerhals and K.P. Wanczek, Int. J. Mass Spectrom. Ion Phys., 46 (1983) 139. Balzen Hochvakuum GmbH, Siemenstrasse 11, D-42 Wiesbaden-Nordenstadt, F.R.G., Model TPU 330. C. Giancaspro, F.R. Verdun and J.F. Muller, Int. J. Mass Spectrom. Ion Processes, 72 (1986) 63. R.L. White and C.L. Wilkins, Anal. Chem., 54 (1982) 2443. T.M. Sack and M.L. Gross, Anal. Chem., 55 (1983) 2419. R.L. Settine, J.S. Kinsinger and S. Ghaderi, Eur. Spectrosc. News, 58 (1985) 16. P. Kofel, unpublished results. P. Kofel, Ph.D. Thesis, University of Bremen, 1987. P. Kofel, M. Allemann, Hp. Kellerhals and K.P. Wanczek, Int. J. Mass Spectrom. Ion Processes, 74 (1986) 1.
237
EXTERNAL TRAPPED ION SOURCE FOR ION CYCLOTRON RESONANCE SPECTROMETRY
P. KOFEL
*, M. ALLEMANN
and HP. KELLERHALS
Spectrospin AG., Fiillanden (Switzerland) K.P. WANCZEK Inorganic Chemistry, (Received
University of Bremen, Bremen (Federal Republic of Germany)
16 July 1987)
ABSTRACT An external trapped ion source employs the stray magnetic field of the superconducting magnet of an ion cyclotron resonance (ICR) spectrometer to accumulate and to trap ions for several hundred milliseconds. A pulsed transfer scheme is used to transfer the ions as a spatially confined cloud to the ICR cell. The trapping and the transfer of the ions are studied. The ions can oscillate between the ion source and the ICR cell because the grounded vacuum system also shows trapping properties. By an accumulation of the ion signals, the signal-tonoise ratio and the sensitivity of the method can be increased strongly. The theoretical signal-to-noise values for an ICR spectrometer coupled to a gas chromatograph are compared for four configurations: single cell, dual cell, external ion source and external trapped ion source.
INTRODUCTION
The applications of ion cyclotron resonance spectrometry have been greatly extended in recent years [l]. In addition to the very successful application of the method to the study of ion/molecule reactions and light-induced phenomena of trapped ions, the first analytical applications are now also being seen. This has been brought about mainly by three instrumental developments: the introduction of pulse spectrometry with a trapped ion ICR cell by McIver [2], of the Fourier transform technique by Comisarow and Marshall [3], and of high field superconducting magnets by Allemann et al. [4]. However, the use of the ICR method is complicated by
* Present address: Department of Chemistry and Guelph-Waterloo Research Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ont. N2L 3G1, Canada.
016%1176/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
238
the very low pressure, below lo-’ mbar, necessary in the analyser cell to obtain very high resolution, one of the major advantages of the technique. Therefore it is difficult, if not impossible, to incorporate techniques such as chromatography which operate with heavy gas loads. The ICR cell is only difficult to access because it is located in the room temperature bore of the superconducting magnet. Three different approaches are described in the literature to overcome one or both of these difficulties. The first is the dual cell introduced by Ghaderi and Littlejohn [5]. One ICR cell is the source cell where the ions are generated, while the second is the analyser cell. With the dual cell, differential pumping is possible and the analyser cell can be kept at low pressure. With this instrument the ions can be trapped in both the source and the analyser cells. However, both cells are difficult to access. The second approach is the tandem quadrupole ICR spectrometer introduced by McIver et al. [6]. The ions are generated outside the magnet and guided by a quadrupole mass spectrometer through the fringing fields into the magnet to the ICR cell. A second quadrupole located directly behind the ion source can be utilized for mass analysis. All three stages of the instrument are differentially pumped. This tandem spectrometer is very versatile but also very complicated. Kofel et al. [7] introduced an external ion source without any guiding structures and showed that this instrument has a high transmission efficiency in spite of the simple construction. In this instrument, with an external source without trapping properties, only a portion of the ions generated in the ion source can be trapped in the ICR cell, even at 100% transfer efficiency. The duty cycle can be characterized by the ratio of two times: the flight time of the ions through the ICR cell and the ionization time applied to the external source. In this work an external trapped ion source is presented which employs the residual magnetic field of the magnet to trap the ions. Therefore accumulation of ions in the external ion source is possible before the ions are injected into the ICR cell. This greatly improves the sensitivity and the dynamic range of the technique. EXPERIMENTAL
The basic ICR spectrometer has been described in detail in the literature [4,9]. Therefore only a brief description will be given here. The vacuum system consists of two chambers which are pumped differentially by two turbomolecular pumps [lo]. The two compartments are connected by an orifice of typically 1 mm in diameter and a pressure differential of a factor of 1000 can be maintained. The base pressure is below 10P9 mbar. The part of the vacuum chamber which contains a cylindrical ICR cell of radius 6 cm
239
A
FXi--
I I G
B
I
4 /IFF _______________--!__________!--_--./-_ I” El i rl 1
z
S
Q
II R
Fig. 1. Schematic diagram of the ICR spectrometer with the external trapping ion source: (A) external trap; (B) ICR cell. The broken line represents the magnet corner. For other details see text.
and length 6.1 cm fits into the room temperature bore of diameter 150 mm of a 4.7 T superconducting magnet. The spectrometer is controlled by an Aspect 3000 minicomputer with a 24-bit word length equipped with an array processor for fast Fourier transform and a fast A/D converter with a sampling frequency of 20 MHz and a buffer memory of 124 K. The external trapped ion source has the same dimensions as the ICR cell. The experimental arrangement is shown in Fig. 1. The trapping ion source was located approximately 10 cm outside the room temperature bore of the magnet and the distance to the ICR cell was 52 cm. The residual magnetic field at the site of the external trap was 0.15 T. The external trap was equipped with a filament F which was operated at a filament current of 3.5 A. The electron energy was 70 eV. At the anode K an electron current of 2 PA was measured. The electron beam could be pulsed with the grid G and for the experiments a pulse of 20 ms duration was employed. Before an experiment all the ions outside the trapped ion source were quenched. All side electrodes (parallel to the direction of the magnetic field) were at the same potential. The two trapping electrodes were at the trapping potential,
Fig. 2. Pulse sequence employed for the external trap. EJ is the ion ejection pulse of the external trap. QE, TR and DE are the quench, trap open and detection pulses of the ICR cell, respectively. At is the difference in duration between the pulses TR and EJ.
240
and had holes 4 mm in diameter (E) and 8 mm in diameter (T). The ions were extracted by pulsing the trapping electrode E. The ICR cell was equipped with a grounded shielding electrode S, a Faraday cup K (anode) and a ramp electrode R. For the injection of the ions the front trapping plate T of the ICR cell was “opened” and “closed” by pulsing it to ground and to the trapping potential, respectively. The duration of this pulse is TR in the pulse sequence and is shown in Fig. 2. The pulse for the ejection of the ions from the external trap EJ starts at the same time as TR but is Aat shorter. By scanning At, time-of-flight ICR can be obtained. This method has been described in detail elsewhere [8]. RESULTS AND DISCUSSION
On their way to the collector K the electrons generate ions in both the external trapping ion source and the ICR cell and in the vacuum system between the two cells. For the study of the external trap, first all the ions in the vacuum system and the ICR cell have to be quenched by pulsing the trapping plates of the ICR cell. To evaluate the optimum conditions of ion transfer, the dependence of the ICR signal intensity on the potential of the external trap and on the transfer voltage, and the influence of ion oscillations have been studied. The molecular ion of benzene, m/z = 78, was used. For the study of the dependence of the relative signal intensities as functions of the external trap voltage and of the transfer voltage, the extraction potential of the external trap and the delay TR between ejecting the ions out of the external trap and closing the ICR cell were optimized for each measurement. Figure 3 shows the relative intensities of m/z = 78 as a function of the potential of the external trap. In the potential range from 0 to 8 V a pronounced optimum is found at 1 V. All the other potentials of the external trap give lower transfer yields. The variation in the potentials of the external trap, V,, and of the ICR cell, Yic., keeping the difference constant at 0.3 V, gives the ion yield as a function of the transfer voltage, which is approximately proportional to the transfer energy. The results are shown in Fig. 4. The transfer yield is increased by using a transfer voltage of a few volts and has a broad maximum. For the determination of the absolute transfer yields an ICR spectrum is recorded with the same experimental settings. The ions are generated in the ICR cell alone, By comparison with the maximum signal in Fig. 4 a yield of 12% was calculated. This indicates that the ions are trapped very effectively in the external trap and are transferred with great efficiency. Figures 5 and 6 show the time dependence of the transfer process. Spectra of the benzene molecular ion, m/z 78, have been recorded at different
241
r
I 0
1
2
Fig. 3. Relative abundances external ion trap.
3
4
of the ion C,Hl
5
, m/z
6
7
8
= 78, as a function
CW
of the potential
of the
values of At, the difference in duration between the ICR cell open and external trap ejection pulses. In Fig. 5 a transfer voltage of 0.3 V and in Fig. 6 a transfer voltage of 8 V has been applied. It is seen from the figures that
0
Fig. 4. Relative
1
abundances
2
3
4
5
6
7
of ion C,Hz , m/z = 78, as a function
8
CVI
of the transfer
voltage.
242
0.3
0.1
05
0.6
018
07
091 25
tCmsl
Fig. 5. Relative abundances of the benzene molecular ion, m/z = 78, as a function of TR. The transfer voltage is 0.3 V.
the ions oscillate between the external trap and the ICR cell if At is large enough. Owing to diffusion, reflection by the magnetic field, the long distance ‘which the ions travel between the two cells and the low magnetic
0. I
08
1.2
1.6
2.0
2.1
2.8
3520
tCmsI
Fig. 6. Relative abundances of the benzene molecular ion, m/z = 78, as a function of TR. The transfer voltage is 8 V.
243
field at the site of the external trap, many ions are lost in the oscillations. The ion packet which is originally concentrated in the external trap diffuses also in the direction parallel to the magnetic field, because of this energy distribution. The first arrival of the ion packet in the ICR cell yields the largest ion intensities at both transfer energies. The duration of maximal signal intensity depends, as expected, on the transfer voltage. For a transfer voltage of E,, = 0.3 V it is 0.3 ms, for E,, = 8.3 V it is 0.1 ms. With time-of-flight measurements the residence time of ions with m/z = 78 in the ICR cell has been determined to be approximately 0.1 ms [8]. Therefore the duration of the maximum in Fig. 6 at the high transfer voltage is determined by the residence time of the ions in the open ICR cell. This is not the case for the low transfer voltage in Fig. 5. Here the ion cloud already has suffered diffusion in its flight direction, because the energy distribution of the ions is more important at low transfer energies. Oscillation of the ion cloud also has been observed for the dual cell by Giancaspro et al. [ll]. In this case the reduction in intensity during the oscillations is much smaller because both cells are located close together in the region of the approximately homogeneous, strong magnetic field. THEORETICAL COMPARISON OF THE SENSITIVITY OF THE EXTERNAL WITH OTHER ICR METHODS IN GC-ICR EXPERIMENTS
TRAP
The coupling of ICR spectrometry with gas chromatography (GC) [12] is difficult because a large amount of carrier gas contains only a small amount of sample, and the heavy gas load increases the pressure in the ICR cell. Therefore trapping time and resolution are degraded. The width of a gas chromatographic peak is of the order of 1 s. Therefore several ICR spectra can be recorded for each gas chromatographic peak. To reduce the heavy gas load, several methods have been published in the literature. One procedure is to split the flow of effluent or to use separation techniques [12]. Only a portion of the gas is allowed to enter the ICR cell; the rest is lost. With a pulsed valve [13], gas is allowed to enter the ICR cell only during the ionization period. Again a large portion of the sample is lost. Use of a pulsed valve is complicated with superconducting magnets. Furthermore, a dual cell configuration [5,14] and external ionization [16] can be employed. In the dual cell the two cells are differentially pumped. They are connected by a common trapping electrode with a small hole. A pressure differential can thus be maintained. The ions are generated and trapped at high pressure in the source cell and they are transferred through the hole from the ion source cell with the high pressure to the analyser cell with the low pressure. By this method the applicability of the GC-ICR instrument is greatly enhanced. The size of the hole, however, limits this method. In order
244
to have a high efficiency in the transfer of the ions from the source cell to the analyser cell the hole has to be large because the ions diffuse from the centre of the cell after generation. To obtain a large pressure gradient between the source and the analyser cell, however, the hole must be as small as possible. For the spectrometer employed in this study the optimum diameter of the hole has been calculated to be approximately 4 mm [16]. With the external source the pressure problem can be solved by adding further differential pumping stages. However, the sensitivity is limited by the short residence time of the ions in the open ICR cell. In an external ion trap the pressure problem can be solved in the same way as with an external source. The sensitivity is greatly enhanced because the ions can be accumulated and transferred to the ICR cell after accumulation. The signal-to-noise ratios in the frequency domain, S/N,, will now be compared theoretically for four configurations: GC capillary with split, introduced’directly into a single ICR cell; dual cell with split; external ion source with continuous extraction; and external trap with discontinuous extraction. The S/N, value for a compound which passes out of the GC and gives upon ionization only a single kind of ion (e.g., the molecular ion) is given by S/N, = E4 (TD)“’ Np,W where TD is the effective transient duration, N is the number of ions generated per millibar, ps is the pressure in millibar in the ion source and is given by
Pq=qdL IV is the sample-to-carrier
w=-
gas ratio given by
N,RT qpvTT
where n is the split ratio, qPv is the carrier gas flow through the capillary, L is the conductance of the ion source, NT is the total number of molecules injected into the GC, TT is the total duration of the GC peak under consideration, R is the gas constant and T the absolute temperature. E, is the sensitivity factor which is constant for each instrument. It is given by E = 9
K’,(N=1, 4, Pop,) VN
245
wherepop1 is
the optimal radius of ion orbit, determined experimentally and V, is the noise voltage (n V(Hz)-‘I*) of the preamplifier. The signal voltage V, for a capacitively coupled preamplifier is
where N is the number of ions of the same mass, orbiting with radius p -=scr with charge q (rotating monopole), and r is the cell radius. The capacity C is the sum of the cell capacity and the capacities of the feedthroughs and leads between preamplifier and ICR cell. SL, is a geometry factor. (For the nomenclature, see ref. 17.) The theoretical results for the four arrangements are listed in Table 1. A total cycle time of 100 ms was chosen. Because the ionization time for the external source is negligible, two scans can be performed in 100 ms. The effective detection time TD is 50 ms. This corresponds to unit resolution at m/z = 1900. Therefore the pressure in the detection ICR cell has to be lower than approximately 2 X lo-’ mbar. The electron current was 1 mA in all the calculations. The number of ions N produced by electron impact can be calculated according to
where e is the elementary charge, Tr is the ionization duration, 1, is the electron current, I, is the length of the trapped ion source (the path length of the ionizing electrons), p is the partial pressure of the neutral gas and S,
TABLE 1 Comparison of the theoretical signal-to-noise ratios for different ICR spectrometric configurations. (For details see text) Parameter
Single cell
Dual cell
External source
External trap
Pressure differential
-
Differential pumping
Differential pumping
T, (s) I, (cm) pq (mW L (1 s-l)
0.05 6 2x10-7
Hole 4 mm in diameter, 100 1 s-i 0.05 6 1.4x10-5 100 1: 3.6 70 10000
3x10-4 0.6 5x10-3 1 1:l 15 2100
0.05 6 5x10-4 10 1:l 2500 350000
n
N* S/N,
30 1: 830 1 140
246
is the ionization yield. Ions formed from the carrier gas are not considered. They can be ejected after closing of the ICR cell, or trapping can be avoided using the time-of-flight method [8]. In Table 1 the ratios of the total numbers of ions which are generated by electron impact during the ionization period are given. The signal-to-noise ratios, S/N,, also shown in Table 1 are proportional to the total numbers of ions. The other fixed parameters were Eq = 0.027 ion- ’ s -‘I2 (broad band mode [16]), q = 5 X 10e3 mbar 1 s-1, N = 3.33 x lo-l2 mol (1 ng sample with molecular mass of 300), T = 300 K, TT = 2 s and therefore W = 8.3 x 10-6. The pressure for external ionization has been chosen so that thermal ions can leave the ion source without collision. The values in Table 1 are theoretical values. They are the maximum values which can be reached without any ion losses through imperfect trapping or during transfer and with neglect of space charge effects. For high resolution the split factor has to be reduced and the acquisition time has to be increased. The table shows that the sensitivity increases drastically in the order single cell, external source, dual cell, external trap. The external source with continuous extraction is placed between the single cell and double cell because the path of the ionizing electrons is short and there is no possibility of ion accumulation. GC measurements are possible with all four arrangements, as has been shown experimentally for the first three; the fourth configuration with an external trap should be much more sensitive than the other three. However, this has to be proved experimentally. Like the external ion source, the external trapped ion source can be operated at higher electron currents compared with an ICR cell with an electron gun located inside the magnet at high magnetic field.
CONCLUSIONS
The ions stored in the external ion trap can be transferred as a spatially confined cloud to the ICR cell. The external ion trap described in this paper employs only the stray field of the superconducting magnet to trap the ions and has therefore to be located very close to the magnet. If the ion trap is moved away from the magnet to allow improved differential pumping, the trapping time in the external trapped ion source is rapidly reduced. In this case either a second magnet or another storing device such as a radio frequency ion trap has to be used. The external trapped ion source combines the advantages of the external ion source and the dual ICR cell. The theoretical limit of the detectable partial pressure is increased by at least one order of magnitude compared with all other ICR cell designs.
247 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
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