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A high mass range quadrupole spectrometer for cluster studies
Znternational Journal of Mass Spectrometry and Ion Processes, 91 (1989) 105-112 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
A HIGH MASS RANGE QUADRUPOLE FOR CLUSTER STUDIES
105
SPECTROMETER
P. LABASTIE Luboratoire de SpectromPtrie Zonique et MolPculaire (associ& au CNRS no. 171), Universitk Lyon 1, B&t. 205, 43 Bd du I1 Nov. 1918- 69622 Vi’illeurbanne Cedex (France) M. DOY Znstitut de Physique ExpPrimentale, Ecole PO&technique F.?d&rale de Lausanne, PHB-Ecublens - CH 1015 Luusanne (Switzerland) (First received 27 June 1988; in final form 6 February 1989)
ABSTRACT By modifying a commercial quadrupole mass spectrometer it has been possible to achieve a mass range up to 9000 u without significant losses of transmission. This apparatus has been used in a mercury cluster photoionization experiment.
INTRODUCTION
High mass mercury clusters have been observed in cluster supersonic beams, by time-of-flight mass spectrometry [l]. However, our aim was to perform photoionization of mercury clusters by VUV synchrotron radiation [2]. Among different “techniques” used in these experiments, the time-offlight has too low a duty cycle and the electrostatic or magnetic mass spectrometers suffer from high transmission losses. Quadrupole mass spectrometers do not share these drawbacks, but the available commercial apparatuses were limited to 2600 u, that is, Hg&. There was, therefore, a need for a much higher mass range quadrupole (e.g., 8000 u for Hg&). PRINCIPLE
OF THE MODIFICATION
The principle of the quadrupole mass spectrometer (QMS) has been devised by Paul et al. [3-51. It consists of four cylindrical rods of radius r (see Fig. 1) and the inscribed circle must have a radius of r (1)
a r=1.145
016%1176/89/$03.50
0 1989 Elsevier Science Publishers B.V.
106
Fig. 1. Schematic cut of the quadrupole rods.
To these rods a constant plus oscillating voltage u(t) is applied u(t) = u+
Ycos(2avt)
(2)
When the ratio U/V is U/V= 0.16784
(3)
only one mass is selected, given by m=
V 7.219v2r20
(4)
where m is in u, r, in cm and v, the frequency, in MHz. By inspection of Eq. 4, it is found that the mass range may be increased or by lowering Y. Since the first solution would either by raising V,,, involve some insulation problems, we chose the second. EXPERIMENTAL
We started with a model 15 Extranuclear QMS [6]. The rod diameter is 3/g in., leading to r,, = 0.416 cm. The operating frequency is 1.2 MHz and the maximum r-f. voltage is about 3 kV. From Eq. 4, it is seen that the mass range extends to about 1700 u. If it was possible to decrease the frequency to 500 kHz, while keeping the r.f. voltage constant, the mass range would be increased by a factor of 6, which is large enough for our purposes. Decreasing the oscillator frequency is very simple and will not be described here. However, the heart of the system is a high-Q coil which forms a resonant circuit with the capacitive load of the rods (Fig. 2). We chose to increase the inductance of the coil by a factor of 2. This is obtained by doubling all the dimensions of the coils (loading and resonant), since the inductance is homogeneous to a length. The remaining increase of the LC product is obtained by adding loading capacitors in parallel with the rods. These are ceramic high voltage capacitors. We also replaced the analog control of the U and V voltages by a digital one. A block diagram is shown
107
Fig. 2. Block diagram of the electric part of the quadrupole: R.F., r.f. oscillator and power amplifier, voltage controlled output power; HTA, high voltage d.c. amplifier; RF DC, r.f. to d.c. converter; EA, error amplifier; DAC, digital-to-analog converter; pP, microcomputer; QUAD, quadrupole rods.
in Fig. 2. A similar device has been used by Saunders [7]. The mass range was 4000 u. Figure 3 shows the schematic diagram of the impedance adapter between the r.f. amplifier and the rods, together with the r.f. to d.c. converter which permits us to monitor the r.f. voltage. The relevant part list is given in Table 1. The output impedance of the r-f. amplifier may be any value between 50 and 50 kQ. The air coils Ll and L2 are made of silvered-copper wire wound on Plexiglas. The capacitors Cl0 and Cl1 may be changed for grossly
-A (Feedback)
RF @---
z---o (Feedback)
PB (Feedback)
Fig. 3. Schematic diagram of the impedance adapter between the r.f. amplifier and the rods, together with the r.f. to d.c. converter for r.f. voltage feedback control and monitoring.
108 TABLE 1 Part list for QMS modification Dl, D2 Cl C2A, C2B C3A, C3B C4, c5 C6, C7 C8, C9 ClO, Cl1 RFCl, RFC2 RFC3, RFC4 Ll L2A, L2B
Any semiconductor diode with fast recovery time and 1000 V inverse voltage 6 nF, 3 kV Ceramic 8-79 pF/2 kV Split-cap 14-75 pF/6 kV 10 pF/3 kV Ceramic 13-25 pF/500 V Adjustable 100 pF/l kV Mica 200 pF/4 kV Ceramic power disc capacitor 10 mH/500 V 2 mH/500 V 75 turns of silvered-copper wire + 1 mm on diameter 106 mm and length 106 mm 38 turns of silvered-copper wire $I 3 mm on diameter 210 mm and length 210 mm; the two coils are spaced 20 mm apart
adjusting the frequency and a fine tune is obtained with the variable capacitor C2. Capacitors C3A and C3B allow balancing of the two circuits. The d.c. voltage is injected through RFCl, RFC2. The r.f. voltage of each circuit is monitored by the diodes Dl, D2 through capacitive divisor bridges C4, C6 and C5, C7. The outputs A and B are also used for a feedback control of the r.f. voltage. We draw attention to the possibility of using this device with any r.f. power generator (200 W) provided the frequency is very stable (1 part in 104) and the voltage is precisely controlled (1 part in 103). In order to avoid ground loops, the very stable digital-to-analog converters (DACs) (Burr-Brown DAC 811) are electrically isolated from the computer bus. The details of the mounting depend on the type of microprocessor and will not be described. We used a Victor Sl microcomputer. The high voltage d.c. amplifiers (HTA in Fig. 2) may be replaced by 0 + 600 V voltage controlled d.c. supplies.
CALIBRATION
Theoretically, an infinite mass resolution is obtained when the U/V ratio is given by Eq. 3, but the transmission is then zero. In order to obtain finite transmission and resolution, V must be slightly decreased. A constant resolution over the whole mass range is achieved with a constant U/V ratio. On the other hand, a constant transmission would involve a constant AV value, where AV is the deviation of V from the infinite resolution value (Eq. 3). Most of the commercial QMS operate somewhere between a constant
109 U/V
and a constant AV mode. With our computer controlled U and V, it is possible to choose the resolution at each mass, by keeping in the computer memory a function V= V(U) for each value of U. However, setting V for the 4096 values of U allowed by the 12 bit converter would be rather lengthy, so that the program allows choosing particular U values for setting and then interpolates between them. The V(U) function has been calibrated on the points corresponding to the mercury cluster masses. For each mass, a value of U is chosen and V is increased up to a zero transmission; then, V is decreased a little and stored
E,r19V
E,=30V
Fig. 4. Mass spectra of mercury clusters, for various ionization energies.
110
in memory. The amount of decrease of V is a matter of convenience. Very different resolution can be obtained from mass to mass, so that the peak intensities may be not at all reliable. This is illustrated in Fig. 4, where the mass spectra show very erratic intensities at low masses. On the other hand accurate intensities can be obtained (at least theoretically) by operating in an accurate, constant AV mode. This is the case for clusters larger than Hg,, on the spectra of Fig. 4. There is a real maximum in cluster intensity for 16 atoms, owing to the use of a conical nozzle. RESULTS
The use of the device described here is primarily for cluster studies. In these studies, one is interested in a high transmission, the resolution being enough to separate two successive clusters. These characteristics are required
eV 9.000
10.000
11.000
4
Fig. 5. Photoionization efficiency curves of mercury clusters, fixed mass; see ref. 9 for details.
111
because the cluster intensities are very low, and no impurity has to be separated. Our first results are with the mass spectra of mercury clusters up to Hg,,, which corresponds to 8400 u. The operating frequency was about 560 kHz. It is possible to decrease the frequency to 500 kHz, and thus increase the mass range. We did not attempt such an improvement since the low intensity of Hg, clusters for x 2 40 makes it of no use. In the spectra of Fig. 4, the cluster ionization is made by electron impact. Double ionized clusters appear for electron energies greater than 20 eV, as was shown in ref. 8. For reasons explained at the beginning of this section, we could not test the resolution of the apparatus. With the high transmission used in our experiment the resolution was typically 80 (100 u at 8000 u). The transmission compares with that of any commercial system. However, we did not test this fact quantitatively because the low masses were saturated (see Fig. 4) and of course no comparison is possible for higher masses. On the other hand, time-of-flight spectra seem to discriminate high masses (see ref. 1) more than in our spectra. However this might well be due to the difference between the two cluster sources. The apparatus was used in a synchrotron radiation photoionization experiment. The details have been published elsewhere [9]. The QMS was used as a mass filter. For each mass, it was possible to obtain the best compromise between transmission and resolution, by adjusting U, V, and the ion lenses voltage. This allowed us to have a good count rate (l-10 s-l) even for Hg,. The photoionization curves are shown in Fig. 5. With these results, we have been able to show the mercury cluster departure from van der Waals structure for a number x of atoms greater than 12 [9]. CONCLUSIONS
By lowering the operating frequency of a commercial mass spectrometer, we obtained a mass range of up to 9000 u. The resolution is adjustable for each mass by computer control. The transmission seems to be good even at high masses. This apparatus has already been used in a mercury cluster photoionization experiment. ACKNOWLEDGEMENT
We thank Dr. W. Saunders for having provided us with unpublished material about his high mass range QMS. One of us (P.L.) gratefully acknowledges the hospitality of the Ecole Polytechnique Fed&ale de Lausanne during his stay.
112 REFERENCES 1 B. Cabaud, A. Hoareau and P. Melinon, J. Phys. D, 13 (1980) 1831. 2 C. Brkchignac, M. Broyer, Ph. Cahuzac, G. Delacretaz, P. Labastie and L. Waste, Chem. Phys. Lett., 120 (1985) 559. 3 W. Paul and H. Steinwedel, Z. Naturforsch., Teil A, 8 (1953) 448. 4 W. Paul and M. Raether, Z. Phys., 140 (1955) 262. 5 W. Paul, H.P. Reinhardt and U. von Zahn, Z. Phys., 152 (1958) 143. 6 Extranuclear Laboratories, Pittsburgh, PA 15238, U.S.A. 7 W. Saunders, Ph.D. Dissertation, Univ. California, 1986. 8 C. Br&ignac, M. Broyer, Ph. Cahuzac, G. Delacretaz, P. Labastie and L. Waste, Chem. Phys. Lett., 118 (1985) 174. 9 C. Bnkhignac, M. Broyer, Ph. Cahuzac, G. Delacretaz, P. Labastie, J.P. Wolf and L. Waste, Phys. Rev. Lett., 60 (1988) 275.
A HIGH MASS RANGE QUADRUPOLE FOR CLUSTER STUDIES
105
SPECTROMETER
P. LABASTIE Luboratoire de SpectromPtrie Zonique et MolPculaire (associ& au CNRS no. 171), Universitk Lyon 1, B&t. 205, 43 Bd du I1 Nov. 1918- 69622 Vi’illeurbanne Cedex (France) M. DOY Znstitut de Physique ExpPrimentale, Ecole PO&technique F.?d&rale de Lausanne, PHB-Ecublens - CH 1015 Luusanne (Switzerland) (First received 27 June 1988; in final form 6 February 1989)
ABSTRACT By modifying a commercial quadrupole mass spectrometer it has been possible to achieve a mass range up to 9000 u without significant losses of transmission. This apparatus has been used in a mercury cluster photoionization experiment.
INTRODUCTION
High mass mercury clusters have been observed in cluster supersonic beams, by time-of-flight mass spectrometry [l]. However, our aim was to perform photoionization of mercury clusters by VUV synchrotron radiation [2]. Among different “techniques” used in these experiments, the time-offlight has too low a duty cycle and the electrostatic or magnetic mass spectrometers suffer from high transmission losses. Quadrupole mass spectrometers do not share these drawbacks, but the available commercial apparatuses were limited to 2600 u, that is, Hg&. There was, therefore, a need for a much higher mass range quadrupole (e.g., 8000 u for Hg&). PRINCIPLE
OF THE MODIFICATION
The principle of the quadrupole mass spectrometer (QMS) has been devised by Paul et al. [3-51. It consists of four cylindrical rods of radius r (see Fig. 1) and the inscribed circle must have a radius of r (1)
a r=1.145
016%1176/89/$03.50
0 1989 Elsevier Science Publishers B.V.
106
Fig. 1. Schematic cut of the quadrupole rods.
To these rods a constant plus oscillating voltage u(t) is applied u(t) = u+
Ycos(2avt)
(2)
When the ratio U/V is U/V= 0.16784
(3)
only one mass is selected, given by m=
V 7.219v2r20
(4)
where m is in u, r, in cm and v, the frequency, in MHz. By inspection of Eq. 4, it is found that the mass range may be increased or by lowering Y. Since the first solution would either by raising V,,, involve some insulation problems, we chose the second. EXPERIMENTAL
We started with a model 15 Extranuclear QMS [6]. The rod diameter is 3/g in., leading to r,, = 0.416 cm. The operating frequency is 1.2 MHz and the maximum r-f. voltage is about 3 kV. From Eq. 4, it is seen that the mass range extends to about 1700 u. If it was possible to decrease the frequency to 500 kHz, while keeping the r.f. voltage constant, the mass range would be increased by a factor of 6, which is large enough for our purposes. Decreasing the oscillator frequency is very simple and will not be described here. However, the heart of the system is a high-Q coil which forms a resonant circuit with the capacitive load of the rods (Fig. 2). We chose to increase the inductance of the coil by a factor of 2. This is obtained by doubling all the dimensions of the coils (loading and resonant), since the inductance is homogeneous to a length. The remaining increase of the LC product is obtained by adding loading capacitors in parallel with the rods. These are ceramic high voltage capacitors. We also replaced the analog control of the U and V voltages by a digital one. A block diagram is shown
107
Fig. 2. Block diagram of the electric part of the quadrupole: R.F., r.f. oscillator and power amplifier, voltage controlled output power; HTA, high voltage d.c. amplifier; RF DC, r.f. to d.c. converter; EA, error amplifier; DAC, digital-to-analog converter; pP, microcomputer; QUAD, quadrupole rods.
in Fig. 2. A similar device has been used by Saunders [7]. The mass range was 4000 u. Figure 3 shows the schematic diagram of the impedance adapter between the r.f. amplifier and the rods, together with the r.f. to d.c. converter which permits us to monitor the r.f. voltage. The relevant part list is given in Table 1. The output impedance of the r-f. amplifier may be any value between 50 and 50 kQ. The air coils Ll and L2 are made of silvered-copper wire wound on Plexiglas. The capacitors Cl0 and Cl1 may be changed for grossly
-A (Feedback)
RF @---
z---o (Feedback)
PB (Feedback)
Fig. 3. Schematic diagram of the impedance adapter between the r.f. amplifier and the rods, together with the r.f. to d.c. converter for r.f. voltage feedback control and monitoring.
108 TABLE 1 Part list for QMS modification Dl, D2 Cl C2A, C2B C3A, C3B C4, c5 C6, C7 C8, C9 ClO, Cl1 RFCl, RFC2 RFC3, RFC4 Ll L2A, L2B
Any semiconductor diode with fast recovery time and 1000 V inverse voltage 6 nF, 3 kV Ceramic 8-79 pF/2 kV Split-cap 14-75 pF/6 kV 10 pF/3 kV Ceramic 13-25 pF/500 V Adjustable 100 pF/l kV Mica 200 pF/4 kV Ceramic power disc capacitor 10 mH/500 V 2 mH/500 V 75 turns of silvered-copper wire + 1 mm on diameter 106 mm and length 106 mm 38 turns of silvered-copper wire $I 3 mm on diameter 210 mm and length 210 mm; the two coils are spaced 20 mm apart
adjusting the frequency and a fine tune is obtained with the variable capacitor C2. Capacitors C3A and C3B allow balancing of the two circuits. The d.c. voltage is injected through RFCl, RFC2. The r.f. voltage of each circuit is monitored by the diodes Dl, D2 through capacitive divisor bridges C4, C6 and C5, C7. The outputs A and B are also used for a feedback control of the r.f. voltage. We draw attention to the possibility of using this device with any r.f. power generator (200 W) provided the frequency is very stable (1 part in 104) and the voltage is precisely controlled (1 part in 103). In order to avoid ground loops, the very stable digital-to-analog converters (DACs) (Burr-Brown DAC 811) are electrically isolated from the computer bus. The details of the mounting depend on the type of microprocessor and will not be described. We used a Victor Sl microcomputer. The high voltage d.c. amplifiers (HTA in Fig. 2) may be replaced by 0 + 600 V voltage controlled d.c. supplies.
CALIBRATION
Theoretically, an infinite mass resolution is obtained when the U/V ratio is given by Eq. 3, but the transmission is then zero. In order to obtain finite transmission and resolution, V must be slightly decreased. A constant resolution over the whole mass range is achieved with a constant U/V ratio. On the other hand, a constant transmission would involve a constant AV value, where AV is the deviation of V from the infinite resolution value (Eq. 3). Most of the commercial QMS operate somewhere between a constant
109 U/V
and a constant AV mode. With our computer controlled U and V, it is possible to choose the resolution at each mass, by keeping in the computer memory a function V= V(U) for each value of U. However, setting V for the 4096 values of U allowed by the 12 bit converter would be rather lengthy, so that the program allows choosing particular U values for setting and then interpolates between them. The V(U) function has been calibrated on the points corresponding to the mercury cluster masses. For each mass, a value of U is chosen and V is increased up to a zero transmission; then, V is decreased a little and stored
E,r19V
E,=30V
Fig. 4. Mass spectra of mercury clusters, for various ionization energies.
110
in memory. The amount of decrease of V is a matter of convenience. Very different resolution can be obtained from mass to mass, so that the peak intensities may be not at all reliable. This is illustrated in Fig. 4, where the mass spectra show very erratic intensities at low masses. On the other hand accurate intensities can be obtained (at least theoretically) by operating in an accurate, constant AV mode. This is the case for clusters larger than Hg,, on the spectra of Fig. 4. There is a real maximum in cluster intensity for 16 atoms, owing to the use of a conical nozzle. RESULTS
The use of the device described here is primarily for cluster studies. In these studies, one is interested in a high transmission, the resolution being enough to separate two successive clusters. These characteristics are required
eV 9.000
10.000
11.000
4
Fig. 5. Photoionization efficiency curves of mercury clusters, fixed mass; see ref. 9 for details.
111
because the cluster intensities are very low, and no impurity has to be separated. Our first results are with the mass spectra of mercury clusters up to Hg,,, which corresponds to 8400 u. The operating frequency was about 560 kHz. It is possible to decrease the frequency to 500 kHz, and thus increase the mass range. We did not attempt such an improvement since the low intensity of Hg, clusters for x 2 40 makes it of no use. In the spectra of Fig. 4, the cluster ionization is made by electron impact. Double ionized clusters appear for electron energies greater than 20 eV, as was shown in ref. 8. For reasons explained at the beginning of this section, we could not test the resolution of the apparatus. With the high transmission used in our experiment the resolution was typically 80 (100 u at 8000 u). The transmission compares with that of any commercial system. However, we did not test this fact quantitatively because the low masses were saturated (see Fig. 4) and of course no comparison is possible for higher masses. On the other hand, time-of-flight spectra seem to discriminate high masses (see ref. 1) more than in our spectra. However this might well be due to the difference between the two cluster sources. The apparatus was used in a synchrotron radiation photoionization experiment. The details have been published elsewhere [9]. The QMS was used as a mass filter. For each mass, it was possible to obtain the best compromise between transmission and resolution, by adjusting U, V, and the ion lenses voltage. This allowed us to have a good count rate (l-10 s-l) even for Hg,. The photoionization curves are shown in Fig. 5. With these results, we have been able to show the mercury cluster departure from van der Waals structure for a number x of atoms greater than 12 [9]. CONCLUSIONS
By lowering the operating frequency of a commercial mass spectrometer, we obtained a mass range of up to 9000 u. The resolution is adjustable for each mass by computer control. The transmission seems to be good even at high masses. This apparatus has already been used in a mercury cluster photoionization experiment. ACKNOWLEDGEMENT
We thank Dr. W. Saunders for having provided us with unpublished material about his high mass range QMS. One of us (P.L.) gratefully acknowledges the hospitality of the Ecole Polytechnique Fed&ale de Lausanne during his stay.
112 REFERENCES 1 B. Cabaud, A. Hoareau and P. Melinon, J. Phys. D, 13 (1980) 1831. 2 C. Brkchignac, M. Broyer, Ph. Cahuzac, G. Delacretaz, P. Labastie and L. Waste, Chem. Phys. Lett., 120 (1985) 559. 3 W. Paul and H. Steinwedel, Z. Naturforsch., Teil A, 8 (1953) 448. 4 W. Paul and M. Raether, Z. Phys., 140 (1955) 262. 5 W. Paul, H.P. Reinhardt and U. von Zahn, Z. Phys., 152 (1958) 143. 6 Extranuclear Laboratories, Pittsburgh, PA 15238, U.S.A. 7 W. Saunders, Ph.D. Dissertation, Univ. California, 1986. 8 C. Br&ignac, M. Broyer, Ph. Cahuzac, G. Delacretaz, P. Labastie and L. Waste, Chem. Phys. Lett., 118 (1985) 174. 9 C. Bnkhignac, M. Broyer, Ph. Cahuzac, G. Delacretaz, P. Labastie, J.P. Wolf and L. Waste, Phys. Rev. Lett., 60 (1988) 275.