RF MASS SPECTROMETER

Abstract: A method of operating a mass spectrometer having a rod set, comprising: directing ions into the rod set, applying an unbalanced RF voltage to the rod set, and applying a low level resolving DC voltage, e.g. 0.3 to 15.5 volts, to the rod set, thus increasing the sensitivity of the mass spectrometer and also improving the resolution. Alternatively, instead of unbalancing the RF voltage on the rod set, suitably phased RF can be applied to an end lens spaced from the exit end of the rod set.

Description: RF MASS SPECTROMETER FIELD OF THE INVENTION This invention relates to a mass analyzer. More particularly, it relates to a rod type mass analyzer which is simple and inexpensive and yet which is able to provide good mass resolution. BACKGROUND OF THE INVENTION Quadrupole mass spectrometers are commonly used to perform mass analysis. These spectrometers, when used in a resolving mode, employ 4 rods which are usually relatively lengthy (e.g., 20 cm) and which are both made and assembled with extreme precision. When used in a resolving mode they are pumped to a relatively high vacuum (e.g. 10' 5 Torr) and both RF and DC voltages are applied to them. While the RF and DC voltages can vary depending on the frequency of operation and the mass range, typical values for the RF are of the order of 1600 volts peak-to- peak at 1 MHz, and for the DC typically ±272 volts peak-to-peak. (These values are typical for a mass range of 600 Daltons and an inscribed radius r 0 for the rod set of 0.415 cm.) The costs of such mass spectrometers, including their associated power supplies and vacuum pumps, are usually extremely high. There has for many years existed a need for a simpler less expensive mass spectrometer, and numerous attempts have been made to fill this need. However while the costs have been reduced, quadrupole and other rod mass spectrometers (e.g., octopoles and hexapoles) have continued to remain extremely expensive and to require very close tolerances and high vacuum pumping equipment, as well as costly power supplies. BRIEF SUMMARY OF THE INVENTION Therefore it is an object of the invention to provide a rod type mass spectrometer which achieves good results but with simpler, shorter, less precisely made resolving rods than have previously been needed, and with less costly vacuum pumping and power supply equipment. In one aspect the invention provides a method of operating a mass spectrometer having a rod set, comprising: a method of operating a mass spectrometer having a rod set which has at least two pole pairs and an exit end, said method comprising directing ions into or forming ions in said rod set, transmitting ions from said exit end of said rod set as transmitted ions, applying RF to said rod set, aligning some of said transmitted ions with one said pole pair and the number of transmitted ions being aligned with said one pole pair being greater than the number of transmitted ions not so aligned, and ejecting the ions aligned with said one pole pair from said exit end with greater kinetic energy than the ions not so aligned. Further objects and advantages of the invention will appear from the following description, taken together with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Fig. 1 is a plot of the well-known a-q operating diagram for quadrupole mass spectrometers; Fig. 2A is a plot showing the distribution of ion axial energies produced by a typical RF-only quadrupole set of rods; Fig. 2B is a plot similar to Fig. 2A but showing the ion energy distribution after the ions have passed through the fringing fields at the exit end of the RF-only quadrupole rods; Fig. 3 is a diagrammatic view showing an RF-only single MS configuration; Fig. 3A is an end view showing how DC is conventionally applied to quadrupole rods; Figs. 4A to 4D are plots showing mass spectra obtained from the Fig. 3 apparatus, both with 0 volts DC on the resolving rods and with 1 volt DC on the resolving rods; Fig. 5 shows another set of mass spectra obtained using the apparatus of Fig. 3, with 0 volts DC and with various low level DC voltages applied to the resolving rods; Fig. 6 is still another view of mass spectra obtained from the Fig. 3 apparatus, showing results obtained with 0 volts DC and with 4 volts and 15.5 volts DC applied to the resolving rods; Fig. 7 is an end view showing how AC is applied to the rods according to the invention; Fig. 8 is a diagrammatic view showing the configuration used for MS /MS analysis according to the invention; Fig. 9 shows a spectrum obtained according to the invention without energy filtering; Fig. 10 shows a mass spectrum obtained using standard balanced RF without DC; Fig. 11 shows a spectrum for the same substance as that of Fig. 10, but obtained using unbalanced RF and low voltage DC; Fig. 12 shows a spectrum obtained using unbalanced RF but no DC; Fig. 13 shows a spectrum for the same substance as that of Fig. 12, but using unbalanced RF with low voltage DC (and with the spectrum of Fig. 12 superimposed thereon); Fig. 14 is a plot showing stopping curves obtained with unbalanced RF and with 0 volts DC and low voltage DC; Fig. 15 is a plot similar to that of Figs. 2A, 2B but showing increased displacement between the ion energy distributions resulting from the use of the invention; Fig. 16 shows two spectra obtained with the use of the invention at two different pressures; Fig. 17 is a computer simulation showing an end view for rods of Fig. 3, and showing the ion distribution at the ends of the rods when balanced RF and no DC is applied; Fig. 18 is a view similar to that of Fig. 17 but showing the ion distribution when low voltage DC is also applied to the rods; Fig. 18A is a view similar to that of Fig. 18 but showing the ion distribution when a larger diameter ion beam enters the rods; Fig. 18B is a view similar to that of Fig. 18A but showing the ion distribution when an even larger diameter ion beam enters the rods; Fig. 19 is a sectional view through two rods and an end lens showing the fringing fields at the exit ends of the rods; Fig. 20 is a diagrammatic view showing use of an extra set of rods in place of the end lens of Fig. 3; Fig. 21 shows three spectra obtained under three different sets of conditions, to illustrate the effects of the invention; Fig. 22 shows two spectra, obtained with in-phase and out-of- phase RF respectively applied to the end lens; Fig. 23 shows stopping curves produced using low voltage DC on the rods of a mass spectrometer and with different levels of RF applied to the end lens; Fig. 24 shows a set of mass spectra obtained using low voltage DC on the spectrometer rods and different RF voltages on the end lens; Fig. 25 shows a mass spectrum illustrating the resolution obtained in a high mass range using the invention; and Fig. 26 shows a set of spectrometer rods and illustrates a modification of the invention using modified ion injection. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Reference is first made to Fig. 1, which shows the well-known operating diagram for a quadrupole mass spectrometer. The parameter a is plotted on the vertical axis while the parameter q is plotted on the horizontal axis. As is well known, a = 8eU/(mω r 0 2 ) q = 4eV/(mω 2 r 0 2 ) where U is the amplitude of the DC voltage applied to the rods, V is the RF amplitude, e is the charge on the ion, m is its mass, ω is the RF frequency, and r 0 is the inscribed radius of the rod set (as explained for example in U.S. patent 5,248,875). In the Fig. 1 operating diagram, ions within the shaded area 10 are stable provided that they are above the operating line 12. The operating line is usually made to run near the tip or peak 14 of the stability diagram, since the resolution of the mass spectrometer is the width LI of the peak above the operating line divided by the width L2 of the base of the stability diagram. This requires as mentioned that substantial RF and DC voltages be applied to the rods. In addition, to optimize the resolution, the RF/DC ratio must be controlled to within very small limits which are mass dependent, so the ratio of RF/DC must be scanned with mass. If the optimal ratio is not maintained, resolution is severely impaired. It is known to operate a quadrupole rod set without DC (RF only), in which case the operating line is along the horizontal axis of the stability diagram and the device acts essentially as an ion pipe, transmitting ions over a wide mass to charge ratio (m/z) range. However ions whose q is .907 become unstable radially, hit the rods, and are not transmitted. In the fringing fields at the entrance or exit of the rods, some component of the radial excitation of the ions is converted into axial excitation. Ions subjected to this influence receive a kinetic energy increase in the axial direction, because of radial /axial coupling in the fringing fields. These ions, of q close to .907, which have greater kinetic energy than ions having a smaller q, can be separated by virtue of their differences in energy and can then be detected. The energy considerations are illustrated in Figs. 2 A and 2B. Fig. 2 A shows at 16 the standard axial energy distribution of ions travelling into an RF only quadrupole rod set, plotted against the number of ions. The width of curve 16 will depend on the energy spread of the ions entering the quadrupole rod set; this energy spread can be made relatively narrow as will be discussed. Fig. 2B shows curve 16 from Fig. 2A and also shows curve 18 representing the distribution of axial energies of ions whose q is about 0.9 and which have therefore received additional axial energy coupled from the fringing fields. If there is a sufficient separation between curves 16, 18, then the ions having the energies represented by curve 18 can be separated from the remaining ions, e.g., by a downstream energy filter, and can be detected. A mass spectrum can be obtained in this way, by scanning the RF voltage applied to the quadrupole rods to bring the q of ions of various masses to near .907, at which time the large radial energies which they acquire yield increased axial energies, so that these ions can be separated. Fig. 3 illustrates apparatus which may be used for obtaining a mass spectrum in the above described way. As shown, sample source 20 (which may be a liquid or gaseous ion source) supplies sample to an ion source 22 which produces ions therefrom and directs them into an interface region 24 which may be supplied with inert curtain gas 26 (usually argon or nitrogen) as shown in U.S. patent 4,137,750. Ions passing through the gas curtain travel through a differentially pumped region 28, at a pressure of about 2 Torr, and enter a quadrupole RF-only rod set Q0 in chamber 30, which is pumped to a pressure of about 8 milli-Torr. Rod set Q0, which is conventional, serves to transmit the ions onward with removal of some gas. In addition, Q0, because of the relatively high pressure therein also serves to collisionally damp or cool the ions to reduce their energy spread, as described in U.S. patent 4,963,736. From chamber 30, the ions travel through orifice 32 in an interface plate 34, and through a set of short RF-only rods 35 into a set of analyzing rods Ql. RF rods 35 serve to collimate the ions travelling into analyzing quadrupole rods Ql. The rods of Q0 may typically be about 20 cm long, while the rods 35 and Ql may typically each be approximately 24mm or 48mm in length. Analyzing rods Ql are supplied with RF through capacitor Cl from power supply 36. The same RF is supplied through capacitors C2, C3 to rods Q0, 35. Conventional DC offsets are also applied to the various rods and to the interface plates from a DC power supply 38. A conventional exit lens 39 and energy filter 40 (consisting of a pair of grids) are located downstream of the analyzing rods Ql, in the ion path, followed by a conventional detector 42. The apparatus described above is relatively conventional (except for the shortness of the rods Ql), and can produce a mass spectrum as the RF on analyzing rods Ql is scanned. As mentioned, ions approaching a q of .907 receive additional axial kinetic energy coupled from their radial energy in the fringing fields at the entrance and exit ends of the analyzing rods Ql and are able to surmount the potential barrier created by the energy filter 40 and can reach the detector 42. However a problem with this arrangement is that the resolution is very poor, and in addition the sensitivity is approximately five times less than with conventional mass spectrometers in which both AC and DC are applied to the resolving rods. It is believed that the reduction in sensitivity is caused because in order for the energy filter 40 to eliminate ions which cause peak broadening, at the same time many ions of significance must also be discarded. It has been found, unexpectedly, that applying a small amount of DC to the analyzing rods Ql produces (when certain RF conditions exist, as will be described) a dramatic increase in performance, far beyond that which would normally be expected. Reference is next made to Figs. 4 A to 4D, which show portions of mass spectra of a mixture of four substances at four different mass peaks. The substances were tetraethyl ammonium hydroxide (ions at m/z 130), dodecyl trimethyl ammonium bromide (ions at m/z 228), tetrahexyl ammonium hydroxide (ions at m/z 354), and tetradecyl ammonium bromide (ions at m/z 578). Curves 50a, 50b, 50c, 50d show the peaks obtained when the resolving rods Ql are operated in conventional RF-only mode (no DC applied). Peaks 52a, 52b, 52c, 52d show the results obtained when one volt DC was applied to the resolving rods Ql. (The DC was applied in the same manner as high voltage resolving DC is normally applied, namely between opposite pairs of rods, as shown for source "DC" in Fig. 3A.) It will be seen that both the resolution and the sensitivity have increased dramatically. Indeed the resolution has improved sufficiently to see isotopic peaks 52b, 52d when a single volt of resolving DC is applied. The sensitivity has improved by a factor of about 4, which brings it close to that of a conventional instrument but with far less cost and much simpler optimization, as will be explained. It will be seen in Figs. 4A to 4D that the peaks 52a to 52d obtained with the use of 1 volt DC are mass shifted from the peaks 50a to 50d obtained when 0 volts DC were applied. This is simply because the calibration is determined by both the RF and DC levels and had not been reset on the instrument. Fig. 5 shows mass spectra obtained from reserpine solution, with m/z approximately equal to 609. Ql was constructed employing two- inch long rods. Curve 54 shows the spectrum obtained when 0 volts DC were applied to the rods Ql (which were therefore operated with RF only). Curve 56a shows the spectrum obtained when 1 volt DC was added to the rods Ql. Curves 56b, 56c show the same spectra when 5 volts and 7 volts DC respectively were applied to rods Ql. It will be seen that as the DC voltage increases, the resolution increases but the sensitivity falls to some extent. Fig. 6 shows a mass spectrum obtained for reserpine with Ql constructed from 24mm long rods. Curve 58 shows the spectrum obtained when 0 volts DC were applied to the rods Ql, while curves 60a and 60b show the spectra obtained when 4 volts and 15.5 volts DC respectively were applied to the rods Ql. The background noise is indicated at 62. Again it will be seen that the resolution increases substantially as the DC voltage is increased, but that the sensitivity is considerably less at 15.5 volts DC than at 4 volts DC. While the rod length is important for a conventional resolving quadrupole mass spectrometer, in which both AC and DC are applied to the rods, rod length is not particularly important with the use of the invention. Relatively short rods will do, as will be explained. The precise amount of DC applied to the rods can vary, as indicated. Experiments indicate that DC in the range of 0.1% to 40% of the normal DC voltage (which may as mentioned typically be 272 volts peak- to-peak at 600 amu) may be used on the analyzing rods when the rods are operating near the tip 14 of the a-q diagram of Fig. 1. A range of between 0.3 and 15.5 volts DC is preferred, and preferably a range of between 1 and 15.5 volts DC is used (since 1 volt produces improved results as compared with 0.3 volts). However, good results were obtained at a DC voltage of up to 40% of the usual DC voltage, or about 109 volts DC. Above that level, both the peak shape degrades and the sensitivity drops off, both relatively sharply. It is also found that in the embodiment described, the RF applied to the rods should be unbalanced and desirably is between 5% and 30% out of balance (for reasons which will be explained). The exact amount of out of balance is a matter of optimization in each case. As shown in Fig. 7, there are normally two RF power supplies, namely power supply RF1 driving one pair of rods 70a, 70b and power supply RF2 driving the other pair of rods 72a, 72b. The 0 to peak voltage of power supply RF1 is desirably between 5% and 30% greater than that of power supply RF2 (or vice versa), i.e. the unbalance is desirably 5% to 30% from 0 to peak or 20% to 60% peak to peak. The drawings provided were achieved with the use of unbalanced RF. Use of the invention has extremely significant advantages in terms of cost and ease of use. In a conventional mass spectrometer using analyzing rods which have AC and DC applied to them, the rods must typically be 20 cm or more in length, metallized ceramic, with roundness tolerances better than 20 micro-inches and straightness tolerances better than 100 micro-inches. Such rods may typically cost $600 each and typically take 240 minutes to assemble. With the use of the invention, much shorter rods can be used, e.g., 2.4 cm metal tubes, with roundness tolerances of ±2/1000 of an inch and straightness tolerances ±2/1000 of an inch. Such rods typically cost $7.00 each (compared with $600 each for conventional rods) and can be assembled in about five minutes (compared with 240 minutes for conventional rods). In addition, since no high voltage DC is needed, the electronics are much simpler and cheaper. Since the DC does not need to be scanned in conjunction with the RF scanning, this additionally simplifies the electronics. (However, if desired the DC can be scanned for other reasons.) Further, the system described can operate at higher pressure (10 -4 Torr, as compared with at least 10 -5 Torr or better for conventional rods), resulting in smaller and less costly vacuum pump requirements. In addition, the instrument is much easier to use since only the RF need be scanned; there is no need to scan the ratio of RF to DC, since resolution is not achieved by adjusting the RF/DC ratio, but instead by adjusting the downstream energy filter. While Fig. 3 shows single MS operation, the instrument described may also be used for MS/MS operation, as shown in Fig. 8, where parts corresponding with those of Fig. 1 are marked with primed reference numerals. In Fig. 8, the ions travel through rod sets QO', 35', and Ql' as before. The ions then travel through a short set of RF only rods 80 which collimate them into a collision cell Q2. The rod offset of RF-only rods 80 is held at 2 to 10 volts more positive than that of rods Ql, creating a voltage barrier which also serves as the energy filter 40. In rod set Q2, located in container 82, collision gas from source 84 is provided. Hence parent ions entering Q2 are fragmented in conventional manner into daughter ions. The daughter ions are directed through analyzing rods Q3 to which RF and the previously described low level DC are applied, and then through energy filter 86 to a detector 42'. While energy filtering provides a simple method of extracting peaks, other methods may be used if desired. Without energy filtering, a "stair step" spectrum is obtained, as shown at 90 in Fig. 9, with different masses represented by different levels 92, 94, 96 in spectrum 90. Mass peaks can be obtained by differentiating the curve 90, as shown in dotted lines at 98, 100 in Fig. 9. However, this method is not preferred, since with the use of this method, the detector 42 receives a larger and more continuous flux of ions and is therefore more likely to burn out. The theory of operation of the invention as it is best understood (and in particular the reasons for the need for unbalanced RF or its equivalent, and the reasons for the applicability of the low voltage DC), and additional embodiments of the invention, will now be discussed. Reference is made to Fig. 10, which shows a spectrum from a conventional set of analyzing rods, such as Ql in Fig. 3, with standard balanced RF applied, and no DC. A peak 110 appears at mass 357.18, out of intensity 8.61e4 (8.61 x 10 4 counts per second (cps)). (AcN solution was used as a solvent, with no acids or buffers, with the same mixture of substances as described in connection with Fig. 4.) Fig. 11 shows a spectrum obtained from the same rods Ql with the same solution as for Fig. 10, when the RF was unbalanced by 30% and ±3 volts DC was applied across respective pairs of rods. The resulting peak 112 corresponds to peak 110 but has been shifted (this is simply a matter of calibration), but the intensity has increased in intensity to 5.70 e5cps, or approximately seven times the intensity of peak 110. Fig. 12 shows another spectrum from rods Ql, using the same solution as for Fig. 11, with unbalanced RF on the rods (the unbalance was approximately 20%), but not using DC. It will be seen that peak 114 has poor shape and low intensity (the intensity is 1.52e5cps). It is generally observed that operating the short analyzing quadrupole with unbalanced RF in the absence of resolving DC results in poor peak shape such as peak 114 (except as will be discussed later). Fig. 13 shows a spectrum similar to that in Fig. 12 (using the same solution), but obtained by using 1 volt DC applied across respective pairs of rods, in addition to the unbalanced RF. The resultant peak 116 had a much narrower (and therefore better) shape and an intensity of 5.07e5cps. For comparison purposes the peak 114 of Fig. 12 is shown in dotted lines in Fig. 13, so that the improvement by using both unbalanced RF and a low voltage DC can be seen. The conclusion from the above experiments was that neither unbalanced RF alone, nor low voltage DC with balanced RF, is sufficient. A combination of both, or their equivalents (to be discussed), is needed for best results. To help assess the reasons for this, stopping curves were produced as shown in Fig. 14. To produce Fig. 14, a barrier DC voltage (plotted on the x-axis of Fig. 14) was applied to the exit lens 39 following Ql, and the intensity (cps) of ions able to pass the exit barrier was plotted on the vertical axis. Curve 118 was produced with the use of unbalanced RF, and 0 volts DC applied to the rods of Ql, while curve 120 was produced with the use of unbalanced RF and 1 volt DC applied to the rods of Ql. It will be seen that when the lens was operated at (for example) 10 volts, there was an increase of about 5.7 times in the intensity of ions able to pass the barrier when both unbalanced RF and low voltage DC were present. It is evident from this that when both unbalanced RF and a low voltage DC are applied, the ions of interest have greater kinetic energy so that more of them are able to pass the barrier created by the biased exist lens 39. The difference in energy distributions is illustrated in Fig. 15, which is the same as Fig. 2b and in which primed reference numerals are used to indicate corresponding elements. As will be seen, the curve 18' of ions having a q of about 0.9 is displaced to a higher energy than was the case in Fig. 2b and is better separated from curve 16' representing ions having a q of less than 0.9. Separation of the respective sets of ions by a downstream energy filter such as filter 40 can therefore more easily be achieved (i.e., low q ions are more efficiently prevented from reaching the detector). Fig. 16 is an overlay of two spectra 122, 124, taken at different pressures in the chamber containing Ql. Spectrum 122 was made at a pressure of 1.7e-5 torr), while spectrum 124 was made at a pressure of 3.4e-4 torr or about 20 times higher than the pressure for spectrum 122. It will be seen that the peak shapes are virtually the same, and that there is little difference in intensity. Since higher pressure operation is therefore possible, cheaper and less bulky vacuum pumps can be used. Figs. 17, 18 help to explain the reasons (as best understood) for the operation of the invention. Fig. 17 is an end-on view (looking towards the exit ends of rods Ql) showing a computer simulation of the distribution of the ions as they exit from the rods (marked as Ql-1, Ql-2, Ql-3, Ql-4), assuming that balanced RF is applied and that no DC is applied. It will be seen that the ions exit in a "cross" pattern 126, symmetrically about the pole pairs of the rods. Fig. 18 shows a plot similar to that of Fig. 17, but with 3 volts DC applied to the rods Ql. The positive rods are the y-axis rods Ql-1, Ql-3, while the negative rods are the x-axis rods Ql-2, Ql-4. It will be seen that the ions (which are assumed to be positive) become aligned with the positive pole pair Ql-1, Ql-3 as indicated at 128. The appearance of Fig. 18 would be similar if standard DC (i.e., at a much higher voltage, e.g., 272 volts) were applied, but there would be far fewer ions since in that case the rods Ql would have a very narrow band pass. However simply to align the ion beam with a pole pair, which is the desired objective here, only a low voltage DC, typically as low as 1 volt, and even as low as 0.3 volts, is needed. The Fig. 18 simulation assumes that a very small diameter collimated ion beam has entered the rods Ql, typically less than approximately 0.1 mm diameter. If the ion beam entering the rods Ql is of larger diameter, then if the rods Ql are short, the ions will become less well aligned with one pole pair, since they do not experience sufficient cycles of the RF before they reach the exit ends of the rods Ql. For example, Fig. 18 A shows a plot similar to that of Fig. 18, using ±3 volts DC applied to the rods, but with a 0.25 mm diameter ion beam entering the rod set Ql. It will be seen that the ions, indicated at 128a, are less well aligned with pole pair Ql-1 - Ql-3. Had the rods been longer than the one inch used in the simulation, the alignment of the ions with pole pair Ql-1 - Ql-3 would have been improved. Similarly, Fig. 18B shows the ion distribution 128b for a 1.4 mm diameter ion beam entering the rod set, with ±3 volts DC applied to the rod set. It will be seen that with a beam of this relatively wide diameter, essentially no alignment with one pole pair is achieved. Again, had the rods been sufficiently long, the ions would have experienced enough cycles of the RF to become aligned with pole pair Ql-1 - Ql-3 by the time they reach the exit ends of the rods Ql. It is known that within the rods Ql, the ions at high q have a secular frequency of radial motion, which frequency is approximately one- half the drive or RF frequency. It is also known that the ions have a smaller motion, referred to as micro motion within the rods, and which is also a radial motion. When the ions enter the fringing field between ends of rods Ql and the exit lens 39, the motion of the ions becomes complex and no analysis presently exists for their motion, nor is it possible easily to visualize the ion motions. However, it is believed that when the RF is unbalanced, then in one plane, i.e., in a plane through one pair of poles, the field gradient will be different than that in a plane through the other pair of rods. In any event, it has been determined that when the RF field is unbalanced such that the highest RF is on the Ql-2 - Ql-4 rod pair (i.e., on the negative DC rods, here defined as the x-rods or x-pole pair), then the ions which are aligned with the Ql-1 - Ql-3 pole pair (i.e., the positive DC pole pair, here defined as the y-rods or y-pole pair) receive the additional kinetic energy described, producing much higher sensitivity. (This discussion assumes positive ions. For negative ions the polarities would be reversed.) It is believed that the reason for this result is that the ions aligned with the y-pole pair are retarded in the fringing field, i.e., they spend more time in the fringing field between the exit ends of rods Ql and the exit lens 39, which will enhance the radial to axial coupling. The field lines for a typical fringing field are shown at 130 in Fig. 19. The greater radial excursions bring the ions to positions radially closer to the rods Ql, where the axial component of the fringing field is the strongest. (It will be seen that the field lines are closer here, as indicated at 132.) Ions closer to the rods are therefore ejected with greater kinetic energy, as shown by the stopping curve 120 in Fig. 14. Figs. 5 and 6 demonstrate that there are additional subtle effects observable by the addition of small amounts of resolving DC to the short analyzing quadrupole. These figures show that increasing amounts of resolving DC lead to enhanced resolution at the expense of sensitivity. This is consistent with a reduction of incoming ion energy with increased resolving DC. It is thought that increases in resolving DC of the appropriate polarity slightly retard the entry of ions into the resolving quadrupole. Such effects have been modeled by Dawson (Int. J. Mass Spectrom. Ion Phys. 17 (1975) 423-445) and found to be important for ion entry in the positive DC quadrants of the entrance fringing fields. This phenomenon, in combination with the modified exit fringing fields achieved via unbalanced drive RF or the application of auxiliary RF to the exit lens (to be described later) may contribute to the high exit kinetic energies observed with this device. Within the rods Ql, the unbalanced RF has no significant effect on the ions and therefore does not interfere with their transmission. The effect achieved by unbalancing the RF applied to the rods Ql can also be achieved by tapping the RF voltage from the RF power supply 36 and applying it to the exit lens 39. The RF applied to the exit lens 39 is phase locked to the main RF applied to Ql and is typically phase adjustable from 0 to 180°, by a control indicated at block 136 in Fig. 3. The RF applied to the exit lens 39 should be in-phase with the RF applied to the pole pair between which the ions are aligned, e.g., rods Ql-1 - Ql-3 in Fig. 18. Applying the RF field to the exit lens 39 in this way has the same effect as unbalancing the RF applied to the rods Ql, in that the suitably phased RF on lens 39 will cause the bulk of the ions exiting the rods Ql (i.e., those ions aligned with the y-axis rods) to spend more time in the fringing fields at the exit ends of the rods and thus to acquire more axial kinetic energy before they are ejected. Instead of a conventional exit lens 39, a set of quadrupole "stubby" (i.e., short) rods Q4 may be used, as shown in Fig. 20. RF can be applied to stubby rods Q4 from the main RF source 36, and the RF on either set of rods Ql, Q4 will be unbalanced appropriately. If desired, rods Q4 can be capacitively coupled to rods Ql (e.g., by a capacitor indicated at C2), in which case the RF on both sets of rods Ql, Q4 will be unbalanced. Alternatively, instead of applying an unbalanced RF voltage to Q4, all four rods of Q4 can have a phase locked, phase adjustable RF voltage applied thereto (i.e., additional to the drive RF), in which case, Q4 will act similarly to the exit lens 39. Reference is next made to Fig. 21, which shows three spectra 140, 142, 144, made from a one micromole reserpine solution. Spectrum 140 was made with balanced RF and no DC applied to the rods Ql, and no RF on the exit lens 39. It will be seen that the intensity was very low. Spectrum 142 was made with ±15 volts DC on the rods Ql, no RF on the exit lens 39 and balanced RF on the rods Ql. The sensitivity was even lower than that of spectrum 140. Spectrum 144 was made using ±15 volts DC on the rods Ql, and 105 volts RF on the exit lens 39, properly phased. It will be seen that the sensitivity increased by about a factor of five from spectrum 140. Fig. 22 shows the effects of varying the phase of the RF applied to the exit lens 39. Spectrum 146 was made with out-of-phase RF applied to exit lens 39, where "out-of-phase" means with respect to the drive RF on the negative or x-rods Ql-2, Ql-4. Spectrum 148 was made with in-phase RF applied to the exit lens 39, i.e., in-phase with respect to the drive RF on the negative or x-rods Ql-2, Ql-4. It will be seen that the sensitivity was much higher when the RF was out-of-phase with the drive RF on the x-rods Ql-2, Ql-4, causing the bulk of the ions (aligned with the y-rods Ql-1, Ql-3) to experience an in-phase field which caused them to spend more time in the fringing fields. Fig. 23 shows stopping curves and illustrates the variation in kinetic energy of ions with variation of the RF amplitude on the exit lens 39. In all cases, balanced RF and ±3 volts DC were applied to the rods Ql. In Fig. 23, curve 150 is the stopping curve when zero volts RF was applied to the exit lens. It will be seen that the axial kinetic energy of the ions was very low. Curves 152, 154, 156, 158 and 160 show 40 volts, 80 volts, 120 volts, 160 volts and 200 volts, respectively, of RF (peak-to-peak) applied to the exit lens 39 and suitably phased. It will be seen that as the RF voltage applied to the exit lens 39 increases, the axial kinetic energy of the ions increases, although the increases become smaller after the RF voltage has been increased to between 80 and 120 volts. Fig. 24 shows spectra obtained from a one micromole reserpine solution, using ±15 volts DC and balanced RF on the rods Ql, and various values of out-of-phase RF on exit lens 39. As would be expected from Fig. 23, it will be seen from Fig. 24 that the intensity increases as the RF on the exit lens 39 increases, but to a limiting value. As the limiting value is approached, peak broadening occurs. Thus, curves 162 to 172 were made at RF voltages of 0 volts, 27 volts, 55 volts, 77 volts, 105 volts and 150 volts RF, respectively (peak-to-peak), on exit lens 39. In all cases, it is believed that sufficient DC should be applied to align the majority of the ions with one pole pair (subject to the comments made below), and then RF is applied phased to retard the aligned ions, so that they acquire greater kinetic energy in the fringing fields. The phased RF can be applied either by unbalancing the RF on the rods Ql, or by applying RF suitably phased to the exit lens 39 or by other suitable techniques. While some ions may be aligned with the other pole pair (the x-pole pair in Fig. 18), and while these ions may be accelerated through the fringing field by the unbalanced RF or by the RF applied to the exit lens, so that they spend less time in the fringing fields and will therefore be ejected with less kinetic energy, only a relatively few ions will be so affected. The majority of the ions, which are aligned with one pole pair (the y-pole pair in Fig. 18), are retarded so as to spend more time in the fringing field and therefore ejected with greater kinetic energy, as desired. The amount of DC applied may be optimized in each case to yield the best intensity and peak shape (while not applying so much DC as to reduce unduly the bandwidth of the rods, thereby reducing the intensity). The fact that identical performance is achieved with unbalanced RF on the rods of Ql, or with auxiliary RF applied to the exit lens 39 when the Ql rods have balanced RF applied to them, is evidence that it is the exit rather than the entrance fringing fields that are important for the observed high kinetic energies of the ions leaving the rods Ql. Fig. 25 shows a typical spectrum 176 obtained in a high mass range using the invention. The spectrum shown is that of erythromycin, using balanced RF on the rods Ql, 130 volts RF on the exit lens 39 and ±9 volts DC on the rods Ql. It will be seen that the peaks shown are sharply defined with relatively high intensity as marked on the drawing. While the ions at the exit end of Ql have been described as being aligned with one pole pair by application of a small DC voltage to Ql, other techniques can be used to align the ions with one pole pair. Two examples are shown in Fig. 26, which shows the rods Ql. In one technique, the ions can be injected parallel to the central axis 180 of rods Ql but spaced radially from the central axis. The line along which the ions are injected is indicated at 182 in Fig. 26. The amount of off-set needed will depend on a number of factors, including particularly the ion beam divergence, the ion energies, and the RF frequency, and will require case- by-case optimization. In many instances, an off-set of 25% of the radius from the centre line to the inner surface of the rods of Ql (r 0 as explained at the beginning of this detailed description) will be sufficient, based on computer simulations. In the other technique, the ions are injected along a line 184 which is oriented at an angle to the central axis 180 of rods Ql. The preferred injection angle will again be optimized on a case-by-case basis, bearing in mind that if the angle is too large, too many ions will be lost to the rods, and if the angle is too small, the ions would not become aligned sufficiently with one pole pair. In many cases, an injection angle of approximately 5° from the central axis 180 will be appropriate, based on computer simulations. Both these techniques will have the effect of preferentially aligning the majority of the ions with one of the pole pairs, so that they can be made to spend more time in the exit fringing fields with the use of suitably phased or unbalanced RF, and thus can be ejected with greater kinetic energy. While the invention has been described as directing ions from an ion source into the resolving rods in question, if desired some or all of the ions can instead be formed within the rods, e.g., by ion reactions or by any other desired means. While preferred embodiments of the invention have been described, it will be appreciated that various modifications will occur to those skilled in the art, and all such changes are intended to be encompassed by the appended claims.

Claims: I CLAIM: 1. A method of operating a mass spectrometer having a rod set which has at least two pole pairs and an exit end, said method comprising directing ions into or forming ions in said rod set, transmitting ions from said exit end of said rod set as transmitted ions, applying an RF voltage to said rod set, aligning some of said transmitted ions with one said pole pair and the number of transmitted ions being aligned with said one pole pair being greater than the number of transmitted ions not so aligned, and ejecting the ions aligned with said one pole pair from said exit end with greater kinetic energy than the ions not so aligned. 2. A method according to claim 1 wherein ions are aligned with said one pole pair adjacent said exit end by applying a low level resolving DC voltage to said rod set. 3. A method according to claim 2 wherein said DC voltage is between 0.3 volts DC and 40% of the DC normally required for said rod set to operate at the tip of the a-q stability diagram for said rod set. 4. A method according to claim 3 wherein said DC voltage is in the range between 1 and 15.5 volts. 5. A method according to claim 1 wherein said rod set has a central longitudinal axis and said ions are aligned with said one pole pair adjacent said exit end by injecting them into said rod set in a direction parallel to and off-set from said central axis. 6. A method according to claim 1 wherein said rod set has a central longitudinal axis and wherein said ions are aligned with said one pole pair adjacent said exit end by injecting them into said rod set at an angle to said central axis. 7. A method according to any of claims 1 to 6 wherein said rod set is a quadrupole rod set. 8. A method according to any of claims 1 to 6 wherein said rod set is a quadrupole rod set and including the step of energy filtering said transmitted ions, and after said step of energy filtering, detecting ions for analysis. 9. A method according to any of claims 1 to 6 wherein said rod set is a quadrupole rod set and including the step of energy filtering said transmitted ions, and after said step of energy filtering, detecting ions for analysis, and wherein said ions aligned with said one pole pair are given greater axial kinetic energy than ions not so aligned by applying an unbalanced RF voltage to said pole pairs. 10. A method according to any of claims 1 to 6 wherein said rod set is a quadrupole rod set and including the step of energy filtering said transmitted ions, and after said step of energy filtering, detecting ions for analysis, and wherein said ions aligned with said one pole pair are given greater axial kinetic energy than ions not so aligned by applying an unbalanced RF voltage to said pole pairs, said unbalance being about 10% to 60% peak-to-peak. 11. A method according to any of claims 1 to 6 wherein said rod set is a quadrupole rod set and including the step of energy filtering said transmitted ions, and after said step of energy filtering, detecting ions for analysis, said mass spectrometer having an exit lens spaced from said exit end of said rod set, said ions aligned with one pole pair being given greater axial kinetic energy than ions not so aligned by applying an RF voltage to said exit lens, the RF voltage applied to said exit lens having a predetermined phase relationship with the RF voltage applied to said rod set. 12. A method according to any of claims 1 to 6 wherein said rod set is a quadrupole rod set and including the step of energy filtering said transmitted ions, and after said step of energy filtering, detecting ions for analysis, said mass spectrometer having a set of secondary rods following the quadrupole rod set, said method including applying an RF voltage to said secondary rods, the RF voltage on at least one of said quadrupole rod set and said secondary rods being unbalanced. 13. A method according to any of claims 1 to 6 wherein said rod set is a quadrupole rod set and including the step of energy filtering said transmitted ions, and after said step of energy filtering, detecting ions for analysis, said mass spectrometer having a set of secondary rods following the quadrupole rod set, said method including applying a first RF voltage to said secondary rods, said first RF voltage having a predetermined relation with the RF voltage applied to said quadrupole rod set, and further applying an additional RF voltage to said secondary rods for said secondary rods to act as an exit lens. 14. A method of operating a mass spectrometer having a rod set, comprising: directing ions into or forming ions in said rod set, transmitting ions from said rod set as transmitted ions, applying an unbalanced RF voltage to said rod set, applying a low level resolving DC voltage to said rod set, and thereby increasing the sensitivity of said mass spectrometer. 15. A method according to claim 14 wherein said DC voltage is between about 0.3 volts DC and 40% of the DC normally required for said rod set to operate at the tip of the a-q stability diagram for said rod set. 16. A method according to claim 15 wherein said DC voltage is in the range between 1 and 15.5 volts. 17. A method according to claim 14, 15 or 16 wherein the unbalance of said RF voltage is between 10% and 60% peak-to-peak. 18. A method according to claim 14, 15 or 16 wherein said rod set is a quadrupole, and including the step of energy filtering ions leaving said rod set and then after said step of energy filtering, detecting said ions for analysis. 19. A method according to claim 14, 15 or 16 wherein said rod set is a quadrupole rod set having at least two pole pairs and an exit end, and including the step of energy filtering said transmitted ions, and after said step of energy filtering, detecting ions for analysis, said mass spectrometer having an exit lens spaced from said exit ends of said rod set, said method further including applying an RF voltage to said exit lens, the RF voltage applied to said exit lens having a predetermined phase relationship with the RF voltage applied to said rod set. 20. A method according to claim 14, 15 or 16 wherein said rod set is a quadrupole rod set and including the step of energy filtering said transmitted ions, and after said step of energy filtering, detecting ions for analysis, said mass spectrometer having a set of secondary rods following said rod set, said method including applying an RF voltage to said secondary rods, the RF voltage on at least one of said rod set and said secondary rods being unbalanced. 21. A method of operating a mass spectrometer having a rod set having an exit end, and an exit lens spaced from said exit end, comprising: directing ions into or forming ions in said rod set, applying an RF voltage to said rod set, applying a low level resolving DC to said rod set, and applying an RF voltage to said exit lens, the RF voltage applied to said exit lens having a phase relative to the phase of the RF voltage applied to said rod set such as to increase the sensitivity of said mass spectrometer. 22. A method according to claim 21 wherein said DC voltage is between about 0.3 volts DC and 25% of the DC normally required for said rod set to operate at the tip of the a-q stability diagram for said rod set. 23. A method according to claim 22 wherein said DC is in the range between 1 and 15.5 volts. 24. A method according to claim 21, 22 or 23 wherein for ions of a selected polarity, one of said pole pairs has said selected polarity, and the RF voltage applied to said end lens is out-of-phase with the RF voltage applied to said one pole pair. 25. A method according to claim 21, 22 or 23 wherein for ions of a selected polarity, one of said pole pairs has said selected polarity, and the RF voltage applied to said end lens is out-of-phase with the RF voltage applied to said one pole pair, said rods being a quadrupole, and including the step of energy filtering ions leaving said rod set and then after said step of energy filtering, detecting said ions for analysis. 26. A method of operating a mass spectrometer having a first resolving rod set and a second auxiliary rod set following said first rod set, comprising: directing ions into or forming ions in said first rod set, applying an RF voltage to said first rod set and an RF voltage to said second rod set, applying a low level resolving DC voltage to said first rod set, the RF voltage applied to at least one of said first and second rod sets being unbalanced, thereby to increase the sensitivity of said mass spectrometer. 27. A method according to claim 26 wherein said DC voltage is between 0.3 volts DC and 40% of the DC normally required for said rod set to operate at the tip of the a-q stability diagram for said rod set. 28. A method according to claim 27 wherein said DC is in the range between 1 and 15.5 volts. 29. A method according to claim 26, 27 or 28 wherein said first rod set is a quadrupole, and including the step of energy filtering ions leaving said second rod set and then after said step of energy filtering, detecting said ions for analysis. 30. A method according to claim 26, 27 or 28 wherein said unbalance of said RF voltage is between about 10% and 60% peak-to-peak. 31. A method according to claim 26, 27 or 28 wherein the RF applied to said first rod set is unbalanced, and including applying auxiliary RF to said second rod set for said second rod set to act as an end lens. 32. A method according to any of claims 1 to 6, 14 to 16, and 21 to 23 wherein said rod set is approximately 24 mm to 48 mm in length, and said ions are directed at said rod set in a narrow collimated beam having a diameter of less than about 0.25 mm. 33. A method according to any of claims 1 to 6, 14 to 16, and 21 to 23 wherein said rod set is approximately 24 mm to 48 mm in length, and said ions are directed at said rod set in a narrow collimated beam having a diameter of less than about 0.1 mm.

Inventors: HAGER JAMES, HAGER, JAMES
Applicants: HAGER JAMES, MDS INC, HAGER, JAMES, MDS INC.
Application year: 1997
Main classification: H01J49
IPC codes: H01J49