Method of mass analyzing a sample by use of a quistor and a quistor designed for performing this method

Abstract: A method of mass analyzing a sample comprises the steps of defining a three-dimensional electrical quadrupole storage field including an RF component and creating sample ions therein so that ions within the mass range of interest are simultaneously trapped and perform ion-mass specific secular movements. In order to analyse the trapped ions, an exciting RF field is generated, the frequency of said exciting RF field being equal to the frequency of secular oscillations of ions having a specific mass. By varying either the frequency of the exciting RF field or the frequency of the secular oscillation of the ions by modifying the quadrupole storage field, the resonance condition for the secular motion is varied in such a way that ions of consecutive masses encounter a resonance of their secular movements with the exciting RF field, so that they take up energy, increase thereby their secular movement, and finally leave the trapping field for being detected. The method is advantageously performed in a QUISTOR having field faults which result in non-harmonic oscillations of trapped ions.

Description: The present invention is directed to a method of mass analyzing a sample according to the preamble of claim 1 and to a QUISTOR designed for performing this method according to the preamble of claim 3, and to a method of producing such a Quistor according to claim 5. This method and a QUISTOR suitable for performing this method are described in US-A-4 540 884. The basic construction of the device called "QUISTOR" ("QUadrupole Ion STORe") or "Ion Trap" is described in US-A-2 939 952. In such QUISTOR, a radio-frequency hyperbolic three-dimensional quadrupole field is generated in which ions of different mass-to-charge ratios can simultaneously be stored. For a detailed introduction into the quadrupole technique including QUISTORs, see Peter H. Dawson (editor), Quadrupole Mass Spectrometry and its Applications, Elsevier, 1976. The QUISTOR consists of a toroidal ring electrode and two end cap electrodes. A high RF voltage of amplitude V stor and frequency f stor is applied between the ring electrode and the two end cap electrodes. Both end cap electrodes are normally connected to the same potential. The radio-frequency voltage across the electrodes forms, at least near the center of the QUISTOR, a hyperbolic three-dimensional quadrupole field which is able to trap ions. As shown in Fig. 1 which illustrates, schematically the design of the ring and end cap electrodes 1 and 2, respectively, cylindrical coordinates are used to describe its configuration. The directions from the center 3 towards the saddle line of the ring electrode 1 are called the r directions and the plane defined by said r directions is called the r plane. The z direction is defined to be normal to the r plane. The ion oscillations by the RF field cause, integrated over time, a resulting force which is directed towards the center 3, and proportional to the distance from the center. This quasi-elastic central force forms, integrated over time, a harmonic oscillator for the ions. The relatively slower harmonic oscillations around the center 3 are superimposed by the faster impregnated RF oscillations. The harmonic oscillations are called the "secular" oscillations of the ions within the QUISTOR field. The exact mathematical description of the movements of ions in a QUISTOR is difficult. Up to now, a solution of the resulting partial equations was only possible for the special case of independent secular movements in the r and z directions. The solution of the corresponding "Mathieu"'s differential equations results in an "ideal QUISTOR" of fixed design: The slope of the asymptotic cone envelope has the "ideal angle" z/r = 1/1,414 (1,414 = square root of 2). Most of the QUISTORs which have been built up to now, follow the design principles of such an "ideal" QUISTOR with hyperbolic surfaces and the above "ideal" angle z/r, although it has been shown experimentally that QUISTORs of quite different design, e.g. with cylindrical surfaces, do store ions by no means less effective. In the special case of an "ideal QUISTOR", the secular oscillations are, by the inherent mathematical assumptions, independent and different in r and z directions. The stability area boundaries for the ion movements in the well-known a/q diagram shown in Fig. 2 can be calculated.The stability area is formed by a net of β r lines (0 < β r < 1) and crossing β z lines (0 < β z < 1). The β-lines describe exactly the secular frequencies: f sec,r = β r * f stor / 2; f sec,z = β z * f stor / 2. In Fig. 2, the stability area for an "ideal" QUISTOR is shown in the a z /q z diagram, together with the iso-β lines. For an ion having a given mass-to-charge-ratio m which is stored inside the stability boundaries given by the operating conditions a and q, there exists a unique secular frequency f sec,z,m (a,q) in the z direction and a (usually different) unique secular frequency f sec,r,m (a,q) in the r direction. "Non-ideal" QUISTORs which are not built according to the above mentioned ideal design criteria, or which show a lack of precision in production, do not have independent r and z secular motions but secular oscillations in one direction which are coupled with the secular oscillations in the other direction. The secular movements influence each other mutually, and, as it is known from coupled oscillators, resonance phenomena appear. Depending on the type of field distortions, several types of natural resonances, like "sum resonances" or "coupling resonances ", exist in a QUISTOR. Up to now, these resonances have been investigated and described only for the cases of superimposed weak multipole fields, see F. v. Busch and W. Paul, Z. Phys. 164 (1961),588. If the quadrupole field is superimposed by a weak multipole field, with one pole fixed in z direction, the conditions for sum resonances are: Type of field: most prominent resonance condition Order of potential terms quadrupole field: none second order, no mixed terms hexapole field: β z + β r /2 = 1 third order with mixed terms octopole field: β z + β r = 1 fourth order, with mixed terms and with equal signs for r⁴, z⁴ terms dodecapole field: β z /2 + β r = 1 sixth order with mixed terms Each electrical field is a first derivative (with respect to r and z) of the electrical potential. The mathematical expression for the electrical quadrupole potential contains only quadratic terms in r and z, and no mixed term. In the case of multipoles, however, terms of higher order and mixed terms appear in the mathematical expression of the electrical potential. The mixed terms represent the mutual influence of the secular movements, and the terms of an order higher than 2 represent non-harmonic additions which make the secular frequencies dependent on the amplitude of the secular oscillations. (For the exact formulae of multipole potentials, see the cited book by Dawson). The trapping field in the center of the QUISTOR is naturally mostly influenced by the shape of those parts of the electrodes which are nearest to the center. Thus, the curvature across the saddle line of the ring electrode and the curvature at the summit of the end cap electrodes influence the trapping field most. These curvatures can be described by inscribed circles 4 and 5, the one having the radius R r for the ring electode 1, and the other having the radius R e for the end cap electrodes 2. In fact, a QUISTOR can be built by an O-ring shaped ring electrode and two spheres as end cap electrodes, just equivalent to a quadrupole mass filter which can be successfully built from four cylindrical rods, (Fig. 1). Then, the ideal QUISTOR is characterized by the following condition for the distance-corrected ratio Q of the described radii: wherein r₀ = smallest distance of the ring electrode 1 from the field center 3 and z₀ = smallest distance of the end cap electrodes 2 from the field center 3. If a non-ideal QUISTOR has end cap and ring electrodes which are both too "sharp" (the radii R r and R e are both too small), or both too "blunt" (the radii are both too large compared with an ideal hyperbolic QUISTOR), its field can be described as an quadrupole field, distorted by the superposition of an octopole field. This is one of the most likely field distortions for QUISTORs. If in such a case, for a given ion mass m, the above sum resonance condition for octopoles is valid, the ion starts to resonate in the field and to take up energy from the RF field in both z and r directions. The oscillation amplitudes increase in both directions. Since the fourth order terms have the same sign in both directions, the frequencies of the oscillations in both directions either increase or decrease together. In both cases, the resonance condition is no longer fulfilled, and the resonance stops. This behaviour can easily be studied by simulations. - Other types of distortions by single multipoles show similar effects. The quadrupole field can be also distorted by too blunt end cap electrodes, and a too sharp ring electrode (Q > 4,000), or vice versa (Q < 4,000). These cases have not yet been investigated. Most prominent additional terms of the electrical potential are pure and mixed terms of fourth order in r and z, as in the case of superimposed octopoles, but with different signs in r and in z directions. The resonance condition β z + β r = 1 , remains valid in this case. Up to a few years ago, the QUISTOR was operated mostly in the so-called "mass selective ion storage mode". After each ionization period, only a preselected single kind of ions was stored by applying corresponding operating conditions near the tip of the stability region, and subsequently measured by ejection through one of the end cap electrodes. A spectrum was acquired by frequent repetitions of this procedure with slightly altered storage conditions for the storage and subsequent detection of ions having different masses. In the context of the present invention, this method is not regarded as a "scan" method. Hereafter, the term "scan method" is used to designate the measurement of the ions in a wide range of ion masses which are generated in a single ionization process, and stored simultaneously in the QUISTOR. Up to now, the "mass selective instability ejection method" described in US-A-4 540 884 is the only ion ejection scan method which has become known. In its simplest form, this method operates the QUISTOR along the line a = 0 of the a z /q z -diagram shown in Fig. 2 only, i.e., the QUISTOR is driven in the "RF only" mode without applying a superimposed DC voltage. In this mode, theoretically all ions having a mass above a lower cut-off mass, can be stored within the QUISTOR. The cut-off ion mass is given by the limit of the stability area on the q axis : wherein q z,1 = 0,91 limit of the stability area for a = 0, (characterized by β z = 1) e = charge of the electron V stor = peak amplitude of the basic RF voltage r₀ = inner radius of the ring electrode ω = 2*π*f stor angular frequency of the basic RF voltage As can be seen from the above formula, m cut-off at the border of the stability area is direct proportional to the amplitude V stor of the basic RF voltage. By scanning V stor to higher values, the storage condition for one ion mass after the other shifts across the stability limit and the movement of the ions become unstable. It is of interest to note that only the ion movement in z direction becomes unstable (β z > 1). The secular oscillations in r direction remain stable (β r = 0,34). Thus, the ions do not longer encounter a backpulling central force in z direction. In contrast, in the z direction the averaged central force reverses its direction and the ions are driven against the end cap electrodes of the QUISTOR. If the tip of one of the end cap electrodes is perforated, a fraction of the ions penetrates through the perforations and can be detected outside the QUISTOR by well-known mass spectrometric means, e.g. by a secondary electron multiplier. This method of creating a mass spectrum, however, has two severe fundamental drawbacks: First, only a small part of the ions hits the perforations in the center of the end cap electrode, whereas a major part of the ions hits the end cap electrode far off from the z axis. The secular frequency movement of the ions in r direction makes the probability very small that an ion resides - at a given time - near the z axis. - It is a difficult task to increase the perforated area and to focus the penetrating ions onto the detection device. Due to the high RF peak voltages V stor , the ions partially have very high kinetic energies when they leave the QUISTOR, and a simultaneous focussing of all ions is thus impossible. Second, ions situated close to the center of the field do not see very much of a field because the field in the center is exactly zero. Ions near the center do not leave the QUISTOR, unless they are hit by another particle, leave the center under the effect of the pulse transfer, encounter a destabilizing field outside the center, and move towards one of the end cap electrodes. In fact, not only the ions near the center are not ejected immediately, but all ions which move almost inside the r plane. At low pressure within the QUISTOR, this process of kicking the ions out of the r plane takes time. At a given scan speed, on the other hand, a long time to leave decreases the spectral resolution. Both problems connected with these drawbacks can be at least partially overcome by the introduction of a damping gas. As shown in US-A-4 540 884, a damping gas (e.g. Helium) increases the spectrum resolution and the ion yield considerably. Both effects can be explained by above considerations. On one hand, the secular movements are damped, and the ions are concentrated near the center. On the other hand, frequent collisions of particles do not allow for long ion residing periods in the field-free center or in the r plane which is free of z field components. According to our investigations, for a QUISTOR with radius r₀ = 1 cm and a storage frequency of 1 MHz, the optimum pressure for Helium as a damping gas is very near to 0,15 Pa (1,5 * 10⁻³ mbar), and the corresponding minimum leaving time for 95 % of the ions of one mass during a linear V stor scan is about 200 µs. It is the object of the present invention to improve the technique of creating a mass spectrum and to eliminate the severe drawbacks of the presently known methods by ion ejection. This object is met by the invention as defined in the characterizing part of claim 1. As may be gathered therefrom, the present invention deals with a new method of creating a mass spectrum. In contrast to the "mass selective instability ejection", this method takes place inside the ion stability region, usually even from such spots inside the stability area where the ion storage stability is especially large. (The storage stability may be defined as resistance against defocussing DC fields. The stability diagram of Fig. 2 reveals that at q z = 0,78 the point of maximum stability can be found because here positive or negative DC voltages of maximum strength can be applied without destroying the storage capability for the respective ions). During the scan the ion movements never become instable but the amplitude of the movement is increased steadily by the resonance effect. This new method needs additional electrical circuitry: An excitation RF voltage with frequency f exc has to be applied across the end cap electrodes of the QUISTOR. If the QUISTOR has field faults which result in non-harmonic oscillations, the scan speed then shows a marked influence onto the quality and resolution of spectrum peaks. Thus, the invention also comprises the QUISTOR as defined by the characterizing part of claim 3. In such a QUISTOR the frequency of the secular oscillations changes with the amplitude of the oscillations. If an ion increases its secular frequency with amplitude (positive terms of fourth order), it will only stay in resonance with the exciting voltage for a longer period of time, if the frequency of the exciting field increases at the same speed during the scan. If the correct scan speed is applied, there is a typical double resonance effect: the secular frequency is in resonance with the exciting frequency, and the increasing rate of the secular frequency is in resonance with the scan speed. Naturally, this type of resonance is not as sharp as the resonance of the secular frequency with the excitation frequency because the scan speed has to hold the ion within resonance for a short period of time only. The resonance maximum is very wide, and deviations by a factor of two do not seriously destroy the effect. This type of scan may be called "mass selective double-resonance scan". A further enhancement of this scan, however, is possible by making use of specific natural resonances in a non-ideal QUISTOR. This type of scan may be designated as "mass selective triple-resonance ejection". As described above, field faults of fourth order produce a coupling resonance effect between the secular oscillations in r and z direction at the location β z + β r = 1 . If the distortions are produced by a QUISTOR obeying one of the conditions for the distance-corrected ratio Q of the inscribed pole radii Q < 4,000 or Q > 4,000 , resonating ions see an increase of their secular movement amplitude in z direction, and a decrease in r direction. Thus, the ions are focussed in z direction during z ejection, and are ideally suited for a high-gain ejection through a small perforated area at the tip of one of the end cap electrodes. It is now possible to apply a fixed excitation frequency and an RF voltage amplitude scan to such a QUISTOR with such field distortions of fourth order, to tune the excitation frequency exactly into such a natural coupling resonance, and to adjust the scan speed to keep the ions in resonance. Three resonance phenomena appear simultaneously: First, the resonating ions take up energy from the excitation field and increase their oscillation amplitudes in z direction. Second, the ion movement in z direction gains additional energy from the coupled movement in r direction. The ions are gathered near the z axis. The secular frequency of the ions becomes slower in z direction, and higher in r direction, and the condition β r + β z = 1 remains nearly fulfilled by a partial compensation. The compensation, however, is only nearly exact, if the amplitudes are similarly large.If the r amplitude is small the r secular frequency changes only very slowly, and the compensation stops. - This resonance concentrates the ions near the z axis and increases largely the ion gain. Third, the V stor scan increases the secular frequencies of a given ion. This compensates the decreasing secular frequency in z direction which stems from the increasing amplitude. If the scan speed is correctly chosen, the ions are held in resonance with the exciting frequency. In a well-tuned system, the ions leave the QUISTOR very near to the z axis. Almost all the ions penetrate the perforations at the tip of the end cap electrode. To direct the ions to the correct end cap electrode, a field fault of third order might be introduced, or a small DC voltage may be applied between both end cap electrodes, in addition to the exciting frequency. The ion yield supersedes that of the damping gas optimized mass selective instability ejection scan by a factor of more than ten, i.e., this type of triple resonance scan makes a tenfold better use of the ions stored in a QUISTOR. The time to leave the QUISTOR is extremely short in the case of the triple resonance: it was possible to produce, with a QUISTOR of selected design, completely resolved spectra at scan speeds of one mass unit in 36 µs only, i.e., all ions of a given mass-to-charge ratio were ejected in 36 periods of the basic RF voltage only, or in only 12 oscillations of the secular frequency. This is about six times faster than the maximum speed for the "mass selective instability ejection scan", each scan exhibiting tenfold the gain. Besides higher yield, faster scan rates, and better line shape, the triple-resonance ejection scan possesses still another advantage over the mass selective instability ejection scan: It needs lower RF voltage amplitudes V for the ejection of the same masses. The mass instability limit is characterized by q z,1 = 0,91, whereas the triple-resonance value is q z,reson = 0,82. Since the q values are reversely proportional to the masses, the triple-resonance scan needs a voltage lower by about 9 % for the same mass. This is a considerable advantage because the generation of the high-voltage RF is one of the most difficult tasks in the design of QUISTORs. In practice, the triple-resonance scan sometimes exhibits a very bad peak shape which is caused by a beat between the exciting high frequency voltage, and a small fraction (in most cases 1/3 or 1/4) of the high frequency storing voltage. In this case, the electrodes of the QUISTOR can be formed with such a distance-corrected ratio Q of the radii that one of the natural resonance frequencies of the secular ion movement coincides exactly with the fraction of the high frequency storage voltage. If the exciting high frequency voltage then is generated from the storage high frequency (e.g. by frequency division), the peak shape of the ions in the spectrum is excellent. One way of carrying out the invention is described in detail below with reference to drawings which illustrate only one specific embodiment of a QUISTOR and the results of measurement performed therewith. In the drawings, Fig. 3 is a schematic crossectional view of a QUISTOR designed and used for performing the method according to the invention and Fig. 4 is a spectrum obtained in the QUISTOR shown in Fig. 3 by means of the triple resonance scan after CI ionization, from a mixture containing tetrachloroethene, toluene and acetone, Fig. 5 is the molecular ion group of tetrachloroethene - enlarged from the spectrum of Fig. 4, Fig. 6 is a single shot enlargement from an extrmeley fast scan, comprising the molecular group of tetrachloroethene similar to Fig. 5 but obtained after EI ioniziation. The QUISTOR mass spectrometer shown in Fig. 3 includes a hyperbolically formed ring electrode 11 and end cap electrodes 12, 13 with an angle 1:1,3784 of the hyperbole asymptotes, giving a Q = 3,610. The electrodes are correctly spaced by insulators 14, 15. The natural resonance frequency f res,z obeying the condition f res,z + f res,r = f stor /2 , matches exactly 1/3 of the storage high frequency f stor . To the ring electrode 11 a storage RF-generator 16 is connected. Similarly, to one of the cap electrodes 13 an exciting RF-generator 17 is connected. Using a storage frequency of f stor = 1 MHz, the exciting frequency is f exc = 333,333 kHz. The latter can be advantageously generated by dividing the frequency of the storage RF-generator 16, so that the exciting RF-generator 17 may be formed by a frequency divider connected to the storage RF-generator 16 as indicated by broken line 18. The optimum voltage of the exciting frequency depends on the scan speed, and ranges from 1 V to about 20 V. With an inner radius of the ring electrode 11 of r r = 1 cm, and with ions stored in the QUISTOR during a preceding ionization phase, a scan of the high frequency storing voltage V stor from a minimum value upwards to 7.5 kV yields a spectrum up to more than 500 atomic mass units in a single scan. Ions may be formed by an electron beam which is generated by a heated filament 21 and a lens plate 22 which focusses the electrons through a hole 23 in the end cap electrode 12 into the QUISTOR during the ionization phase, and stops the electron beam during other time phases. During the scan period, ions are ejected through the perforations 24 in the end cap electrode 13 and measured by a multiplier 25. With this QUISTOR, a chemical ionization (CI) spectrum of acetone, toluene, and tetrachloroethene was measured. The full spectrum covered the mass range from 39 u to 500 u, the section from 39 u to nearly 180 u being shown in Fig. 4. The spectrum was taken by means of an RF voltage amplitude scan. Fig. 4 is the spectrum of a single shot taken in 33 ms. A spectra repetition rate of 25 spectra/s left time for 250 µs of quenching, 1 ms of ionisation and 5 ms of CI reaction. Fig. 4 illustrates the high resolution and the high sensitivity which is achieved with this method even in a single shot. This fact is emphasized by the enlarged section from Fig. 4 shown in Fig. 5, which section comprises the molecular group of tetrachloroethene. As a second example, Fig. 6 shows the tetrachloroethene comprising enlargement of a single shot spectrum similar to Fig. 4, but taken in 8 ms from mass 30 u to mass 180 u with a repetition rate of 100 spectra/s and making use of EI ionisation.

Claims: The method of mass analyzing a sample by means of a QUISTOR which comprises the steps of a) generating an electrical field comprising an ideal three-dimensional electrical quadrupole storage field and comparatively small field components of higher multipole order including an RF component, with frequency f stor and amplitude Vs tor , wherein ions within a mass range of interest can oscillate with secular frequencies 0≦f sec ≦f stor /2 ; b) introducing sample ions into or creating sample ions within the quadrupole storage field; c) generating an exciting RF field with the frequency f exc , d) changing parameters of the electrical field causing ions of consecutive masses to encounter consecutive resonances of their mass specific secular oscillations with the exciting RF field; e) detecting the ions of consecutive masses as they leave the storage field, f) providing an output signal indicative of the ion masses, characterized in that the frequency f exc of the exciting RF voltage matches a natural sum or coupling resonance frequency created by the field components of higher multipole order. The method according to claim 1 in which the changing speed for the parameters of the electrical field is chosen such that the shift of the secular frequency with increasing amplitude, caused by the field components of higher multipole order, is at least partially compensated for by an opposite shift of the secular frequency caused by a resulting change of the parameters themselves. A QUISTOR comprising a toroidal ring electrode (11), first and second end cap electrodes (12, 13) mounted in coaxial relationship to and axially spaced from said ring electrode (11) for the generation of an electric field for the storage ions characterized in that the radii of the curved end electrodes and of the curved ring electrode, both defined in the points nearest to the field center, in order to generate higher order multipole components of the electric storage field resulting in natural resonances of the secular movements, obey the condition 0,500 < Q < 3,990 or 4,010 < Q < 25,0 , where R e = radius of the end electrodes in the points nearest to the field center R r = radius of the ring electrode in the points nearest to the field center r₀ = smallest distance of the ring electrode from the field center z₀ = smallest distance of the end electrodes from the field center. The QUISTOR as in claim 3, characterized in that the shapes of said ring and said end cap electrodes (11, 12, 13) deviate from a hyperbolic shape having an ideal angle of arctan (√2) of the asymptotes. Method of producing a QUISTOR according to claim 3 or 4 in which the angle of the asymptotes of the QUISTOR electrodes are chosen so that the natural coupling resonance frequency of the secular ion movements coincides with a low fraction 1/n (n = small integer number greater than 2) of the storage frequency, and in which the excitation voltage with frequency f exc = f stor /n is coupled in frequency and phase to the storage voltage with frequency f stor . The method as in claim 5 in which n = 3.

Inventors: FRANZEN JOCHEN, GABLING REEMT HOLGER, HEINEN GERHARD, WEISS GERHARD, FRANZEN, JOCHEN, GABLING, REEMT HOLGER, HEINEN, GERHARD, WEISS, GERHARD
Applicants: BRUKER FRANZEN ANALYTIK GMBH
Application year: 1988
Main classification: H01J49
IPC codes: H01J49