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Patent US7141789 - Method and apparatus for providing two-dimensional substantially quadrupole ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsA method and apparatus for manipulating ions using a two-dimensional substantially quadrupole field, and a method of manufacturing and operating an apparatus for manipulating ions using a two-dimensional substantially quadrupole field are described. The field has a quadrupole harmonic with amplitude...http://www.google.com/patents/US7141789?utm_source=gb-gplus-sharePatent US7141789 - Method and apparatus for providing two-dimensional substantially quadrupole fields having selected hexapole componentsAdvanced Patent SearchPublication numberUS7141789 B2Publication typeGrantApplication numberUS 10/943,069Publication dateNov 28, 2006Filing dateSep 17, 2004Priority dateSep 25, 2003Fee statusPaidAlso published asCA2539221A1, EP1668665A1, EP1668665A4, US20050067564, WO2005029533A1Publication number10943069, 943069, US 7141789 B2, US 7141789B2, US-B2-7141789, US7141789 B2, US7141789B2InventorsDonald J. Douglas, Chuan-Fan Ding, Frank LondryOriginal AssigneeMds Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (38), Non-Patent Citations (54), Referenced by (9), Classifications (13), Legal Events (8) External Links: USPTO, USPTO Assignment, EspacenetMethod and apparatus for providing two-dimensional substantially quadrupole fields having selected hexapole componentsUS 7141789 B2Abstract A method and apparatus for manipulating ions using a two-dimensional substantially quadrupole field, and a method of manufacturing and operating an apparatus for manipulating ions using a two-dimensional substantially quadrupole field are described. The field has a quadrupole harmonic with amplitude A2 and a hexapole harmonic with amplitude A3. The amplitude A3 of the hexapole component of the field is selected to improve the performance of the field with respect to ion selection and ion fragmentation.
FIELD OF THE INVENTION This invention relates in general to quadrupole fields, and more particularly to quadrupole electrode systems for generating improved quadrupole fields for use in mass spectrometers.
BACKGROUND OF THE INVENTION The use of quadrupole electrode systems in mass spectrometers is known. For example, U.S. Pat. No. 2,939,952 (Paul et al.) describes a quadrupole electrode system in which four rods surround and extend parallel to a quadrupole axis. Opposite rods are coupled together and brought out to one of two common terminals. Most commonly, an electric potential V(t)=+(U−V cos Ωt) is then applied between one of these terminals and ground and an electric potential V(t)=−(U−V cos Ωt) is applied between the other terminal and ground. In these formulae, U is a DC voltage, pole to ground, and V is a zero to peak AC voltage, pole to ground, and ω is the angular frequency of the AC. The AC component will normally be in the radio frequency (RF) range, typically about 1 MHz.
In constructing a linear quadrupole, the field may be distorted so that it is not an ideal quadrupole field. For example round rods are often used to approximate the ideal hyperbolic shaped rods required to produce a perfect quadrupole field. The calculation of the potential in a quadrupole system with round rods can be performed by the method of equivalent charges�see, for example, Douglas et al., Russian Journal of Technical Physics, 1999, Vol. 69, 96�101. When presented as a series of harmonic amplitudes A0, A1, A2 . . . An, the potential in a linear quadrupole can be expressed as follows:
ϕ ( x , y , z , t ) = V ( t ) � ϕ ( x , y ) = V ( t ) ∑ n ϕ n ( x , y ) ( 1 ) Field harmonics φn, which describe the variation of the potential in the X and Y directions, can be expressed as follows:
a x = - a y = a = 8 eU m ion Ω 2 r 0 2 and q x = - q y = q = 4 eV m ion Ω 2 r 0 2 ( 7 ) where e is the charge on an ion, mion is the ion mass, Ω=2πf where f is the AC frequency, U is the DC voltage from a pole to ground and V is the zero to peak AC voltage from each pole to ground. If the potentials are applied with different voltages between pole pairs and ground, then in equation (7) U and V are � of the DC potential and the zero to peak AC potential respectively between the rod pairs. Combinations of a and q which give stable ion motion in both the X and Y directions are usually shown on a stability diagram.
As well, when linear quadrupoles are operated as a mass filter the DC and AC voltages (U and V) are adjusted to place ions of one particular mass to charge ratio just within the tip of a stability region, as described. Normally, ions are continuously introduced at the entrance end of the quadrupole and continuously detected at the exit end. Ions are not normally confined within the quadrupole by stopping potentials at the entrance and exit. An exception to this is shown in the papers Ma'an H. Amad and R. S. Houk, �High Resolution Mass Spectrometry With a Multiple Pass Quadrupole Mass Analyzer�, Analytical Chemistry, 1998, Vol. 70, 4885�4889, and Ma'an H. Amad and R. S. Houk, �Mass Resolution of 11,000 to 22,000 With a Multiple Pass Quadrupole Mass Analyzer�, Journal of the American Society for Mass Spectrometry, 2000, Vol. 11, 407�415. These papers describe experiments where ions were reflected from electrodes at the entrance and exit of the quadrupole to give multiple passes through the quadrupole to improve the resolution. Nevertheless, the quadrupole was still operated at low pressure, although this pressure is not stated in these papers, and with the DC and AC voltages adjusted to place the ions of interest at the tip of the first stability region.
In contrast, when linear quadrupoles are operated as ion traps, the DC and AC voltages are normally adjusted so that ions of a broad range of mass to charge ratios are confined. Ions are not continuously introduced and extracted. Instead, ions are first injected into the trap (or created in the trap by fragmentation of other ions, as described below, or by ionization of neutrals). Ions are then processed in the trap, and are subsequently removed from the trap by a mass selective scan, or allowed to leave the trap for additional processing or mass analysis, as described. Ion traps can be operated at much higher pressures than quadrupole mass filters, for example 3�10−3 torr of helium (J. C. Schwartz, M. W. Senko, J. E. P. Syka, �A Two-Dimensional Quadrupole Ion Trap Mass Spectrometer�, Journal of the American Society for Mass Spectrometry, 2002, Vol. 13, 659�669; published online Apr. 26, 2002 by Elsevier Science Inc.) or up to 7�10−3 torr of nitrogen (Jennifer Campbell, B. A. Collings and D. J. Douglas, �A New Linear Ion Trap Time of Flight System With Tandem Mass Spectrometry Capabilities�, Rapid Communications in Mass Spectrometry, 1998, Vol. 12, 1463�1474; B. A. Collings, J. M. Campbell, Dunmin Mao and D. J. Douglas, �A Combined Linear Ion Trap Time-of-Flight System With Improved Performance and MSn Capabilities�, Rapid Communications in Mass Spectrometry, 2001, Vol. 15, 1777�1795. Typically, ion traps operate at pressures of 10−1 torr or less, and preferably in the range 10−5 to 10−2 torr. More preferably ion traps operate in the pressure range 10−4 to 10−2 torr. However ion traps can still be operated at much lower pressures for specialized applications (e.g. 10−9 mbar (1 mbar=0.75 torr) M. A. N. Razvi, X. Y. Chu, R. Alheit, G. Werth and R. Blumel, �Fractional Frequency Collective Parametric Resonances of an Ion Cloud in a Paul Trap�, Physical Review A, 1998, Vol. 58, R34�R37). For operation at higher pressures, gas can flow into the trap from a higher pressure source region or can be added to the trap through a separate gas supply and inlet.
Recently, there has been interest in performing mass selective scans by ejecting ions at the stability boundary of a two-dimensional quadrupole ion trap (see, for example, U.S. Pat. No. 5,420,425 (Bier et al., issued May 30, 1995); J. C. Schwartz, M. W. Senko, J. E. P. Syka, �A Two-Dimensional Quadrupole Ion Trap Mass Spectrometer�, Journal of the American Society for Mass Spectrometry, 2002, Vol. 13, 659�669; published online Apr. 26, 2002 by Elsevier Science Inc.). In the two-dimensional ion trap, ions are confined radially by a two-dimensional quadrupole field and are confined axially by stopping potentials applied to electrodes at the ends of the trap. Ions are ejected through an aperture or apertures in a rod or rods of a rod set to an external detector by increasing the AC voltage so that ions reach their stability limit and are ejected to produce a mass spectrum.
Mass spectrometry (MS) will often involve the fragmentation of ions and the subsequent mass analysis of the fragments (tandem mass spectrometry). Frequently, selection of ions of a specific mass to charge ratio or ratios is used prior to ion fragmentation caused by Collision Induced Dissociation (CID) with a collision gas or other means (for example, by collisions with surfaces or by photodissociation with lasers). This facilitates identification of the resulting fragment ions as having been produced from fragmentation of a particular precursor ion. In a triple quadrupole mass spectrometer system, ions are mass selected with a quadrupole mass filter, collide with gas in an ion guide, and mass analysis of the resulting fragment ions takes place in an additional quadrupole mass filter. The ion guide is usually operated with AC only voltages between the electrodes to confine ions of a broad range of mass to charge ratios in the directions transverse to the ion guide axis, while transmitting the ions to the downstream quadrupole mass analyzer. In a three-dimensional ion trap mass spectrometer, ions are confined by a three-dimensional quadrupole field, a precursor ion is isolated by resonantly ejecting all other ions or by other means, the precursor ion is excited resonantly or by other means in the presence of a collision gas and fragment ions formed in the trap are subsequently ejected to generate a mass spectrum of fragment ions. Tandem mass spectrometry can also be performed with ions confined in a linear quadrupole ion trap. The quadrupole is operated with AC only voltages between the electrodes to confine ions of a broad range of mass to charge ratios. A precursor ion can then be isolated by resonant ejection of unwanted ions or other methods. The precursor ion is then resonantly excited in the presence of a collision gas or excited by other means, and fragment ions are then mass analyzed. The mass analysis can be done by allowing ions to leave the linear ion trap to enter another mass analyzer such as a time-of-flight mass analyzer (Jennifer Campbell, B. A. Collings and D. J. Douglas, �A New Linear Ion Trap Time of Flight System With Tandem Mass Spectrometry Capabilities�, Rapid Communications in Mass Spectrometry, 1998, Vol. 12, 1463�1474; B. A. Collings, J. M. Campbell, Dunmin Mao and D. J. Douglas, �A Combined Linear Ion Trap Time-of-Flight System With Improved Performance and MSn Capabilities�, Rapid Communications in Mass Spectrometry, 2001, Vol. 15, 1777�1795) or by ejecting the ions through an aperture or apertures in a rod or rods to an external ion detector (M. E. Bier and John E. P. Syka, U.S. Pat. No. 5,420,425, May 30, 1995; J. C. Schwartz, M. W. Senko, J. E. P. Syka, �A Two-Dimensional Quadrupole Ion Trap Mass Spectrometer�, Journal of the American Society for Mass Spectrometry, 2002, Vol. 13, 659�669; published online Apr. 26, 2002 by Elsevier Science Inc.). Alternatively, fragment ions can be ejected axially in a mass selective manner (J. Hager, �A New Linear Ion Trap Mass Spectrometer�, Rapid Communications in Mass Spectrometry, 2002, Vol. 16, 512�526 and U.S. Pat. No. 6,177,668, issued Jan. 23, 2001 to MDS Inc.). The term MSn has come to mean a mass selection step followed by an ion fragmentation step, followed by further ion selection, ion fragmentation and mass analysis steps, for a total of n mass analysis steps.
Similar to mass analysis, CID is assisted by moving ions through a radio frequency field, which confines the ions in two or three dimensions. However, unlike conventional mass analysis in a linear quadrupole mass filter, which uses fields to impart stable trajectories to ions having the selected mass to charge ratio and unstable trajectories to ions having unselected mass to charge ratios, quadrupole fields when used with CID are operated to provide stable but oscillatory trajectories to ions of a broad range of mass to charge ratios. In two-dimensional ion traps, resonant excitation of this motion can be used to fragment the oscillating ions. However, there is a trade off in the oscillatory trajectories that are imparted to the ions. If a very low amplitude motion is imparted to the ions, then little fragmentation will occur. However, if a larger amplitude oscillation is provided, then more fragmentation will occur, but some of the ions, if the oscillation amplitude is sufficiently large, will have unstable trajectories and will be lost. There is a competition between ion fragmentation and ion ejection. Thus, both the trapping and excitation fields must be carefully selected to impart sufficient energy to the ions to induce fragmentation, while not imparting so much energy as to lose the ions. In some instruments (J. Hager, �A New Linear Ion Trap Mass Spectrometer�, Rapid Communications in Mass Spectrometry, 2002, Vol. 16, 512�526), with some modes of operation, it is desirable to use a linear quadrupole rod set as an ion trap to resonantly excite ions for MS/MS and in other modes to use the same rod set as a mass filter.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Referring to FIG. 1, there is illustrated a quadrupole rod set 10 according to the prior art. Quadrupole rod set 10 comprises rods 12, 14, 16 and 18. Rods 12, 14, 16 and 18 are arranged symmetrically around axis 20 such that the rods have an inscribed circle C having a radius r0. The cross sections of rods 12, 14, 16 and 18 are ideally hyperbolic and of infinite extent to produce an ideal quadrupole field, although rods of circular cross-section are commonly used. As is conventional, opposite rods 12 and 14 are coupled together and brought out to a terminal 22 and opposite rods 16 and 18 are coupled together and brought out to a terminal 24. An electrical potential V(t)=+(U−V cos Ωt) is applied between terminal 22 and ground and an electrical potential V(t)=−(U−V cos Ωt) is applied between terminal 24 and ground. When operating conventionally as a mass filter, as described below, for mass resolution, the potential applied has both a DC and AC component. For operation as a mass filter or an ion trap, the potential applied is at least partially-AC. That is, an AC potential will always be applied, while a DC potential will often, but not always, be applied. As is known, in some cases just an AC voltage is applied. The rod sets to which the positive DC potential is coupled may be referred to as the positive rods and those to which the negative DC potential is coupled may be referred to as the negative rods.
a x = - a y = a = 8 eU m ion Ω 2 r 0 2 and q x = - q y = q = 4 eV m ion Ω 2 r 0 2 ( 7 ) where e is the charge on an ion, mion is the ion mass, Ω=2πf where f is the AC frequency, U is the DC voltage from a pole to ground and V is the zero to peak AC voltage from each pole to ground. Combinations of a and q which give stable ion motion in both the X and Y directions are shown on the stability diagram of FIG. 2. The notation of FIG. 2 for the regions of stability is taken from P. H. Dawson ed., �Quadrupole Mass Spectrometry and Its Applications�, 1976, Elsevier, Amsterdam, 19�23 and 70. The �first� stability region refers to the region near (a,q)=(0.2, 0.7), the �second� stability region refers to the region near (a,q)=(0.02, 7.55) and the �third� stability region refers to the region near (a,q)=(3,3). It is important to note that there are many regions of stability (in fact an unlimited number). Selection of the desired stability regions, and selected tips or operating points in each region, will depend on the intended application.
ω x = ( 2 n + β x ) Ω 2 ( 9 ) ω y = ( 2 n + β y ) Ω 2 ( 10 ) where n=0, �1, �2, �3 . . . 0≦βx≦1, 0≦βy≦1, and βx and βy are determined by the Mathieu parameters a and q for motion in the X and Y directions respectively (equation 7).
When higher field harmonics are present in a linear quadrupole, so called nonlinear resonances may occur. As shown for example by Dawson and Whetton (P. H. Dawson and N. R. Whetton, �Non-Linear Resonances in Quadrupole Mass Spectrometers Due to Imperfect Fields�, International Journal of Mass Spectrometry and Ion Physics, 1969, Vol. 3, 1�12) nonlinear resonances occur when
A 2 ( x 2 - y 2 r 0 2 ) + A 3 ( x 3 - 3 x y 2 r 0 3 ) = � constant ( 12.1 ) Assuming r0=1 and constant=1, this yields
A 2(x 2 −y 2)+A 3(x 3−3xy 2)=�1 (12.2)
y = � A 3 x 3 + A 2 x 2 ∓ 1 3 A 3 x + A 2 ( 12.3 ) For a quadrupole, including, say, a 2% hexapole component, A2=0.98, A3=0.02, and equation (12.3) can be rewritten as follows:
y = � 0.02 x 3 + 0.98 x 2 ∓ 1 0.06 x + 0.98 ( 12.4 ) Referring to FIG. 3, four curves for the four possible combinations of the � and ∓ are shown to illustrate the shape of the electrodes suitable for providing substantially quadrupole fields, each having a selected hexapole component.
To produce a quadrupole with added hexapole field, electrodes with the shapes given by equation 12.3 can be manufactured. This is expensive. However, a hexapole field can also be added to a quadrupole set having round rods. Specifically, an angular displacement of one rod introduces higher harmonics and the greatest of these is the A3 term, as described by Douglas et al., in Russian Journal of Technical Physics, 1999, Vol. 69, 96�101 at FIG. 5. However, while a substantial hexapole component is added, there are significant contributions from other higher order quadrupoles.
The amplitudes of the harmonics produced by rotating two Y rods toward the X rods are shown in FIG. 8 for angles between 0 and 20 degrees. For this calculation the ratio of rod radius, r, to field radius, r0, was R/r0=1.1487 because this ratio produces low levels of the higher harmonics when the rotation is zero (i.e. without an added hexapole) (R. E. March and R. J. Hughes, Quadrupole Storage Mass Spectrometry, John Wiley and Sons, Toronto, 1989, page 42). The method of calculation of the harmonic amplitudes is given by Douglas et al., in Russian Journal of Technical Physics, 1999, Vol. 69, 96�101. It can be seen that a significant hexapole component (amplitude A3) is produced. As well a significant dipole component A1, having both DC and AC subcomponents, is added to the field. However, the AC subcomponent of the dipole component is at the frequency of the quadrupole AC and will not excite ions. Because the hexapole is added by displacing two rods, not by changing rod diameters, similar results are obtained for a broad range of ratios r/r0, although with other ratios the higher harmonic amplitudes can be somewhat higher. FIG. 9 shows in more detail the harmonic amplitudes for rotations between 0 and 5.0 degrees. A hexapole amplitude of up to 0.075 can be produced, while amplitudes of higher multipoles remain small. For example with a rotation of the two Y rods of 3.0 degrees the amplitudes are A0=3.73�10−5, A1=−3.68�10−2, A2=1.0011, A3=4.64�10−2, A4=2.77�10−3, A5=−8.18�10−3, A6=−1.098�10−3, A7=−1.43�10−3, A8=−1.54�10−4, A9=5.00�10−4 and A10=−2.29�10−3.
112 � 1.0943
114 � 0.9099
When ions in a pure quadrupole field are excited with a dipole field, the excitation voltage requires a frequency given by equations 9 or 10. As shown in M. Sudakov, N. Konenkov, D. J. Douglas and T. Glebova, �Excitation Frequencies of Ions Confined in a Quadrupole Field With Quadrupole Excitation�, Journal of the American Society for Mass Spectrometry, 2000, Vol. 11, 10�18, when ions are excited with a quadrupole field the excitation angular frequencies are given by
ω ( m , k ) =  m + β  Ω K ( 13 ) where K=1, 2, 3 . . . and m=0, �1, �2, �3 . . . Of course, when the quadrupole field has small contributions of higher field harmonics added, the excitation fields, dipole or quadrupole, may also contain small contributions from the higher harmonics.
When a simple quadrupole field, lacking any higher order terms, is generated by an electrode system, and when (1) there is no excitation of ion motion, (2) there is a collision gas preset, and (3) the ions have a q value that is not near a stability boundary, then the ions generally have a declining quantity of kinetic energy. Ions move through the two-dimensional quadrupole field and lose energy in the radial and axial directions as discussed for example in D. J. Douglas and J. B. French, �Collisional Focusing Effects in Radio Frequency Quadrupoles�, Journal of the American Society for Mass Spectrometry, 1992, Vol. 3, 398�408. As a consequence, the ions are confined and move toward the centerline of the quadrupole, and fragmentation is minimal. As the ions oscillate in the field, their kinetic energy varies between zero and a maximum value that decreases with time. The kinetic energy averaged over each period of the ion motion decreases with time.
V eff ( x , y ) = q A 2 2 V 4 ( x 2 + y 2 r o 2 ) + q A 2 A 4 V 1 ( x 4 - y 4 r 0 4 ) + � ( 14 ) In equation 14 terms of the form xn ym have been omitted because they do not change the calculation of the frequency shift for the X motion when Y=0. Consider motion in the X direction when Y=0. The force on an ion is
F x = - ⅇ ∂ V eff ∂ x = - e q A 2 2 V 2 x 4 r 0 2 - 4 e q A 2 A 4 V x 3 r 0 4 ( 15 ) This gives the equation of motion for x as
x � = ⅆ 2 x ⅆ t 2 , ω 0 2 = e q A 2 2 V rf 2 4 m ion r 0 2 , α = 0 and β = 4 e q A 2 A 4 V rf m ion r 0 4 . Landau and Lifshitz (L. Landau and E. M. Lifshitz, Mechanics, Third Edition, Pergamon Press, Oxford 1966, pages 84�87) have shown that when motion is determined by equation 16, there is a shift in the resonant frequency from ω0 by an amount given by
V eff = qA 2 2 ( x 2 + y 2 ) 4 r 0 2 V + 3 qA 2 A 3 x 3 4 r 0 3 V + 9 qA 3 2 ( x 4 + y 4 ) 16 r 0 4 V + � ( 19 ) where again terms in xn ym have been omitted because they do not change the calculated frequency shift for X motion when y=0. This leads to the equation of motion for an ion
x � + ω 0 2 x = - 9 eqA 2 A 3 4 m ion r 0 3 Vx 2 - 36 eqA 3 2 16 m ion r 0 4 Vx 3 ( 19.1 ) In comparison to equation 16
α = 9 2 ( A 3 A 2 ) ω 0 2 r 0 and β = 9 A 3 2 2 A 2 2 ω 0 2 r 0 2 ( 19.2 ) The frequency shift from the α term is
Δ ω α = - 5 12 81 A 3 2 4 A 2 2 a 2 r 0 2 ω 0 ( 19.3 ) If A3=0.020 and A2=1.00, a=r0, then the frequency shift from this term is Δωα=−3.38�10−3ω0. The frequency shift from the β term is
Δ ω β = 3 β 8 ω 0 a 2 = 27 A 3 2 16 A 2 2 a 2 r 0 2 ω 0 ( 19.4 ) and for the same values of the parameters is Δωβ=+6.75�10−4ω0.
The combined frequency shift for X motion is −2.71�10−3ω0 or about 22 times less than that from a 2% octopole field.
y � + ω 0 2 y = - 36 eqA 3 2 16 m ion r 0 4 Vy 3 ( 19.5 ) and there is a shift up in frequency
Δ ω y = 27 A 3 2 16 A 2 2 a 2 r 0 2 ω 0 ( 19.6 ) When A3=0.020, A2=1.00 and a=r0, this shift is +1.35�10−3ω0, or about four times less than the total shift in the X frequency.
The above-described quadrupole fields having significant hexapole components can be useful as quadrupole mass filters. The term �quadrupole mass filter� is used here to mean a linear quadrupole operated conventionally to produce a mass scan as described, for example, in P. H. Dawson ed., Quadrupole Mass Spectrometry and its Applications, Elsevier, Amsterdam, 1976, pages 19�22. The voltages U and V are adjusted so that ions of a selected mass to charge ratio are just inside the tip of a stability region such as the first region shown in FIG. 2. Ions of higher mass have lower a,q values and are outside of the stability region. Ions of lower mass have higher a,q values and are also outside of the stability region. Therefore ions of the selected mass to charge ratio are transmitted through the quadrupole to a detector at the exit of the quadrupole. The voltages U and V are then changed to transmit ions of different mass to charge ratios. A mass spectrum can then be produced. Alternatively the quadrupole may be used to �hop� between different mass to charge ratios as is well known. The resolution can be adjusted by changing the ratio of DC to AC voltages (U/V) applied to the rods.
It has been expected that for operation as a mass filter, the potential in a linear quadrupole should be as close as possible to a pure quadrupole field. Field distortions, described mathematically by the addition of higher multipole terms to the potential, have generally been considered undesirable (see, for example, P. H. Dawson and N. R. Whetton, �Non-linear Resonances in Quadrupole Mass Spectrometers Due to Imperfect Fields�, International Journal of Mass Spectrometry and Ion Physics, 1969, Vol. 3, 1�12, and P. H. Dawson, �Ion Optical Properties of Quadrupole Mass Filters�, Advances in Electronics and Electron Optics, 1980, Vol. 53, 153�208). Empirically, manufacturers who use round rods to approximate the ideal hyperbolic rod shapes, have found that a geometry that adds small amounts of 12-pole and 20-pole potentials, gives higher resolution and gives peaks with less tailing than quadrupoles constructed with a geometry that minimizes the 12-pole potential. It has been shown that this is due to a cancellation of unwanted effects from the 12- and 20-pole terms with the optimized geometry. However the added higher multipoles still have very low magnitudes (ca. 10−3) compared to the quadrupole term (D. J. Douglas and N. V. Konenkov, �Influence of the 6th and 10th Spatial Harmonics on the Peak Shape of a Quadrupole Mass Filter with Round Rods�, Rapid Communications in Mass Spectrometry, 2002, Vol. 16, 1425�1431).
The results of simulations of RF/DC performance when �2% hexapole was added to a nominally quadrupolar potential are shown in FIG. 10. The curve 400 shows the transmission and peak shape through a pure quadrupole field. The curves 402 and 404 show the transmission through a quadrupole field with added hexapole with amplitudes A3=+0.020 and A3=−0.020 respectively. The peak shapes corresponding to A3=−0.020 and A3=+0.020 are identical as expected from the discussion above. FIG. 11 shows trajectories for one ion through fields with A3=+0.020 and A3=−0.020. FIG. 11 a shows the X motion with A3=+0.020 and FIG. 11 b shows the X motion with A3=−0.020. The trajectories would be identical if the sign of X was changed. The Y motion is shown in FIG. 11 c and is identical for A3=+0.020 and A3=−0.020.
As a further alternative, and instead of scanning either the AC voltage applied to rods 238 or the auxiliary AC voltage applied to end lens 242, a further supplementary or auxiliary AC dipole voltage or quadrupole voltage may be applied to rods 238 (as indicated by dotted connection 257 in FIG. 13) and scanned, to produce varying fringing fields which will eject ions axially in the manner described. Alternatively, dipole excitation may be applied between the X pair and at the same time additional dipole excitation may be applied between the Y rod pair. This is of particular advantage when the trapping field provided by the AC voltage applied to the rods has an added hexapole component. That is, with a conventional rod set, only about 20% of the ions confined in the linear trap can be axially ejected; the remaining 80% appear to be lost by striking the rods (J. Hager, �A New Linear Ion Trap Mass Spectrometer�, Rapid Communications in Mass Spectrometry, 2002, Vol. 16, 512). However, as described above, with a linear quadrupole having an added hexapole field, a greater excitation voltage is required to cause ions to strike the rods, and ions can be continuously excited without striking the rods. Thus, the percentage of ions that are axially ejected is increased and the percentage of ions that strike the rods is reduced.
Depending on the context, it is sometimes better to have unbalanced AC applied between the rods. In other contexts, it is also advantageous to have DC between the rods, typically 0.5 to 50 volts (see J. Hager, �Performance Optimization and Fringing Field Modification of a Twenty-Four Millimeter Long RF Only Quadrupole Mass Spectrometer�, Rapid Communications in Mass Spectrometry, 1999, Vol. 13, 740; see also U.S. Pat. No. 6,177,668). It depends on the context. Accordingly, it is advantageous to have as many different modes of operation as possible, as different modes of operation may be preferred in different contexts.
In principle, any of the three trapping voltages can be combined with any of the three methods of applying DC between the rods, which could be used with any of the nine excitation modes. Thus, there are 3�3�9=81 modes of operation for positive ions. With each of these modes, either the AC amplitude is scanned to bring ions sequentially into resonance with the AC excitation field or fields, or else the frequency of the modulation is scanned so that again, when such frequency matches a resonant frequency of an ion in the fringing fields in the vicinity of the exit lens, the ion will absorb energy and be ejected axially for detection. Thus there are 81�2=162 methods of scanning to mass selectively eject ions axially.
[ A 2 ( x 2 - y 2 r 0 2 ) + A 3 ( x 3 - 3 x y 2 r 0 3 + A 4 ( x 4 - 6 x 2 y 2 + y 4 r 0 4 ] = � constant ( 19.8 ) An example is shown in FIG. 14 which shows the electrodes for a quadrupole with A2=+0.96, A3=+0.02 and A4=+0.02. Solutions of equation 19.8 will give exactly the field of equation 19.7. However it is preferable to construct the electrodes with round (cylindrical) electrodes because these can be manufactured to high precision at lower cost.
As described in US patent application �Improved Geometry for Generating a Substantially Quadrupole Field, Michael Sudakov, Chuan-Fan Ding and D. J. Douglas, U.S. application Ser. No. 10/211,238, filed Aug. 5, 2002, an octopole component can be added to a quadrupole field by constructing the rod set with the rods of one pair different in diameter from the other pair. For example if the Y rods have greater diameter than the X rods, there is a positive octopole component (A4>0) and all other higher multipoles remain small.
FIGS. 16 to 20 inclusively show the amplitudes of the higher spatial harmonics for rotation angles, θ, between about 0.5 and 3.5 degrees. The ratios of Y rod radius to X rod radius in these figures are ry/rx of 1.20, 1.40, 1.60, 1.80, and 2.00 respectively. For each angle of rotation, a ratio of the voltage applied to X rod 312 relative to X rod 314 and Y rods 316 and 318 was chosen to make A1 small. The angle was then adjusted slightly to make A1<1�10−5 i.e. to make A1 very close to zero. Thus, FIGS. 16 to 20 show the amplitudes of the harmonics for the case where A1≈0. FIGS. 16 to 20 show that an octopole component in the range +0.02 to +0.06 can be provided. If desired, a larger octopole component could be added. The octopole component is mostly determined by the ratio of rod radii, and changes little with rotation angle. At the same time, the rotation introduces a hexapole component in the range 0 (at θ=0) to +0.06 for the range of angles illustrated. When the larger rods are rotated toward the smaller rods, the hexapole and octopole components have the same sign (positive in this case).
Referring to FIG. 21 there is illustrated in a sectional view, another set of quadrupole rods including Y rods that have undergone a rotation through an angle θ about a quadrupole axis 420. The set of quadrupole rods includes X rods 412 and 414, Y rods 416 and 418, and quadrupole axis 420. The Y rods have radius ry and the X rods radius rx. All rods are a distance r0 from the central axis 420 and ry=r0. In this case the radius of the X rods is greater than the radius of the Y rods (rx>ry). The Y rods have been rotated towards the X rod 412. FIGS. 22 to 26 show the amplitudes of the higher harmonics for different rotation angles for ratios rx/ry of 1.20, 1.40, 1.60, 1.80, and 2.0 respectively for the quadrupole of FIG. 21. For each angle of rotation, a ratio of the voltage applied to X rod 412 relative to X rod 414 and Y rods 416 and 418 was chosen to make A1 small. The angle was then adjusted slightly to make A1<1�10−5�i.e. to make A1 very close to zero. Thus, FIGS. 22 to 26 show the amplitudes of the harmonics for the case where A1≈0. FIGS. 22 to 26 show an octopole component in the range −0.02 to −0.06. If desired, a larger octopole component could be added. The octopole component is mostly determined by the ratio of rod radii, and changes little with rotation angle. At the same time, the rotation introduces a hexapole component in the range 0 (at θ=0) to +0.06 for the range of angles illustrated. However in this case the octopole and hexapole components have opposite signs (A3>0 and A4<0).
A quadrupole mass filter which has both octopole and hexapole fields added, can be used for mass analysis, provided the signs of the added multipoles and applied DC are correct. Simulations of peak shapes have been done for a quadrupole with A3 and A4 terms of both signs. The simulations were done as described in the article �Influence of the 6th and 10th Spatial Harmonics on the Peak Shapes of a Quadrupole Mass Filter With Round Rods�, D. J. Douglas and N. V. Konenkov, Rapid Communications in Mass Spectrometry, Vol. 16, 1425�1431, 2002.
FIG. 27 shows a peak shape 490 for a pure quadrupole field and a peak shape 492 for a quadrupole field (amplitude A2=1) with an added octopole field of amplitude A2=+0.020, and a>0. As described in US patent application �Improved Geometry for Generating a Substantially Quadrupole Field�, Michael Sudakov, Chuan-Fan Ding and D. J. Douglas, U.S. application Ser. No. 10/211,238, filed Aug. 5, 2002, which is incorporated herein by reference, such an added octopole field can be created by using a rod set with Y rods greater in diameter than the X rods. For positive ions, provided the positive DC is applied to the X rods, the peak shape with the added octopole field has transmission and resolution similar to that of a pure quadrupole field. A slightly lower value of a is required for the same transmission and resolution.
Thus (1) and (4) are equivalent physically and (2) and (3) are equivalent physically. They differ only in that the directions of x and y are interchanged. As shown in US patent application �Improved Geometry for Generating a Substantially Quadrupole Field�, Michael Sudakov, Chuan-Fan Ding and D. J. Douglas, U.S. application Ser. No. 10/211,238, filed Aug. 5, 2002, for positive ions, good peak shape and transmission are only obtained with cases (1) and (4), and for negative ions with (2) and (3). Peak 492 of FIG. 27 corresponds to case (1). When a hexapole component is also added, all four cases differ.
Peak Peak FIG. A2 A4 α A3 without A3 with A3 27 1 +0.020 +0.2365 0 good 28 1 +0.020 +0.2365 �0.020 good Good, improved transmission 29 1 +0.020 −0.2460 +0.020 split good 30 1 +0.020 −0.2460 �0.020 split good 31 1 −0.020 +0.2470 �0.020 split split 32 1 −0.020 −0.2360 �0.020 good split As discussed, the sign of A3 does not affect the peak shape. The four possibilities are shown in FIGS. 28 (A4,a)=(+,+), 30 (A4,a)=(+,−), 31 (A4,a) =(−,+) and 32 (A4,a)=(−,−). It can be seen that when a hexapole component is added, good peak shape is obtained only when A2 and A4 have the same sign, regardless of the sign of a.
FIG. A2 A4 α A3 Peak without A4 Peak with A4 27 1 +0.020 +0.2365 0 good 28 1 +0.020 +0.2365 �0.020 good good 29 1 +0.020 −0.246 +0.020 poor good 30 1 +0.020 −0.246 �0.020 poor good 31 1 −0.020 +0.247 �0.020 good split 32 1 −0.020 −0.236 �0.020 poor split With positive ions, when there is a hexapole present good peak shape and transmission can be obtained provided the positive DC is applied to the X rods (a>0), as described. If the positive DC is applied to the Y rods (a<0), the transmission and resolution are poor. However, if the positive DC is applied to the Y rods and if a positive octopole component is added to the field (FIG. 30) good peak shape and transmission are restored. If a negative octopole component is added (FIG. 32) a badly split peak is produced.
Other variations and modifications of the invention used with axial ejection are possible. For example the rod set may be used as an ion trap for mass selective axial ejection combined with another ion trap to improve the duty cycle as shown in FIG. 2 of U.S. Pat. No. 6,177,668. The rod set with axial ejection may also be operated at lower pressure such as 2�10−5 torr, as shown in FIG. 4 of U.S. Pat. No. 6,177,668. In addition the rod set with axial ejection may be used as a collision cell to produce fragment ions, followed by axial ejection of the fragment ions for mass analysis. Fragment ions may be formed by injecting ions at relatively high energy to cause fragmentation with a background gas or by resonant excitation of ions within the rod set. In some cases it is desirable to operate the same rod set used for axial ejection as a mass filter with mass selection of ions at the tip of the stability diagram (J. Hager, �A New Linear Ion Trap Mass Spectrometer�, Rapid Communications in Mass Spectrometry, 2002, Vol. 16, 512). Rod sets with added hexapole fields can be operated as mass filters as described above.
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Ltd.Methods and systems for providing a substantially quadrupole field with a higher order component* Cited by examinerClassifications U.S. Classification250/292, 250/288, 250/281, 250/290, 250/396.00RInternational ClassificationG21K1/08, H01J49/00, H01J3/26, H01J49/42Cooperative ClassificationH01J49/4215, H01J49/4225European ClassificationH01J49/42D1Q, H01J49/42D3LLegal EventsDateCodeEventDescriptionMay 28, 2014FPAYFee paymentYear of fee payment: 8Apr 9, 2013ASAssignmentOwner name: APPLIED BIOSYSTEMS, INC., CALIFORNIAFree format text: LIEN RELEASE;ASSIGNOR:BANK OF AMERICA, N.A.;REEL/FRAME:030182/0677Effective date: 20100528May 3, 2010FPAYFee paymentYear of fee payment: 4Mar 31, 2010ASAssignmentOwner name: APPLIED BIOSYSTEMS, LLC,CALIFORNIAFree format text: RELEASE BY SECURED PARTY;ASSIGNOR:BANK OF AMERICA, N.A.;US-ASSIGNMENT DATABASE UPDATED:20100331;REEL/FRAME:24160/955Effective date: 20100129Free format text: RELEASE BY SECURED PARTY;ASSIGNOR:BANK OF AMERICA, N.A.;US-ASSIGNMENT DATABASE UPDATED:20100406;REEL/FRAME:24160/955Free format text: RELEASE BY SECURED PARTY;ASSIGNOR:BANK OF AMERICA, N.A.;US-ASSIGNMENT DATABASE UPDATED:20100504;REEL/FRAME:24160/955Free format text: RELEASE BY SECURED PARTY;ASSIGNOR:BANK OF AMERICA, N.A.;REEL/FRAME:24160/955Free format text: RELEASE BY SECURED PARTY;ASSIGNOR:BANK OF AMERICA, N.A.;REEL/FRAME:024160/0955Owner name: APPLIED BIOSYSTEMS, LLC, CALIFORNIAFeb 19, 2010ASAssignmentOwner name: APPLIED BIOSYSTEMS (CANADA) LIMITED,CANADAFree format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:MDS INC. 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