EP0459602A2 - Mass spectrometric high-frequency quadrupole cage with superposed multipole fields - Google Patents

Abstract

An ion cage mass spectrometer, also known as a quistor or ion trap, having an annular electrode and two end-cap electrodes, voltage supplies for generating an ion-storing RF quadrupole field, means for generating ions of the substances which are to be investigated by mass-spectrometric means inside or outside the ion cage, if appropriate, means for introducing the ions into the ion cage, means for detecting those ions which emerge from the ion cage, characterised in that the exact quadrupole potential Pq = (A2/4z0<2>) * (r<2>-2z<2>) * [U - Vcos(Wt)] has superposed on it, by special shaping of the electrodes, exactly or approximately, a six-pole potential Ps = (A3/4z0<3>) * (3r<2>z-2z<3>) * [U - Vcos(Wt)], or an eight-pole potential P0 = (A4/4z0<4>) * (r<4> + 8z<4>/3-8r<2>z<2>) * [U - Vcos(Wt)], or a linear combination of the two, where   r = distance from the z-axis,   z = distance from the plane z = 0,   z0 = distance of an end cap from the centre z = 0,   A2 = intensity of the quadrupole field,   A3 = intensity of the six-pole field,   A4 = intensity of the eight-pole field,   U = value of the DC voltage   V = peak value of the AC voltage,    omega = angular frequency of the AC voltage,   t = time. <IMAGE>

Description

The invention relates to an ion cage mass spectrometer, a quistor, an ion trap or the like according to the preamble of patent claims 1 and 6.

From DE-PS 944 900 a mass spectrometer is known in which the electrodes are arranged so that the surfaces of the ring electrode and the end cap electrodes form a one-piece rotary hyperboloid or a two-part rotary hyperboloid, the end cap electrodes being conductively connected to one another and between the ring electrode and a time-varying voltage is applied to the end cap electrodes. If a potential U + V ^· sin (ωt) is generated between the ring electrode and the end cap electrodes, ions remain, their specific charge e / m is in a certain range between the electrodes, while the others hit the electrodes. The superimposition of direct and high-frequency fields in such mass spectrometers is called a quadrupole memory field. The ion movement forms a good approximation of a spatial overlay of two independent harmonic oscillators. The forces of the storage field, which acts on the ions, oscillate in the ion cage formed in this way. The force integrated over half of the so-called secular period approximately fulfills the condition of a harmonic oscillator, so that such a system is also called a pseudo-harmonic oscillator. Two such pseudoharmonic oscillator systems form the aforementioned ion cage, which is also referred to as a quistor or ion trap (for terminology: Dawson, "Quadrupole Mass Spectrometry", Elsevier, Amsterdam, 1976; Mahrs / Hughes "Quadrupole Storage Mass Spectrometry", John Wiley & Sons, New York, 1989). The two pseudoharmonic oscillator systems of the quistor consist of a cylinder-symmetrical system, which shows the same behavior regardless of the coordinate in the direction of the cylinder axis (z-axis), and a plane system, the behavior of which is independent of the distance r from the cylinder axis.

In both pseudo-harmonic oscillator systems, ie in the r and z directions, the ions vibrate with so-called "secular frequencies", which are completely independent of one another. The secular frequencies can be determined using known formulas. Since the secular frequencies in the r and z directions and the storage frequency only have a common divisor in rare situations, the movement patterns of the ions are usually very complicated.

An ion cage can be used as a mass spectrometer. The well-known basic principle of mass spectrometry consists in the proportions of ions with different masses relative to one another ascertain. For this purpose, so-called scan methods are used, which carry out the measurement of the different types of ions one after the other by varying measurement or filter conditions. Various scanning methods are known for the ion cage.

At this point, however, only the method of mass-selective ejection of the ions from the cage is interesting. For this purpose, the ions of successive masses are ejected sequentially in time from the cage and fed to a detection system, so that the measurement signals of the ions can be processed to a mass spectrum in a known manner.

As previously known, mass-selective ejection can be carried out in three different ways. First, the ions can be ejected by changing the storage conditions in the ion cage such that the ions move mass by mass beyond the edge of the stability range, become unstable, and leave the ion cage ("Mass-selective instability scan", US Pat. No. 4,440 884). Secondly, the secular frequency of successive ion masses can be excited by an externally applied high-frequency voltage in such a way that they absorb kinetic energy in resonance and thus leave the cage ("mass-selective resonance scan by excitation frequency", US Pat. No. 4,736,101). And thirdly, the ions can be introduced into a device-specific nonlinear resonance condition in which they absorb kinetic energy and leave the cage (“mass-selective scan by means of nonlinear device resonance”, US Pat. No. 4,882,484).

In all applications of the ion cage as a mass spectrometer, it is desirable that the ejection process of non-specific ions takes place as quickly as possible.

A mass spectrometer is already known from US Pat. No. 4,882,484 Generic type known, in which the non-linear resonances of an octupole field superimposed on the quadrupole field are used to accelerate the creation of the mass spectrum. This publication does not provide a generally applicable reference to the structure and shape of the multipole field overlay of the quadrupole field.

The known quadrupole cage can not only be used to identify individually supplied substances based on their primary spectra, but can also be used to identify mixture components by tandem mass spectrometry, whereby daughter ion spectra are generated. First, an ion type, the parent ion, is selected; all other types of ions are removed from the cage. Then the parent ion is fragmented by collision with a gas introduced into the cage. To do this, the parent ion must be accelerated to increase the collision energy above the fragmentation threshold. It is easiest to excite ion oscillation in the z direction by an alternating voltage between the end cap electrodes which is in resonance with the corresponding secular frequency.

However, the excitation is critical in the known quadrupole cages. In the quadrupole field, the amplitude of the secular motion increases linearly with time, and eventually the ions will collide with the end cap electrodes. Fine tuning is required between a low excitation voltage and a high collision gas density, and a yield of about 30 to 50% of daughter ions can be achieved; the rest of the parent ions are lost.

The invention is therefore based on the object of developing the generic mass spectrometer in such a way that in order to increase the ability and the detection power while further - resolving - accelerating the measurement of the mass spectrum a general rule for the appropriate type of multi-field overlay is given, the ion losses from the spectrometer being reduced by unwanted resonances in use for tandem mass spectrometry and the yield should be increased in the case of shock-induced fragmentation.

This object is achieved by a mass spectrometer of the type mentioned by the features listed in the characterizing part of claim 1. Particularly advantageous embodiments of the invention according to claim 1 are the subject of the dependent claims.

The invention is based on the surprising finding that it is possible to sharpen the temporal smearing of the ejection process in a multipole superposition according to the invention, be it in a mathematically exact description or according to the approximate formula of claim 6, thereby accelerating the creation of the mass spectrum. Furthermore, ion losses are reduced and the yield of daughter ions is improved. The superposition of z-asymmetrical multipole fields intensifies the ejection by the non-linear resonance effects that then occur.

It has been found that it is generally not necessary to superimpose higher order multipole fields than octupole fields on the base quadrupole field, although this is fundamentally possible and is within the spirit of the invention. It should be mentioned that the occurrence of nonlinear resonances and their subsequent effects by FV Busch and W. Paul are described in the "Zeitschrift für Physik" 164, pages 588 to 594 (1961). Here it is found that the non-linear resonances in the mass spectrometer caused by field errors are so weak that they do not impair its functionality, only to split mass lines can lead in the spectrum. Advantageous effects of the non-linear resonances are not recognized, so that it cannot be derived from this document how these could lead to an improvement in the properties of the mass spectrometer.

The surface shape of the electrodes is chosen in the invention so that the effect of the desired multipole field overlay results. In the mathematically exact embodiments of the invention, the exact dimensions of the electrodes are determined by the relative strength A₃ of the sextupole field or the relative strength A₄ of the octupole field in relation to the strength A₂ of the quadrupole field. The strengths of the sextupole field or the octupole field with respect to the quadrupole field can be between approximately 0% and 20%, it being particularly advantageous if the proportion of the superimposed fields is between 0.5% and 4.5%; the proportion is particularly preferably between 1% and 3%.

According to the formulas specified according to the invention, the electrodes can easily be shaped in such a way that mathematically exact superimpositions of the quadrupole field with predetermined contributions of the octupole field or the sextupole field are obtained. The deviations due to the superimposed fields are mainly noticeable in the outer areas of the spectrometer area, while an almost exact quadrupole field is present in the area of the center.

It should be noted that the manufacture of electrodes according to the regulation of the invention in one embodiment, as is the subject of claim 6, is carried out by successively adding higher-order thermal springs in w, once the measure p 1 for the proportion of the octupole field, the measure p₂ are specified for the portion of the sextupole field or the correction portion p₃ of the octupole field. It is again advantageous if p₁, p₂ and p₃ are between 0% and 20%, these variables should not, however, take the value 0 at the same time, so that in any case a superimposed heat contributes.

Fig. 1

den Längsschnitt durch Elektrodenanordnung eines Massenspektrometers gemäß der vorliegenden Erfindung, wobei ein Oktupolfeld als Multipolfeld höherer Ordnung einem Basis-Quadrupolfeld überlagert ist,

Fig. 2

einen Längsschnitt durch die Elektrodenanordnung, wobei ein Sextupolfeld überlagert ist, und

Fig. 3

einen Längsschnitt durch die Elektrodenanordnung, wobei sowohl ein Oktupol- als auch ein Sextupolfeld überlagert sind.

The invention is explained in detail below on the basis of exemplary embodiments with reference to the schematic drawing. It shows:

Fig. 1

2 shows the longitudinal section through the electrode arrangement of a mass spectrometer according to the present invention, an octupole field as a higher order multipole field being superimposed on a base quadrupole field,

Fig. 2

a longitudinal section through the electrode arrangement, wherein a sextupole field is superimposed, and

Fig. 3

a longitudinal section through the electrode arrangement, both an octupole and a sextupole field are superimposed.

Figure 1 shows the arrangement of two end cap electrodes 1, 2, which are each arranged at a distance z₀ from the equatorial plane 4. The descriptive coordinate system is chosen so that the equatorial plane 4 coincides with the coordinate plane z = 0. Between the end cap electrodes 1, 2 there is a ring electrode 3 such that the entire arrangement of the electrodes 1, 2, 3 is axially symmetrical, the axis of symmetry coinciding with the z axis of the coordinate system. The distance of the ring electrode 3 from the center z = 0 in the equatorial plane 4 is denoted by r₀. The electrode arrangement is chosen so that r₀ / z₀ = √ 1.8 is. The octupole field generated by the electrode shape has a strength A₄ / A₂ of 2%, measured in the equatorial plane 4 at the ring electrode 3. The overlaid field causes non-linear forces both in the z direction and as a function of r, the distance from the z -Axis generated. As a result, the secular frequencies become dependent on the secular amplitudes and either increase or at. In both cases, however, a resonance catastrophe of the secular amplitude is prevented. The increasing secular oscillation shifts in frequency and phase through the octupole field and reaches a maximum amplitude when the phase shift is 90 °, after which the amplitude decreases again. Therefore, like all other "even" multipole fields, the octupole field has a surprisingly positive effect. Almost all ion losses due to resonance effects are prevented, whatever the cause of the resonance.

• (1) Resonanzen zwischen den Endkappenelektroden 1, 2, die durch eine Anregungsfrequenz hervorgerufen werden,
• (2) nichtlineare Resonanzen aus Überlagerungen von gegenüber der Speicherfrequenz verschobenen Frequenzen oder hervorgerufen durch Multipolfelder, die durch ungenaue Anordnung der Elektroden erzeugt werden oder auch durch Oberflächenladungen auf den Elektroden.

Normally disturbing resonances can be

• (1) resonances between the end cap electrodes 1, 2 caused by an excitation frequency,
• (2) non-linear resonances from superpositions of frequencies shifted with respect to the storage frequency or caused by multipole fields which are generated by inaccurate arrangement of the electrodes or also by surface charges on the electrodes.

The only exception is the so-called octupole sum resonance, in which the ion absorbs energy in both the r direction and the z direction.

With the electrode arrangement from FIG. 1, it is also possible to avoid the disadvantages of the prior art with regard to the generation of daughter ions. If an octupole field is superimposed on the base quadrupole field, the excitation voltage can be selected such that the parent ions never reach the end cap electrodes 1, 2. Yields of daughter ions in the order of 80 to 100% of the parent ions are possible.

An octupole field that normally blocks the resonance reactions of ions can still have positive effects on the resonance reaction during a scan. When the secular frequency reaches the external excitation frequency, due to the coupling of the secular frequency and the secular amplitude, the effects of the increase in the sampling frequency and the decrease in the amplitude are compensated, whereby the ion is expelled from the mass spectrometer.

FIG. 2 shows an electrode arrangement comprising end cap electrodes 1, 2 and ring electrode 3, in which the electrodes are shaped in such a way that a sextupole field is superimposed on the base quadrupole field. The dimensioning of the electrodes is otherwise the same as that in FIG. 1, in particular r₀ / z₀ = √ 1.8 . The dotted lines 5, 6 indicate the corresponding electrode structure in which a pure quadrupole field would be present. It can be seen that deviations only occur in the outer regions of the electrode arrangement, while an almost exact quadrupole field results in the interior. By superimposing the sextupole field, the secular frequency remains essentially unchanged in the z direction, while frequency splitting takes place in the r direction. The sextupole field produces a strong nonlinear resonance at a frequency that is exactly one third of the storage frequency. If an excitation voltage is now applied in phase and at this frequency, the ion oscillation is first increased by this excitation voltage, which leads to a linear increase in the secular amplitude, then the oscillation will increase exponentially through the sextupole resonance. The hexapole resonance can therefore be used for mass-selective ejection of the ion. The ejection process is therefore exacerbated by overlaying the sextupole field. Good results are achieved when the proportion A₃ of the overlying sextupole field is 2% of the quadrupole field.

FIG. 3 shows an electrode arrangement in which both a superimposed octupole field and a superimposed one Sextupol field have been generated, the octupole portion is 2% and the sextupole portion is 6%. The combination of the two superimposed fields has the result that the advantages of both systems are realized in the arrangement. The loss of ions is reduced by the octupole effect, the non-linear resonance of the sextupole field promotes the ejection of the ions and sharpens the ejection process. It has been found that the best results are achieved if the proportion A₃ of the overlaid sextupole field is twice as large as the proportion A₄ of the overlaid octupole field.

The features of the invention disclosed in the above description, in the drawing and in the claims can be essential both individually and in any combination for realizing the invention in its various embodiments.

REFERENCE SIGN LIST

1

End cap electrode

2nd

End cap electrode

3rd

Ring electrode

4th

Equatorial plane

5

dotted line (quadrupole structure)

6

dotted line (quadrupole structure)

Claims (6)

Ion cage mass spectrometer, also called quistor or ion trap, with a ring electrode and two end cap electrodes, voltage supplies for generating an ion-storing RF quadrupole field, means for generating ions of the substances to be examined by mass spectrometry inside or outside of the ion cage, optionally means for introducing the ions into the Ion cage, means for the detection of such ions emerging from the ion cage, characterized in that the exact quadrupole potential

${\text{P}}_{\text{q}}\text{= (A₂ / 4z₀²) * (r²-2z²) * [U - Vcos (ωt)]}$

a special sextupol potential due to the special shape of the electrodes

${\text{P}}_{\text{s}}\text{= (A₃ / 4z₀³) * (3r²z-2z³) * [U - Vcos (t)],}$

or an octupole potential

${\text{P}}_{\text{O}}\text{= (A₄ / 4z₀⁴) * (r⁴ + 8z⁴ / 3-8r²z²) * [U - Vcos (t)],}$

or a linear combination of the two is superimposed with r = distance from the z-axis, z = distance from the plane z = 0. z₀ = distance of an end cap from the center z = 0, A₂ = strength of the quadrupole field, A₃ = strength of the sextupol field, A₄ = strength of the octupole field, U = DC voltage value, V = peak value of the AC voltage, ω = angular frequency of the AC voltage, and t = time. Mass spectrometer according to claim 1, characterized in that an overlay of exact sextupol and octupole fields by a surface shape of the end cap electrodes (1.2) r _k (z) and the ring electrode (3) r _r (z) according to the equations

is given with the abbreviations

$\text{d = 4 * z² - (3A₃ / 2A₄) * z * z₀ - (A₂ / 2A₄) * z₀²,}$

${\text{e}}_{\text{k}}{\text{= (2A₂ / A₄) * z₀² * z² + (2A₃ / A₄) * z₀ * z³ - (8/3) * z⁴ + P}}_{\text{k}}\text{,}$

${\text{e}}_{\text{r}}{\text{= (2A₂ / A₄) * z₀² * z² + (2A₃ / A₄) * z₀ * z³ - (8/3) * z⁴ + P}}_{\text{r}}\text{,}$

where P _k and P _{r are} proportional to the desired peak alternating potentials on the electrodes (1, 2 and 3). Mass spectrometer according to claim 2, characterized in that the surface shapes of the end cap electrodes (1,2) r _k (z) and the ring electrode (3) r _r (z) according to the equations

are given with the abbreviations

$\text{d = 4 * z² - (3A₃ / 2A₄) * z * z₀ - (A₂ / 2A₄) * z₀²,}$

and

Mass spectrometer according to claim 2 or 3, characterized in that

$\text{0.002 <= A₄ / A₂ <= 0.08,}$

and

$\text{0 <= A₃ / A₂ <= 0.16.}$

Mass spectrometer with superimposed exact sextupole field according to claim 1, characterized in that the surface shapes of the end cap electrodes (1,2) r _k (z) and the ring electrode (3) r _r (z) according to the equations

${\text{r}}_{\text{k}}\text{= √}\overline{\text{(2z²}}\overline{\text{-2z₀² * g (z))}}$

and
${\text{r}}_{\text{r}}\text{= √}\overline{\text{(2z²}}\overline{\text{+}}\overline{\text{2z₀² * g (z))}}$

are given with
$\text{g (z) = (A₂ + A₃) / (A₂ + 3 * A₃ * z / z₀),}$

and
$\text{0.001 <= A₃ / A₂ <= 0.2.}$

Mass spectrometer according to claim 1 with approximate sextupole and octupole fields, characterized in that the multipole fields by surface shapes of the electrodes (1,2, 3) according to the equations

${\text{e.g.}}_{\text{r}}{\text{(r) = (w}}_{\text{r}}{\text{+ (P₁ * w}}_{\text{r}}{\text{) + (p₂ * w}}_{\text{r}}{}^{\text{2nd}}{\text{) + (P₃ * w}}_{\text{r}}{}^{\text{3rd}}\text{)),}$

${\text{e.g.}}_{\text{k}}{\text{(r) = (w}}_{\text{k}}{\text{+ (p₁ * w}}_{\text{k}}{\text{) + (p₂ * w}}_{\text{k}}{}^{\text{2nd}}{\text{) + (p₃ * w}}_{\text{k}}{}^{\text{3rd}}\text{)),}$

With ${\text{w}}_{\text{r}}{\text{= w}}_{\text{r}}\text{(r) = ((r² - r₀²) / 2),}$

${\text{w}}_{\text{k}}{\text{= w}}_{\text{k}}\text{(r) = ((r² + r₀²) / 2),}$

and

0 ≦ p₁ ≦ 0.2 (roughly approximate octupole portion), or
0 ≦ p₂ ≦ 0.2 (approximate portion of sextupol), and / or
0 ≦ p₃ ≦ 0.2 (more approximate octupole portion),
but not p₁, p₂, p₃ disappearing at the same time, are generated.

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