EP0321819B1 - Method for the massspectrometric analysis of a gas mixture, and mass sprectrometer for carrying out the method - Google Patents

Description

The invention relates to a method for mass spectroscopic examination of a gas mixture using a mass spectrometer with an ion trap, which is designed as a quistor with a ring electrode and two end electrodes closing the chamber delimited by the ring electrode, of which end electrodes at least one with an extension of the axis of rotation of the ring electrode arranged perforation, in which method the following steps are carried out:

Applying an RF voltage of such amplitude and frequency and possibly such a DC voltage to the ring electrode that a three-dimensional RF quadrupole field is generated within the ion trap, which is suitable for ions whose charge / mass ratio is in a predetermined range, to catch and save in the ion trap,

Introducing or generating ions of the gas mixture into or within the ion trap and storing those ions in the ion trap whose charge / mass ratio is in the predetermined range,

Changing at least one of the field parameters formed by the amplitude, the frequency and possibly the DC voltage in such a way that ions with a monotonically changing charge / mass ratio become unstable one after the other and the ion trap in the direction of the axis of rotation of its ring electrode through said perforation in the Leave end electrode, and

Measuring and recording the intensity of the ion current leaving the ion trap as a function of the change in the field parameters.

The invention also relates to a mass spectrometer.

Basic explanations about the use of a quistor in mass spectrometry can be found in a book published by PH Dawson with the title: "Quadrupol mass spectrometry and its applications", Amsterdam-Oxford-New York 1976, in particular pages 181 to 190 and pages 203 to 219 The specific process from which the invention is based is described in EP-A-0 113 207. In this known method, the limits of the range of the charge / mass ratio, for which stable storage conditions prevail in the quistor, are shifted by varying the amplitude of the HF voltage, so that the trapping conditions disappear one after the other for ions with increasing or decreasing mass and the Ions are able to leave the quistor in the direction of the axis of rotation of the ring electrode. The ions leaving the quistor are registered by means of an electron multiplier so as to obtain the spectrum of the gas sample contained in the quistor.

A special property of the quistor is that the ions in the center of the RF field are not exposed to any field strength that could give them a movement component to leave the ion trap. To remedy this deficiency, a collision gas is admitted into the ion trap, the pressure of which is set in such a way that the ions are driven far enough from the center of the ion trap by the optimal number of impacts that they can leave the ion trap. Since this gas simultaneously increases the yield by damping the ion movement transverse to the direction of expulsion, it is also called "damping gas".

All known embodiments of the ion trap follow the so-called "ideal" quistor in their construction. The construction of such an "ideal" quistor consists of a ring electrode in the form of a hyperbolic toroid and two rotating hyperbolic end electrodes, the asymptotic angle of the hyperbolas being exactly 1: √2. A quistor with this structure is characterized in that the ion trajectories in the quistor can be calculated by solving the Matthieu differential equations. The ion trajectories for other forms of the ion trap, however, have not yet been predictable. To date, it is not even possible to calculate the exact potential distribution in differently shaped ion traps in such a way that a tolerably fast computer simulation of the movements is possible.

The results with these "ideal" ion traps show that the ions need about 200 periods of the HF voltage during the spectra recording under optimal pressure conditions of the damping gas and optimal scanning conditions in order to be able to leave the ion trap by about 95%. The line shape therefore shows a slow tailing after a steep increase to a maximum, which prevents optimal resolution of the spectrum.

The line shape is also affected by space charge effects if there are too many ions in the quistor. As a work by J.W. Eichelberger et al in "Analytical Chemistry" 59, page 2732, 1987, can be seen, this space charge effect even leads increasingly to scientific misinterpretations.

Accordingly, the invention has for its object to further develop the method of the type mentioned in such a way that an improvement in the line shape and thus an improvement tion of the resolving power is achieved in the mass spectrometric examination of gas mixtures using such a mass spectrometer.

mit

• R_e = Radius des Scheitelquerschnittes der Endelektroden
• R_r = Radius des Scheitelquerschnittes der Ringelektrode
• _Zo = Abstand der Scheitel der Endelektroden vom Zentrum des Quistors
• r_o = Abstand des Scheitels der Ringelektrode vom Zentrum des Quistors.

This object is achieved according to the invention by the method according to claim 1. To implement the method, a quistor is used in which the distance-related ratio Q of the radii of the inscribed electrode apexes satisfies the condition Q ≦ 3,990, where

With

• R _e = radius of the apex cross section of the end electrodes
• R _r = radius of the apex cross section of the ring electrode
• _Zo = distance of the apices of the end electrodes from the center of the quistor
• r _o = distance of the apex of the ring electrode from the center of the quistor.

In the "ideal" quistor described above, the distance-related ratio Q of the radii of the inscribed electrode apexes has exactly the value Q = 4. It is surprising that the reduction in the ratio Q to a value Q 3.990 results in the mass-selective ejection of the ions sequential instability of the ion pathways can be decisively improved. So far it has been taken for granted that the "ideal" quistor is not only characterized by its calculability, but would also prove to be ideal in terms of its memory properties and its other behavior. For example, it is known from the Dawson book mentioned at the outset that so-called sum resonances of the ion movements in the quistor, which lead to memory losses, are due to extremely small deviations of the quistor configuration from the "ideal" form.

The measure according to the invention not only shortens the time which the ions need to leave the trap, but also improves the line shape, increases sensitivity and detection capacity by improving the signal / noise ratio and reduces the influence of the space charge. The shortening of the time it takes for the ions to leave the ion trap allows an increase in the number of spectra recordings per unit of time, which again increases the sensitivity.

The effect of the measure according to the invention can be explained by the fact that the potential acting most strongly on the ions inside the quistor is located at those points on the electrodes which are closest to the center, ie to the storage space for the ions. These points are the apexes of the end electrodes and the apex line of the ring electrode. In the case of hyperbolic electrodes, these points each have the greatest curvature. Therefore, the ratios of the radii of curvature of the electrodes at the vertices and the distances between these vertices, as expressed in the ratio Q defined above, which can be briefly referred to as the distance-related circular ratio, are of crucial importance for the behavior of the quistor. Relatively small deviations from the ratio Q = 4.000, as is the case with the ideal quistor, are of great influence.

mit

• R_e = Radius des Scheitelquerschnittes der Endelektroden
• R_r = Radius des Scheitelquerschnittes der Ringelektrode
• _Zo = Abstand der Scheitel der Endelektroden vom Zentrum des Quistors
• r_o = Abstand des Scheitels der Ringelektrode vom Zentrum des Quistors.

The invention also relates to a mass spectrometer which is suitable for examining a gas mixture according to the method according to the invention and has an ion trap which is designed as a quistor with a ring electrode and two end electrodes closing the chamber delimited by the ring electrode, of which end electrodes at least one with one perforation arranged in the extension of the axis of rotation of the ring electrode. With this mass spectrometer, the distance-related ratio Q of the radii of the inscribed electrode apexes again satisfies the condition Q 3.990, where

With

• R _e = radius of the apex cross section of the end electrodes
• R _r = radius of the apex cross section of the ring electrode
• _Zo = distance of the apices of the end electrodes from the center of the quistor
• r _o = distance of the apex of the ring electrode from the center of the quistor.

The relationship given above allows many design options. In a preferred embodiment of the invention, of the dimensions of the quistor which determine the distance-related ratio Q, the distance r _{o of} the apex of the ring electrode from the center of the quistor has a value which ensures that the amplitude of the RF voltage applied to the ring electrode has the value largest mass of interest is still captured by means of the storage field, the distance Z _{o of} the apex of the end electrodes from the center of the quistor is given the value of the ratio Q z _o = r _o / 4√Q and there are finally the radii R _e and R _r Vertex cross sections selected so that R _e x R _r = r _o xz _oo With this type of quistor design, the values r _o and Q which are particularly important for the behavior of the quistor are preselected and the other values are determined taking into account the rules given, whereby for the choice of R _e and R _{r there are} freedoms that take into account other influencing factors, especially in manufacturing from a technical point of view.

• Fig. 1 einen Querschnitt durch einen nach der Erfindung ausgebildeten Quistor in schematischer Darstellung,
• Fig. 2 das Stabilitätsdiagramm des Quistors nach Fig. 1,
• Fig. 3 ein Diagramm, das die Zeit, welche die Ionen zum Verlassen des Quistors benötigen, als Funktion des Verhältnisses Q für drei verschiedene Scangeschwindigkeiten veranschaulicht, und
• Fig. 4 die Wiedergabe von unter unterschiedlichen Bedingungen aufgenommenen Spektren.

The invention is described and explained in more detail below with reference to the embodiment shown in the drawing. The features to be gathered from the description and the drawing can be used in other embodiments of the invention individually or in combination in any combination. Show it

• 1 shows a cross section through a quistor designed according to the invention in a schematic representation,
• 2 shows the stability diagram of the quistor of FIG. 1,
• 3 is a graph illustrating the time it takes for the ions to exit the quistor as a function of the ratio Q for three different scanning speeds, and
• 4 shows the reproduction of spectra recorded under different conditions.

The quistor shown in FIG. 1 has a ring electrode 4 and two end electrodes 3, 5, each arranged on one side of the ring electrode, which terminate the chamber delimited by the ring electrode 4 on both sides of the ring electrode. The end electrodes 3 and 5 are each supported on the ring electrode 4 by ring-shaped insulators 7, 8. The ring-shaped insulators 7, 8 also form a tight connection between the outer sections of the ring electrode 4 and the end electrodes 3, 5. An inlet line 11 opens into the ring electrode 8 and makes it possible to introduce a damping gas into the ion trap. The upper end electrode 3 in FIG. 1 has a central opening 10, which is opposed on the outside of the end electrode 3 by a hot cathode 1 for generating an electron beam and a blocking lens 2 serving to control the electron beam. The lower end electrode 5 in FIG. 1 has a perforation 9 in the area of its center, through which ions can leave the quistor. A secondary electron multiplier 6 is arranged on the outside of the lower end electrode 5 and makes it possible to detect the ions leaving the quistor through the perforation 9.

Both the ring electrode 4 and the end electrodes 3 and 5 have strictly hyperbolic surfaces, which means that their contours are hyperbolas in the cross section shown in FIG. 1. The asymptotic angle of both the ring electrode 4 and the hyperbola producing the end electrodes 3, 5 is 1: 1.360. The inner radius r _{o of} the ring electrode is 1.00 cm. Otherwise, the dimensions are chosen so that the distance-related ratio Q defined above has the value Q = 3.422, that is to say a value which is significantly below Q = 4.000. While the end electrodes 3, 5 are at ground potential, an RF voltage with a frequency of 1.0 MHz is applied to the ring electrode 4, which can be varied in the range from 0 V to 7.5 kV. At a voltage of 7.5 kV, the range of the charge / mass ratio of the ions that are captured and stored by the quistor, with a simple ionization, includes ions with mass numbers 1 to 500 µ, where u is the atomic mass unit. Accordingly, by changing the RF voltage in the range from 0 V to 7.5 kV, a mass range from 1 to 500 µ can be covered in one scan. The characteristic stability diagram for this is shown in FIG. 2. In it they are coordinate values q proportional to the field strength V / m of the alternating field and the coordinate values a of the field strength U / m of the constant field. Since the DC voltage U in the quistor shown as an exemplary embodiment has the value U = 0, the stability range is traversed along the line 21 by changing the RF voltage.

The device for generating an electron beam provided in the quistor according to FIG. 1 allows the ions to be generated in the quistor itself by in the ionization phase, the duration of which can be determined by means of the blocking lens 2, an electron beam from the hot cathode 1 through the opening 10 is focused in the quistor. Typical ionization times for an electron beam of 100 µA strength are in the range from 10 µs to 100 ms, depending on the concentration of the substance to be examined.

The diagram according to FIG. 3 illustrates the time it takes for ions to leave the quistor and accordingly manifests itself as a line width as a function of the distance-related circular ratio Q. The three curves of the diagram according to FIG. 3 correspond to different scanning speeds speeds indicated at the bottom of Fig. 3. Damping gas was used under optimal pressure conditions. It is readily apparent that the release capability increases significantly for Q <4,000.

Fig. 4 shows the spectrum of the molecular ion group of tetrachloroethene for different values of the distance-related ratio circuit Q. The spectra were recorded as Dampfungsgas mbar with different scanning speeds over each 300 mass units using air at a pressure of 4.10- ^4. In the upper spectra a, c and e the scan time was 100 ms each, while in the lower spectra b, d, and f the scan time was 20 ms each. The spectra a and b were in a quistor with the distance-related circular ratio Q = 4.4, the middle spectra c and d in a quistor with Q = 4.0 and finally the right spectra e and f in a quistor with Q = 3, 6 added. The quistors used had the dimensions shown in the following table (in cm):

From these dimensions, the distance r _o determines the field strength V / m of the alternating field and thus the highest mass that can be detected with a scan, given the amplitude of the RF voltage applied to the ring electrode. From this point of view, the value of r0 = 1 cm, which was fixed for all three quistors, made the above-mentioned scan of 300 mass units possible. The values of z _o were calculated to be z _o = r _o / 4√Q, while R _e and R _r were chosen such that R _e x R _r = r _o xz _oo

The dramatic improvement in the resolving power and the signal-to-noise ratio between the spectra according to FIG. 4a and FIG. 4f underlines the significant technical progress that the invention brings about. It should be particularly emphasized that the increase in the scanning speed, which enables the reduction of the distance-related circular ratio Q to values Q <4.000, also leads to a disproportionate increase in the signal / noise ratio and thus to the significantly increased resolution.

Another advantage is that the influence of the space charge is significantly reduced for values of Q <4.000. Even if the signal strengths were reduced by a factor of 100, no significant change in line shape and line width could be observed.

The reason for the observable improvements is the occurrence of a resonance of the secular movement of the ions exactly at the instability limit, which accelerates the increase in the amplitude of the secular movement and thus increases the speed of the ion ejection. The ejection is therefore only partly due to the instability of the orbits and partly due to the additional energy absorption of the ions from the storing HF field, which is made possible by the resonance.

So far, negative influences due to resonance phenomena have not been found, as long as work was carried out essentially without using a DC voltage field. A preferred embodiment therefore provides for the omission of the DC voltage field. In principle, however, it would also be possible to use a DC voltage field and to vary the DC voltage field in order to change the stability range.

It is understood that the invention is not limited to the exemplary embodiment shown, but many deviations therefrom are possible without leaving the scope of the invention. In particular, the use of a large number of different quistors is possible, the dimensions of which can be modified in many ways without violating the condition that the distance-related circular ratio Q is always less than or at most equal to 3.990.

Claims (3)

1. Method for mass-spectroscopic investigation of a gas mixture by using a mass spectrometer having an ion trap which is constructed as a quistor with an annular electrode and two end electrodes which seal the chamber bounded by the annular electrode and of which at least one is provided with a perforation arranged in an extension of the axis of rotation of the annular electrode, in which method the following steps are carried out:

application of an RP voltage of such amplitude and frequency and, possibly, of such a direct voltage to the annular electrode that there is generated inside the ion trap a three-dimensional RF quadrupole field which is suitable for capturing ions, whose charge/mass ratio lies in a prescribed range, and storing them in the ion trap,

introducing or generating ions of the gas mixture into or inside the ion trap, and storing in the ion trap ions whose charge/mass ratio lies in the prescribed range,

varying at least one of the field parameters formed from the amplitude, the frequency and, possibly, the direct voltage in such a way that ions having a monotonically varying charge/mass ratio sequentially become unstable and leave the ion trap in the direction of the axis of rotation of its annular electrode through the said perforation in the end electrode and

measuring and recording the intensity of the ion current leaving the ion trap as a function of the variation in the field parameters, characterised in that

in order to carry out the method use is made of a quistor in which the distance-related ratio Q of the radii of the inscribed electrode apex circles satisfies the condition Q 3.990, it being the case that

where

R_e = radius of the apex cross-section of the end electrodes

R_r = radius of the apex cross-section of the annular electrode

_Zo = distance of the apexes of the end electrodes from the centre of the quistor and

r_o = distance of the apex of the annular electrode from the centre of the quistor.

2. Mass spectrometer having an ion trap which is constructed as a quistor with an annular electrode and two end electrodes sealing the chamber bounded by the annular electrode and of which at least one is provided with a perforation arranged in an extension of the axis of rotation of the annular electrode, for investigating a gas mixture according to the method according to Claim 1,
characterised in that

the distance-related ratio Q of the radii of the inscribed electrode apex circles satisfies the condition Q < 3.990, it being the case that

where

R_o = radius of the apex cross-section of the end electrodes

R_r = radius of the apex cross-section of the annular electrode

_Zo = distance of the apexes of the end electrodes from the centre of the quistor and

r_o = distance of the apex of the annular electrode from. the centre of the quistor.

3. Mass spectrometer according to Claim 2, characterised in that of the dimension of the quistor which determine the distance-related ratio Q the distance r_o of the apex of the annular electrode from the centre of the quistor has a value for which it in ensured that given the amplitude of the RP voltage applied at the annular electrode the largest mass of interest is still captured by means of the storage field, for a prescribed value of the ratio Q the distance z_o of the apexes of the end electrodes from the centre of the quistor is z_o = r_o/4√Q, and, finally, the radii R_e and R_r of the apex cross-sections are selected such that R_e x R_r = r_o x z_o.

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