EP0290712A1 - Mass spectrometer - Google Patents

Abstract

In the eight-pole arrangement (5) used for holding and guiding ions having a low kinetic energy, an E-field in the desired ion movement direction is superimposed on the radially acting RF field, as a result of which, despite the kinetic energy of the ions remaining low, safe, rapid deflection to the mass spectrometer quadrupole (3) takes place and the response time and overall sensitivity of the arrangement can be significantly increased. <IMAGE>

Description

The invention relates to a mass spectrometer arrangement with an ion source, an adjoining one, receiving a gas mixture and surrounded by an octopole arrangement with a high-frequency 8-pole field acting perpendicular to the main direction of movement of the primary ions supplied by the ion source. as well as a primary mass spectrometer quadrupole connected to the reaction space in the main direction of movement of the low kinetic energy.

Such arrangements are known and are used at least in laboratory operation for two different applications:

On the one hand, the input stages of so-called drift tube arrangements (e.g. SIDT-Quasistatic Selected-Ion Drift Tubes) can be designed in this way, whereby the primary ions emerging from an exchangeable ion source enter an octopole storage zone, where they are exposed, for example, to a neutral gas or React gas mixture with production of new ions in known excitation states. From the octopole arrangement or the reaction space surrounding it, the ions enter the mass spectrometer quadrupole, where the type of ion to be investigated is selected in a known manner which is of no further interest here and then introduced into the actual drift chamber via a venturi-type inlet.

On the other hand, the arrangement described at the outset can also be used directly as a mass spectrometer analyzer, in which case the gas mixture in the reaction space is examined by producing certain product ions with the primary ions, which, after being selected in the mass spectrometer quadrupole, are directly used by an ion detector can be supplied, which allows a mass spectrometric analysis in a known manner.

Mit:
    r      Radius der Oktopol-Anordnung
    v_r      Geschwindigkeit der Ionen in r-Richtung
    ω      Frequenz des HF-Feldes
    e      Elementarladung
    V_O      Amplitude des HF-Feldes
    m      Masse der IonenThe octopole arrangement is used in both cases to hold or guide the ions, which at low kinetic energies (in general kinetic energies are understood to be less than 100 eV) due to their own space charge pressure (Coulomb repulsion between the charges) a high divergence in itself of their movement. The requirement for low kinetic kinetic energies is in turn based on the fact that selective, fragment-free ionization - i.e. the generation of a certain type of product ions in a certain gas with a certain primary ion beam with the greatest possible efficiency - only allows small kinetic energies in the ion / molecule center of gravity . The primary ion beam entering the octopole arrangement with low kinetic energy is thus held by a high-frequency 8-pole field perpendicular to the direction of movement of the primary ions in the reaction space, whereby essentially two conditions must apply to the applied high-frequency field:

With:
r Radius of the octopole arrangement
v _r Velocity of the ions in the r direction
ω frequency of the RF field
e Elementary charge
V _O amplitude of the RF field
m mass of ions

Thus charged particles move in a time-independent effective potential, which is zero in the center - i.e. along the axis - and rises towards the individual electrode rods like a parabolic function of a higher order against infinity. This potential counteracts the space charge pressure of the ions and thus keeps the ion beam in a cross section which is predetermined by the dimension of the interior of the octopole arrangement.

A major problem in connection with the known arrangements of the type mentioned at the outset was the fact that the product ions remaining or resulting after a reaction of the primary ions with the neutral molecules of the gas mixture generally do not provide any information about the necessary direction of flight to the mass spectrometer quadrupole, because during ion impact, depending on the relative mass of the particles involved, part of the amount of the momentum is transferred from the ion to the neutral molecule, but not the direction of the momentum. Accordingly, the product ions move in random directions, i.e. the directional distribution corresponds to the translational Boltzmann distribution.

Now about the yield of product ions reaching the mass spectrometer quadrupole, which is of course very low on the basis only of diffusion processes for the statistical compensation of concentration differences, and above all measurements of product ions from dynamic or highly dynamic processes and through the reaction space not allowed to move moving gas mixtures or only with mostly intolerably large delays, an extractor lens with a potential that is negative compared to the octopole arrangement could be arranged after the reaction space in order to direct the ions from the reaction space into the mass spectrometer quadrupole focus. Under certain circumstances it would also be conceivable to place the quadrupole itself at a potential which is negative compared to the octopole arrangement. Both measures are not expedient because very high voltages would have to be applied to this extractor lens or the quadrupole for the necessary electrical field penetration into the reaction space or into the octopole arrangement. However, these high voltages of the order of magnitude of over 100 eV would in an extremely disadvantageous manner accelerate the ions to such high kinetic energies that the function of the downstream mass spectrometer quadrupole was severely impaired, ie its resolving power would be greatly reduced.

The object of the present invention is to improve an arrangement of the type mentioned at the outset in such a way that the disadvantages of the known arrangements are avoided and in particular that the supply rate and supply speed of product ions formed in the reaction space to the mass spectrometer quadrupole are increased in a simple manner.

This is achieved in accordance with the present invention in that the octopole arrangement is divided in the longitudinal direction into individual regions which are electrically insulated from one another and which, viewed from the ion source to the mass spectrometer quadrupole, are each at a negative DC voltage potential which rises in relation to the previous region. In this way, a DC voltage potential is generated at the nth octopole range, which is negative compared to the (n - 1) th range but positive compared to the (n + 1) th range. In the center or in the axis of the octopole arrangement, a homogeneous electric field is generated in the axial direction to the mass spectrometer quadrupole, which serves as the driving force for the product ions. Because the random movement energy of the product ions is generally not greater than 1 eV, very low field strengths are sufficient, as a result of which disturbances in the mass spectrometer quadrupole due to high kinetic energies of the incoming product ions are reliably avoided.

According to a preferred embodiment of the invention, the total DC voltage effective at the octopole is in the range 5 to 30 V.

According to a further embodiment of the invention, it is provided that the direct voltage field strength at the octopole is uniformly 0.5 to 3 V / cm, preferably 1 V / cm.

Since the random kinetic energy of the product ions is generally not greater than 1 eV, a field strength of 1 V / cm is in itself sufficient. This gives e.g. with a 10 cm long octopole arrangement, product ions with energies of at most 10 eV, which does not cause any disturbance for the mass spectrometer quadrupole, since its energy acceptance is usually around 40 eV.

According to a preferred further embodiment of the invention, the individual octopole regions are connected to a common DC voltage supply via a resistor chain, which enables a simple structure of the arrangement and enables a controlled field profile over the entire length of the octopole arrangement.

• Fig. 1 zeigt in schematischer,geschnittener Darstellung eine Massenspektrometer-Anordnung nach der vorliegenden Erfindung,
• Fig. 2 einen schematischen Schnitt durch einen einzelnen Oktopol-Bereich entlang der Linie II-II in Fig. 1 in anderem Maßstab,
• Fig. 3 ein Potential V/Radius r-Diagramm der Anordnung nach Fig. 2,
• Fig. 4 eine in drei einzelne Bereiche aufgeteilte Okto­pol-Anordnung zur Verwendung in einer Massenspektro­meter-Anordnung nach der Erfindung und
• Fig. 5 ein Potential/Radius/Längserstreckung-Diagramm einer Oktopol-Anordnung gemäß Fig. 4.

The invention is explained in more detail below with reference to the drawing.

• 1 shows a schematic, sectional illustration of a mass spectrometer arrangement according to the present invention,
• Fig. 2 shows a schematic section through an individual Octopole area along the line II-II in Fig. 1 on a different scale,
• 3 shows a potential V / radius r diagram of the arrangement according to FIG. 2,
• Fig. 4 shows an octopole arrangement divided into three individual areas for use in a mass spectrometer arrangement according to the invention and
• 5 shows a potential / radius / longitudinal extension diagram of an octopole arrangement according to FIG. 4.

The mass spectrometer arrangement according to FIG. 1 has an ion source 1 of any design that is not of interest here, a reaction chamber 2 adjoining the ion source 1, and a mass spectrometer connecting the primary ions supplied by the ion source 1 to the reaction chamber 2 in the main direction of movement z Quadrupole 3 on. The reaction chamber 2, to which a gas or gas mixture can be supplied via a connection 4, is surrounded by an octopole arrangement 5 with a high-frequency 8-pole field acting perpendicular to the main direction of movement z of the primary ions, which is used for collection and mounting or guidance of the primary ions, which have a very low kinetic energy of typically below 100 eV. The product ions selected in the mass spectrometer quadrupole 3, which are likewise of no further interest here, come to an ion sensor 6, which may also be of a known or customary type and whose function and mode of operation are of no greater interest here.

For the sake of simplicity of illustration, various additional devices, such as diaphragms or lenses for the ion beam or vacuum pumps and seals and the like, which are partly essential for the function of the arrangement in normal operation, are not shown, since they are of minor importance in connection with the present invention.

The octopole arrangement 5 is divided in the longitudinal direction - ie in the main direction of movement z of the primary ions emerging from the ion source 1 in a beam - into three individual regions 7, 8, 9 which are electrically insulated from one another and which - from the ion source 1 to the mass spectrometer - Quadrupole 3 seen - lie on the negative direct voltage potential that rises in each case with respect to the previous region.

The individual eight rods of the area 7 of the octopole arrangement 5 are indicated in section in FIG. 2. Four of them are contacted together, a high-frequency alternating field (potential V≈) being applied in a manner not shown here, which causes a high-frequency guide field V _eff in the interior between the eight rods of the octopole arrangement, which is indicated in the diagram in FIG. 3 . The edges or surfaces of two opposite rods are marked with r _o and -r _o in FIG. 3 - it can be seen that the effective potential in the center of the arrangement (r = 0) is zero and towards the octopole electrode rods how a parabolic function of higher order rises towards infinity. This potential V _eff counteracts the space charge pressure (Coulomb repulsion between the charges) of the ions which have only very low kinetic energy and thus holds them in a cross section which is predetermined by the dimension of the octopole arrangement 5.

The division of the entire octopole arrangement 5, which can also be seen in FIG. 4, into three individual regions 7, 8, 9 which generate a direct current field which is superimposed on the high-frequency guide field, serves to separate the primary ions from the primary ions in collisions with the gas particles Gas supplied via port 4 or

Bring the gas mixture generated product ions as quickly and completely as possible from the reaction chamber 2 and feed them to the mass spectrometer quadrupole 3 (FIG. 1) for selection or subsequent detection. 4 also schematically shows the potential connections at the areas 7, 8, 9 of the octopole arrangement 5; the high-frequency guide field or the potential V≈ is the same for all three areas; the superimposed negative DC voltage potential increases from U₁ to U₂ to U₃, the corresponding field strength being, for example, 1 V / cm. The total potential V _Tot over the length of the octopole arrangement can be seen from FIG. 5. The potential of the RF field is a parabolic trough, which is quasi raised by the - here uniform over the entire length - potential of the superimposed E field at the entrance of the octopole arrangement. From the illustration in FIG. 5 it is also easy to see figuratively how the ions on the one hand are held in the reaction space defined in the interior of the octopole arrangement and on the other hand are rapidly discharged towards the mass spectrometer quadrupole but with little kinetic energy.

Deviating from the shown division of the octopole arrangement 5 into three individual regions, a division into two or, as such, any number of regions could naturally also take place which, as mentioned, are at different DC potentials.

Claims (5)

1. mass spectrometer arrangement, with an ion source, an adjoining the latter, receiving a gas mixture and surrounded by an octopole arrangement with a high-frequency 8-pole field acting perpendicular to the main direction of movement of the primary ions supplied by the ion source, and a reaction chamber in the main direction of movement of the primary ions having low kinetic energy adjoining the reaction space, the mass spectrometer quadrupole, characterized in that the octopole arrangement (5) is divided in the longitudinal direction into individual regions (7, 8, 9) which are electrically insulated from one another, which - by seen the ion source (1) to the mass spectrometer quadrupole (3) - on each rising relative to the previous area, negative DC voltage potential (U₁, U₂, U₃). 2. Arrangement according to claim 1, characterized in that the total effective DC voltage at the octopole (5) is in the range of 5 - 30 V. 3. Arrangement according to claim 1 or 2, characterized in that the direct voltage field strength at the octopole is uniformly 0.5 to 3 V / cm. 4. Arrangement according to claim 3, characterized in that the direct voltage field strength at the octopole is evenly 1 V / cm. 5. Arrangement according to one of claims 1 to 4, characterized in that the individual octopole regions (7, 8, 9) are connected to a common DC voltage supply via a resistor chain.

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