Radiant energy – Ionic separation or analysis – Ion beam pulsing means with detector synchronizing means
Reexamination Certificate
2001-03-02
2004-03-02
Wells, Nikita (Department: 2881)
Radiant energy
Ionic separation or analysis
Ion beam pulsing means with detector synchronizing means
C250S282000, C250S281000, C250S292000
Reexamination Certificate
active
06700117
ABSTRACT:
The invention relates to a method and a device which reduces the phase space volume of ions in an ion beam in such a way that their injection into a downstream time-of-flight mass spectrometer optimizes the performance of that spectrometer. The performance of the time-of-flight mass spectrometer, i.e. the sensitivity of the spectrometer, the temporal resolution for fast concentration changes of the examined substances, and particularly the mass resolving power, relates critically to the transmission of the ions.
The invention consists of completely decelerating the ions by means of collisions with a damping gas in an RF ion guide system, guiding them to the end of the ion guide system by active forward thrust, extracting them by a drawing lens system, and forming an ion beam with a low phase space volume. In particular, the ion guide system can take the form of a pair of wires coiled in a double helix and be surrounded by an envelope which is filled with the damping gas.
PRIOR ART
Time-of-flight mass spectrometers with orthogonal injection of a primary ion beam have a so-called pulser at the beginning of the flight path, which accelerates a section of the primary ion beam, i.e. a thread-like ion package, at right angles to the previous direction of the beam. A band-shaped secondary ion beam is created in which light-weight ions fly fast and heavier ions fly more slowly, and the flight direction of which is between the previous direction of the primary ion beam and the perpendicular direction of acceleration. Such a time-of-flight mass spectrometer is preferably operated in conjunction with a velocity-focusing reflector which reflects the band-shaped secondary ion beam over its entire breadth and deflects it to an also extended detector
The mass resolution of such a time-of-flight mass spectrometer depends quite essentially on the spatial distribution and velocity distribution of the ions of the primary beam in the pulser.
If all the ions are flying exactly along an axis behind one another and if the ions do not have any velocity components at right angles to the primary ion beam, an infinitely high mass resolving power can, theoretically and very plausibly, be achieved because all the ions having the same mass are flying exactly in the same front and reach the detector at exactly the same time. If the primary ion beam has a finite cross section but none of the ions has a velocity component at right angles to beam direction, spatial focusing of the pulser can in turn theoretically bring about an infinitely high mass resolution (W. C Wiley and I. H. McLaren “Time-of-Flight Mass Spectrometer with Improved Resolution” Rev. Scient. Instr. 26, 1150, 1955). The high mass resolution can even be achieved if there is a strict correlation between the ion location (measured from the beam axis of the primary beam in the direction of acceleration) and the perpendicular ion velocity in the primary beam in the direction of acceleration. If, however, there is no such correlation, i.e. if the ion locations and perpendicular ion velocities are statistically distributed without any correlation between the two distributions, high mass resolution can no longer be achieved.
The primary ion beam has therefore to be conditioned relative to spatial and velocity distribution in order to achieve a high mass resolution in the time-of-flight mass spectrometer.
In the simplest case such a conditioning can be achieved with two coaxial apertured diaphragms with very small holes, which only admit beam ions which are flying along very parallel axes and axes which are close to one another. In this case the conditioning takes place at the expense of ion transmission, and therefore at the expense of the sensitivity of such a mass spectrometer. Generally speaking, such a solution with low sensitivity is undesirable.
The six-dimensional space of spatial and pulse coordinates is called the “phase space”. In an ion beam the spatial and pulse coordinates of all the ions fill out a certain part of the phase space and that part is called the “phase space volume”. Conditioning the primary beam therefore always means reducing phase space volume, at least in the coordinates at right angles to beam direction. A reduction in phase space volume cannot be achieved according to physical laws with ion-optical means but only by cooling the ion plasma of the ion beam, e.g. by cooling in a damping gas. Such cooling of the ions by a damping gas (at the expense of time) is known, for example, from high frequency quadrupole ion traps.
Time-of-flight mass spectrometers with orthogonal ion injection are preferably used for scanning high-resolution mass spectra with a fast spectrum sequence in order to be able to follow a separation of substances in fast methods of separation, capillary electrophoresis or microcolumn chromatography, for example, without any time smearing. Consequently, apart from high mass resolution, a high temporal resolution of subsequent substances is desirable. The cooling of the ions should therefore, if possible, take place by a continuous method which does not cause any mixing of earlier and later ions.
For time-of-flight mass spectrometers with preferably orthogonal injection an instrumental arrangement recently has became known from U.S. Pat. No. 6,011,259 (Whitehouse, Dresch and Andrien) in which multipole rod systems are used as ion guide systems (“multipole ion guides”), which guide ions from vacuum-external ion sources to the mass spectrometer and thus are also used for the selection of suitable parent ions and their fragmentation. The gas penetrating into the vacuum system together with the ions (usually nitrogen) is used as the collision gas for fragmentation, which also damps part of the motion of the ions but cannot be used systematically to reduce the phase space volume of the ions. Multipole rod systems used as ion guide systems do not have any active ion forward thrust; that is why in such systems the velocity must not be damped completely or else they can no longer pass through the ion guide system without mixing. On the other hand, they can be used as storage with requirement time-controlled outflow of the ions, but earlier and later ions mix and disturb the temporal resolution of fast chromatography and electrophoresis.
These multipole field ion guide systems consist of at least 2 pairs of straight pole rods which are evenly distributed over the surface of a cylinder and whose rods are alternately supplied with the two phases of an RF voltage. If there are two pairs of rods this is referred to as a quadrupole field, and if there are more than two pairs of rods they are referred to as hexapole, octopole, decapole, dodecapole fields etc. An ion-guiding dipole field with only one pair of rods cannot be generated. The fields are frequently termed 2-dimensional because in each cross section through the rod array the field distribution is the same. Consequently, field distribution only changes in two dimensions.
The RF multipole rod systems have become known as guide fields for ions between ion sources and ion consumers, particularly for feeding ions generated outside of the vacuum to RF or ICR ion traps inside vacuum systems.
The rod systems used for guiding ions are generally very slim in order to concentrate the ions in an area with a very small diameter. They can then advantageously be operated at low RF voltages and represent a good starting point for further ion-optical ion imaging. The clear cylindrical interior often only has a diameter of about 2 to 4 millimeters and the rods are less than 1 mm thick. The rods are usually fitted into grooves which are located inside of ceramic rings. The requirements for inside diameter uniformity, i.e. rod spacing, are relatively high. For this reason the system is not easy to make and it is also sensitive to vibrations and shock. The rod systems bend very easily and then they can no longer be adjusted.
On the other hand, U.S. Pat. No. 5,572,035 (Franzen) describes various ion guide systems which are completely different from the multipole rod systems describe
Bruker Daltonik GmbH
Wells Nikita
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