Radiant energy – Ionic separation or analysis – Cyclically varying ion selecting field means
Reexamination Certificate
2002-12-06
2004-01-06
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Ionic separation or analysis
Cyclically varying ion selecting field means
Reexamination Certificate
active
06674071
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to ion-guide systems for the transfer, cooling, fragmentation, selection and temporary storage of ions.
BACKGROUND OF THE INVENTION
In mass spectrometers with out-of-vacuum ion generation, it is necessary first to inject the ions into the vacuum system through apertures or capillaries and then to transmit them via various differential pump stages to the actual mass separation system, the mass spectrometric ion analyzer.
Ion transmission has long been achieved using so-called ion guides, which are generally in the form of radio-frequency carrying multi-pole systems such as quadrupole, hexapole or octopole systems consisting of long, thin parallel pole rods. Other systems are also known, e.g. the radio-frequency double helix. By using terminating diaphragms at both ends maintained at an ion-repulsion dc potential, all these systems can also be used as temporary storage devices so that, for example, ions can be injected into a pulsed mass analyzer at the correct times. Pulsed mass spectrometers in this sense include ion-trap mass spectrometers, ion-cyclotron resonance spectrometers and time-of-flight mass spectrometers with orthogonal ion injection.
The ion-guide systems consist of a number of pairs of rods (or pairs of helixes). The two phases of a two-phase radio-frequency voltage supply are applied to two neighboring rods in each case. Barriers of a so-called pseudo-potential are formed between the rods. These barriers hold the ions within the rod system. However, the pseudo-potential barriers are not very high and ions with energies greater than about ten electron volts are able to surmount them.
Radio-frequency ion-guide systems with rod-shaped electrodes have since been adopted for almost all mass spectrometers which operate with out-of-vacuum generated ions such as electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI). These types of ionization are preferably linked to a device which temporally separates the analyte mixtures by liquid chromatography or capillary electrophoresis. However, ion-guide systems can also be used for ions which have been generated in the vacuum system itself. For example, these types of ion-guide systems are used for ions which are produced by matrix-assisted laser desorption and ionization (MALDI) when they are destined for an ion-trap mass spectrometer (ITMS) or an ion-cyclotron resonance spectrometer (ICRMS or Fourier-transform mass spectrometer FTMS).
In U.S. Pat. No. 5,179,278 (D. J. Douglas), a device and method are described for supplying externally generated ions to an ion trap. In this case, the ions can be temporarily stored and freed of unwanted ions beforehand. The feed system used is an ion-guide system in the form of a multipole, i.e. a quadrupole, hexapole, octopole or higher multipole, with rod-shaped electrodes arranged in parallel to produce a two-dimensional radio-frequency multipole field. According to the claims in the patent, the multipole field is used both for the temporary storage of ions during the time the ions are analyzed in the ion trap and for preselection. Preselection is achieved by resonance ejection of the unwanted ions from the multipole system by the special application of an additional ac voltage to two opposite electrode rods or rod pairs. This method enables unwanted ion species to be removed individually by choosing the frequency of the supplementary ac voltage.
Out-of-vacuum ion generation means that the ions have to be introduced into the vacuum system. Here, a combination of injection capillaries, an initial differential pump stage, a skimmer, a second differential pump stage and a multipole system to capture the divergent, dispersing ions behind the skimmer has proved to be successful, even though by no means all of the ions introduced into the vacuum can be captured with this system. A higher-order multipole system (with a larger number of rods) is the preferred system for capturing a high proportion of the ions emerging from the skimmer at a wide angle. At the very least, a hexapole system or, better still, an octopole system is used for this purpose. With divergent ion bundles, these multipole systems are more efficient for ion capture than quadrupole systems because the reflection at the gridded wall system is better. However, many ions are lost even before the skimmer.
In the first ion-guide system after the skimmer, there is still significant residual pressure of the order of 10
−1
to 100 Pascals which causes the kinetic energy of ions moving both in the direction of and across the axis to fall very rapidly. The ions tend to collect at the axis of the ion-guide system. With the special addition of a damping gas such as helium to the first or following ion guide systems, the ion beam can also be conditioned by cooling.
In this context, conditioning of the ion beam means decelerating the movement of ions and collecting them in the potential minimum of the pseudo potential at the axis or near the axis of the ion-guide system. With suitable diaphragm systems at the end of the ion guide, the ions can then be drawn from the ion-guide system and formed into a relatively fine, almost parallel ion beam. The conditioning process results in a reduction of the six-dimensional phase space volume which describes the distribution of ions in the position and momentum space. This type of conditioning by reducing the phase space volume, cannot be achieved using ion-optical methods (a consequence of Liouville's theorem). The phase space volume can only be reduced by so-called cooling processes, e.g. gas cooling or laser cooling. An ion-guide system which conditions the ions by gas cooling for injection into a mass-selecting quadrupole filter was described in U.S. Pat. No. 4,963,736 (D. J. Douglas and J. B. French).
However, the ion-guide systems are not only used for transmitting ions to the mass analyzer. When filled with gas, they can also be used for collisionally induced fragmentation. In this case, ions are injected with higher energies into an ion guide system filled with a collision gas in a certain pressure regime. The fragmentation process is referred to by the abbreviation CID (collisionally induced decomposition). Here too, whether they are fragmented or not, the ions are cooled in the collision gas. The fragmentation process in this ion-guide system (including the frequently used quadrupole system) is the more effective the greater the molecular weight of the collision gas; however, the heavier gases cannot be used since, on collision, the gas molecules frequently deflect the ions sideways so that they are then able to overcome th pseudo-potential barriers between the rods and get lost from the ion guide.
Another arrangement for the time-of-flight mass spectrometers with orthogonal ion injection is disclosed in U.S. Pat. No. 6,011,259 (Whitehouse, Dresch and Andrien) where multipole systems in the form of multipole ion guides are used both for transmitting the ions from out-of-vacuum ion sources to the mass spectrometer and for selecting and fragmenting suitable parent ions. In this case, the gas (usually nitrogen) penetrating into the vacuum system from the external electrospray source at the same time is used as a collision gas for fragmenting the ions and for damping some of the ion movement. Here, the forward movement of the ions must not be damped completely because the multipole rod systems which are used as the ion-guide systems do not provide any active means of forward motion for the ions. The velocity therefore must not be fully damped because otherwise the ions will only be able to leave the ion system by slow diffusion processes. Although these systems can be used to store the ions so that their emptying can be time-controlled according to demand, the ions generated earlier mix with the ions generated later and this interferes with the high temporal resolution of substances separated by rapid chromatography or electrophoresis. On the other hand, ions which have not been decelerated to zero velocity in the gas
Brekenfeld Andreas
Franzen Jochen
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