An atmospheric pressure ionization mass spectrometer, which uses an ion source for ionizing ions under approximately atmospheric pressure by an appropriate ionization method, such as electrospray ionization (ESI), atmospheric chemical ionization (ACPI), inductively coupled plasma ionization (ICP) or atmospheric pressure matrix laser assisted ionization (AP-MALDI), generally includes a multi-stage differential pumping system to maintain a high-vacuum atmosphere within a vacuum chamber in which a mass analyzer (e.g. a quadrupole mass filter or a time of flight mass spectrometer) is provided. In this type of mass spectrometer, it is necessary to efficiently transport ions under a low-vacuum atmosphere with a gas pressure of approximately 1-104 Pa. For this purpose, various types of ion transport optical systems (which may also be referred to as ion guides or ion lenses) with different forms and configurations have been proposed and supplied for practical uses.
In some cases, the term “ion transport optical system” is used to refer only to an electrode unit for creating an electric field within a space which the ions pass through. However, the resulting electric field not only depends on the configuration of the electrode unit; it is also affected by the voltages applied to the electrodes. Accordingly, the term “ion transport optical system” is hereinafter used to refer to a system that includes both the electrode unit and a voltage-applying unit (circuit) for applying voltages to the electrodes.
One conventionally known type of ion transport optical systems is the so-called “ion funnel”, which is disclosed in WO97/49111 and other documents. As shown in FIG. 9, the electrode structure of the ion funnel basically consists of an array of ring electrodes arranged at equal intervals along the ion-transport direction, with each electrode having a circular aperture at the center thereof through which ions can pass. Not all of these electrodes have the same aperture diameter; their aperture diameter gradually decreases in the ion-transport direction, with the electrode at the ion-entrance end having the largest aperture diameter and the electrode at the ion-exit end having the smallest aperture diameter. A pair of radio-frequency voltages having a phase difference of 180 degrees (that is, with reverse phases) are applied to any pair of ring electrodes neighboring each other in the ion-transport direction. As a result, a radio-frequency electric field for confining ions is created in the inner space of the ring-electrode array (this space is called the “ring-electrode inner space” in this specification). Additionally, a direct-current (DC) voltage is applied from the voltage-applying unit to each of the electrodes to create a potential gradient that promotes the travel of ions in the ion-transport direction.
The ring-electrode inner space is in the form of a funnel that is tapered in the ion-transport direction. Therefore, the radio-frequency electric field created in this space has a relatively strong spatial-focusing effect for converging ions into the vicinity of the central axis (ion-beam axis) of the ring electrodes. In the case where the ion funnel is used under a low-vacuum atmosphere of approximately 102-104 Pa, a focusing effect due to collisional cooling also works on the ions since there is a considerable amount of residual gas. Due to these effects, the ion beam has an extremely small beam diameter when it is emitted from the ring electrode at the exit end of the ion-transport direction, and the emitted ions have low emittance. Another advantage exists in that the ions can be efficiently transported since the radio-frequency electric field is evenly formed in the circumferential direction around the central axis and thereby suppresses the leakage of ions through the spaces between the neighboring electrodes, which occurs in the case of a multi-pole rod configuration.
Examples of atmospheric pressure ionization mass spectrometers using the previously described type of ion funnels are disclosed in U.S. Pat. Nos. 6,107,628, 6,803,565 and 6,583,408. In these mass spectrometers, the electrode unit of the ion funnel is disposed in a low-vacuum chamber next to the ionization chamber in which electrospray ionization is performed. The ions produced in the ionization chamber are sent through a capillary pipe into the low-vacuum chamber, where the ions are injected along the ion-transport direction into the circular aperture of the ring electrode located at the front end, and a thin beam of ions is emitted from the ring electrode located at the farthest end.
As just described, the ion funnel has outstanding ion-transport efficiency and ion-converging capability. However, this device has the following problems.
In a mass microscope (which may also be called an imaging mass spectrometer) using AP-MALDI as disclosed in Harada et al. “Kenbi shitsuryou Bunseki Souchi Ni Yoru Seitai Soshiki Bunseki (Analysis of Living Tissue Using Mass Microscope”, Shimadzu Hyouron (Shimadzu Review), Shimadzu Hyouron Henshuu-bu, Vol. 64. No. 3/4, Apr. 24, 2008, the sample to be analyzed is placed on a horizontal plane within a sample chamber maintained at approximately atmospheric pressure for the convenience of microscopic observation of the sample with an optical microscope. Therefore, the ions produced from the sample by laser irradiation need to be extracted upwards. On the other hand, an ion-transport optical system (RE ion guide), an ion trap and a time-of-flight mass analyzer are horizontally arranged in the vacuum chamber, where ions are transported in the substantially horizontal direction. Accordingly, the capillary pipe, which functions as an interface connecting the sample chamber and the vacuum chamber, has its entrance directed downwards and its exit directed horizontally. This design is realized by almost perpendicularly bending the capillary pipe at the middle point thereof. Such a design also applies to the case where an ion funnel is used as the ion-transport optical system, in which case the ions are almost horizontally ejected from the exit of the bent capillary pipe and injected into the apertures of the ring electrodes.
There is a pressure difference between the entrance and exit ends of the capillary pipe. This pressure difference produces a gas stream, which carries ions into the capillary pipe, transports them to the exit end, and ejects them into the vacuum chamber. However, if the capillary pipe is significantly bent in the previously describe manner, the gas stream is disturbed at the bent portion, making the ions collide with the inner wall of the pipe and possibly causing a considerable loss of ions. This problem is particularly serious since the inner diameter of the capillary pipe is small to restrict conductance for several reasons, e.g. to maintain the low gas pressure inside the vacuum chamber or to allow the use of a low-power pump as the pump for evacuating the vacuum chamber. Using such a thin capillary pipe increases the influence of the disturbance of the gas stream and results in a considerable ion loss. Thus, even if the ion funnel can efficiently transport ions, the overall ion-transport efficiency cannot be easily improved since a significant amount of ions is lost in the previous stage.
The ions ejected from the exit of the capillary pipe are introduced through the aperture of the ring electrodes into the ring-electrode inner space together with the gas stream. In the ion funnel, the ring electrodes are arrayed at small intervals, so that the gas hardly diffuses through the gap between the neighboring ring electrodes. Therefore, a significant part of the gas flows through the ring-electrode inner space, to be ejected from the small aperture of the ring electrode located at the exit end. As a result, the gas pressure around the exit of the ion funnel becomes higher than the surrounding pressure, which deteriorates the degree of vacuum atmosphere in the subsequent stage where the ion-transport optical system and the mass analyzer are provided. To solve this problem, a mass spectrometer is disclosed in U.S. Pat. No. 6,583,408, in which a disk-shaped electrode is provided on the ion-beam axis within the ring-electrode inner space so that the gas stream will collide with this electrode and become deflected outwards. However, adding this electrode makes the electrode structure more complex. Furthermore, the additional electrode is likely to become contaminated and disorder the electric field in the ring-electrode array.
On the other hand, in a mass spectrometer using an ICP ion source as disclosed in Japanese Unexamined Patent Application Publication No. 2008-192519, an off-axis ion-transport optical system is used to remove elements that will cause a background noise, such as the light or neutral particles emitted from the ion source. In the case of the ion funnel, the off-axis structure can be created, for example, by gradually shifting the axis of each ring electrode. However, this method may possibly disorder the radio-frequency or DC electric field and thereby considerably deteriorate the ion-transport efficiency.
The present invention has been developed to solve the previously described problems, and one objective thereof is to provide a mass spectrometer in which a high level of analysis sensitivity is achieved by improving the overall ion-transport efficiency while making use of the advantages of the ion funnel. Another objective of the present invention is to provide a mass spectrometer which is designed to suppress the gas pressure around the rear end of the ring-electrode array having a funnel structure while making use of the advantages of the ion funnel. Still another objective of the present invention is to provide a mass spectrometer which is designed to obtain the effect of the off-axis structure while making use of the advantages of the ion funnel.