CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/JP2018/021010, filed May 31, 2018.

TECHNICAL FIELD

The present invention relates to a mass spectrometer, and more particularly to a mass spectrometer including an ion transport optical element such as a multipole ion guide.

BACKGROUND ART

Generally, a mass spectrometer generates ions derived from a sample component in an ion source, and transports the generated ions to a mass separator by an ion transport optical element called an ion lens or an ion guide. Then, the mass separator separates the ions according to a mass-to-charge ratio m/z and a detector detects the ions. As the mass separator, a quadrupole mass filter, a time-of-flight type mass separator, or the like is often used.

One of problems that occurs in such a mass spectrometer is a charge-up phenomenon (charging).

For example, in a mass spectrometer using an atmospheric pressure ion source such as an electrospray ion source, sample droplets in a state where a solvent is not sufficiently vaporized by the ion source are sent to a rear stage and adhere to and deposit on a surface of the ion transport optical element. When dirt or foreign matter adheres to the surface of the ion transport optical element to form an insulating layer, a charge-up easily occurs when ions (charged particles) collide with a portion where the insulating layer is formed. Further, an ion transport optical element such as a quadrupole mass filter or an ion guide is fixed at a predetermined position in a space by being held by a structure made of an insulating material such as ceramic. When ions come into contact with such an insulating structure, a charge-up also occurs.

When the charge-up becomes severe, an electric field formed in an ion passage space by a voltage applied to the ion transport optical element is disturbed, which makes it difficult for the ions to pass or to properly converge or accelerate. Accordingly, the amount of ions that eventually reaches the detector decreases. As a result, detection sensitivity of the ions decreases.

Patent Literature 1 discloses one method for eliminating or reducing the above-mentioned charge-up. Generally, in a quadrupole mass filter, a pre-rod electrode is arranged immediately before a main rod electrode that forms a quadrupole electric field (electric field in which a radio-frequency electric field and a direct-current electric field are superimposed) for separating ions according to the mass-to-charge ratio so as to reduce turbulence of an edge electric field of the quadrupole electric field. In the mass spectrometer described in Patent Literature 1, in order to reduce a charge-up on a surface of the pre-rod electrode and an insulating structure holding the pre-rod electrode, during a waiting time between one measurement and the next measurement, the polarity of the direct-current bias voltage applied to the pre-rod electrode is reversed for a short time from the polarity of the direct-current bias voltage at a time of measurement before and after the waiting time. When the polarity of the direct-current bias voltage is temporarily reversed in this way, the polarity becomes the same as the polarity of the electric charges that constitute the charge-up. Therefore, the electric charges accumulated on the surface of the insulating structure are released and the charge-up is eliminated.

CITATION LIST

Patent Literature

Patent Literature 1: WO 2014/181396 A1

SUMMARY OF INVENTION

Technical Problem

However, according to a study based on the experiment of the present inventor, although the method described in Patent Literature 1 is effective in improving detection sensitivity in many cases and is presumed to be effective in reducing the charge-up, in some cases, the effect may not always be sufficient.

The present invention has been made to solve such a problem, and an object of the present invention is to provide a mass spectrometer that can more reliably eliminate or reduce the charge-up even when the charge-up cannot be sufficiently eliminated by the above-mentioned conventional method, thereby avoiding a decrease in the detection sensitivity and the like.

Solution to Problem

The present invention, which has been made to solve the above-mentioned problem, is a mass spectrometer having one or more ion transport optical elements each of which includes a plurality of electrodes arranged so as to surround an ion optical axis between an ion source that generates ions derived from a sample component and a detector which detects ions separated according to a mass-to-charge ratio, and transports ions while converging the ions by an effect of a radio-frequency electric field formed by the plurality of electrodes, the mass spectrometer including:

a) a direct-current voltage generator configured to apply direct-current voltages of different polarities to electrodes adjacent to each other around the ion optical axis included in at least one of the one or more ion transport optical elements; and

b) a controller configured to control an operation of the direct-current voltage generator so as to apply a predetermined direct-current voltage to each of the plurality of electrodes included in the at least one of the one or more ion transport optical elements from the direct-current voltage generator during a measurement preparation period during which no measurement is substantially performed between one measurement and a next measurement, and stop application of the direct-current voltage during a measurement period during which a measurement is performed.

The “ion transport optical element” described here typically refers to an ion guide including a plurality of rod electrodes. Further, a quadrupole mass filter generally performs an ion separation operation according to a mass-to-charge ratio. However, when applying only a radio-frequency voltage or a radio-frequency voltage and a direct-current bias voltage without applying a direct-current voltage for mass separation to the rod electrodes constituting the quadrupole mass filter, the quadrupole mass filter operates substantially in the same manner as the ion guide. Therefore, the quadrupole mass filter in a driving state in which the mass separation operation is not performed corresponds to the “ion transport optical element” described here.

For example, in a triple quadrupole mass spectrometer having front and rear quadrupole mass filters with a collision cell in between, in some cases, an MS1mass spectrometry mode is executed to let ions pass free through the front quadrupole mass filter and mass-separate the ions in the rear quadrupole mass filter, and, in other cases, another MS1mode is executed to mass-separate the ions in the front quadrupole mass filter and let the ions pass free through the rear quadrupole mass filter. In such cases, the front quadrupole mass filter or the rear quadrupole mass filter is substantially the above-mentioned “ion transport optical element”.

In a quadrupole-time-of-flight type (Q-TOF type) mass spectrometer having a quadrupole mass filter in a front stage and a time-of-flight type mass separator in a rear stage with a collision cell in between, in some cases, an MS1mode is executed to let ions pass free through the front stage quadrupole mass filter and mass-separate the ions in the rear stage time-of-flight mass separator. In such a case, the front stage quadrupole mass filter is substantially the above-mentioned “ion transport optical element”.

In the case of the method described in Patent Literature 1, the polarity of the direct-current bias voltage applied to the rod electrodes or the like constituting the ion guide for eliminating the charge-up is temporarily reversed, but at this time, the polarity of the direct-current potential of the rod electrodes adjacent to each other around the ion optical axis is the same. Therefore, there is no potential difference between the rod electrodes adjacent to each other around the ion optical axis, and no potential gradient is generated. Therefore, for example, in an annular rod holder holding a plurality of rod electrodes, among electric charges accumulated in a portion between the adjacent rod electrodes, it is presumed that although the electric charges existing very close to the rod electrodes move in a direction away from the rod electrodes, the electric charges remain without being removed from the portion between the adjacent rod electrodes.

On the other hand, in the present invention, the controller controls the direct-current voltage generator, and during the measurement preparation period during which no measurement is substantially performed, applies direct-current voltages of different polarities to electrodes adjacent to each other around the ion optical axis among a plurality of electrodes included in at least one of one or more ion transport optical elements. Therefore, a potential gradient is generated between the rod electrodes adjacent to each other around the ion optical axis, and as described above, the electric charges accumulated in the portion between the adjacent rod electrodes in the annular rod holder is smoothly moved by the above-mentioned potential gradient, and properly removed from the portion between adjacent rod electrodes. As a result, the charge-up, which has not been sufficiently eliminated by the conventional method, can be more reliably eliminated.

In one embodiment of the present invention, in the above-mentioned MS1mode, the measurement is repeated a predetermined number of times, and data obtained by each of the predetermined number of measurements are accumulated to create a mass spectrum in a predetermined mass-to-charge ratio range.

The above-mentioned controller is configured to control an operation of the direct-current voltage generator so as to apply a predetermined direct-current voltage to each of a plurality of electrodes included in a quadrupole mass filter from the direct-current voltage generator during a measurement preparation period between the predetermined number of measurements for obtaining one mass spectrum and the predetermined number of measurements for obtaining another mass spectrum, and stop the application of the direct-current voltage during a measurement period.

According to this configuration, the direct-current voltage application operation for eliminating the charge-up described above is performed every predetermined number of measurements, so that the measurement is always performed in a good state in which the charge-up is eliminated. As a result, it is possible to acquire a mass spectrum with high accuracy and sensitivity.

Advantageous Effects of Invention

According to the present invention, even when a charge-up cannot be sufficiently eliminated by the above-mentioned conventional method, the charge-up can be more reliably eliminated or reduced. As a result, favorable mass spectrometry results can be obtained while avoiding a decrease in detection sensitivity.

