MASS SPECTROMETER AND CONTROL METHOD THEREOF

To provide a mass spectrometer capable of shortening the time required to obtain the mass spectrum over the wide range of the mass-to-charge ratios. A mass spectrometer includes an ionizing unit that generates ions from a sample, a mass filter that separates the ions according to a mass-to-charge ratio, and a detecting unit that detects the ions separated by the mass filter. The mass spectrometer further includes an ion guide that transports the ions to the mass filter, and a control unit that generates a mass spectrum and a mass chromatogram using detection signals obtained by performing a sweep control and a step control, the sweep control gradually increases a high frequency voltage to be applied to the ion guide, and the step control constantly keeps the high frequency voltage.

TECHNICAL FIELD

The invention relates to a mass spectrometer provided with an ion guide portion and particularly to a control for a voltage to be applied to the ion guide portion.

BACKGROUND ART

The mass spectrometer is a device for analyzing a sample using a mass spectrum obtained by separating and detecting ions generated from the sample according to a mass-to-charge ratio m/z that is a ratio of mass m and charge z. Many mass spectrometers are provided with ion guides that utilize the convergence function of ion by a high frequency electric field to efficiently transport the generated ions to a mass filter for separating the ions according to the mass-to-charge ratio.

Since the ions are transported while oscillating by the high frequency electric field of the ion guide, the range of the mass-to-charge of the ions allowed to pass through the ion guide depends on the magnitude of the high frequency voltage to be applied to the ion guide. Therefore, to obtain a mass spectrum over a wide range of the mass-to-charge ratios, there is used a method of performing several times of measurements while varying the magnitude of the high frequency voltage and integrating the mass spectra corresponding to the different mass-to-charge ratio ranges obtained from the respective measurements. However, when the mass-to-charge ratio range gets wider, the peak intensity relatively decreases in a region of a low mass-to-charge ratio, compared with the case of a narrower mass-to-charge ratio range.

Patent Literature 1 discloses a mass spectrometer that reduces the decrease of the peak intensity in the low mass-to-charge ratio region. Specifically, it is disclosed that even when the mass-to-charge ratio ranges are different, a high frequency voltage to be applied to the ion guide is set so that the ratio of the measurements at a high frequency voltage where the transmission efficiency of the ions is relatively high, may be even in the low mass-to-charge ratio region.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

Patent Literature 1, however, performs a plurality of measurements while varying the magnitude of the high-frequency voltage to be applied to the ion guide, which requires a long time to obtain a mass spectrum over a wide range of the mass-to-charge ratios.

Therefore, it is an object of the present invention to provide a mass spectrometer and the control method capable of shortening the time required to obtain the mass spectrum over the wide range of the mass-to-charge ratios.

Solution to Problem

To achieve the above object, the invention is a mass spectrometer having an ionizing unit that generates ions from a sample, a mass filter that separates the ions according to a mass-to-charge ratio, and a detecting unit that detects the ions separated by the mass filter, characterized by further including an ion guide that transports the ions to the mass filter and a control unit that generates a mass spectrum and a mass chromatogram using detection signals obtained by performing a sweep control for gradually increasing a high frequency voltage to be applied to the ion guide and a step control for constantly keeping the high frequency voltage.

The invention is a control method of a mass spectrometer having an ionizing unit that generates ions from a sample, a mass filter that separates the ions according to a mass-to-charge ratio, and a detecting unit that detects the intensity of every separated ion, characterized in that a mass spectrum and a mass chromatogram are generated using detection signals obtained by performing a sweep control for gradually increasing a high frequency voltage to be applied to the ion guide that transports the ions to the mass filter and a step control for constantly keeping the high frequency voltage.

Advantageous Effects of Invention

According to the invention, it is possible to provide a mass spectrometer and the control method capable of shortening the time required to obtain the mass spectrum over the wide range of the mass-to-charge ratios.

DESCRIPTION OF EMBODIMENTS

Hereinafter, referring to the attached drawings, a mass spectrometer and the control method according to the invention will be described with a preferred embodiment. The mass spectrometer is a device for analyzing a sample using a mass spectrum obtained by separating and detecting ions generated from the sample according to a mass-to-charge ratio m/z that is a ratio of mass m and charge z.

First Embodiment

With reference toFIG.1, an example of the whole structure of the mass spectrometer according to a first embodiment will be described. The mass spectrometer is provided with an ionizing unit101, a counter plate102, an off-axis unit104, an ion guide105, a mass filter107, a detector109, and a control unit110. Hereinafter, the respective units will be described.

The ionizing unit101is a device that generates ions from a sample. For example, a solution containing the sample is poured into a capillary with a high voltage applied there, charged droplets are generated by spraying the solution from the distal end of the capillary, and the charged droplets are heated and vaporized, hence to generate the ions of the sample.

The counter plate102has holes through which ions are taken, to form an electric field for capturing the ions. Further, a gas flows in the opposite direction to the ion taking direction to suppress the capture of neutral particles other than the ions. The ions captured in the counter plate102are guided to the off-axis unit104through a first fine pore103.

The off-axis unit104deflects the ions by the electric field to pass the above downstream, thereby removing the neutral particles other than the ions. The deflected ions by the off-axis unit104are guided to the ion guide105.

The ion guide105is a device that transports the ions to the mass filter107in the subsequent stage. The ions passing through the ion guide105are guided to the mass filter107through a second fine pore106. The ion guide105is formed, for example, with even number of four or more lot electrodes arranged in parallel along the ion proceeding direction, and a high frequency voltage of the same intensity and different polarity is applied to the adjacent lot electrodes. The high electric field formed in the ion guide105by the application of the high frequency voltage oscillates the ions, and the magnitude of the oscillation of the ions depends on the mass-to-charge ration of the ion and the magnitude of the high frequency voltage. In short, the ion transmittance that is the ratio of the ions passing through the ion guide105varies according to the mass-to-charge ratio of the ion and the magnitude of the high frequency voltage.

FIG.2shows an example of the ion transmittance varying according to the ion mass and the high frequency voltage. The vertical axis ofFIG.2is the ion transmittance and the horizontal axis is the high frequency voltage. As shown inFIG.2, the magnitude of the high frequency voltage that results in a high ion transmittance depends on the ion mass; in the case of a light ion, the ion transmittance is high at a smaller high frequency voltage and in the case of a heavy ion, it is high at a larger high frequency voltage. Here, the relationship shown inFIG.2may be previously recorded and read out according to the necessity.

The mass filter107is a device that separates the ions according to the mass-to-charge ratio m/z that is the ratio of mass m and charge z. The ions passing through the mass filter107are guided to the detector109through a third fine pore108. The mass filter107is formed, for example, with four lot electrodes arranged in parallel along the ion proceeding direction, and a high frequency voltage of the same intensity and different polarity and a direct current are applied to the adjacent lot electrodes. The range of the mass-to-charge of the ions passing through the mass filter107is restricted to the magnitude of the high frequency voltage and the direct current voltage.

FIG.3shows a stable region in which the oscillation of the ions converges and an unstable region in which the above diverges in the mass filter107, in a coordinate system with the axes of the high frequency voltage V and the direct current voltage U. Since the stable region varies according to the mass of the ions, it is necessary to set the high frequency voltage V and the direct current voltage U according to the mass of the ions to be measured. A mass spectrum can be obtained by constantly keeping the ratio of the high frequency voltage V and the direct current voltage U, in other words, by continuously changing the two voltages along the scanning straight line in the figure.

The detector109is a device that detects the ions separated according to the mass-to-charge ratio, including a conversion dynode, a scintillator, a photomultiplier, and the like. The detection signal output by the detector109is transmitted to the control unit110.

The control unit110is a device that controls each unit and is formed, for example, by a computer. The control unit110generates a mass spectrum in which the ion intensities are plotted for every mass-to-charge ratio and a mass chromatogram in which the ion intensity of a specified mass-to-charge ratio is recorded with time, based on the detection signal transmitted from the detector109. The generated mass spectrum and mass chromatogram are displayed on a monitor and used for analysis of the sample. Further, the control unit110controls the high frequency voltage to be applied to the ion guide105so that the mass spectrum in a wide range of the mass-to-charge ratio can be obtained only by one measurement.

An example of a control pattern of the high frequency voltage to be applied to the ion guide105will be described usingFIGS.4A and4B. The control unit110performs a sweep control for gradually increasing the high frequency voltage to be applied to the ion guide105and a step control for constantly keeping the high frequency voltage. The number of the times of the sweep control and the step control is not limited but, as shown inFIG.4A, the sweep control may be performed three times and the step control twice, or as shown inFIG.4B, each sweep control and step control may be once.

While performing the sweep control and the step control on the ion guide105, the high frequency voltage V and the direct current voltage U to be applied to the mass filter107are controlled to continuously change along the scanning straight line as shown inFIG.3, hence to generate the mass spectrum. During the sweep control and the step control for the ion guide105, when the high frequency voltage V and the direct current voltage U of the mass filter107are kept constant, only the ions of a specified mass-to-charge ratio are detected, hence to generate the mass chromatogram.

When the sweep control is performed on the ion guide105, the ions in a wide range of the mass-to-charge ratio can reach the mass filter107, which allows only one measurement to obtain the mass spectrum, thereby reducing the time required for the measurement.

Further, by performing the step control at a proper timing, the measurement accuracy can be improved. For example, at the timing when the gradually-increasing high frequency voltage by the sweep control reaches the range in which a change of the ion transmittance is small, the sweep control is switched to the step control. By switching to the step control, the ions can be stably transported to the mass filter107, which improves the measurement accuracy. The range of the high frequency voltage in which the change of the ion transmittance is small may be obtained from the data showing the relationship between the ion transmittance and the high frequency voltage as shown inFIG.2.

Another example of a control pattern of the high frequency voltage to be applied to the ion guide105will be described usingFIG.5. InFIG.5, the whole scanning range is divided into three scanning ranges, and in each scanning range, the sweep control and the step control are performed arbitrary number of times. For example, when the range of the mass-to-charge ratios 5 to 1000 is measured in the whole scanning range, the above ratios 5 to 100 are measured in the first scanning range, the above ratios 100 to 500 in the second scanning range, and the above ratios 500 to 1000 in the third scanning range. As shown inFIG.2, since the high frequency voltage showing the high ion transmittance varies according to the ion mass, the high frequency voltage of the ion guide105is set preferably according to the mass-to-charge ratio of measurement target. Specifically, in the first scanning range of measuring the range of the mass-to-charge ratios 5 to 100, a comparatively smaller high frequency voltage is set, and in the third scanning range of measuring the range of the mass-to-charge ratios 500 to 1000, a comparatively larger high frequency voltage is set. Here, each scanning range is preferably set larger than the mass-to-charge ratio range of the measurement target.

The mass-to-charge ratio range and the scanning range will be described with reference toFIGS.6A to6C.FIG.6Ashows the mass spectrum measured when the first scanning range is set at 5 to 130 as for the measurement target mass-to-charge ratio range 5 to 100. In short, the first scanning range is wider than the measurement target mass-to-charge ratio range by 30. Further,FIG.6Bshows the results of the measurement with the second scanning range 70 to 530 set for the mass-to-charge ratio range 100 to 500. In short, the second scanning range is wider than the measurement target mass-to-charge ratio range by 60. Further,FIG.6Cshows the results of the measurement with the third scanning range 470 to 1000 set for the mass-to-charge ratio range 500 to 1000. In short, the third scanning range is wider than the measurement target mass-to-charge ratio range by 30. By setting the scanning range wider than the measurement target mass-to-charge ratio range, it is possible to reduce the detection leakage of the ions.

FIG.7shows an example of the mass spectrum in the mass-to-charge ratio range 5 to 1000 generated by integrating the measurement results as shown inFIGS.6A to6C. In a region where the scanning ranges overlap with each other, the data of the higher ion intensity is adopted.

As set forth hereinabove, by performing the sweep control and the step control on the ion guide105, it is possible to reduce the time required for obtaining the mass spectrum in a wide range of the mass-to-charge ratios. Further, it is possible to improve the measurement accuracy by switching from the sweep control to the step control at a proper timing.

As mentioned above, the embodiment of the invention has been described. The invention is not restricted to the above embodiment but the components may be modified without departing from the spirit of the invention. Further, a plurality of the components described in the above embodiment may be properly combined. Furthermore, some of the components may be deleted from the whole components described in the above embodiment.

LIST OF REFERENCE SIGNS