JP5412246B2 - Spectral signal correction method in quadrupole mass spectrometer - Google Patents

Description

  The present invention relates to a spectral signal correction method in a quadrupole mass spectrometer, and more particularly to a spectral signal correction method in a quadrupole mass spectrometer that effectively removes noise components to obtain an accurate spectral signal. For example, it is used in the field of GC-MS (gas chromatograph mass spectrometer), GC-MS with a pretreatment device, or LC-MS (liquid chromatograph mass spectrometer).

  A quadrupole mass spectrometer (QMS) is a device that allows an ionized fluid to pass through a quadrupole and obtains a spectrum of ions that have passed through the filter function. Normally, in QMS, RF (high frequency) and DC (direct current) are always driven at a certain ratio in accordance with Mathieu's formula to function as a mass filter. Ions are detected by a detector through a mass filter, digitally processed by an A / D converter, and digitally processed by a computer.

  FIG. 4 is a conceptual diagram of the configuration of a quadrupole mass spectrometer. Particles such as molecules and atoms enter the ionization box 2. Thermoelectrons are emitted by the heated filament 3 and collide with particles in the ionization box 2 to form ions. The formed ions are accelerated by a repeller (ion acceleration lens) 1 and emitted from an ionization box 2. Reference numeral 4 denotes a trap electrode for capturing electrons emitted from the filament 3. A carrier gas for feeding particles is introduced into the ionization box 2.

  Ions emitted from the ionization box 2 are focused by the subsequent lens 5 and enter the quadrupole (QP) 10. The quadrupole 10 is composed of four electrodes as shown in the figure, and the same voltage is applied to the electrodes facing each other. The applied voltage is a high frequency (RF) and a direct voltage (DC). The high frequency generator 6 and the DC power source 7 are connected in series and applied to the electrodes. A voltage that is 90 ° out of phase with this voltage is generated by the phase shifter 8 and applied between an electrode pair different from the previous electrode pair.

  Here, the high frequency generator 6 and the DC power source 7 are configured so that the frequency and the voltage value can be varied, respectively, and by applying these voltages, the quadrupole 10 functions as a mass filter. . Ions that have passed through the quadrupole 10 are irradiated to a conversion dynode (CD) 12 through a post filter 11. Note that the mass filter is configured so that RF and DC are swept by step feed for each mass unit, for example. As a result, the mass number of ions passing through the filter changes stepwise from the low mass side to the high mass side (or in the reverse direction), and mass sweep is performed. Here, a step in which ions having a certain mass number are passed is defined as a mass step.

  Here, the post filter 11 changes its potential between a step wait period and a data acquisition period. As a result, electrons e corresponding to the incident ions are emitted from the surface of the CD 12 and detected by the detector 13. The detected ion signal is converted into digital data by an A / D converter (not shown) and sent to an APU (Acquisition Processing Unit) (not shown) for data processing.

FIG. 5 is a time chart of data collection of the conventional apparatus. The operation is shown below.
1) Molecules are ionized by thermoelectrons generated from the filament 3, focused by an appropriate lens 5, and introduced into the QP 10.

  2) The ions introduced into the QP 10 are mass filtered by the RF / DCs 6 and 7 applied to the QP 10. The applied RF / DC is applied at a voltage required at each mass step by an APU (not shown). FIG. 6 is a diagram for explaining the operation of the conventional apparatus. The same components as those in FIG. 4 are denoted by the same reference numerals. The detection signal for each mass step detected by the detector 13 is converted into digital data by the subsequent A / D converter 14 and then input to the APU 15.

The APU 15 receives the output of the A / D converter 14, collects data, and performs signal processing. At the same time, a data acquisition command is given to the detector 13.
3) At each mass step, the APU 15 sends a command to the signal detection system and waits for a step-wait period.

  4) After waiting for the step-wait period, the APU 15 sends a command to the signal detection system, and collects the ion detection signal during the designated accumulation period.

  5) The detected ion detection signal is digitized by the A / D converter 14, subjected to various processes by the APU 15, and converted into data as the ion intensity of the mass being measured.

6) Data collection is performed by repeating the operations of 2) to 4) in the specified mass number range (Low-mass to High-mass).
In this type of conventional device, the combination of secondary ion mass spectrometry (SIMS) and ion scattering analysis (ISS) facilitates the identification of the composition of the sample, enables quantitativeness, and is highly sensitive and efficient. A complex analyzer that enables analysis of a sample is known (for example, see Patent Document 1). Further, there is known a particle separation apparatus that improves the S / N ratio by reliably blocking light and neutral particles without reducing analytical sensitivity (see, for example, Patent Document 2). Alternatively, in a quadrupole mass spectrometer that applies a superimposed voltage of a high-frequency voltage and a DC voltage to a quadrupole electrode and detects an ion current from the ion source through the quadrupole electrode with a detector, a plurality of microchannels Are arranged two-dimensionally, and a plate formed with an opening at a position through which the central axis of the quadrupole electrode passes is arranged on the central axis of the quadrupole electrode. A quadrupole mass spectrometer is known (see, for example, Patent Document 3).

JP-A-5-89817 (paragraphs 0013 to 0015, FIG. 1) JP-A-8-64169 (paragraphs 0012 to 0017, FIGS. 1 and 3) Japanese Patent Laid-Open No. 10-144254 (paragraphs 0016 to 0020, FIGS. 1 and 2)

  However, since the prior art is converted into data including noise due to neutral molecules, it is considered that the degree of coincidence and sensitivity of the spectrum are affected. In addition, the conventional technique has a problem in that it is costly because neutral molecules are physically removed using a complicated optical system such as off-axis.

  The present invention has been made in view of such problems, and appropriately measures noise during a scan cycle and reflects it in data processing, and copes with changes in the sample and carrier gas that change during the measurement. An object of the present invention is to provide a spectral signal correction method in a quadrupole mass spectrometer capable of removing neutral particle noise.

In order to solve the above problems, the present invention adopts the following method.
(1) Ion invention of claim 1 wherein the molecular to be measured, the atom is ionized, and detects the resulting ions at the detector through the four Jukyoku mass filter, which passes the quadrupole mass filter the number of mass steps alter the sweeps, and processing the detection signals obtained with the mass number swept computer, the quadrupole mass spectrometer was set to obtain the MS spectrum, the quadrupole A noise acquisition period in which ions are not incident on the detector is provided for each step in which the mass number of ions passing through the mass filter is changed stepwise, and the noise signal obtained from the detector in the noise acquisition period is be characterized in that to perform the spectrum correction by the ions in each step is subtracted from the ion detection signal obtained by detecting by said detector .

(2) The invention according to claim 2 is a step wait time in which an ion detection signal is not taken out from the detector immediately after the change for each step in which the mass number of ions passing through the quadrupole mass filter is changed stepwise. An ion detection signal is taken out from the detector after a while, and the noise acquisition period is provided in the step weight period .

The present invention has the following effects.
(1) According to the invention of claim 1, a noise acquisition period in which ions are not incident on the detector is provided for each step in which the mass number of ions passing through the quadrupole mass filter is changed stepwise, Noise can be removed by subtracting the noise signal obtained from the detector during the noise acquisition period from the ion detection signal obtained by detecting ions by the detector at each step , and The neutral particle noise corresponding to the sample changing during measurement and the fluctuation of the carrier gas can be removed.

(2) According to the invention described in claim 2 , the step of not extracting the ion detection signal from the detector immediately after the change for each step of stepwise changing the mass number of ions passing through the quadrupole mass filter. The noise component can be removed by taking out the ion detection signal from the detector after waiting time and providing the noise acquisition period in the step weight period .

It is a time chart which shows operation | movement of Example 1 of this invention. It is a time chart which shows operation | movement of Example 2 of this invention. It is a time chart which shows operation | movement of Example 4 of this invention. It is a composition conceptual diagram of a quadrupole mass spectrometer. It is a time chart of data collection of the conventional device. It is operation | movement explanatory drawing of a conventional apparatus.

Examples of the present invention will be described in detail below.
Example 1
FIG. 1 is a time chart showing the operation of the first embodiment of the present invention. The basic configuration of Example 1 is the same as in FIGS. 4 and 6, and a noise acquisition (Noise-Acq) period for noise collection is provided after accumulation (accumulation). In this period, the potential of the post filter 11 before the detector 13 shown in FIG. 4 is changed. The operation of the first embodiment will be described below.

1) The molecules are ionized by the thermoelectrons generated from the filament 3 and introduced into the quadrupole (QP) 10 by an appropriate lens 5.
2) The ions introduced into the QP 10 are mass filtered by the RF / DCs 6 and 7 added to the QP 10, and the applied RF / DC is applied by the APU 15 at a voltage required at each mass step.

3) In each mass step, considering the response of RF / DC6, 7 and the flight time of ions, wait for the step weight time.
4) After waiting for the step wait, an ion detection signal is collected by the detector 13 as shown in the acquisition during the designated accumulation period.

5) The detected ion detection signal is digitized by the A / D converter 14.
6) In the noise acquisition period after the accumulation period, the voltage of the post-filter 11 is changed from a regular potential to a noise acquisition potential (Noise Acq potential) (actually, 0V or regular). Invert the polarity of the potential).

  7) The potential of the post filter 11 is changed so that ions cannot reach the detector 13. As a result, since ions cannot be detected, it is possible to detect noise derived from neutral molecules, and to detect noise derived from neutral molecules during the noise acquisition period. That is, since the neutral molecule is not ionized and is not affected by the potential of the post filter 11, it can reach the detector 13 and can be detected as noise data.

8) The noise signal detected in 7) is digitized by the A / D converter 14.
9) The ion signal detected in 5) by the processing of the APU 15 and the noise signal detected in 8) are compared and calculated as the ion intensity of the mass being measured. As an example of the comparison calculation process, a method of subtracting noise from the ion signal to obtain the ion intensity of the mass being measured can be considered.

10) The operations of 3) to 9) are repeated for the designated mass number range (low mass to high mass) to collect data.
According to this embodiment, the noise component can be removed by performing a comparison operation between the signal detected in the spectrum acquisition period and the noise acquisition period and the noise.
(Example 2)
FIG. 2 is a time chart showing the operation of the second embodiment of the present invention. In the first embodiment, a noise acquisition period is provided for each mass step. However, in the second embodiment, as shown in FIG. 2, noise acquisition is performed at the beginning and end of scanning. is there. In this way, in the second embodiment, noise acquisition is provided at the beginning and end of scanning, and correction calculation is performed using these noise acquisition data as noise acquisition data for all steps. It is. For example, the entire scan may be corrected using the noise acquisition data at the beginning of the scan, or the entire scan may be corrected using the noise acquisition data at the end of the scan.

Furthermore, the average of the first noise acquisition date and the last noise acquisition data can be taken and used as the noise acquisition data of all the mass steps. According to this embodiment, a noise acquisition period is provided at the beginning and end of each square position, and the noise acquisition data detected in this period is used as noise data in each square step, and the noise acquisition period is set. By using data, the noise component can be removed, although the accuracy is slightly reduced.
(Example 3)
In the second embodiment, the case where the noise acquisition period is provided at the beginning and the end of the scan has been described. However, the present invention is not limited to this, and a noise acquisition period may be provided at a necessary square position as appropriate. According to this embodiment, it is possible to correct the acquisition data measured in the spectrum acquisition period using the noise data obtained at each position as the noise data in each mass step, although the accuracy is slightly reduced. The noise component can be removed.
Example 4
In the first to third embodiments, the case where the noise acquisition period is provided at the end of each mass step, that is, after the accumulation period is shown. bad. Therefore, noise acquisition is performed using a step weight period in which ions are not originally detected. The operation will be described below. The hardware configuration shown in FIGS. 4 and 6 is used.

1) The molecules are ionized by the thermoelectrons generated from the filament 3 and introduced into the quadrupole (QP) 10 by an appropriate lens 5.
2) The ions introduced into the QP 10 are mass filtered by the RF / DCs 6 and 7 added to the QP 10, and the applied RF / DC is applied by the APU 15 at a voltage required at each mass step.

3) In each mass step, considering the response of RF / DC6, 7 and the flight time of ions, wait for the step weight time.
4) Insert a noise acquisition period in the step wait period, and change the potential of the post filter 11 from the regular potential to the noise acquisition potential.

  5) The potential of the post filter 11 is changed so that ions cannot reach the detector 13. In this state, since ions cannot be detected, it becomes possible to detect noise derived from neutral molecules, and noise derived from neutral molecules is detected during the noise acquisition period.

6) The noise detected in 5) is converted into digital data by the A / D converter 14.
7) After waiting for a step wait, an ion detection signal is collected as indicated by acquisition during the designated accumulation period.

8) The detected ion detection signal is converted into digital data by the A / D converter 14.
9) The APU 15 compares the noise data detected in 6) with the ion signal data detected in 8), and is converted into data as the ion intensity of the mass being measured. As a specific example, the noise signal-free ion signal data is obtained by subtracting the noise data measured during the noise acquisition period from the data measured during the accumulation period.

10) Data collection is performed by repeating the operations of 3) to 9) in the designated mass number range (low mass to high mass).
According to this embodiment, since the noise acquisition period is provided in the step wait period in which ions are not detected, the data acquisition efficiency can be improved.
(Example 5)
In (Embodiment 1) to (Embodiment 4), in order to perform noise acquisition, the potential of the post filter 11 is changed. However, even with the potential of QP10 or another lens, noise acquisition is performed from the regular potential. The same can be expected by changing to potential. The means for removing charged particles in order to detect neutral noise, that is, the charged particle blocking mechanism is not limited to the post filter, and a new lens may be arranged in the current quadrupole mass spectrometer. Alternatively, it can be left to the currently arranged lens.

  In addition, since the lens 5 immediately after the ionization chamber (ionization box) 2 is used as a lens for blocking charged particles, most charged particles cannot enter the QP 10. Rather, it can be measured and separated as electronics noise, and can be reflected in the data.

The effects of the present invention described above are listed as follows.
1) Neutral molecular noise can be removed from the data without changing the optical mechanism such as off-axis.

  2) Noise caused by neutral particles changes during measurement, and the present invention aims to apply noise reduction during measurement. Therefore, passively and appropriately respond to measurement conditions such as the amount of carrier and sample introduced. can do.

  3) Similarly, it is considered that neutral noise caused during measurement attenuates small signals that are close to the detection limit with a uniform level of noise cut, but this is introduced in the present invention. Since the objective is to cut noise according to neutral particles, the attenuation of small signals can be suppressed.

  4) Even when the lens voltage is swept dynamically to correct the mass number-dependent error (spectrum), etc., the amount of noise derived from neutral molecules will change, but small signals can be obtained by detecting noise in real time. Can be prevented from decaying.

5) The spectrum can be improved by noise reduction.
6) With the spectrum improvement, the degree of coincidence of library spectra can be improved.

7) The S / N ratio can be improved.
8) In (Example 4), noise detection can be performed without impairing ion detection efficiency.

DESCRIPTION OF SYMBOLS 1 Repeller 2 Ionization box 3 Filament 4 Trap 5 Lens 6 High frequency power supply 7 DC power supply 8 Phase shifter 10 Quadrupole electrode (QP)
11 Post filter 12 CD (conversion dynode)
13 Detector

Claims (2)

Molecules to be measured, the atom is ionized, and detects the resulting ions at the detector through the four Jukyoku mass filter, sweep steps alter the mass number of the ions passing said quadrupole mass filter In the quadrupole mass spectrometer in which the detection signal obtained with the mass number sweep is processed by a computer to obtain an MS spectrum,
A noise acquisition period in which ions are not incident on the detector is provided for each step in which the mass number of ions passing through the quadrupole mass filter is changed stepwise, and the noise obtained from the detector during the noise acquisition period Spectral signal correction method in quadrupole mass spectrometer characterized in that signal is subtracted from ion detection signal obtained by detecting ions by said detector at each step . After each step of changing the mass number of ions passing through the quadrupole mass filter in a stepwise manner, after a step wait time during which no ion detection signal is extracted from the detector immediately after the change, the ion detection signal from the detector. The method for correcting a spectrum signal in a quadrupole mass spectrometer according to claim 1 , wherein the noise acquisition period is provided in the step weight period .

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