JP2018004298A - Mass spectrometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily determine sample molecular ions in a mass spectrum obtained using only an ion source by an EI method.SOLUTION: The ionization voltage of an ion source 2 by an EI method is switched between a standard value (70 eV) and a lower value than that and mass spectra for the same sample are acquired. An intensity value normalization unit 72 normalizes each intensity value, with the intensity value of base peak in each mass spectrum being 100, and a differential mass spectrum creation unit 73 subtracts a standard EI mass spectrum from low energy ionized mass spectrum after normalization and finds a differential mass spectrum. A sample molecular ion determination unit 74 determines a sample molecular ion from the differential mass spectrum and two original mass spectra. As the ratio of the sample molecular ion to a fragment ion increases when the ionization voltage is low, a fragment ion peak in the differential mass spectrum is reduced and it is made easy to confirm the sample molecular ion.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a mass spectrometer, and more particularly to a mass spectrometer equipped with an ion source based on an electron ionization method.

  When ionizing a compound in a gas sample in a mass spectrometer, an electron ionization (EI) method or a chemical ionization (CI) method is generally used.

  Generally, in the electron ionization method, the sample molecules are ionized by accelerating the thermal electrons generated from the filament and bringing them into contact with the sample molecules. In the electron ionization method, in most cases, the ionization voltage is set to 70 V, that is, the energy applied to the thermoelectrons is set to 70 eV. This is because the ionization efficiency by the electron ionization method in many atoms and molecules is maximized between 50 and 150 eV (see Non-Patent Document 1, etc.). In addition, mass spectral patterns of various compounds recorded in many spectrum libraries (databases) provided in general are measured under the condition of 70 eV. When identifying compounds by so-called pattern matching, There are also circumstances where it is desirable to perform measurements under the same conditions.

  Since the ionization energy of a general organic compound is in the range of about 8 to 14 eV and lower than the energy received by contact with thermionic electrons, this excess energy breaks the bonds in the compound and generates many fragment ions. The Therefore, structural information of the compound can be obtained from the mass spectrum obtained by the electron ionization method. However, in the mass spectrum obtained by the electron ionization method, since there are many fragment ion peaks, it is often difficult to find a peak corresponding to the molecular ion itself of the target compound. Therefore, it is difficult to determine the molecular weight of the target compound from the peak on the mass spectrum.

  On the other hand, in the chemical ionization method, accelerated thermoelectrons are brought into contact with a reagent gas to ionize the reagent gas, and the sample molecules are ionized by a chemical reaction between the reagent gas ions and the sample molecules. Since this chemical reaction is basically a proton-donating reaction or a proton desorption reaction, a large number of ions with protons added or ions with protons desorbed are generated. Since the mass of the proton is known, it is generally possible to estimate the molecular weight of the target compound from the mass-to-charge ratio of peaks corresponding to proton addition ions and proton desorption ions.

However, when it is desired to obtain the structural information of the compound and to know the molecular weight, it is necessary to perform both mass analysis under the electron ionization method and mass analysis under the chemical ionization method. When using the chemical ionization method, it is necessary to prepare a reagent gas, and the analysis work is complicated. In general, since a dedicated ion source different from the electron ionization method is required, a laborious work of exchanging the ion source is required.
Furthermore, the peak of the sample molecular ion itself does not appear on the mass spectrum obtained by the chemical ionization method, and the peak of various adduct ions may appear. It can be difficult.

Kusai, “Electron Monochomator”, JEOL Ltd., JEOL News, Vol. 33, No. 1, 11, 2001 “Basics of GCMS Analysis 2.1.6. Electron Energy and EI Spectrum”, [online], Shimadzu Corporation, [May 19, 2016 search], Internet <URL: http: //www.an.shimadzu. co.jp/gcms/support/faq/fundamentals/ei.htm>

  The present invention has been made to solve the above-mentioned problems, and the object of the present invention is to provide a mass spectrum capable of identifying compound molecular ions in a sample while using an ion source for electron ionization. The object is to provide a mass spectrometer that can be obtained.

In order to solve the above problems, the present invention provides a mass spectrometer including an ion source based on an electron ionization (EI) method.
a) an analysis control unit for performing mass spectrometry on the same sample while switching the ionization voltage in the ion source to a plurality of stages;
b) For a plurality of mass spectra obtained for different ionization voltages under the control of the analysis control unit, normalize the intensity values so that the intensity values of the base peaks in each mass spectrum are the same. A difference mass spectrum creation unit for obtaining a difference mass spectrum indicating a difference in intensity value at
The compound molecular ion in the sample can be specified using the difference mass spectrum and the plurality of mass spectra before or after normalization.

  In the mass spectrometer according to the present invention, it is preferable that one mass analysis performed under the control of the analysis control unit is a standard ionization voltage in the electron ionization method, that is, 70 V (thermionic energy is 70 eV). It is recommended that the other mass analysis be performed under an appropriate ionization voltage lower than that. This appropriate ionization voltage can be determined experimentally, for example.

  As disclosed in Non-Patent Document 2, in the electron ionization method, the mass spectrum changes when the ionization voltage is changed. In general, when the ionization voltage is further lowered below the standard value of 70 V, for example, about 50 V, both sample molecular ions and fragment ions are produced, but sample molecular ions relative to fragment ions are relatively reduced. The proportion of increases. The mass spectrometer according to the present invention utilizes this phenomenon to identify sample molecular ions from the difference in intensity values between a plurality of mass spectra obtained under a plurality of different ionization voltages, or to obtain information for that purpose. To do.

That is, in the mass spectrometer according to the present invention, after acquiring a plurality of mass spectra for different ionization voltages under the control of the analysis control unit, the differential mass spectrum creation unit determines the intensity value of the base peak in each mass spectrum. Normalize the intensity values so that they are the same. This is because the difference in the total ion amount, which differs depending on the ionization voltage, is made uniform based on the intensity value of the base peak. Then, the difference mass spectrum creation unit obtains the difference mass spectrum by taking the difference in the intensity values of the same mass-to-charge ratio among the plurality of mass spectra whose intensity values are normalized.
At this time, in general, the mass spectrum whose intensity of the peak of the sample molecule ion is relatively higher than that of the fragment ion (usually the mass spectrum whose ionization voltage is lower than the standard value) is not so. The difference mass spectrum may be obtained by subtracting the mass spectrum (normally, the mass spectrum whose ionization voltage is a standard value). However, if the intensity value becomes negative as a result of subtraction, the intensity value may be set to zero.

  In general, when the ionization voltage is a standard value, since the amount of one fragment ion among a plurality of fragment ions is larger than the amount of sample molecule ions, the peak of one fragment ion is not a sample molecule ion. It becomes the base peak. If the same fragment ion peak is also the base peak in the mass spectrum obtained under an appropriate ionization voltage lower than the standard value, the peak at the same mass-to-charge ratio (base peak) in the two mass spectra. Normalization is performed so that the intensity values are uniform. Further, as described above, when the ionization voltage is lowered below the standard value, the ratio of the sample molecule ions becomes relatively large. Therefore, in the mass spectrum obtained under an appropriate ionization voltage lower than the standard value, fragment ions are obtained. Then, the peak of the sample molecular ion may become the base peak.

  When the peak at the same mass-to-charge ratio is the base peak in the two mass spectra, the base peak disappears on the differential mass spectrum. In addition, the intensity value becomes considerably small even if the fragment ion peaks other than the base peak disappear or do not disappear. On the other hand, when the ionization voltage is lowered, the ratio of the sample molecular ion to the fragment ion is relatively large, so that the decrease in the peak intensity of the sample molecular ion is small even if the difference is taken. As a result, the peak of the sample molecular ion can be easily specified on the differential mass spectrum. Further, even when the mass-to-charge ratios of the base peaks in the two mass spectra are different, by taking the difference, the intensity of the peak of the fragment ion becomes considerably small, while the decrease in the intensity of the peak of the sample molecular ion is small. Therefore, the peak of the sample molecular ion can be easily specified on the differential mass spectrum.

  In addition, if the peak of the sample molecular ion is the base peak in the two mass spectra, there should be no peak having a mass-to-charge ratio larger than the base peak on the differential mass spectrum (why If so, the fragment ion mass-to-charge ratio is smaller than the sample molecular ion). Therefore, in that case, the base peak in the original mass spectrum can be specified as the peak of the sample molecular ion.

  As described above, in the mass spectrometer according to the present invention, sample molecular ions can be specified based on the differential mass spectrum and a plurality of mass spectra before or after normalization. Such an operation may be performed by the analyst himself or may be automatically determined based on the intensity value of each mass spectrum. That is, the mass spectrometer according to the present invention further includes a molecular ion specifying unit that specifies a peak corresponding to the molecular ion of the compound in the sample based on the difference mass spectrum and at least one of the plurality of mass spectra. It is good also as a structure.

  In the mass spectrometer according to the present invention, the sample molecular ion is easily confirmed on the obtained mass spectrum without using the chemical ionization method, that is, using only the ion source by the electron ionization method, and the target compound is obtained. Can be known. Thereby, it is not necessary to use a reagent gas for ionization, and it is possible to save the trouble of exchanging the ion source with an ion source for chemical ionization.

1 is a schematic configuration diagram of a mass spectrometer according to an embodiment of the present invention. The flowchart of the control and process for sample molecular ion specification in the mass spectrometer of a present Example. The figure which shows an example of the mass spectrum obtained under the standard ionization voltage and the ionization voltage lower than it. The figure which shows the mass spectrum after normalizing the intensity value of the mass spectrum shown in FIG. 3, and the difference mass spectrum calculated | required from them. The figure which shows the other example of the mass spectrum after normalizing an intensity value, and the difference mass spectrum calculated | required from them.

  A mass spectrometer which is one embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic configuration diagram of the mass spectrometer of the present embodiment.

  In the mass spectrometer of the present embodiment, an ion source 2, an ion transport optical system 3, a quadrupole mass filter 4, and an ion detector 5 are arranged inside a chamber 1 that is evacuated by a vacuum pump (not shown). Has been. The ion source 2 is an ion source capable of electron ionization, and includes a box-shaped ionization chamber 21 to which a sample gas is supplied through a sample gas supply pipe 26, a filament 23 and an acceleration electrode 24 provided outside the ionization chamber 21. A thermoelectron generator 22 and a trap electrode 25 that collects thermoelectrons. The ion source voltage generation unit 8 applies a voltage to the thermoelectron generation unit 22 and the trap electrode 25 in accordance with instructions from the control unit 9. The energy of the thermoelectrons is determined by the voltage applied from the ion source voltage generator 8.

  When the thermoelectrons generated and accelerated by the thermoelectron generator 22 come into contact with the sample molecules or atoms in the sample gas supplied into the ionization chamber 21 through the sample gas supply pipe 26, the sample molecules and atoms are ionized. The generated ions exit the ionization chamber 21 and are converged by the ion transport optical system 3 and introduced into the space in the long axis direction of the quadrupole mass filter 4. A voltage obtained by superimposing a DC voltage and a high-frequency voltage is applied to the quadrupole mass filter 4 from a power source (not shown), and only ions having a mass-to-charge ratio corresponding to the applied voltage pass through the space in the major axis direction, It reaches the ion detector 5 and is detected. A detection signal from the ion detector 5 is converted into digital data by an analog-digital converter (ADC) 6 and input to a data processing unit 7. The data processing unit 7 includes a data storage unit 71, an intensity value normalization unit 72, a differential mass spectrum creation unit 73, and a sample molecule ion identification unit 74 as a block configuration characteristic of the apparatus of this embodiment. The control unit 9 controls the operation of each unit including the ion source voltage generation unit 8 and serves as a user interface by the input unit 10 and the display unit 11.

  It should be noted that at least a part of the functions of the data processing unit 7 and the control unit 9 can be realized by executing dedicated processing / control software preinstalled in the personal computer on the computer. .

  Next, characteristic functions in the mass spectrometer of the present embodiment will be described with reference to FIG. FIG. 2 is a flowchart of control and processing for specifying sample molecular ions in the mass spectrometer of the present embodiment.

  In the mass spectrometer of the present embodiment, the control unit 9 controls each unit to perform mass analysis on the same sample under two different ionization voltages. Specifically, first, the ion source voltage generation unit 8 was controlled so that the ionization voltage was set to 70 V which is a standard value, and scan measurement was performed in a predetermined mass-to-charge ratio range with respect to the sample under the ionization voltage. The mass spectrum data is temporarily stored in the data storage unit 71. Subsequently, the ion source voltage generation unit 8 is controlled so that the ionization voltage becomes a predetermined value (for example, about 10 to 20 V) lower than the standard value, and scan measurement is performed on the same sample under the ionization voltage. The obtained mass spectrum data is temporarily stored in the data storage unit 71 (step S1). The predetermined value lower than the standard value is preferably determined appropriately depending on the type of the target compound. Therefore, it may be determined experimentally beforehand.

  In the following description, a mass spectrum obtained under an ionization voltage of 70 V is referred to as a standard EI mass spectrum, and a mass spectrum obtained under a lower ionization voltage is referred to as a low energy ionization mass spectrum.

  3A is an example of a standard EI mass spectrum, and FIG. 3B is an example of a low energy ionization mass spectrum for the same sample. Usually, when the ionization voltage is lowered below the standard value, the ionization efficiency is lowered, so that the total ion amount is reduced. On the other hand, although both the fragment ions and the sample molecule ions are reduced, since the method of reducing the fragment ions is larger, the sample molecule ions (m / z 120 in FIG. 3) with respect to the fragment ions (for example, m / z 100 in FIG. 3). The rate increases.

  The intensity value normalization unit 72 reads the two mass spectrum data from the data storage unit 71. In each mass spectrum, the intensity value of the base peak (the peak having the maximum intensity in the mass spectrum) is assumed to be 100 (which is of course a reference value and may be 1 or another value). The intensity value of the peak is normalized (step S2). 4A and 4B are mass spectra after normalizing the mass spectra shown in FIGS. 3A and 3B, respectively. In this case, the peak of the fragment ion whose m / z is 100 in any mass spectrum is the base peak.

  Next, the difference mass spectrum creation unit 73 subtracts the mass spectrum that is not so from the mass spectrum on which the confirmation of the sample molecular ions is easy (the ratio of the sample molecular ions to the fragment ions is large), and more specifically, the same m / The difference mass spectrum is obtained by subtracting the intensity value of the z peak. In the case of the ionization voltage, the former is a low energy ionization mass spectrum, and the latter is a standard EI mass spectrum (step S3). However, if the intensity value becomes negative due to subtraction, it is replaced with a zero value.

  When such a process for obtaining the differential mass spectrum is performed, the base peak disappears when the m / z of the base peak is the same as in the example shown in FIGS. 4 (a) and 4 (b). In addition, in the standard EI mass spectrum after normalization, a peak whose intensity value is larger than that of the low energy ionization mass spectrum after normalization also disappears. Usually, the amount of fragment ions is higher in the standard EI mass spectrum than in the low energy ionization mass spectrum, so the fragment ion peaks other than the base peak often disappear. On the other hand, although the intensity value of the peak of the sample molecule ion having a large relative ratio to the fragment ion in the low energy ionization mass spectrum is reduced by subtraction, the intensity value hardly becomes zero. As a result, the peak of the sample molecular ion clearly appears in the differential mass spectrum.

  Subsequently, the sample molecular ion specifying unit 74 specifies the sample molecular ion in the following procedure based on the difference mass spectrum, the standard EI mass spectrum after normalization, and the low energy ionization mass spectrum after normalization. (Step S4).

  First, it is determined whether the m / z of the base peak in the standard EI mass spectrum after normalization and the low energy ionization mass spectrum after normalization are the same (step S5). If they are the same (Yes in step S5), the base peak always disappears in the differential mass spectrum as described above. Therefore, it is determined whether or not there is a peak whose m / z is larger than the base peak on the differential mass spectrum (step S6).

  If there is no peak whose m / z is larger than the base peak (No in step S6), the base peak on the standard EI mass spectrum (or low energy ionization mass spectrum) is specified as the peak of the molecular ion of the target compound (step). S7). On the other hand, when there is a peak having a larger m / z than the base peak on the differential mass spectrum (Yes in step S6), it can be estimated that the base peak is a fragment ion peak. On the differential mass spectrum, the base peak disappears, and the peak intensity of the other fragment ions is greatly reduced, whereas the decrease in the peak intensity of the sample molecular ion is small. Therefore, the peak having the maximum intensity value among the peaks on the differential mass spectrum is specified as the peak of the molecular ion of the target compound (step S8).

In addition, in the case of No in step S5, although the original base peak may remain on the differential mass spectrum, the intensity of the fragment ion peak including the base peak greatly decreases, whereas the sample molecule The way of decreasing the intensity of the ion peak is small. Proceeding to step S8, the peak having the maximum intensity value among the peaks on the differential mass spectrum is specified as the peak of the molecular ion of the target compound.
Thus, the molecular ion peak of the target compound can be identified in the obtained mass spectrum, and the molecular weight of the target compound can be obtained from the mass-to-charge ratio.

  In the example of FIG. 4, the peak of the same fragment ion is the base peak in both the standard EI mass spectrum and the low energy ionization mass spectrum, and the base peak disappears in the differential mass spectrum. Although the peak of the lower m / z fragment ion remains on the differential mass spectrum, the sample molecular ion peak has a higher intensity value than this, so the sample molecular ion is identified on the differential mass spectrum. be able to.

  On the other hand, in the example of FIG. 5, the peak of the sample molecular ion is the base peak in both the standard EI mass spectrum and the low energy ionization mass spectrum. For this reason, the peak of the sample molecular ion is not observed in the differential mass spectrum, but there is no peak whose m / z is larger than this base peak. Therefore, it can be specified that the original base peak is the peak of the sample molecular ion.

  In the above-described embodiment, the identification of the sample molecular ion using the difference mass spectrum after the creation of the difference mass spectrum is performed automatically, that is, in the data processing unit 7. The resulting mass spectrum may be displayed on the screen of the display unit 11 so that the analyst can identify the sample molecular ion by looking at it.

  Further, the above-described embodiment is an example of the present invention, and it is obvious that any modification, change, or addition as appropriate within the scope of the present invention is included in the scope of the claims of the present application.

DESCRIPTION OF SYMBOLS 1 ... Chamber 10 ... Input part 11 ... Display part 2 ... Ion source 21 ... Ionization chamber 22 ... Thermal electron production | generation part 23 ... Filament 24 ... Acceleration electrode 25 ... Trap electrode 26 ... Sample gas supply tube 3 ... Ion transport optical system 4 ... Quadrupole mass filter 5 ... Ion detector 6 ... Analog to digital converter 7 ... Data processing unit 71 ... Data storage unit 72 ... Intensity value normalization unit 73 ... Differential mass spectrum creation unit 74 ... Sample molecule ion identification unit 8 ... Ion Source voltage generation unit 9 ... control unit

Claims (3)

In a mass spectrometer equipped with an ion source by electron ionization,
a) an analysis control unit for performing mass spectrometry on the same sample while switching the ionization voltage in the ion source to a plurality of stages;
b) For a plurality of mass spectra obtained for different ionization voltages under the control of the analysis control unit, normalize the intensity values so that the intensity values of the base peaks in each mass spectrum are the same. A difference mass spectrum creation unit for obtaining a difference mass spectrum indicating a difference in intensity value at
A mass spectrometer capable of specifying a compound molecular ion in a sample using the difference mass spectrum and the plurality of mass spectra before or after normalization. The mass spectrometer according to claim 1,
One mass analysis performed under the control of the analysis control unit is performed under a standard ionization voltage in an electron ionization method. The mass spectrometer according to claim 2,
A mass spectrometer comprising: a molecular ion specifying unit that specifies a peak corresponding to a molecular ion of a compound in a sample based on the difference spectrum and at least one of the plurality of mass spectra.

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