JP3837264B2 - Ion trap mass spectrometry method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a mass spectrometric method for dioxins using an ion trap mass spectrometer, and in particular, by selectively cleaving ions (chemical noise) derived from coexisting interference components and elution components from the column and out of the analysis region. The present invention relates to an ion trap mass spectrometry method that enables highly sensitive analysis of dioxins.
[0002]
[Prior art]
Dioxins are those in which the hydrogen atom of the dioxin skeleton is replaced by a chlorine atom, and there are many isomers. Among dioxins, dioxins in which hydrogen at positions 2, 3, 7, and 8 are substituted with chlorine are the strongest in terms of carcinogenicity and toxic teratogenicity, and there is concern about their diffusion into the environment. . In order to suppress the diffusion of dioxins into the environment, it is important to analyze dioxins that are extremely dangerous even in very small amounts. Environmental pollutants such as dioxins are present in very small amounts in very complex systems. Therefore, analysis of these substances requires pretreatment that is very complicated and requires labor and time. Furthermore, the analysis of these substances is required to have high sensitivity and to be highly selective so that the substance to be analyzed can be distinguished from interfering substances. Therefore, a gas chromatograph direct-coupled mass spectrometer (GC / MS) in which a gas chromatograph as a separation means is placed in front of the mass spectrometer is used for analysis of dioxins.
[0003]
Dioxins are currently widely used, but here are used as a general term for polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs).
[0004]
In order to measure a larger number of samples in a short time and to perform simple analysis, analysis using a small mass spectrometer QMS or ion trap mass spectrometer has been attempted.
[0005]
The ion trap mass spectrometer is described in US Pat. No. 2,939,952, Japanese Patent No. 1,321,036, Japanese Patent Publication No. 8-21365, and Japanese Patent Publication No. 8-21365. An ion trap mass spectrometer is described in JP-A-6-96727.
[0006]
[Problems to be solved by the invention]
The biggest problem in the analysis of dioxins is that many interfering substances (for example, chlorinated pesticides such as PCB and DDT) cannot be eliminated even by pretreatment and are present in the sample. As shown in the upper part (a) of FIG. 5, these interfering substances cannot be separated even in the capillary column of the gas chromatograph, but are eluted at the same retention time as dioxin and detected by a mass spectrometer. This obstruction is generally called chemical noise. Chemical noise appears over the entire mass range and the dioxin signal cannot be identified. When the molecular weight region of dioxin is enlarged and observed, a mass spectrum as shown in the upper part of FIG. 5 is obtained. These chemical noise and dioxin signals cannot be distinguished by the resolution of the ion trap mass spectrometer. That is, chemical noise is superimposed on the dioxin signal. In this mass spectrum, chemical noise is indicated by a white bar, and dioxin signals are indicated by hatched bar graphs. Since chemical noise varies from sample to sample and from analysis to analysis, analysis of dioxins is uncertain as long as chemical noise is present. In order to distinguish this chemical noise from dioxin, a high-resolution double-focusing mass spectrometer using a large magnetic field and electric field is used. This high-resolution double-focusing mass spectrometer performs separation detection based on a slight mass difference between PCB, DDT, and dioxin.
[0007]
As described above, the ion trap mass spectrometer is different from the QMS and the double convergence mass spectrometer, and performs mass analysis by switching the operation mode with time.
[0008]
The high-resolution double-focusing mass spectrometer used in the analysis of dioxins is a large device and is very expensive. Furthermore, the operation requires high skill and experience. The throughput was not satisfactory.
[0009]
For this reason, there has been a strong demand for analyzing dioxins with a small mass spectrometer such as a quadrupole mass spectrometer (QMS) or an ion trap mass spectrometer that is inexpensive and easy to operate. However, due to the low resolution of these mass spectrometers, it is not possible to separate and detect interfering substances (chemical noise) and dioxins that are analytes. Therefore, dioxin analysis using an ion trap mass spectrometer is considered to have two or more orders of magnitude lower sensitivity (higher lower limit of quantification) than analysis using a high resolution mass spectrometer.
[0010]
An object of the present invention is to achieve high-sensitivity analysis such as dioxin by an ion trap mass spectrometer that reduces chemical noise, is inexpensive, and is easy to operate.
[0011]
[Means for Solving the Problems]
The present invention improves the accuracy of dioxin analysis by removing the chemical noise superimposed on the signal instead of separating and detecting the chemical noise and the dioxin signal.
[0012]
The present invention relates to a method for mass spectrometry of dioxins using an ion trap mass spectrometer, and a broadband auxiliary alternating current having a voltage lower than a voltage at which the dioxin is cleaved at an end cap electrode during ionization and ion accumulation in an ion trap operation sequence. Was applied.
[0013]
Another invention relates to a method for mass spectrometry of dioxins using an ion trap mass spectrometer, wherein a voltage lower than a voltage at which dioxins are cleaved at an end cap electrode after ionization and ion accumulation in an ion trap operation sequence. A broadband auxiliary alternating current was applied, and then the main high frequency voltage was swept to obtain a mass spectrum.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment according to the present invention will be described below with reference to the drawings.
[0015]
An ion trap mass spectrometer will be described with reference to FIG.
[0016]
The sample solution is injected into the inlet 31 of the gas chromatograph 23, and after vaporization, is sent to the capillary column 32 by the carrier gas He. Here, the sample gas is separated for each component due to the difference in distribution between the liquid phase applied to the inner surface of the column and the gas (He). The separated sample component is sent to the ion trap mass spectrometer 33 housed in the evacuated container through the gas guide pipe 16. The ion trap mass spectrometer 33 includes a filament power source 1, a filament 2, an electronic gate 5, an end cap electrode 6, a ring electrode 7, an end cap electrode 8, a detector 12, a power source, a data processing device 14, and the like. The ring electrode 7 having a paraboloid of revolution and the two end cap electrodes 6 and 8 adjacent from both sides of the rotational symmetry axis form the heart of the ion trap mass spectrometer. A main high frequency is applied between the end cap electrodes 6 and 8 and the ring electrode 7. As a result, a quadrupole high-frequency electric field that traps ions in the space 9 is generated in a space (ion trap space) 9 surrounded by three electrodes.
[0017]
Substances that interfere with dioxins when analyzing dioxins using a gas chromatograph-coupled ion trap mass spectrometer are the liquid phases of columns eluted from chlorinated pesticides such as PCBs, DDT, and DDE, and capillary columns of gas chromatographs. (This is called column bleed).
[0018]
Dioxins are chemically very stable compounds. Chemical noise is reduced by collision-induced dissociation (CID) using an ion trap mass spectrometer, utilizing the difference in stability between dioxin ions and ions derived from chemical noise.
[0019]
As shown in FIG. 1, the ions 10 trapped in the ion trap space 9 in the ion trap mass spectrometer 33 are stably trapped while vibrating at a natural frequency ω corresponding to the mass of the ions. . The natural frequency ω can be obtained from the equation (1).
[0020]
ω = βΩ / 2 (1)
Here, Ω is the frequency of the main high frequency applied to the ring electrode, and β is a constant depending on the mass. For ions with the smallest mass that can be trapped in the ion trap, β = 1, and for ions with the largest mass, β = 0. That is, β takes a value from 0 to 1. If the frequency of the main high frequency applied to the ring electrode 7 is 1 MHz, ω is a value from 0 to 500 kHz from equation (1). Small mass ions vibrate at high frequencies, and large mass ions vibrate (slowly) at low frequencies.
[0021]
Now, when auxiliary AC is applied from the auxiliary AC power source 21 through the transformer 19 to the two end cap electrodes 6, 8, a dipole electric field is generated in the ion trap space 9. When the frequency of the dipole electric field and the natural frequency of the ions coincide, the ions are in a resonance state, and energy is absorbed from the dipole electric field, and the amplitude of the natural vibration of the ions increases rapidly. The ions collide with He gas atoms having a pressure of about 0.1 Pa filling the ion trap space and lose a part of their kinetic energy. In the process of repeated resonance and collision, part of the kinetic energy is converted and stored in the internal energy of ions. When the internal energy rises and exceeds the bond energy between atoms in the ion, the ion is cleaved and becomes a fragment ion (called daughter ion or product ion) with a small mass. This process is called Collision Induced Dissociation CID.
[0022]
Because of CID, the auxiliary AC voltage applied to the end cap electrode is about 0.5V to 1.5V. When this auxiliary AC voltage is 3 V or higher, ions are resonantly excited and their trajectories exceed the ion trap space, and the ions collide with the inner wall surfaces of the end cap electrodes 6 and 8 or the center of the end cap electrodes 6 and 8. It is discharged out of the ion trap through the hole.
[0023]
The voltage value of the auxiliary alternating current for CID varies depending on the compound to be analyzed and is therefore carefully selected. According to the inventor's experimental results, dioxins are 1.3 to 1.5 V, PCBs are 1.1 to 1.2 V, ions of chlorinated pesticides such as DDT and DDE, and multiple ions derived from column bleed are about 1 V. CID is generated by the application of auxiliary AC. Conversely, dioxin does not cause CID at less than 1.3V. For example, when an auxiliary AC of 1.2 V is applied, ions derived from PCB, DDT, DDE, and column bleed become daughter ions or grandchild ions by CID. On the other hand, the molecular ion of dioxin maintains almost the same strength as before CID.
[0024]
FIG. 2 shows an operation sequence diagram of the present invention. FIG. 3 shows a flowchart of the present invention.
[0025]
The mass analysis of the ion trap proceeds while changing the analysis mode one after another as time passes.
[0026]
(1) Ionization step (t ₀ to t ₁ , t ₃ to t ₄ ...) And chemical noise CID
In the ionization, first, the voltage of the main high frequency is set low so that ions in a wide mass range can be trapped in the ion trap space 9. +200 V is applied to the electron gate 5 to guide the voltage to the ion trap space. The sample gas introduced into the ion trap space 9 collides with the electrons 4 introduced from the pores of the end cap and is ionized. The generated ions 10 are trapped in a stable ion trap space 9 by a quadrupole high-frequency electric field.
[0027]
+200 V is applied to the electron gate 5, and ionization is started. At this time, the main high frequency voltage is set to a low voltage. Thereby, ions in a wide mass range can be accumulated in the ion trap space 9. When the frequency of the main high frequency applied to the ring electrode 7 of the ion trap is 1 MHz, auxiliary alternating current (white noise) including all high frequency components of 1 to 500 kHz is applied during ionization. Thereby, all the ions 10 generated by ionization and trapped in the ion trap space 9 are excited at once by the auxiliary alternating current of white noise. The white noise auxiliary AC voltage is a voltage (1.0 to 1.2 V) at which an interfering substance such as PCB is cleaved and dioxins are not cleaved. This application is displayed as auxiliary AC on in FIG. The ionization time is determined by the amount of the sample to be introduced, but is about 1 ms to 1 second. During this time (from t ₀ to t ₁ ), the auxiliary alternating current of white noise ranging from 1 to 500 kHz is continued. During the ionization time (from t ₀ to t ₁ ), compounds other than dioxins are subjected to resonance excitation, and chemical bonds between atoms in the ions are broken to generate daughter ions. Furthermore, the generated daughter ions are further subjected to resonance excitation by auxiliary alternating current and cleaved to become the next generation grandchild ions. Cleave until finally stable ions. Many ions having the same mass as dioxins are successively reduced in mass by broadband resonance as described above. Here, when the main high frequency is 1 MHz, the frequency band included in the applied auxiliary alternating current is 1 to 500 kHz. However, since the maximum molecular weight of dioxin is 456, the natural frequency corresponding to the mass 500 may be 500 kHz.
[0028]
(2) Step of mass spectrometry (t ₁ to t ₂ , t ₄ to t ₅ ...)
When the ionization time ends (becomes t ₁ ), −200 V is applied to the electron gate 5 to prevent electrons from entering the ion trap space 9. Auxiliary AC is turned off. Next, the main high frequency voltage supplied from the main high frequency power supply 15 and applied to the ring electrode is swept in accordance with an instruction from the data processing device 14. The ions in the ion trap space 9 become unstable with respect to the mass in order from the low mass, and are discharged out of the trap through the pores of the end cap. The detector 12 detects the ions 11 and obtains a mass spectrum by the data processor 14 through the DC amplifier 13.
[0029]
This mass spectrum collection may be performed by the methods shown in Japanese Patent No. 1,321,036 and Japanese Patent Publication No. 8-21365.
[0030]
(3) Reset If a predetermined mass range or sweep is performed, the main high frequency power supply is reset to zero. Thereby, all the ions remaining in the ion trap are eliminated. becomes t ₃ becomes second time scan is started again ionized returns to (1). This process is repeated to obtain a mass spectrum.
[0031]
By applying the white noise auxiliary AC simultaneously with the ionization, the dioxin signal buried in the chemical noise as shown in the upper diagram (b) of FIG. 4 is improved as shown in the mass spectrum of the lower diagram (b) of FIG. Many chemical noise ions that exceed the signal of dioxins are excited and cleaved by auxiliary alternating current. The mass spectrum after application of the auxiliary alternating current is shown in the lower part of FIG. The molecular ion of dioxin and the fragment ion from which COC1 is desorbed from dioxin are clarified. Chemical noise in the high mass region is cleaved to become daughter ions and shifts to the low mass region. These daughter ions do not overlap with dioxin ions and do not interfere with the analysis.
[0032]
FIG. 5 shows a mass spectrum obtained by enlarging the molecular weight region of dioxin. The upper diagram (A) shows a mass spectrum obtained without applying auxiliary AC. Chemical noise is superimposed on the dioxin signal. Therefore, the molecular ion pattern does not reflect the chlorine isotope ratio. Of course, quantitative analysis is not possible as it is. The lower part (b) of FIG. 5 shows a mass spectrum when an auxiliary alternating current is applied simultaneously with ionization as described above. The chemical noise superimposed on dioxin is cleaved and shifts from the molecular region of dioxin to the low mass region. For this reason, the ion current detected here reflects the dioxin concentration and enables high-accuracy quantitative analysis, or lowers the lower limit of quantification (high sensitivity analysis).
[0033]
(Second embodiment)
Since dioxin is a very stable compound, a maximum auxiliary AC voltage must be applied for its CID. In addition, it is necessary to continue excitation for a long time for the CID of dioxins. Agricultural chemicals such as DDT and DDE can be easily cleaved by irradiating auxiliary AC of about 1V for several milliseconds. In the case of PCB, if 1.2V auxiliary AC is irradiated for 5 ms or more, chlorine atoms dissociate in PCB by CID. On the other hand, since dioxins have a stable structure, they cannot be easily cleaved, and irradiation with an auxiliary alternating current of 1.5 V must be continued for 20 milliseconds or more.
[0034]
Using this feature, another method of chemical noise reduction is possible.
[0035]
FIG. 6 shows another operation sequence diagram of the present invention. FIG. 7 shows a flowchart. Basically, reference is made to the description relating to FIGS.
[0036]
(1) Ionization time The main high frequency voltage is set low, the electron gate 5 is opened, and the electrons are guided to the ion trap space 9 to start ionization.
[0037]
(2) Chemical noise elimination After ionization ion accumulation (t ₀₁ ), the electron gate 5 is closed to stop ionization. Here, a 1.5 V broadband auxiliary alternating current is irradiated for a period of 10 ms to 20 ms. With a voltage of 1.3 V, irradiation of 20 ms or more can be performed. During this process, many ions other than dioxins existing in the ion trap space are cleaved.
[0038]
(3) After the mass spectrometry time t ₁ becomes auxiliary AC and off, to obtain a mass spectrum by sweeping the fundamental RF voltage.
[0039]
(4) Reset When sweeping to a predetermined mass, the main high frequency voltage is reset to zero. As a result, ions remaining in the ion trap are eliminated.
[0040]
Acquisition of one mass spectrum is completed in (1) to (4). Again, repeat (1) to (4) to continue the measurement.
[0041]
As shown in FIG. 6, it is not necessary to apply auxiliary alternating current with the start of ionization. However, during ionization, auxiliary alternating current with a specific frequency can be applied to resonantly emit chemical noise of a specific mass. . Further, as in the first embodiment, a wide-range auxiliary alternating current may be applied to reduce chemical noise in a wide mass range.
[0042]
【The invention's effect】
According to the present invention, chemical noise during dioxin measurement can be reduced, and the lower limit of quantification can be greatly reduced.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of an ion trap mass spectrometer to which an embodiment of the present invention is applied.
FIG. 2 is an operation sequence diagram showing one embodiment of the present invention.
FIG. 3 is a flowchart of an embodiment of the present invention.
FIG. 4 is a mass spectrum diagram.
FIG. 5 is a mass spectrum diagram.
FIG. 6 is an operation sequence diagram showing another embodiment of the present invention.
FIG. 7 is a flowchart of another embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Filament power supply, 2 ... Filament, 3 ... Grid electrode, 4 ... Electron, 5 ... Electron gate, 6 ... End cap electrode, 7 ... Ring electrode, 8 ... End cap electrode, 9 ... Ion trap space, 10 ... Trapped 11 ... Ion, 12 ... Detector, 13 ... DC amplifier, 14 ... Data processing device, 15 ... Main high frequency power supply, 16 ... Gas guide pipe, 17 ... Grid power supply, 18 ... Electronic gate power supply, 19 ... Transformer, DESCRIPTION OF SYMBOLS 20 ... Signal line, 21 ... Auxiliary AC power source, 22 ... Signal line, 23 ... Gas chromatograph, 31 ... Inlet, 32 ... Capillary column, 33 ... Ion trap mass spectrometer.

Claims (6)

In a method for mass spectrometry of dioxins using an ion trap mass spectrometer equipped with a ring electrode and an end cap electrode,
During ionization and ion accumulation in the ion trap operation sequence, an auxiliary alternating current having a voltage of 0.5 V or more and less than 1.3 V, which is lower than the voltage at which dioxins are cleaved, is applied to the end cap electrode, and then the ring electrode is applied. An ion trap mass spectrometry method characterized in that a mass spectrum is obtained by sweeping an applied main high frequency voltage.   2. The ion trap mass spectrometry method according to claim 1, wherein the frequency band included in the applied auxiliary alternating current is from 1 kHz to 1/2 of the main high frequency.   2. The ion trap mass spectrometry method according to claim 1, wherein the lower limit value of the frequency band of the auxiliary alternating current to be applied is a natural frequency of ions having a mass of 500.   2. The ion trap mass spectrometry method according to claim 1, wherein the voltage of the auxiliary AC applied is 1 V or more and less than 1.3 V.   In a method for mass spectrometry of dioxins using an ion trap mass spectrometer equipped with a ring electrode and an end cap electrode, a voltage of 1.5 V is applied to the end cap electrode during ionization and ion accumulation during an ion trap operation sequence. A mass spectrum is obtained by applying the auxiliary alternating current of 10 msec to 20 msec and then sweeping the main high frequency voltage applied to the ring electrode to obtain a mass spectrum. In a method for mass spectrometry of dioxins using an ion trap mass spectrometer that has a ring electrode and an end cap electrode and forms an ion trap space by applying a main high frequency voltage to the ring electrode,
Introducing ions into the ion trap space to ionize the sample;
Applying an auxiliary alternating current of 1.3 V to the end gap electrode for 20 milliseconds or more;
And a step of obtaining a mass spectrum by sweeping a main high frequency voltage applied to the ring electrode.

Images