JP2545940B2 - Mass spectrometer - Google Patents

Description

TECHNICAL FIELD The present invention relates to a quadrupole mass spectrometer for detecting negative ions.

(Prior Art) When detecting negative ions in a quadrupole mass spectrometer, a secondary electron multiplier is used as a negative ion detector and a positive high voltage is applied to the first stage of the secondary electron multiplier. And attracts negative ions. However, in this case, in addition to the negative ions to be measured, electrons and ions generated by the ion source, secondary electrons generated by collision of electrons with the quadrupole electrode, etc. are detected by the secondary electron multiplier, Since the background is increased, the S / N ratio becomes worse. Usually, as a method of reducing this background, a magnetic field is bent between the exit of the quadrupole electrode part and the first stage of the secondary electron multiplier to bend the orbits of the light electrons so that they do not enter the secondary electron multiplier. I am trying. According to the literature, this magnetic field strength is required to be about 500 Gauss. If this magnetic field strength is large, the background due to electrons will decrease, but there will be the adverse effect that even small-mass ions will not be detected due to the effect of this magnetic field, and leakage magnetic flux will cause a problem in the secondary electron multiplier tube. There is also an adverse effect such as a decrease in electron detection sensitivity due to the bending of the electron orbit.

(Problems to be Solved by the Invention) The present invention eliminates electrons other than negative ions to be measured, detects negative ions of small mass, and prevents secondary electron multipliers from being adversely affected. The purpose is to

(Means for Solving the Problems) The center of the ion entrance hole of the ion detector is displaced from the central axis of the quadrupole mass analyzer, and the rear part of the quadrupole mass analyzer and the ion detector are arranged. Between the ion entrance hole of
An electrode that bends the negatively charged particle orbit in the direction of the ion entrance hole and a magnetic field generation unit that generates a magnetic field that bends the negatively charged particle orbit in the same direction from the rear portion of the mass analyzer are provided.

(Operation) The present invention detects only negative ions in the mass band to be measured among the charged particles emitted from the ion emission end at the rear of the quadrupole with the ion detector. It is intended to improve the ratio and the measurement sensitivity.
As the method, the electric field and the magnetic field are overlapped so that the outgoing direction of the ions emitted from the quadrupole part is bent in the same direction, and the moving direction of the low-mass electrons is reversed so that they do not enter the ion detector. The magnetic field strength required to do so is reduced. The ion detector was arranged so as to be displaced from the center line of the quadrupole so that the negative ions whose orbits were bent by both the electric field and the magnetic field were incident on the ion detector. Thus, the present invention, by performing the bending action of the negatively charged particles by both the electric field and the magnetic field, even if the magnetic field strength is lowered by assisting the action of the magnetic field with the electric field,
It has become possible to effectively remove electrons.

(Embodiment) FIG. 1 shows a configuration of a portion from an ion emission end to an ion detector of a quadrupole mass spectrometer in one embodiment of the present invention. In FIG. 1, R1 and R2 are a pair of quadrupole electrode rods, P1 is an ion emission hole plate, and an emission hole PE is provided on the same line as the center line of the quadrupole electrode portion, and a positive electrode of several tens of volts is provided. Voltage is being applied. P2 is an ion entrance hole plate to the ion detector, and an entrance hole PN is provided at a position shifted upward from the center line,
A positive voltage of several tens of V is applied from the same potential as the ion emission hole plate P1 or P1. D is a cylindrical deflector electrode which is arranged coaxially with the entrance hole PN, and a negative voltage of several tens of V is applied from the ion exit hole plate P1. MG is a magnet provided so as to sandwich the exit hole PE from the left and right, and applies a magnetic field so that negatively charged particles such as incident electrons bend in the direction of the exit hole plate P1. MY is a magnet yoke, EMP is for detecting ions 2
It is the first stage dynode of the secondary electron multiplier.

In the configuration shown in FIG. 1A, the electrons and negative ions that have passed through the emission hole PE pass near the lower portion of the deflector electrode D, and are thus repelled by the negative potential of the deflector, and the trajectory is bent upward. The negatively charged particles are also bent upward by the magnetic field generated by the magnet MG, and the electrons having a small mass are largely bent by the magnetic field, the traveling direction is reversed, and the particles are attracted and removed by the ion exit hole plate P1. Since negative ions have a larger mass than electrons, they are less bent by the magnetic field, are attracted to the positive potential of the ion entrance hole plate P2, and enter the detector EMP.

Considering now that there is no deflector electrode and electrons are removed only by the magnetic field, in order to make the ion exit hole plate P1 absorb the electrons that have exited the quadrupole mass analysis section, the traveling direction of the electrons is changed by 180 ° by the magnetic field. There is a need. Deflector electrode D
, The electron and ion orbits are bent by the same amount (if the acceleration energy is the same). Now, if the angle of the bend is 45 °, the magnetic field of the electron orbit for absorbing the electrons into the ion emission hole plate P1. A bending angle of 90 ° is sufficient, and compared with the case of only a magnetic field, in both the electric field and the magnetic field, the effect of electron removal is obtained with a weak magnetic field. The mass dispersion is also small, and all ions can pass through the ion entrance hole PN of the ion entrance hole plate P2 without being cut.

FIG. 2 shows the measurement results measured by the apparatus of the above embodiment. Figure 2 shows the PFTBA peak of M / Z452 and the background appearing near M / Z700 at the same time.
The vertical magnification of the recording is displayed with the PFTBA peak being 1 and the background being magnified 100 times. In the figure, the background is shown only near M / Z700, but there is similar background intensity over the entire mass range. In FIG. A, when the magnetic flux density is 0, B has a magnetic flux density of about 90 gauss. As is clear from the figure, the background is reduced to about 1/5 even when a magnetic field of about 90 Gauss, which is a magnetic field strength of about 1/5 of the conventional one, is reduced compared to the case where no magnetic field is generated. It can be seen that the invention is sufficiently effective in reducing the background even in a low magnetic field.

In the embodiment, the magnetic field is generated by the permanent magnet, but it goes without saying that it may be an electromagnet. Also,
The deflector electrode D is also a cylindrical electrode in the embodiment, but may be two flat plate electrodes facing each other.

(Effect of the Invention) According to the present invention, the background can be removed by electrons due to a weak magnetic field, so that the detection sensitivity of small mass ions is not cut, and the secondary electron multiplier tube due to leakage magnetic flux is used. The decrease in detection sensitivity of was eliminated.

[Brief description of drawings]

FIG. 1 shows an embodiment of the present invention, in which FIG.
FIG. 2B is a front cross-sectional view of the main part, and FIG. 2 is a graph showing a comparison between the detection peak and the background in the embodiment when there is no magnetic field and when there is a 90 gauss magnetic field. R1, R2 ... Electrode rod, P1 ... Ion exit hole plate, P2 ... Ion exit hole plate, PE ... Ion exit hole, PN ... Ion entrance hole, D ... Deflector electrode, EMP ... Ion detector , MG …… Magnet, MY …… Magnet yoke.

Claims (1)

(57) [Claims] 1. An ion incident hole of an ion detector, which is disposed so that the center of the ion incident hole of the ion detector is offset from the central axis of the quadrupole mass analyzer, and the ion incident hole of the rear of the quadrupole mass analyzer and the ion detector. An electrode that bends the negatively charged particle orbit in the direction of the ion entrance hole, and a magnetic field generation unit that generates a magnetic field that bends the negatively charged particle orbit in the same direction as the above from the mass spectrometer rear portion. Mass spectrometer.