JPH0580786B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an improvement in a scanning method with a constant peak width in a quadrupole mass spectrometer.

(Prior art) As shown in Fig. 4, a quadrupole mass spectrometer applies a DC voltage U and a high frequency voltage V in a superimposed manner between the quadrupole, and places a voltage at the center of the quadrupole on the paper surface of the figure. It is a configuration in which an ion beam is input in the vertical direction and ions of a specific mass are extracted from the exit slit side, and in a certain combination of DC voltage U and high frequency voltage V applied to the electrode, only ions of a specific mass are quadrupled. Mass spectrometry is performed because ions of other masses can reach the exit slit through the center of the pole, and ions of other masses cannot reach the exit slit due to divergent trajectories. Figure 5A shows the stability and instability of the trajectory of an ion of a certain mass as it passes between quadrupoles on a plane with high frequency voltage V on the horizontal axis and DC voltage U on the vertical axis. The region below one chevron-shaped curve (stability curve) is the stable region of the trajectory of an ion of a certain mass, and the region above is the region where the trajectory is unstable, that is, the trajectory diverges. This curve changes as the mass changes, as shown by 1, 2, and 3 in the figure, and the smaller the mass, the more the peak shifts to the left and the height becomes lower.
The peak apex is along a single straight line l passing through the origin. Here, in the U-V plane, passing through the origin O, the fifth
When the values of the DC voltage U and the high-frequency voltage are changed according to the scanning locus along the straight line L in Figure A, each ion mass in a narrow area surrounded by the peak of this stability curve and the straight line L. can pass between the quadrupole and the exit slit in a stable orbit, so the fifth
A mass spectrum as shown in Figure B is obtained. As is clear from Figure 5A, the shapes of the triangular parts above straight line L of the stability curves of ions of each mass are approximately similar, so the width of the peak in the mass spectrum obtained in this case is large. If M is the mass and ΔM is the peak width, the resolution M/ΔM is constant. That is, on the UV plane, the DC voltage U
If mass scanning is performed with the high-frequency voltage and the high-frequency voltage varying along a straight line passing through the origin, scanning with constant resolution will be performed. In this scanning method with constant resolution, as is clear from FIG. 5A, the smaller the mass, the narrower the peak width, and the lower the ion detection sensitivity. Furthermore, the mass spectrum appears in leaps and bounds based on the proton mass number (=1), and this is the same even in areas with large masses, so if the resolution is constant, adjacent peaks cannot be distinguished from each other in areas with large masses. If you want to be able to identify a mass number difference of 1 at the maximum mass in the measurement range, the resolution must be set high when the maximum mass is large, and the sensitivity will be extremely low in areas with low mass. .

For this reason, a scanning method in which the peak width of a mass spectrum is constant is generally used, which will be described below, and this is one of the characteristics of a quadrupole mass spectrometer. 6th
As shown in Figure A, the straight line C that cuts the vicinity of the apex of the ion stability curve for each mass with a constant width ΔM is U-V
When the DC voltage U is a certain constant value at V=O on a plane
Have Uo. That is, a constant bias Uo to the current voltage U
, so that the relationship between U and V is a straight line.
By changing ΔM, scanning can be performed with constant ΔM, and the mass spectrum at this time becomes as shown in Figure 6B. With this scanning method, the sensitivity is constant regardless of the size of the mass, and adjacent peaks can be distinguished from each other even in areas with large masses. Considering that mass spectra appear one at a time as mentioned earlier, it is best to set the peak width ΔM of the mass spectrum as large as possible to the extent that a mass number difference of 1 can be identified in order to increase sensitivity. Therefore, there is no point in reducing ΔM unnecessarily. Therefore, the slope of Uo and the U, V relationship straight line in FIG. 6A is usually set so that ΔM=1.

(Problems to be Solved by the Invention) The above-described scanning method with a constant peak width has the following problems. On the UV plane, the left tails of the stability curves for each mass all converge at the origin, and as seen in Figure 6A, U in a scan with a constant peak width,
If we try to set ΔM close to 1, the V locus will fall below the stability curve for all masses from near mass number 1 to ions are detected, and ions with mass number 1 cannot be separated and detected.

(Means for Solving Problems) As shown in FIG.
In the process of bending upward near , and bringing V closer to O, U does not become O, but is shaped so that U has a constant value Up at V=O.

(Function) When the above-mentioned scanning trajectory is used, the scanning trajectory passes below the mass stability curve and above the other two or more mass stability curves, so the mass spectrum becomes as shown in Figure 1B, and the mass The peak a in equation 1 indicates the peak of ions with a mass number of 1, not all ions.

(Embodiment) FIG. 2 shows a circuit according to an embodiment of the present invention. Q is connected as a pair of two opposing electrodes of each quadrupole, and a voltage of opposite polarity is applied between the two pairs. RE is a high frequency oscillator, the output of which is applied to a pair of quadrupole electrodes Q via a capacitor C. The output of RF is converted into DC by rectifier circuit D, the DC output voltage is applied to subtraction circuit SB, a constant voltage Uo is subtracted, and the output of SB is applied to quadrupole Q via diode d1 and chiyoke coil L. Applied to a pair. The output of the rectifier circuit D is compared with a reference voltage V by a comparator CP, and the high frequency oscillator RF is controlled so that the high frequency output voltage (peak value) is equal to the reference voltage V by the output signal of the comparator CP. Mass scanning is performed by changing the reference value V using this feedback system. Since the DC voltage U applied to the quadrupole Q is the rectified RF output, it maintains a linear relationship with V, and is given a bias Uo by the subtraction circuit SB. In addition, a DC power supply is connected between the diode d1 and the high frequency choke L via the diode d2.
Up is connected. As the high frequency voltage V applied to the pair of quadrupole electrodes Q increases, the DC voltage U applied to the quadrupole electrodes increases. This U
When is higher than the DC power supply Up, d2 is cut off and U changes as V changes, but the value of U changes.
When it becomes lower than Up, d1 enters the cutoff state and the DC voltage applied to the quadrupole becomes a constant value Up. The DC voltage U applied to the other pair of quadrupole electrodes Q is the voltage that is output via the diode d1' after the polarity of the output of the subtraction circuit SB is inverted by the inversion circuit In. Voltage U is DC voltage Up′ (=Up)
When the voltage is below, d1' becomes cut off, and a constant voltage Up' is applied via d2'.

In the embodiment described above, when the AC voltage V applied to the quadrupole becomes below a certain value, the DC voltage U becomes a constant value.
Fixed to Up.

In the embodiment of FIG. 3, as shown in FIG. 1A, in the region where the high frequency voltage V is below V', the DC voltage U changes with a gentle slope, and remains constant at V=O.
It is designed so that it becomes Up. The difference from the embodiment shown in Fig. 2 is the DC power supplies Up and Up' in Fig. 2, which are the addition circuit AD, which has a smaller amplification factor than the subtraction circuit SB.
The voltage obtained by adding a constant voltage dU to the output of the rectifier circuit D is determined by OR consisting of diodes d1, d2, etc.
The point is that it is input to the circuit. Other 2nd
Components corresponding to those in the figure are given the same reference numerals, and individual explanations will be omitted.

(Effects) According to the present invention, as described above, a quadrupole mass spectrometer performs scanning by setting the width of each peak in a mass spectrum to a unit mass, and can also separate and detect a peak with a mass number of 1. Therefore, highly sensitive ion detection is possible over the entire mass range.

[Brief explanation of the drawing]

Figure 1 is a graph explaining the present invention, Figure 2 is a circuit diagram of one embodiment of the present invention, Figure 3 is a circuit diagram of another embodiment, and Figure 4 is a quadrupole mass spectrometer. FIG. 5 is a diagram illustrating the types of voltages applied to the poles, FIG. 5 is a diagram illustrating constant resolution scanning, and FIG. 6 is a diagram illustrating conventional scanning with a constant peak width.

Claims (1)

[Claims] 1 Using a quadrupole mass spectrometer, scan the DC voltage U and high-frequency voltage V applied to the quadrupole on a scanning trajectory such that the width ΔM of the mass spectrum is the unit mass number, and the same trajectory is such that the high-frequency voltage becomes O. A mass spectrometry method characterized by giving a recursion so that the DC voltage has the same polarity as the other part of the scanning locus from the vicinity of the mass number 1 to the mass number O so that the DC voltage U does not become O before it becomes .