JP2529219B2 - Circuit used with mass spectrometer quadrupole mass filter - Google Patents

Description

FIELD OF THE INVENTION The present invention relates to a quadrupole mass filter used in a mass spectrometer.

BACKGROUND OF THE INVENTION One type of mass analyzer widely used for qualitative and quantitative analysis of chemicals uses one or more quadrupole mass filters. In such an analysis system, a quadrupole mass filter incorporating four conductive metal rods is supported by a mounting portion formed of an insulating material. These rods are energized by a combination of DC and AC voltages for selective mass focusing. An example of a mass filter is "Temperature stable multi-mass filter and its method (Temperature Stab
le Multiple Mass Filter and Method There for) ”. US Pat. No. 4,032,782. This patent discloses a method of maintaining filter stability by thermal matching of rods and mounts.

Problems to be Solved by the Invention In order to obtain and decode an accurate reading of the analysis performed by mass spectrometry, the mass peak waveform obtained by scanning should be smooth and affect the spectral quality of the data. It is strongly desired that there be no spurious divisions or pits that would otherwise occur. In known systems, it is known that such spurious splitting and depression of mass peaks frequently occur and adversely affect the decoding of the obtained data.

Applicants have discovered that by unbalancing the high frequency voltage applied to the quadrupole rod pair, spurious splits and dips are substantially reduced.

Therefore, an object of the present invention is to provide a quadrupole mass filter having a smooth mass peak and free of spurious signals and deformation.

Yet another object of the present invention is to provide a quadrupole system with improved ion transport and thus sensitivity, especially for high mass ions.

In accordance with the present invention, a quadrupole mass filter used in a mass analyzer uses a radio frequency (rf) voltage that is applied to a quadrupole rod in an unbalanced manner. To be unbalanced, the relative values of the capacitive and / or inductive circuit components connected to the rod are selected to provide the desired unbalanced high frequency voltage to the quadrupole mass filter system. . This high frequency voltage is measured and controlled so as to obtain a predetermined unbalanced value.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The quadrupole mass filter shown in the accompanying drawings comprises two pairs of conductive rods 10a, 10c and 10 arranged in a manner to generate a hyperbolic electric field for passing ions of the material under test.
It incorporates b and 10d. These rods 10 are made of molybdenum and are connected to an electric circuit that generates a DC voltage and a high frequency AC voltage. The network includes a tuning circuit that controls the magnitude of the high frequency voltage and the DC voltage applied to the rods of the filter. This tuning or high frequency resonance circuit is a circuit with low loss and high Q, and the phase relationship of the high frequency voltage supplied to the two pairs of rods 10a, 10c and 10b, 10d is substantially 180 °. These rods serve to generate a time-varying electrostatic field for focusing the mass in a narrow band.

In operation, the mass control voltage is derived from a control device such as a computer or sweep signal generator, and the input resistance 12
And is sent to the addition point 14 via. This control voltage is used as a reference to set the mass to which the mass analyzer responds. The feedback voltage Vfb is obtained from the measuring device 38, which is connected to the rod 10 and the tuning circuit as described below. If a non-zero voltage appears at summing point 14, this voltage is amplified by error voltage amplifier 16 and the amplified voltage is sent to the control input of radio frequency signal generator 18. The high frequency signal generator 18 supplies a high frequency signal having a frequency in the range of 1.0 to 2.0 MHz to the tuning circuit, for example. This tuning circuit has inductances 20 and 22.
An inductive network consisting of: and a capacitive network consisting of capacitive elements 34 and 36. A central coil 24, preferably one or two turns, is connected to the output of the signal generator 18 and is centered between the two inductances 20 and 22 to provide high frequency power to the tuning circuit. .

The capacitive network of the tuning circuit comprises capacitive elements 30 and 32 that represent the capacitance of the system wiring, mass filters and cables. The capacitances 34 and 36 connected to the test points TP1 and TP2 are selected so that the circuit resonates at the desired operating frequency and the high frequency voltage measured at these test points TP1 and TP2 is in a predetermined unbalanced state. Capacitances 34 and 36 may be fixed or variable capacitors of selected value. In any case, the value of the capacitance meets the requirements for resonance and high frequency imbalance.

One end of each of the inductances 20 and 22 is connected to a capacitance element 26 and 28, respectively, and these capacitance elements are connected to a reference potential such as ground. The other ends of the inductances 20 and 22 are connected to test points TP1 and TP2, respectively, which are connected to capacitors 34 and 36, respectively. The connection point of the inductance 20 and the capacitance element 34 is connected to the opposing rods 10a and 10c, while the connection point of the inductance 22 and the capacitance element 36 is connected to the opposing rods 10b and 10d. The inductance and capacitance elements form, for example, an LC resonant circuit that operates at high high frequency voltages up to 3000 volts peak.

The need to adjust the capacitance value of the tuning circuit to obtain high frequency imbalance is determined by a high frequency voltage measuring device such as an oscilloscope, since some capacitive elements of the tuning circuit do not know the exact values. This measuring device is connected to the test points TP1 and TP2 and a pair of rods 10.
Capacitors 34 and 36 are adjusted to obtain the desired imbalance. These adjustments may be performed manually or automatically depending on the measurement values obtained by the measuring device. Alternatively, inductors 20 and 22 may be formed with different numbers of turns on either side of the center tap to form the desired high frequency voltage imbalance, or may be adjustable variable inductances.

Output feedback signal Vf of negative polarity from measuring device 38
b represents the difference in high frequency voltage between the rod pair 10a, 10c and the rod pair 10b, 10d. This feedback voltage is supplied to the summing point 14 via the feedback resistor 40,
Combined with positive polarity mass control voltage. As mentioned above,
If the sum of the two voltages is non-zero, then an error signal is generated which is processed by the feedback loop including the tuning circuit and the measuring device 38 to compensate for the error and to add the sum of the junction points 14. The voltage is forced to go to zero.

Also, the feedback voltage Vfb generated by the detector or measuring device 38 is used to generate positive and negative DC voltages which are applied to the rod 10 to perform a quadrupole mass filtering action. The feedback voltage is sent to the resolution control circuit 42, which controls the ramping and blocking of the DC signal and thus allows the mass resolution to be adjusted appropriately. The DC voltage is sent to the DC rod polarity reversible switch S1 via a parallel channel, one of which is provided with a phase inverting amplifier 44. This switch S1 can be manually operated or, if desired, computer controlled.
The polarity of the DC voltage is reversed so that positive or negative ions can be detected by a quadrupole filter. In actual operation, it is desirable to quickly switch between positive and negative ion analysis, in which case computer control is used.

The positive and negative DC signals pass through voltage amplifiers 46 and 48, respectively, and are connected to inductances 22 and 20 and bypass capacitors.
It is sent to the connection point with 28 and 26 and goes through a tuning circuit to make a rod pair.
Sent to 10.

The use of switch S1 at the inputs of voltage amplifiers 46 and 48 is the preferred approach, since it does not require switching high voltages, but it does allow two poles to provide a positive or negative output signal from each amplifier. Dc rod voltage amplifier is required. In another embodiment, a switch S2 is used at the output of the voltage amplifiers 46 and 48, each amplifier need only provide a unipolar DC signal, one positive and the other negative.

In the disclosed embodiment, a DC rod voltage is applied to the quadrupole rod via an inductive coil structure. In such a case, the center tap of coil 24 should be isolated from the system ground and the bypass capacitor grounded.
The high frequency circuit is completed by using 26 and 28. These capacitors serve to complete the high frequency circuit while preventing excess high frequency voltage from reaching the DC rod voltage amplifiers 46 and 48.

It has been found that unbalancing the high frequency voltage delivered to the rods results in a smooth mass peak shape, which provides a significant improvement in mass spectrometric operation, especially in the quantitative analysis of high mass chemical materials. There is a clear separation between adjacent mass peaks and the spurious signal does not affect the accuracy of the mass analyzer reading as in known systems. In order to obtain the desired unbalanced state, one or both of the capacitive elements are adjusted so that their values are different. Similarly, the inductive element may be tuned to different values to obtain the desired unbalanced state of the high frequency voltage,
Both inductance and capacitance may be adjusted. In the preferred embodiment, the high frequency voltage on one rod pair, e.g.
It is made about 1.4 times the voltage at b and 10d.

FIG. 2 shows the masses 69, 219, 414, 502 when the radio frequency voltages between the rod pairs 10a-10c and 10b-10d are balanced.
3 shows a similar mass spectrum when the radio frequency voltage between the pair of rods is not balanced. There is no fluctuation seen in the mass peak under voltage equilibrium, and the SN ratio is also improved. Table 1 is the equilibrium voltage data for the experiment of FIG. 2 and Table 2 is the unbalanced voltage data for the experiment of FIG. The voltage was measured between the rod pair and the grounded ion source. The substance to be measured is perfluorotributylamine PFTBA.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing the device of the present invention. FIG. 2 shows a mass spectrum when the radio frequency voltage between the rod pairs is balanced, and FIG. 3 shows a mass spectrum when the radio frequency voltage between the rod pair is not balanced. 10 …… rod, 14 …… addition point 16 …… error voltage amplifier 18 …… high frequency signal generator 20,22 …… inductance 30, 32,34,36 …… capacitive element 38 …… measurement device 42 …… resolution control Circuit 44 …… Phase inverting amplifier 46, 48 …… Voltage amplifier

Claims (1)

(57) [Claims] 1. A first pair of conductive rods and a second pair of conductive rods that create an electrostatic field that varies with time to focus a narrow band of ion masses, for each pair of conductive rods. In a circuit used with a quadrupole mass filter of a mass spectrometer including a voltage source for applying a radio frequency voltage of the same frequency, in a circuit used, the amplitude of the radio frequency voltage applied to the conductive rod of one pair is greater than the conductive rod of the other Means for adjusting the amplitude of a radio frequency voltage applied to said pair, thereby creating a radio frequency voltage imbalance between said pair of conductive rods to improve ion transfer through said quadrupole mass filter. Circuit used with the quadrupole mass filter of the mass spectrometer.