Quadrupole mass spectrometer
As a control parameter given to a direct-current (DC) voltage generator which generates a DC voltage for ion selection, a “mass-related offset” for allowing an adjustment of the offset for each mass-to-charge ratio is provided in addition to the “gain” and “common offset” which respectively determine the gradient and position of a scan line drawn on a stability diagram during a mass-scan operation. In an automatic adjustment operation using a standard sample, under the control of an automatic regulator, the “gain” and “common offset” are initially set, after which the “mass-related offset” for each mass-to-charge ratio is determined so that the mass-resolving power will be substantially uniform, and these data are stored in a control data memory. In an analysis of a sample of interest, a quadrupole voltage controller controls the DC voltage generator and a radio-frequency (RF) voltage generator according to the control parameters read from the memory.
1. A quadrupole mass spectrometer comprising:
an ion source for ionizing a sample;
a quadrupole mass filter composed of four rod electrodes;
a quadrupole driver for producing a composite voltage, which comprises a direct-current voltage and a radio-frequency voltage corresponding to the mass-to-charge ratio of an ion to be allowed to pass through the quadrupole mass filter, and for applying the composite voltage to the quadrupole mass filter; and
a detector for detecting an ion that has passed through the quadrupole mass filter,
wherein the quadrupole driver comprises:
a) a memory for storing voltage-setting data corresponding to the mass-to-charge ratio and for storing a gain, a common offset and a mass-related offset as control parameters for varying the direct-current voltage corresponding to the mass-to-charge ratio during a mass-scan operation, where the gain determines the ratio of the direct-current voltage to the amplitude of the radio-frequency voltage, the common offset determines a different offset voltage according to a scan speed, independently of the mass-to-charge ratio, and the mass-related offset specifies a different offset voltage for each of a plurality of mass-to-charge ratios within a mass-scan range; and
b) a direct-current voltage generator for generating a direct-current voltage to be applied to the quadrupole mass filter by adding at least three voltages during a mass-scan operation, the three voltages including: a voltage generated by retrieving from the memory the voltage-setting data according to a change in the mass-to-charge ratio, performing a digital-to-analogue conversion of the voltage-setting data, and multiplying the resultant analogue signal by a gain retrieved from the memory; a voltage generated by a digital-to-analogue conversion of the common offset obtained from the memory according to a scan speed at that point in time; and a voltage generated by a digital-to-analogue conversion of the mass-related offset obtained from the memory according to the change in the mass-to-charge ratio.
2. The quadrupole mass spectrometer according to claim 1 , further comprising a regulator for supplying the ion source with a sample containing a known kind of component, for selecting each of a plurality of mass-to-charge ratios of the ions to be allowed to pass through the quadrupole mass filter, for monitoring the detection signal produced by the detector while varying the mass-related offset given to the direct-current voltage generator with the mass-to-charge ratio fixed at a selected value, and for determining a value of the mass-related offset for each of the mass-to-charge ratios so that a mass-resolving power will be substantially the same at any of the mass-to-charge ratios.
3. The quadrupole mass spectrometer according to claim 1 , wherein the quadrupole driver further comprises:
a radio-frequency voltage generator for generating two radio-frequency voltages having a phase difference of 180 degrees; and
a quadrupole voltage controller for reading the gain, the common, and the mass-related offset from the memory and sending these appropriately to the radio-frequency voltage generator and the direct-current voltage generator.
4. The quadrupole mass spectrometer according to claim 1 , wherein the quadrupole driver further comprises:
a transformer for adding the radio-frequency and direct-current voltages; and
a wave detector for monitoring the radio-frequency voltage applied to the quadrupole mass filter.
5. The quadrupole mass spectrometer according to claim 1 , wherein for each of the mass-to charge ratios, the mass-related offset is decreased or increased such that the values of the mass-related offset are adjusted so that the differences in mass-resolving power for any of the mass-to-charge ratios will be within a predetermined range.
6. The quadrupole mass spectrometer according to claim 1 ,
wherein the direct-current voltage generator comprises:
a first D/A converter for converting the voltage-setting data into an analogue voltage;
a second D/A converter for converting the voltage-setting data into an analogue voltage and multiplying this voltage by a coefficient corresponding to a given gain;
a third D/A converter for converting a given value of the common offset into an analogue voltage; a fourth D/A converter for converting a given value of the mass-related offset into an analogue voltage;
a first adder for adding the analogue voltages outputted from the third and fourth D/A converters;
a second adder for adding the analogue voltage outputted from the first adder and the analogue voltage outputted from the second D/A converter;
a third adder for adding the analogue voltage outputted from the second adder and the analogue voltage outputted from the first D/A converter;
an inverting amplifier for inverting the polarity of the analogue voltage outputted from the third adder;
a fourth adder for adding a DC bias voltage Bias to the analogue voltage outputted from the third adder; and
a fifth adder for the DC bias voltage Bias to the analogue voltage outputted m the inverting amplifier.
7. The quadrupole mass spectrometer according to claim 6 , wherein the first, second, third, fourth, and fifth adders are able to add two inputs with a ratio of 1:1, but also to add them with any appropriate ratio; and the adders are also able to add a fixed value to further shift the voltage level.