FIG. 6 schematically shows the structure of the prior art mass spectrometer. This instrument has an ion source 23 producing ions. These ions are mass-analyzed by an electric field 24 and a magnetic field 25. The ions then pass through a slit 26 and reach a secondary electron multiplier 27, where they are detected. The output signal from the multiplier 27 is supplied via both an amplifier 17 and an A/D converter (ADC) 19 to a controller 22 incorporating a computer, where the signal is stored. The controller 22 controls a power supply 11 via a D/A converter (DAC) 12, the power supply 11 developing an accelerating voltage for the ion source 23 and an electric field voltage. The controller 22 also controls a magnetic field power supply 20 via a D/A converter (DAC) 21.
Where a mass analysis is made by magnetic field scanning, the controller 22 sends a scanning signal to the magnetic field power supply 20 via the DAC 21 while maintaining the accelerating voltage and the electric field voltage constant.
The controller 22 also controls a secondary electron multiplier power supply 15 via a D/A converter (DAC) 16. The controller sets the gain of the amplifier 17 via a gain controller 18. Furthermore, the controller reads the magnetic field strength from the output signal from a magnetic field strength detector circuit 13 via an A/D converter (ADC) 14. The magnetic field scanning is controlled according to the magnetic field strength read in this way.
As is well known in the art, in the mass spectrometer constructed as described above, the mass-to-charge ratio m/z, the magnetic field strength B in the magnetic field 25, and the ion-accelerating voltage V given by the ion source 23 generally satisfy the relation EQU m/z=K(B.sup.2 /V) (1)
where K is a constant. Accordingly, if the magnetic field is scanned at a fixed value of the accelerating voltage V, then we have EQU m/z=K.sub.1 B.sup.2 ( 2)
where K.sub.1 is a constant. Also, since the relation EQU B=K.sub.2 t (3)
holds (where t is time and K.sub.2 is a constant), we have EQU m/z=K.sub.3 t.sup.2 ( 4)
where K.sub.3 is a constant. It is common practice to determine the masses of the substances under investigation by creating a table of reference peaks (i.e., masses) and positions at which they appear (or magnetic field strengths or emergence times) and performing calculations, based on this table. In this case, therefore, the operator usually measures the mass numbers of individual peaks during the preparation of the table and enters the measured mass numbers into the controller 22.
In the above-described mass spectrometer, the peaks appearing on a mass spectrum obtained by the conventional ionization technique are mostly derived from ions having the elementary electric charge. However, on a mass spectrum obtained by electrospray ionization techniques which have become widespread in recent years, there exist many peaks derived from ions having electric charges that are integral multiples of the elementary electric charge. Ions of this kind are hereinafter referred to as polyvalent ions. The integers are hereinafter referred to as valence numbers.
FIG. 2 is a diagram schematically illustrating a mass spectrum of polyvalent ions. The horizontal axis of the mass spectrum is usually expressed in m/z (i.e., ion mass m divided by ion valence number). On this axis, polyvalent ions having valence number n derived from a substance having mass m appear at positions given by EQU x=(m+nH)/n=m/n+H (5)
where H is the mass of hydrogen, for example. In the case of electrospray technique, n hydrogen ions are attached to molecules with mass m, thus resulting in the aforementioned polyvalent ions. In this way, on a mass spectrum of polyvalent ions, many peaks attributed to ions having the same mass but different valence numbers arise, thus complicating the spectrum.