Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J49/00—Particle spectrometers or separator tubes
  • H01J49/26—Mass spectrometers or separator tubes
  • H01J49/34—Dynamic spectrometers
  • H01J49/42—Stability-of-path spectrometers, e.g. monopole, quadrupole, multipole, farvitrons
  • H01J49/4205—Device types
  • H01J49/424—Three-dimensional ion traps, i.e. comprising end-cap and ring electrodes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J49/00—Particle spectrometers or separator tubes
  • H01J49/26—Mass spectrometers or separator tubes
  • H01J49/34—Dynamic spectrometers
  • H01J49/42—Stability-of-path spectrometers, e.g. monopole, quadrupole, multipole, farvitrons
  • H01J49/426—Methods for controlling ions
  • H01J49/427—Ejection and selection methods
  • H01J49/4285—Applying a resonant signal, e.g. selective resonant ejection matching the secular frequency of ions

Definitions

• the present invention is directed to a method of mass analyzing a sample according to the preamble of claim 1.
• a QUISTOR is comprised of a ring electrode and of end electrodes which together define a storage space.
• the conditions are discussed in detail which allow a stable trapping of ions within the storage space of a Quistor. These conditions comprise the application of an RF-voltage having a so-called “Trapping Frequency” to the ring electrode of the Quistor.
• the phenomenon is discussed that the ions perform an oscillating movement which is called secular movement.
• EP-A-0 113 207 also discloses a method which comprises the variation of the magnitude of the storage RF-voltage in order to modify the value of the mass-to-charge ratio of stable trap­ping conditions so that the trapping conditions vanish suc­cessively for ions with growing masses, and the ions become free to leave the ion trap.
• the ions which leave the Quistor in the direction of the rotational axis of the ring electrode, are registered by means of an electron multiplier in order to evaluate the spectrum of the sample included in the Quistor.
• DE-A-3 533 364 also discloses the method to eliminate all ions which are produced when a sample contained in the Quistor is ionized, with the exception of those ions which belong to a watched pollution, by varying concurrently the magnitude of the RF-signal and of a DC-voltage both applied to the ring electrode in such a way that all ions are eliminated which have a mass-to-charge ratio which is different from that of the watched species of ions.
• the method described in EP Application No. 88 105 837.3 consists in generating in addition to the quadrupole storage field an exciting RF-field having a frequency which is different from the frequency of the RF component of the quadrupole storage field and in modifying the electrical field condition defined by said exciting RF-field and said quadrupole storage field in such a way that ions of consecutive masses encounter a resonance of their secular movements with the exciting RF field, thereby taking up energy and increasing the amplitude of their secular movement until they finally leave the storage field.
• the ad­vantage of this method consists in that a mass selective reso­nance ejection of the ions which are present in the stability region of the quadrupole storage field takes place. This method is very selective and results in a quantitative elimination of the ions having a specific mass-to-charge ratio.
• a mass spectrometer having a Quistor as an ion trap has the special advantage that a Quistor does not need to be exposed to a homogeneous magnetic field. Thus no magnet is needed and the spectrometer may have a simple construction and be easyly handled. On the other hand, it has the drawback that space charge effects limit the performance of the device. It is pos­sible to store about 108 to 109 ions within the Quadrupole storage field of a Quistor having a ring electrode with an inner diameter of about 2 cm. However, with such a high ion density, the available methods of ion ejection do not allow the sepa­ration of different ions with the desired resolution.
• the present invention makes use of one of the basic principles described in the elder European Application No. 88 195 847.3 as a means for eliminating all ions from the trap which have a mass-to-charge ratio which is different from that of the ions to be traced.
• the special ad­vantage of this method consists in that the steps of providing ions and eliminating all those ions therefrom which have a mass-to-charge ratio out of interest may be continued until the number of ions having the mass-to-charge ratio of interest has been increased to a level which is sufficient for an easy de­tection of the ions, and this irrespective of steps b) and c) being performed simultaneously or alternately.
• the ionisation and elimination steps are performed alter onlynately and if there is only one unwanted ion species present, it is sufficient to excite the ionized sample with an RF signal having only one defined frequency. Thereby, the secular motion of the undesired ions is excited so that these ions take up energy, increase their secular motion and are finally eliminated so that the ion density within the trap is reduced.
• a second ionization step produces both ions of the traced pollu­tion and unwanted ions. If then the unwanted ions are again eliminated by exciting their secular motion, the ions having a mass-to-charge ratio of interest remain trapped so that their number in the trap is increased. By repeating these steps, the density of ions of interest may be raised until the limit is reached which produces an optimum relation between signal strength and resolution.
• an elimination step may be performed for each one of said species before the next ionization step in the above mentioned method.
• more than ion species is eliminated by the use of a broad-band RF excitation voltage which comprises the secular frequencies of all disturbing ions. In this case, ions having a continous mass range are eliminated.
• Puls excitation is another method of eliminating unwanted ions.
• An appropriate excitation RF-puls may be calculated from the frequencies and amplitudes of the plurality of signals which would be necessary to eliminate the unwanted ions by means of Fourier Transformation.
• the provision of ions and the elimination of those of the provided ions which have masses out of interest may be performed simultaneously, which means that ion creation and ion ejection takes place in the same time interval.
• the ejection time should last a little longer than the ionization time to ensure the complete removal of the unwanted ions.
• the advantage of this method is the shorter time needed to eliminate the unwanted ions as compared to the process making use of alternate steps described above.
• Providing the Quistor with ions may be performed in two basi­cally different ways, i.e. either by forming the ions inside the Quistor or by forming them outside thereof and then intro­ducing them into the Quistor as an ion beam.
• all ionization methods which are common in mass spectro­metry may be used, as e.g. Electron Ionisation (EI), Chemical Ionisation (CI), Charge Exchange (CE), Photo Ionisation (PI), Field Desorption (FD), Fast Atom Bombardement (FAB), and Secon­dary Ion Mass Spectrometry (SIMS).
• the invention may be performed with mass spectrometers as described in the prior art.
• These mass spectro­meters include an ion trap in form of a Quistor comprising a ring electrode and two end cap electrodes, which ring and end cap electrodes delimit a storage space.
• the performance of the method requires the connection of an exciting RF-generator to the end cap electrodes which allows the generation of at least one RF signal having a frequency which is equal to the frequency of the secular motion in the direction of the axis of the ring electrode of one species of ions to be eliminated, so that this species of ions which is of no interest may be eliminated by exciting its secular motion.
• This RF-generator may be de­signed as a pulse generator producing output signals having a predetermined spectrum.
• Such RF-pulse generators are common in the art of mass spectroscopy. Indeed, in mass spectrometers, all steps having a rather short duration so that all events may be considered as being pulse like.
• the mass spectrometer described in the elder European Patent Application No. 88 105 847.3 is especially suited to perform the invention. It includes a Quistor which is comprised of a ring electrode and of cap electrodes which deviate from the generally used ideal hyperbolic shape having an angle 1:1,414 of the asymptotes in order to provide field faults which result in non-harmonic oscillations of the ions which are trapped within the Quistor. These non-harmonic oscil­lations result in very sharp resonances which improve the se­lectivity of the ion ejection and thus the resolution. Thus, it is of special advantage to use the Quistor described in said European application No. 88 105 847.3 for performing this invention.
• the claimed method may be performed with any common mass spectrometer having a Quistor as an ion trap and since the principle of the invention is easy to be understood, it is considered as being superfluous to further explain the invention by means of specific examples.
• the invention makes use of one part of the basic method disclosed in said application as a means for the selective elimination of ions.
• this method is now used for the purpose of enhancing the density of trapped ions having a specified mass-to-charge ratio. This is the rea­son, why the contents of this elder application has been in­corporated into this disclosure by reference.
• the method of eliminating ions by exciting their secular motion by means of an RF voltage applied to the end cap electrodes can be applied in the course of all methods for running mass spectrometers which include a Quistor.
• the ionisation time has commonly a duration in the ⁇ s- to ms- range.
• the RF voltage connected to the ring electrode has a frequency in the range of about 1 MHz and an amplitude of up to some kV.
• the frequency of the secular motion varies from about 1/100 to about 1/2 of the trapping frequency, and thus varies between about 10 and about 500 kHz.
• the vol­tage necessary to excite the secular motion is much lower than the voltage of the storage RF-signal, and has a value from about 0,1 to 100 V.
• the values giving optimum results may be determined by experiments.
• the ions of one or more substances of interest may be accumulated until the maximum ion density is reached so that they produce a clear signal and may be definitely detected.
• the kind and number steps of ne­cessary to eliminate unwanted ions and to reach the maximum density of the ions of interest allows to estimate the concen­tration of the pollution. Experiments on samples having known concentrations of the polluting substance may be used to cali­brate the device.

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• Chemical & Material Sciences (AREA)
• Analytical Chemistry (AREA)
• Electron Tubes For Measurement (AREA)
• Other Investigation Or Analysis Of Materials By Electrical Means (AREA)

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Cited By (21)

Citations (3)

• 1988
  • 1988-10-07 EP EP88116599A patent/EP0362432A1/de not_active Withdrawn

Patent Citations (3)

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