Charge detection mass spectrometry
Disclosed herein are various methods and apparatus for performing charge detection mass spectrometry (CDMS). In particular, techniques are disclosed for monitoring a detector signal from a CDMS device to determine how many ions are present in the ion trap ( 10 ) of the CDMS device. For example, if no ions are present the measurement can then be terminated early. Similarly, if more than one ion is present, the measurement can be terminated early, or ions can be removed from the trap ( 10 ) until only a single ion remains. Techniques are also provided for increasing the probability of there being a single ion in the trap ( 10 ). A technique for attenuating an ion beam is also provided.
1. A method of charge detection mass spectrometry comprising:
monitoring a detector signal from a charge detector of a charge detection mass spectrometry device during a first ion trapping event within an ion trap of the charge detection mass spectrometry device to determine how many ions are present within the ion trap during the first ion trapping event;
wherein when it is determined that no ions are present within the ion trap during the first ion trapping event, the method comprises terminating the first ion trapping event early and initiating a second ion trapping event;
whereas when it is determined that more than one ion is present within the ion trap during the first ion trapping event, the method comprises either:
(i) terminating the first ion trapping event early and initiating a second ion trapping event; or
(ii) ejecting or otherwise removing one or more of the ions from the ion trap.
2. The method of claim 1 , comprising ejecting or otherwise removing all of the ions from the ion trap and initiating a second ion trapping event.
3. The method of claim 1 , comprising ejecting or otherwise removing one or more of the ions from the ion trap so that only a single ion remains within the ion trap.
4. The method of claim 1 , where the number of ions present within the ion trap of the charge detection mass spectrometry device is determined based on the number of masses recorded in a spectrum by the charge detection mass spectrometry device and/or based on the total charge detected by the charge detection mass spectrometry device.
5. The method of claim 1 , wherein the geometry of the ion trap is configured such that ion trajectories become unstable when more than one ion is present resulting in the ejection of all but one ion.
6. The method of claim 1 wherein a substantially quadratic potential is applied to the or each ion trap such that ions undergo substantially harmonic motion within the ion trap.
7. The method of claim 1 , wherein the signals from the charge detection mass spectrometry device are processed using forward fitting and/or Bayesian signal processing techniques.
8. A charge detection mass spectrometry device comprising:
an ion trap for holding one or more ions to be analysed;
one or more charge detector(s) within the ion trap for determining a charge for the one or more ions to be analysed; and
control circuitry for monitoring a detector signal from the charge detector(s) during a first ion trapping event to determine how many ions are present within the ion trap during the first ion trapping event;
wherein when it is determined that no ions are present within the ion trap during the first ion trapping event, the control circuitry is configured to terminate the first ion trapping event early and initiate a second ion trapping event;
whereas when it is determined that more than one ion is present within the ion trap during the first ion trapping event, the control circuitry is configured to either:
(i) terminate the first ion trapping event early and initiate a second ion trapping event; or
(ii) eject or otherwise remove one or more of the ions from the ion trap.