AcroMass TechnologiesProductMass SpectrometerCharge-Sensing Particle Detector
Charge-Sensing Particle Detector
  • Minimum vacuum requirement
  • Minimum high voltage bias requirement
  • Relatively inert to ion mobility
  • Low power consumption (< 0.5 W)
  • Low noise level (~ 3 mVrms at sampling rate of 200k Hz)
  • High sensitivity of charge particles (~62 e⁄mV when event width is below 30 μs)
  • High dynamic range of ion quantitation (> 20 dB)
  • Bipolar charge measurement
Introduction
The charge-sensing particle detector is a highly charge-sensitive device intended to detect molecular ions directly. Compared with conventional ion detectors, the charge-sensing particle detector possesses various exceptional features.
Appearance
Working Principle and Properties

The charge-sensing particle detector (CSPD) senses the field variation and represents it as a peak signal. The figure below illustrates an equivalent circuit of the CSPD. When the charged particles approach the Faraday tray, image current, which is induced and accumulated in the feedback capacitor (C), is few pico farad only. The 1st-stage output, which is proportional to the voltage across the capacitor, is then shaped into a narrow peak (the final output).

The time constant of the capacitor and the release resistor (R) is 10 ms so it takes 50 ms at least to release the charges accumulated in the capacitor. As a result, if the time duration between two events is shorter than 50 ms, the increment of the cross-voltage capacitor can be less due to a different discharging rate. However, once the event width is distinctly shorter than the RC time constant (e.g. 100 μs), this effect can be neglected.

At the point of the 1st stage output, we can input a waveform which looks like an error function whose distribution of incoming charges is a Gaussian distribution in time. A series of error functions are used as test functions and the result demonstrates the properties of the shaping circuit. The figure below shows the response strength to the event width of a flying ion cloud approaching the Faraday tray and it represents the scenario that the CSPD reaches a plateau if the event width is less than 30 μs. That is, the incoming ion cloud is better to have an event duration less than 30 µs to get the best detection sensitivity. The figure below also illustrates the linearity of the CSPD while the event width remains. Besides, the tailing of a peak after the shaping circuit is ~200 μs. Thus the off-duration between events cannot be shorter than 200 μs. Otherwise, the shaping circuit shall not respond the second event correctly.

The background noise of CSPD is ~3 mVrms at a sampling rate of 200 kHz. In other words, under this acquisition condition, the CSPD has a detection limit of ~600 electrons if the event width is below 30 μs.

Briefly, to use the CSPD properly, we recommend that the width of the event shall be less than 30 μs and the off-duration between events shall be longer than 200 μs. Moreover, once the peak heights of events are linearly comparable, their widths have to be less than or equal to 30 μs. Individual test report will be provided along with the shipping detector.

Excellent Results
The figure shown above is a mass spectrum of biomolecule cytochrome c, which is resolved by unstable ejection and additional synchronization aid from auxiliary pulses. Beside the primary peak of pure cytochrome c, the detector is able to finesse tails of every matrix-effected peak and prevent ambiguous overshoot issues.
2019 Annual Conference, American Society for Mass Spectrometry
Third party application: CSPD in SLIM at PNNL