MCPa: Using Microchannel Plate in Ion Detection

Function of MCP in Ion Detection:

  • MCPs are described as two-dimensional sensors that can detect ions in a vacuum.
  • Beyond mere detection, MCPs also amplify the detected signals.
  • The core principle of amplification involves a potential gradient across the MCP channels. An incident particle, after causing the emission of secondary electrons, leads to an exponential increase in electrons as they travel through the channels and repeatedly strike the inner walls, resulting in a large output signal.
  • Some MCP assemblies are designed with a center hole allowing incident ions to pass through and irradiate a sample; the MCP then detects the signal generated from the sample. This type is noted for its ability to detect a very small amount of ion.

Properties Needed for MCPs Used in Ion Detection:

Several properties and specific designs are highlighted as beneficial or required for MCPs used in ion detection, particularly in applications like mass spectrometry:

  • Signal Amplification (Gain): As a fundamental function, the MCP must amplify the weak signal from an incident ion into a measurable output. The gain is determined by the channel length to diameter ratio (α) and the secondary emission factor of the wall material. The number of MCP stages (1 to 3) can be selected to obtain the necessary gain.
  • Fast Time Response: For applications like Time-of-Flight Mass Spectrometry (TOF-MS), which relies on measuring the arrival time of ions, a fast response time is crucial. MCP assemblies for TOF measurement are available with fast response times ranging from 450 ps to 1.5 ns (FWHM). Specific models listed under "Fast time response" are indicated as suitable for Mass spectrometry (TOF-MS, MALDI-TOF, imaging MS).
  • Durability and Stability (Robust MCPs): In TOF-MS, the stability and durability of the MCP assembly are important for minimizing time jitter, which directly affects mass resolution. Robust MCPs are less likely to warp or crack and are stable in shape, contributing to reduced time jitter.
  • Large Open Area Ratio (Funnel MCPs): Funnel MCPs have a larger Open Area Ratio (OAR) due to their funnel-shaped input channels. This design allows more signals (including ions) to enter each channel, enabling effective and accurate signal detection. An ion detection experiment demonstrated that a Funnel-MCP detected more than 90% of the signal compared to about 40% for a standard MCP.
  • Ability to Operate at Higher Pressure: Certain MCP assemblies are designed for ion detection at higher pressures, up to 1 Pa. These designs, such as those with a triode structure, suppress ion feedback noise, allowing stable operation in environments where achieving very high vacuum is difficult. 
  • Vacuum Conditions: While some types can operate at higher pressures, standard MCP operation for optimal performance, including ion detection, requires a high vacuum condition below 1.3 × 10-4 Pa. Degassing the MCP in a high vacuum for more than 24 hours before supplying voltage is also recommended, as gas adsorption can affect performance.

In summary, MCPs function as ion detectors by converting incident ions into a cascade of secondary electrons that are then amplified within the channels. For effective ion detection, particularly in time-sensitive applications like TOF-MS, key properties include high gain, a fast response time to minimize time jitter, durability and stability, a large open area ratio, and the ability to operate under specific vacuum conditions, with certain types designed for higher pressure environments.