Time-of-flight Mass Spectrometry - Detectors

Detectors

A time-of-flight mass spectrometer (TOFMS) consists of a mass analyzer and a detector. An ion source (either pulsed or continuous) is used for lab-related TOF experiments, but not needed for TOF analyzers used in space, where the sun or planetary ionospheres provide the ions. The TOF mass analyzer can be a linear flight tube or a reflectron. The ion detector typically consists of microchannel plate detector or a fast secondary emission multiplier (SEM) where first converter plate (dynode) is flat. The electrical signal from the detector is recorded by means of a time to digital converter (TDC) or a fast analog-to-digital converter (ADC). TDC is mostly used in combination with orthogonal-acceleration (oa)TOF instruments.

Time-to-digital converters register the arrival of a single ion at discrete time "bins"; a combination of threshold triggering and constant fraction discriminator (CFD) discriminates between noise and ion arrival events and converts 1–2 nanosecond long Gaussian-shaped electrical pulses of different amplitudes from MCP into common-shape pulses (e.g., pulses compatible with TTL logic circuitry) sent to TDC. Using CFD provides a time point correspondent to a position of peak maximum independent of gain peal amplitude caused by variation of the MCP or SEM. Fast CFDs typically have dead time of 1–2 seconds thus preventing repetitive triggering from the same pulse.

The TDC is an ion counting detector – it can be extremely fast (down to a few picosecond resolution), but its dynamic range is limited due to its inability to properly count the events when more than one ions simultaneously hit the detector. The outcome of limited dynamic range is that the number of ions detected in one spectrum is somewhat small. This problem of limited dynamic range can be alleviated using multichannel detector design: an array of mini-anodes attached to a common MCP stack and multiple TDC, where each TDC records signals from individual mini-anode. To obtain peaks with statistically acceptable intensities, ion counting is accompanied by summing of hundreds of individual mass spectra (so-called hystograming). To reach a very high counting rate (limited only by duration of individual TOF spectrum, ~ 200 microsecond in long or multipath TOF setups), a very high repetition rate of ion extractions to the TOF tube is used. Orthogonal acceleration TOF mass analyzers operate at 20–30 kHz repetition rates. In combined mass spectra obtained by summing a large number of individual ion detection events, each peak is a histogram obtained by adding up counts in each individual bin. Because the recording of the individual ion arrival with TDC produces only a single time point (e.g., a time "bin" correspondent to the maximum of the electrical pulse produced in a single-ion detection event), the TDC eliminates the fraction of peak width in combined spectra determined by a limited response time of the MCP detector. This propagates into better mass resolution.

Modern ultra-fast 4 GSample/sec analog-to-digital converters digitize the pulsed ion current from the MCP detector at discrete time intervals (250 picoseconds). Typical 8-bit or 10-bit 4 GHz ADC has much higher dynamic range than the TDC, which allows its usage in MALDI-TOF instruments with its high peak currents. To record fast analog signals from MCP detectors one is required to carefully match the impedance of the detector anode with the input circuitry of the ADC (preamplifier) to minimize "ringing". Mass resolution in mass spectra recorded with ultra-fast ADC can be improved by using MCP detectors with shorter response times.