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The Development of a High Speed Detector for Time-of-Flight Mass Spectrometry 49th ASMS Conference May 2001 Instrumentation Session Poster Paper 019 Bruce N. Laprade, Raymond Cochran and Ronald Starcher BURLE Electro-Optics Inc. Sturbridge MA, 01566 1-800-648-1800

Introduction Time of Flight Mass Spectroscopy has become the most widely used technique for identifying very large organic molecules. This technique has become the method of choice for most drug discovery and polymer applications. The Time of Flight technique is frequently chosen because it is the only technique capable of the high mass sensitivity needed for many substances. Recently, table top RGA, ICPMS, GC and LCMS instruments have emerged.

Introduction (Continued) The Time-Of-Flight mass spectrometry technique is an old technique which has seen a resurgence in popularity due to cost reductions in electronics and the advent of high temporal resolution detectors. The availability of high temporal resolution detectors has enabled shorter flight tubes to be used, which lead to smaller vacuum systems and lower overall instrument costs.

Introduction (Continued) In the operation of a typical MALDI TOF Instrument, analyte molecules, dispersed among matrix material are ionized by a nitrogen laser. (Figure 1) The resultant ions are held (delayed extraction) and then ejected down a flight tube through the application of high voltage pulses. Mass separation occurs during the flight (typically 1 meter) to the detector, with the lower mass ions arriving first, followed by progressively larger mass ones. Upon arrival at the detector, the electron multiplier will produce a charge pulse corresponding to the arrival time of each ion. (Figure 2) A high speed digitizer is then used to record the arrival times of the ions, from which the mass of the ion can be determined. Three types of electron multipliers have been historically used in TOF-MS. Single Channel Electron multipliers (SEM), Discrete Dynodes (DD), and Microchannel Plate (MCP) based. Single Channel Electron Multipliers are not used in modern instruments because of limitations in temporal resolution (20-30 ns FWHM) and dynamic range. Discrete dynode electron multipliers exhibit good dynamic range, but are used in moderate and low resolution applications because of relatively poor pulse widths (Typically 6-10 ns FWHM). MCP based detectors are used in high resolution applications because they provide the highest (650 ps) temporal resolution, however they are limited in dynamic range to about 20 mhz/cm2 of active area.

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Discussion Improving the temporal resolution of the ion detector will enable resolution of lost peaks (Figure 8) The temporal resolution of a microchannel plate (Figure 9) based detector is ultimately determined by the electron transit time (Figure 10) through the channel and the anode impedance. The electron transit time of a microchannel plate can be decreased by shortening the channel length. In order to maintain proper operating conditions, the channel diameters must be reduced proportionally.

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Objectives The Primary Objective of this Development Project was to Significantly Increase the Temporal Resolution of the Conventional Time of Flight Mass Spectrometer Detector. In order to increase the temporal resolution, the MCP pore size must be significantly reduced. Reengineering the fabrication process was necessary in order to overcome obstacles materials fiber draw, wafer grind and polish, activation and test.

Experimental Method Microchannel Plates are fabricated by a series of fiber draw operations involving a two glass system. Precision alterations were made to the draw parameters resulting in the successful fabrication of 18mm active diameter format MCPs with 2.3 micron (Figure 11) channel diameters. Interdiffusion obstacles were overcome by the development of a new process. New grinding and polishing techniques were developed in order to produce microchannel plates as thin as 80 microns. A cartridge assembly  (Figure 12) containing two microchannel plates, a grounded mesh and an output biasing system was developed in order to accommodate the thin microchannel plates.

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Results – Pulse Width

Conclusions A low profile, time-of-flight detector for mass spectrometry incorporating the smallest pore microchannel plates ever developed has been successfully produced. The temporal resolution of this detector is over twice that of the best detector currently available. The availability of this detector can significantly reduce instrument size and cost, while improving mass resolution.