Enhanced detection of high-mass proteins by using an active pixel detector

Since their emergence in the 1980s, matrix-assisted laser desorption/ionization (MALDI)[1] and electrospray ionization (ESI)[2] have been widely applied to the analysis of proteins and have been key drivers in the growth of the proteomics field.[3] A significant difference between the two ionization techniques is that MALDI, unlike ESI, typically produces singly or doubly charged ions for large molecules and therefore requires mass analyzers and ion detection approaches suitable for these species with higher m/z ratios. MALDI is often coupled with time-of-flight (TOF) mass analysers because of their theoretically unlimited mass range. However, in practice, such systems are ultimately limited by the ability to efficiently detect singly charged ions with high m/z ratios. Ion detection in TOF-mass spectrometry (TOFMS) is traditionally accomplished by using the ion-to-electron conversion abilities of microchannel plates (MCPs). However, the ion-to-electron conversion efficiency of an MCP is well known to decrease with decreasing ion momentum (i.e., at higher mass).[4] Thus, although many high-mass ions can be generated, they may not be efficiently detected and their observed ion abundance may appear artificially low or possibly not be detected at all. To overcome this limitation, a variety of alternative non-MCP-based ion detection systems have been developed and have been demonstrated for m/z values up to 1 MDa.[4b, 5] Herein we describe the first reported application of the Timepix active pixel detector[6] to a commercial MALDI linear TOF instrument that provides ion acceleration voltages up to 25 kV. This combination of high ion acceleration voltages and a highly sensitive pixelated detector are shown to allow detection of ions up 400 kDa with high m/z ratios by using MCP-based detection with significant enhancements in signal-to-noise ratios compared with conventional detection approaches