Mass spectra of atactic polystyrene were collected into the mega-dalton mass range with a matrix-assisted laser desorption ionization time of flight (MALDI TOF) mass spectrometer, which incorporates a cryodetector comprised of an array of 16 superconducting tunnel junctions (STJ). The STJ cryodetector, theoretically, has no loss in signal response at any mass compared with the reduced signal found at high mass when using a conventional secondaryionization detector. Since ion detection at high m/z is one of the fundamental limitations of mass spectrometry (MS), the cryodetector was used to explore the high m/z limit of the MALDI TOF technique for the analysis of two polymer types. Mass spectra were collected for polystyrene at M n 170, 400, 900, and 2000 kDa and polymethyl methacrylate (PMMA) at M n 62.6 kDa and 153.7 kDa. For polystyrene, the data showed a trend toward increased aggregation and charge state with mass. The M n 2 MDa polystyrene data revealed a peak at m/z 2.2 MegaTh and a charge state analysis revealed that these ions were primarily polystyrene aggregates with a mass of ϳ4 MDa. This aggregate assignment was possible because the cryodetector response allows for the determination of a charge state up to about four. The contribution of each charge state for a selected peak can be determined in this fashion. This analysis revealed the preferential formation of doubly charged even-numbered aggregates over oddnumbered aggregates for high molecular mass polystyrene. A potential mechanism for the aggregation process for doubly charged species is discussed. Karas and Hillenkamp [3]. The MALDI technique was a major breakthrough in mass spectrometry and it has seen widespread use in chemical and biological research because it offers several key advantages. First, it is a relatively "soft" ionization technique that can produce intact low charge state molecular ions at low laser power. Second, like electrospray ionization (ESI), MALDI is suitable for the ionization of large nonvolatile molecules including synthetic polymers and biomacromolecules, which cannot be brought into the gas phase and ionized intact by most other more energetic introduction and ionization techniques. Third, the MALDI technique is more robust than ESI for samples that contain high concentrations of salts and buffers. And finally, because MALDI can be operated in a pulsed ionization mode, it can be conveniently coupled to a time-of-flight (TOF) mass spectrometer. The theory and applications of the MALDI method are reviewed elsewhere [4 -6].It is reasonable to assume that the MALDI technique may have an upper m/z limitation. For example, the laser desorption ionization (LDI) technique, is limited to ϳ1000 Da for intact biological molecules and a few kDa for synthetic polymers [7]. Compared with LDI, MALDI shows reduced fragmentation of macromolecules [8] and the mass range has been extended to several 100 kDa. This extension of the mass range is attributed to the abundant matrix, usually in a ratio of at least 1000:1 with the analyte th...