Polymethyl methacrylate (PMMA) 4000 was measured by using the JMS-S3000 SpiralTOF. The [M+H]+ peaks for PMMA with the basic monomer units of 100u (Fig.1) were observed for this sample. The full PMMA mass spectrum and an expanded view around m/z 4000 are shown in Fig. 2(a) and (b), respectively. The resolving power at m/z 4,000 was approximately 45,000 (FWHM). Also, the mass differences between the 39, 40, and 41- mers had a very good match with the theoretical mass number (100.0524) of the PMMA repeat unit (C5H8O2). A comparison between the 40-mer’s observed and simulated isotopic patterns is shown in Fig. 2(c). The observed isotopic pattern is in very good agreement with the calculated isotopic distribution.