Straight chain polydimethylsiloxanes of various molecular weight were characterized with LC/MS (liquid chromatograph/mass spectrometer) by atmospheric pressure chemical ionization (APCI) technique. The m/z 237 ion was observed as a base peak only when solvents containing methanol were used. The mass spectrum was different from those of EI and Cl and was found characteristic in the APCI technique. The structure of m/z 237 ion was considered to ion-molecule complex of CH3+ and hexamethylcyclotrisiloxane. APCI mass spectra for cyclic siloxanes were measured to confirm the structure of m/z 237 ion. A detailed structure of m/z 237 ion was optimized by ab initio MO study. Diluted polydimethylsiloxane (10,000 cSt) solutions were directly introduced into APCI/MS and were detected as m/z 237 ion. The detection limit was 0.1 μg/ml. Correlation coefficient of 0.999 was obtained between concentration and intensity of m/z 237 ion.
