Negative-ion low-temperature fast-atom bombardment mass spectrometry (LT-FAB-MS) is applied to. Only in the case of the dimeric C 120 O has the use of atmospheric-pressure chemical ionization (APCI) and electrospray ionization (ESI) been reported. 7 Thus, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) represents the only mass spectrometric technique for most of the dimers so far. 4,[8][9][10][11][12][13] However, MALDI-TOFMS exhibits limitations in the case of the rather unstable C 120 5,6 and it does not provide the unambigous identification of C 120 OS 13 because this cannot be distinguished from C 120 O 3 under these conditions.Good solubility of the sample in the matrix is a prerequisite for FAB-MS. [17][18][19][20] Consequently, the problems with FAB-MS arise from the extremely low solubility of the [60]fullerene dimers in conventional FAB matrices. Recently, a technique was introduced which employs volatile solvents, e.g. methanol, dichloromethane and toluene, as matrix in FAB-MS. 21 This is achieved by cooling the solvent/analyte mixture with liquid nitrogen and performing the measurement under low-temperature (LT) conditions. As a result, a solvent ideally suited for the analyte can be chosen regardless of its volatility. Independently, Shiea et al. published an alternative approach to the use of LT liquid secondary ion mass spectrometry. Using their method, they obtained mass spectra of the phosphaoxetane-type intermediate of the Wittig reaction.
22The aim of this paper is to outline the application of negative-ion LT-FAB-MS to the title compounds in both low-resolution (LR) and high-resolution (HR) mode.
EXPERIMENTALA JEOL JMS-700 sector instrument (JEOL, Tokyo, Japan) was used for LT-FAB-MS. The FAB gun was operated with Xe at an emission current of 8 mA and at an accelerating voltage of 6 kV. On the stainless steel FAB probe tip the solid [60]fullerene compounds were dissolved in 2-3 mL of a 10 : 1 mixture of 1-chloronaphthalene (Cl-naph) and 3-nitrobenzyl alcohol (3-NBA). The sample was subsequently frozen by immersion into liquid nitrogen for 15 s 21,23 and then immediately transferred into the vacuum lock to keep condensation of atmospheric water to its surface low. Spectra were monitored during thawing of the sample in the ion source which was kept at 40°C. Final spectra were obtained by averaging 3-7 scans. For low-resolution measurements (R = 2000) the magnet was scanned over the mass range of interest, i.e. m/z 100-1000 for the monomers, m/z 600-1800 for the dimers, in about 12 s per cycle. For high-resolution measurements (R = 5000), scans of the accelerating voltage were used to cover the narrow range necessary to include two reference peaks. Internal calibration of the mass scale was achieved using [(H 2 SO 4 ) n HSO 4 ] À cluster ions generated from concentrated sulfuric acid which was applied to the position opposite to the analyte on a dual-target FAB probe.C 60 was purchased from Hoechst (Hoechst, Germany). Characterization of C 120 OS was part...
