Optimized Alkali-Metal Cationization in Secondary Ion Mass Spectrometry of Polyethylene Glycol Oligomers with up to m/z 10000: Dependence on Cation Species and Concentration

In secondary ion mass spectrometry (SIMS), the detection of large organic molecules is accomplished using cluster ion bombardment. Ion formation often proceeds via cationization, through the attachment of (alkali) metal ions to the molecule. To study this process, the emission of secondary ions sputtered from polyethylene glycol (PEG) samples with molecular weights (MW) of 1000-10000 was examined. They were mixed with alkali-metal trifluoroacetic acid (X-TFA, where X = Li, Na, K, or Cs) in a wide range of concentrations to investigate the efficiency of cationization for 10 keV Ar-2000(+) cluster irradiation. Typically, cationized molecular ions [M + X](+) (with repeat units n of up to similar to 250, corresponding roughly to m/z 11000) and some characteristic fragment species were observed in the mass spectra. For all alkali cations, the oligomer intensities increase strongly with the molecular composition ratios X-TFA/PEG in the samples, and values of 5-10 seem to be optimal. With increasing molecular weight, the intensity of oligomer ions relative to the total number of ions decreases; as the latter remains rather constant, this implies that more fragment species are formed. The ion yields (detected ions per primary ions) of cationized [M + Na](+) oligomers sputtered from a PEG decrease very strongly with their size n: from 5.2 x 10(-6) at n = 21 (MW similar to 1000) to 4.5 x 10(-1)0 at n similar to 245 (MW similar to 11000). By contrast, the total yields Y-tot(+) show only a small variation for these different specimens, from 1.3 x 10(-5) to 3.7 x 10(-5).