A new method is proposed for determining chlorine traces in monolayer films adsorbed on solid substrates. The method is based on secondary-ion mass spectrometry (SIMS) with the formation of positive chlorine-containing molecular ions. In contrast to the traditional method making use of inert gases, the secondary ions are generated by bombarding the chlorine-containing substrate with a beam of methane molecules. This leads to the formation of CCl 2 + ions with molecular masses of 82, 84, and 85 amu. These peaks are reliably observed in the form of a doublet not masked by the peaks of ions of the substrate material and the other background reaction products.
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