Molecular cluster emission in sputtering of amino acids by argon gas-cluster ions

• Abundant emission of amino-acid cluster ions by Ar cluster irradiation.
• Argon cluster ion bombardment of biological solids leads to soft desorption processes.
• Orthogonal TOF-SIMS provides efficient ion detection of large molecular species.

Ten pure amino acid specimens (alanine, arginine, asparagine, glutamine, glutamic acid, isoleucine, leucine, phenylalanine, threonine, and tyrosine) and three binary mixtures (phenylalanine/tyrosine, phenylalanine/arginine, and arginine/tyrosine) were bombarded by 10 keV argon gas-cluster ions Ar1000+ and the sputter-induced emission of positive secondary ions was investigated by orthogonal time-of-flight secondary ion mass spectrometry (TOF-SIMS). A prolific flux of protonated molecular cluster ions (Mn+H)+ was observed, with up to n = 22 for alanine. In most cases, the cluster yields Yn were found to follow an overall exponential decay with cluster size n, Yn ∝ exp(−βn), although a non-monotonous decrease was found for some clusters. The parameter β varies between ∼0.2 and ∼0.65 for the different amino acids; it seems to scale inversely with the respective sputtering yields of the materials. Generally, the yields of dimers relative to monomers are surprisingly high, amounting to 20–170% whereas those of trimers and tetramers are in the range of 10–90%. In amino acid samples with an (unintentionally) enhanced Na content, Na-cationized cluster (Mn+Na)+ were found in addition to (Mn+H)+. For both molecular cluster species the yields exhibit a very similar dependence on the cluster size n. From the binary mixtures, the emission of both homo- and heteromolecular clusters was observed; their size distributions seem to indicate that the two components were not homogeneously distributed within the samples.

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