Secondary-ion emission of amino acids

Abstract

Secondary-ion mass spectroscopy (SIMS) is a hydrogen, isotope and compound sensitive analytical technique of extremely high absolute sensitivity. Continuing earlier measurements for carboxylic acids, adsorbed alcohols etc., we have carried out a systematical investigation of secondary-ion emission from metal-supported amino acids, containing various functional groups (e.g., alanine, phenylalanine, cysteine, arginine). In order to avoid damage effects we applied extremely small primary-ion current densities in the 10⁻⁹ A·cm⁻² range.

The main results of our investigations can be summarized as follows:

• - All investigated amino acids produce high-intensity secondary-ion parent peaks (M+1)⁺ and (M−1)⁻.

• - In addition positive as well as negative fragment ions representative for the different functional groups are emitted with high yields.

• - For 2.5 keV Ar⁺-ions the absolute yields for the parent ions and the most important fragment ions are in the range of 0.1; the damage cross section is >10⁻¹⁴ cm² for all investigated acids. The resulting absolute sensitivities are below 10⁻⁶ of one monomolecular layer or <10⁻¹² g.

We infer from these results that static SIMS is an excellent tool for trace detection, structural investigation and surface reaction studies of amino acids.