Controlling the specific enrichment of multi-phosphorylated peptides on oxide materials: aluminium foil as a target plate for laser desorption ionization mass spectrometry

A specific multi-phosphopeptide enrichment method is presented to complement current phosphopeptide isolation strategies that normally bias mass spectrometry analysis towards the detection of mono-phosphopeptides. An adsorption model is developed that shows how the physical parameters of adsorption such as specific surface area, adsorption time, etc. control the extraction of mono- and multi-phosphorylated peptides on oxide materials such as alumina. Commercial alumina particles, alumina supported on aluminium particles and alumina on aluminium foils were used to enrich multi-phosphopeptides. The synthesized phosphopeptides and tryptic digests of casein and milk were employed as samples to validate the adsorption simulation. Both experimental and theoretical results show how the selective enrichment of multi-phosphopeptides can be achieved when using extractors with a high specific affinity for phosphate groups but with a relatively small loading capacity or specific surface area. We also show that a commercial aluminium foil represents an ideal substrate to enrich multi-phosphorylated peptides for laser desorption ionisation mass spectrometry.