Signal Enhancement in Electrospray Laser Desorption/Ionization Mass Spectrometry by Using a Black Oxide-Coated Metal Target and a Relatively Low Laser Fluence

The electrospray laser desorption/ionization (ELDI) method is actively used for direct sample analysis and ambient mass spectrometry imaging. The optimizing of laser desorption conditions is essential for this technology. In this work, we propose using a metal target with a black oxide (Fe₃O₄) coating to increase the signal in ELDI-MS for peptides and small proteins. The experiments were performed on an LTQ-FT mass spectrometer equipped with a home-made ELDI ion source. A cutter blade with black oxide coating was used as a target. A nitrogen laser was used with the following parameters: 337 nm, pulse duration 4 ns, repetition rate 10 Hz, fluence ∼ 700 J m⁻². More than a five times signal increase was observed for a substance P peptide when a coated and a non-coated metal target were compared. No ion signal was observed for proteins if the same fluence and the standard stainless steel target were used. With the assistance of the Fe₃O₄ coated metal target and a relatively low laser fluence (≤ 700 J m⁻²), proteins such as insulin, ubiquitin and myoglobin were successfully ionized. It was demonstrated that the Fe₃O₄-coated metal target can be used efficiently to assist laser desorption and thus significantly increase the analyte signal in ELDI-MS. A relatively low laser fluence (≤ 700 J m⁻²) was enough to desorb peptides and proteins (up to 17 kDa) with the assistance of the Fe₃O₄-coated metal target under ambient conditions.