Matrix-assisted laser desorption/ionization (MALDI) is a soft ionization technique widely used in the analysis of biomolecules. To perofrm MALDI experiments the analyte has to be mixed with a molecule (called matrix) that absorbs the laser light and promotes the analyte desorption/ionization (D/I). Unfortunately it also generates strong interfering signals in the low mass spectral region that, often, hinder the detection and structural characterization of low molecular weight analytes. In MALDI both the matrix and the molar ratio [analyte]/[matrix] are still found using empirical approaches and the inhomogeneous analyte-matrix co-crystallization produces poor shot-to-shot and sample-to-sample signal reproducibility.
To overcome these limitations, several nanostructured materials have been proposed as suitable substrates for matrix-free laser desorption/ionization (LDI): the possibility to avoid matrix use will ease both sample preparation and mass spectrum interpretation.
Here we demonstrate an alternative, easy and fast method to develop chips for matrix-free LDI. In fact we have demonstrated that the fluoro-silanization of a bulk, conductive and UV absorbing substrates, is the only step needed to activate the surfaces and to achieve matrix-free operation with the same detection limit (femtomolar) obtained using nanostructured functionalized substrates.
Our results suggest that the D/I process is mainly driven by interfacial effects, since we demonstrate that neither the chemistry of the underlying substrate nor the surface nanostructuring have a role in the D/I process.
The fact that our approach do not require any kind of nanostructures, along with the evidence that several materials, once fluorinated, can be used as substrates (e.g. the routine metallic sample holder), make this method a promising technique for the detection of low molecular weight analytes.