Electrospray-assisted laser desorption/ionization (ELDI) is a soft ionization method for mass spectrometry (MS). It combines features of both electrospray ionization (ESI) and laser desorption. The ELDI process is based on merging the ESI-generated charged droplets with particles laser-desorbed from sample deposits, and generates multiply charged macromolecules under ambient conditions.

We have constructed ELDI sources with a N₂-ultraviolet laser and an optical parametric oscillator (OPO) infrared laser. We have demonstrated that ELDI-MS can be used to directly analyze peptides and proteins from dried or wet samples, and with or without the addition of a conventional MALDI matrix. ELDI is amenable to proteins up to 80 kDa, as well as complex tryptic digest mixtures, biological fluids, and even tissues. The limit of detection of ELDI was determined to be 250 fmol for peptides (bradykinin) and 500 fmol for medium-sized proteins (29 kDa carbonic anhydrase) with the IR laser, which is superior compared to similar ambient desorption/ionization methods.

ELDI production of multiply charged polypeptides allows laser-desorbed species to readily access the advantages of tandem mass spectrometry for producing sequence-informative product ions from both bottom-up and top-down MS protein identification strategies. We have demonstrated that ELDI-MS and MS/MS are amenable for protein identification from tryptic peptide mixtures (i.e., bottom-up proteomics). Top-down ELDI-MS characterization with up to 5 stages of MS (MS ⁵) provides unambiguous sequence information for medium-sized proteins, such as carbonic anhydrase.

Based on the merging and mixing of charged solvent droplets and laser-desorbed particles, a “Reactive”-ELDI methodology was demonstrated to support on-line reactions during the ELDI process. Preliminary Reactive-ELDI data showed that protein-ligand complexes can be formed in the gas-phase during the ELDI ionization process. Reactive-ELDI also allowed on-line disulfide bond reduction using dithiothreitol, which not only proves that the laser-desorbed particles merge with the charge droplets to effect chemical reactions prior to on-line MS detection, but also provides the possibility for direct on-line top-down protein identification of disulfide bond-containing proteins for proteomic study, side-stepping laborious, time-consuming sample preparation steps such as in-solution reduction and alkylation.