In order to understand whether the ablation laser would cause chemical reactions within the trapped organic molecules during resonant laser ablation of water ice containing organic molecules, a two-layer approach was devised. The first layer consisted of D2O ice containing organics that are inactive for the laser wavelength used (2.94 microns), and shown not to ablate under these conditions. When an additional layer of H2O ice was deposited on top of the D2O layer, both H2O and D2O layers, and the organics embedded in the D2O layer, became ablated due to resonant excitation of the H2O ice layer that transferred energy to the D2O layer. This showed that the organic matter is not damaged.
H2O ice absorbs the infrared laser pulse photons at 2.94 microns during a few nanoseconds pulse duration. Subsequently, within the H2O ice layer, which is typically about 1 to 3 micrometers thick, phase-explosion occurs, sending pressure waves in every direction. These pressure waves (or shockwave, based on its velocity in the medium) travel to the material that is below the H2O ice layer (in this case, D2O ice) and transfer the pressure waves into the D2O ice layer. Subsequently, both the H2O ice and the D2O ice containing the trapped material are ejected into vacuum, where they are ionized through resonance enhanced multiphoton ionization (REMPI), followed by time of flight mass spectrometry.
Chemical composition of materials — in particular, biological — is done using a well-known technique called matrix-assisted laser desorption and ionization (MALDI). Using this technique, the analyte (molecules of interest) is dispersed in matrix medium that can be resonantly excited and ionized by the same laser. Such a method needs extensive sample preparation. The new method would avoid such a sample preparation into a laser-active matrix medium because the laser-active ablation initiation material (H2O ice) is deposited on top of a sample.
This new method increases the flexibility and removes much of the sample preparation efforts. This two-step laser ablation and ionization mass spectrometry (2SLAIMS) technique allows any surface to be tested for surface contaminant/composition without extracting material from the surface and preparing samples that need to meet the stringent conditions of MALDI mass spectrometry. Using 2S-LALIMS, it should now be possible to analyze the surface composition by bringing the instrument to the sample in its native form.
This work can mature into a handheld 2S-LAIMS instrument that can be used to routinely detect surface composition of many materials; in particular, organics, trace metals, or biomolecules.