Novel two-step laser ablation and ionization mass spectrometry (2S-LAIMS) of actor-spectator ice layers: Probing chemical composition of D2O ice beneath a H2O ice layer

Abstract

In this work, we report for the first time successful analysis of organic aromatic analytes imbedded in D2O ices by novel infrared (IR) laser ablation of a layered non-absorbing D2O ice (spectator) containing the analytes and an ablation-active IR-absorbing H2O ice layer (actor) without the analyte. With these studies we have opened up a new method for the in situ analysis of solids containing analytes when covered with an IR laser-absorbing layer that can be resonantly ablated. This soft ejection method takes advantage of the tenability of two-step infrared laser ablation and ultraviolet laser ionization mass spectrometry, previously demonstrated in this lab to study chemical reactions of polycyclic aromatic hydrocarbons (PAHs) in cryogenic ices. The IR laser pulse tuned to resonantly excite only the upper H2O ice layer (actor) generates a shockwave upon impact. This shockwave penetrates the lower analyte-containing D2O ice layer (spectator, a non-absorbing ice that cannot be ablated directly with the wavelength of the IR laser employed) and is reflected back, ejecting the contents of the D2O layer into the vacuum where they are intersected by a UV laser for ionization and detection by a time-of-flight mass spectrometer. Thus, energy is transmitted from the laser-absorbing actor layer into the non-absorbing spectator layer resulting its ablation. We found that isotope cross-contamination between layers was negligible. We also did not see any evidence for thermal or collisional chemistry of PAH molecules with H2O molecules in the shockwave. We call this "shockwave mediated surface resonance enhanced subsurface ablation" technique as "two-step laser ablation and ionization mass spectrometry of actor-spectator ice layers." This method has its roots in the well-established MALDI (matrix assisted laser desorption and ionization) method. Our method offers more flexibility to optimize both the processes--ablation and ionization. This new technique can thus be potentially employed to undertake in situ analysis of materials imbedded in diverse media, such as cryogenic ices, biological samples, tissues, minerals, etc., by covered with an IR-absorbing laser ablation medium and study the chemical composition and reaction pathways of the analyte in its natural surroundings.

Photochemistry of Polycyclic Aromatic Hydrocarbons in Cosmic Water Ice: The Role of Pah Ionization and Concentration

“…As will be shown below, the principle PAH-related photoproducts initially formed include PAH cations, alcohols, and ketones. After first generating PAH cations, UV photolysis of PAHs in water ice is expected to produce aromatic alcohols as the first chemical photoproducts within the ice; with increasing photolysis time, these aromatic alcohols are then expected to transform into ketones (Gudipati 2004;Gudipati & Allamandola 2006;Ashbourn et al 2007;Guennnon et al 2011aGuennnon et al , 2011bGudipati & Yang 2012;Yang & Gudipati 2014). For example, the UV irradiation of phenol, the simplest aromatic alcohol, induces the breaking of the O−H bond to form a phenoxyl radical (Giuliano et al 2012); see Figure 2.…”

“…First, consider PAH photolysis in water ice. Recent, elegant, laser desorption/ionization-TOF experiments (Gudipati & Yang 2012;Yang & Gudipati 2014), coupled with earlier UV-VIS spectroscopic studies show that immediately upon photolysis, in situ PAH ionization and stabilization is facile and rapid, with ionization yields and final products within the ice strongly dependent on ice composition, temperature, photolysis duration (Gudipati & Allamandola 2003, 2006Bouwman et al 2010Bouwman et al , 2011b, and PAH concentration (Bouwman et al 2011a;Cuylle et al 2014). The reasons for strong PAH ionization and possible PAH cation reaction pathways with H 2 O radical photoproducts (H and OH) in UV photolyzed water ice are discussed in great detail elsewhere (Gudipati & Allamandola 2003;Woon & Park 2004;Gudipati & Allamandola 2006;Bouwman et al 2010Bouwman et al , 2011a.…”

“…However, until quite recently, it was not at all clear if these species formed in the low temperature ice during irradiation or afterward, during warm-up, when reactive species could diffuse through the ice. Therefore, while they might be important interstellar ice constituents, significantly influencing astrochemistry and IR observations as discussed in Section 5.1 above, they may only play a role in regions where irradiated ices can warm to well above 100 K. This question is now settled thanks to the novel two-color, laser-desorption, laser-ionization-TOF experiments by Gudipati & Yang (2012) and Yang & Gudipati (2014). They have shown that the addition of OH and O are the primary chemical reactions that occur in PAH:H 2 O ices subjected to Lyα radiation at 5 K. This contrasts with the efficiency of hydrogenation which, they find, is higher at 100 K than at 5 K. Consider also the knowledge summarized in Section 4.1 that PAH cation formation is the first step upon exposure to UV radiation and that aromatic alcohols, ketones and quinones are also formed in low temperature PAH:H 2 O ices irradiated with near-UV radiation (Guennoun et al 2011a(Guennoun et al , 2011b.…”

Laser desorption time-of-flight mass spectrometry of ultraviolet photo-processed ices

“…Indeed, recently a number of new developments have been reported. [22][23][24][25][26][27][28][29][30][31][32] Successful experiments have been reported in which TPD has been combined with vacuum UV (VUV) single photo-ionisation time-of-flight mass spectrometry (TOF-MS), measuring the mass from desorbing species from the ice. 22,23 Diagnostic techniques such as Substrate/Matrix Assisted Laser Desorption Ionisation Time-ofFlight Mass Spectrometry ((S/M)ALDI-TOF-MS) have been introduced.…”

“…A similar approach was recently applied, to investigate radiation-induced processes of Polycyclic Aromatic Hydrocarbons (PAHs), embedded in interstellar ice analogous, using Resonant Enhanced Multi Photon Ionisation (REMPI) as ionisation source. 24,25 Alternatively, the laser energy can be coupled into the substrate to induce the thermal desorption of ice constituents. 28,29,[33][34][35][36] The desorbed ice constituents are subsequently ionised, for example, using electron impact ionisation, and analysed with a TOF-MS.…”