Laser-Induced Acoustic Desorption Atmospheric Pressure Photoionization via VUV-Generating Microplasmas

Abstract

Abstract. We demonstrate the first application of laser-induced acoustic desorption (LIAD) and atmospheric pressure photoionization (APPI) as a mass spectrometric method for detecting low-polarity organics. This was accomplished using a Lyman-α (10.2 eV) photon generating microhollow cathode discharge (MHCD) microplasma photon source in conjunction with the addition of a gas-phase molecular dopant. This combination provided a soft desorption and a relatively soft ionization technique. Selected compounds analyzed include α-tocopherol, perylene, cholesterol, phenanthrene, phylloquinone, and squalene. Detectable surface concentrations as low as a few pmol per spot sampled were achievable using test molecules. The combination of LIAD and APPI provided a soft desorption and ionization technique that can allow detection of labile, low-polarity, structurally complex molecules over a wide mass range with minimal fragmentation.

Mechanisms of Acoustic Desorption of Atomic Clusters and Exfoliation of Graphene Multilayers

“…The strained surface features are then assumed to release the stored elastic energy in response to an acoustic stimulus, thus providing the needed extra energy for the desorption of molecules in LIAD. Although the latter idea has received a wide acceptance in the LIAD community, ,,− we note that the connection between the release of the elastic energy stored in the strained films/islands and the desorption of intact molecular analytes has been demonstrated neither experimentally nor computationally so far.…”

“…Acoustic desorption is a process where the interaction of acoustic waves with surfaces causes the ejection/desorption of atoms, molecules, or atomic clusters. − First reported as a scientific curiosity, , this process has been recognized as a useful tool for the acoustic control of surface reactions and catalytic activity of surfaces, − as well as for “cold” desorption of heat-sensitive organic molecules for mass spectrometry analysis. − The mass spectrometry applications of the acoustic desorption commonly rely on the ability of short laser pulses to generate strong acoustic waves in irradiated targets , and are often referred to as laser-induced acoustic desorption (LIAD).…”

“…In its simplest form, LIAD has a layout where the molecules of interest (analytes) are deposited on the front side of a thin metal foil facing the mass spectrometer, while the laser irradiates the back side of the film launching an acoustic pulse toward the front surface and leading to the desorption of the analytes. ,− Placing the irradiated side of the metal foil in direct contact with an optically transparent solid substrate of liquid layer has been shown to increase the efficiency of acoustic desorption, , which can be attributed to higher amplitudes of compressive pressure pulses generated under conditions of spatial confinement in both ablative and nonablative irradiation conditions. − An alternative setup, where an absorbing metal layer (liquid mercury) is confined between two sapphire plates, has also been demonstrated to be effective for the generation of strong acoustic pulses leading to desorption of electrons, ions, and molecules. , Note that, while metal foils are used most commonly for the generation and transmission of acoustic pulses in LIAD, the acoustic desorption has also been reported from a Si substrate and the molecular sample itself …”

Laser Desorption/Ablation Postionization Mass Spectrometry: Recent Progress in Bioanalytical Applications

“…According to the research by Zhang et al LID efficiency of 1064 nm for small dye molecules was not as high as that of visible and UV lasers because the photochemical process surpasses the photothermal process for the desorption of molecules with π conjugate structures (Zhang et al, 2012b). The ns‐pulsed 1064 nm laser and a visible laser at 532 nm have also been employed for laser‐induced acoustic desorption (LIAD) of nonvolatile and thermally labile compounds in techniques such as LIAD‐APPI, LIAD‐ESI, and LIAD‐APPI (Cheng et al, 2009a; Gao et al, 2011; Benham et al, 2016). In these studies, analytes deposited onto a metal foil were evaporated upon irradiating the backside of the foil with high‐intensity laser pulses.…”

“…Another variant of LA‐APPI is LIAD‐APPI (Benham et al, 2016). LIAD is accomplished by pointing a nanosecond pulsed laser at 1064 nm on the backside of a Ti foil.…”