Laser-induced acoustic desorption
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damental aspects Introducing large, thermally labile compounds into the gasphase environment of mass spectrometers without changing their structures has always been a challenge. To address this issue, various soft desorption/ionization techniques have been developed, such as matrix-assisted laser desorption ionization1 (MALDI), electrospray ionization2 (ESI), desorption electrospray ionization3 (DESI), and laser-induced acoustic desorption4 (LIAD) coupled with various ionization methods.5,6 Among these techniques, LIAD is unique in that it desorbs neutral molecules instead of ions. A typical LIAD device focuses a high-energy laser beam on the backside of a thin metal foil (typically ∼10 μm). A thin layer of a solid sample is deposited on the front side of the foil. The laser irradiation causes desorption of intact, neutral sample molecules into the gas phase from the metal surface (if the conditions are such that thermal degradation of the sample does not take place). Many different ionization methods and mass analyzers have been utilized to ionize and analyze the gaseous molecules. LIAD exhibits several major advantages over MALDI, ESI, and DESI, including the following:6,7 (1) LIAD can be used to desorb large saturated hydrocarbons with no polar functional groups into the mass spectrometer for ionization, while these compounds cannot be desorbed using MALDI,
ESI or DESI; (2) the translational energy and average velocity of the desorbed molecules are lower than for gas-phase ions generated using MALDI, thus allowing for more efficient ion– molecule reactions; and (3) sample desorption is decoupled from ionization, making LIAD a more versatile technique than MALDI, ESI, or DESI as many different ionization methods can be coupled with LIAD. However, the plume densities (109–1010 cm3) generated upon some LIAD experiments have been reported to be less than those obtained upon MALDI, although still high enough for many experiments.8 Due to these and other advantages, LIAD has become a common desorption technique, with its broadening field of applications summarized later in this article. As for the working principle of LIAD, the exact mechanism(s) of LIAD is (are) still unknown, and different mechanisms may be operational under different experimental conditions. Substantial efforts have been made thus far to understand these mechanism(s), including both computational modeling and experimental measurements. Several models have been proposed and tested; the most important ones are discussed next.
Thermal waves Although thermal waves are generated upon laser shots on a metal and may contribute to sample desorption, this is
Xin Ma, Department of Chemistry, Purdue University, USA; [email protected] Yuyang Zhang, Department of Chemistry, Purdue University, USA; [email protected] Hao-Ran Lei, Department of Chemistry, Purdue University, USA; [email protected] Hilkka I. Kenttämaa, Department of Chemistry, Purdue University, USA; [email protected] doi:10.1557/mrs.2019.105
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• VOLUME 44 • MAY 2019 Kean • www.mrs.org/bulletin ©
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