Comparison of Internal Energy Distributions of Ions Created by Electrospray Ionization and Laser Ablation-Liquid Vortex Capture/Electrospray Ionization

Abstract

Recently a number of techniques have combined laser ablation with liquid capture for mass spectrometry spot sampling and imaging applications. The newly developed noncontact liquid-vortex capture probe has been used to efficiently collect material ablated by a 355 nm UV laser in a continuous flow solvent stream in which the captured material dissolves and then undergoes electrospray ionization. This sampling and ionization approach has produced what appears to be classic electrospray ionization spectra; however, the 'softness' of this sampling/ionization process versus simple electrospray ionization has not been definitely determined. In this work, a series of benzylpyridinium salts were employed as thermometer ions to compare internal energy distributions between electrospray ionization and the UV laser ablation/liquid-vortex capture probe electrospray combination. Measured internal energy distributions were identical between the two techniques, even with differences in laser fluence (0.7-3.1 J cm(-2)) and when using UV-absorbing or non-UV-absorbing sample substrates. These data, along with results from the analysis the biological molecules bradykinin and angiotensin III indicated that the ions or their fragments formed directly by UV laser ablation that survive the liquid capture/electrospray ionization process were likely to be an extremely small component of the total ion signal observed. Instead, the preponderate neutral molecules, clusters, and particulates ejected from the surface during laser ablation, subsequently captured and dissolved in the flowing solvent stream, then electrosprayed, were the principal source of the ion signal observed. Thus, the electrospray ionization process used controls the overall 'softness' of this technique.

Laser dissection sampling modes for direct mass spectral analysis

“…An experimentally verified imaging resolution of 0.62 μm in the analysis of phase‐separated polymers was reached, making it one of the highest spatial resolution ambient ionization MSI techniques yet reported. In addition, thermometer ion and empirical studies have shown that for small organic and biological molecules, as well as medium size proteins (e.g., bradykinin and cytochrome c , 1060 and ~12,000 Da, respectively), the LA‐LVC/ESI technique is a 'soft' ionization method despite the use of the UV laser (349 nm) . This makes the technique suitable for a wide range of biological imaging applications.…”

“…In addition, thermometer ion and empirical studies have shown that for small organic and biological molecules, as well as medium size proteins (e.g., bradykinin and cytochrome c, 1060 and~12,000 Da, respectively), the LA-LVC/ESI technique is a 'soft' ionization method despite the use of the UV laser (349 nm). [18] This makes the technique suitable for a wide range of biological imaging applications.…”

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

“…Using a 355 nm laser with 10 Hz repetition frequency and 7 ns pulse width, profiling of stamped ink and a selected lipid in mouse brain tissue resulted in imaging resolutions of 6 and 50 μm, respectively. Despite the utility of an UV laser, LA‐LVC/ESI has been demonstrated to be a “soft” technique, yielding mass spectra of bradykinin and angiotensin III similar to those obtained with conventional ESI‐MS (Cahill et al, 2015). To improve the imaging performance of this method, a commercial optical microscope has been integrated into the vertically aligned LA‐LVC/ESI system to generate highly focused UV laser beam for laser ablation and provide bright‐field and fluorescence microscopy capabilities (Cahill, Kertesz, & Van Berkel, 2015).…”

Spatially resolved absolute quantitation in thin tissue by mass spectrometry

“…This arrangement typically results in greater particle collection efficiency by LA-LCE than by LA-EESI methods. There are several permutations of these approaches including LA-LVC [22,[132][133][134], LA-sample transfer [135][136][137], LA-solvent capture by aspiration [138,139], and ambient infrared laser ablation mass spectrometry (AIRLAB-MS) [140]. Unlike with LA-EESI approaches, analyte extraction is not particle size dependent in LA-LCE.…”