Transforming a Mid-infrared Laser Profile from Gaussian to a Top-Hat with a Diffractive Optical Element for Mass Spectrometry Imaging

Abstract

Many mass spectrometry imaging (MSI) applications such as infrared matrix-assisted electrospray ionization (IR-MALDESI) employ an infrared (IR) laser with a Gaussian profile where laser irradiance is highest in the center and decreases exponentially. To enable full ablation of a square region of interest, oversampling is often needed, which results in nonuniform ablation and leads to decreased image quality. A diffractive optical element (DOE) was integrated into the optical path to generate homogeneous intensity distributions while maintaining laser energy above the ablation threshold, to enable complete sample removal from laser pulses without oversampling. 2D and 3D imaging with the DOE inserted show clear and sharp ablation patterns with satisfactory biological signals gained. Further improvements will optimize the beam profile and generate a square top-hat laser beam for MSI application at higher spatial resolution.

Maximized Spatial Information and Minimized Acquisition Time of Top-Hat IR-MALDESI-MSI of Zebrafish Using Nested Regions of Interest (nROIs)

“…29 A solid state 2970 nm laser (JGM Associates, Inc., Burlington, MA) fires downward toward the sample. In the down tube, the laser is first expanded by a −75 mm planoconcave beam expander (LC5401-E, Thorlabs; Newton, NJ), collimated by a +250 mm plano-convex lens placed 175 mm below (LA5255, Thorlabs), reshaped into a top-hat beam by a custom ZnSe DOE (HOLO-OR, FO-1933 #2), 7 and focused onto the sample by a Schwarzschild-like reflective objective (LMM15X-P01, Thorlabs). 10 The laser energy at the focal plane was measured with a Nova II power meter and energy sensor (Ophir Optronics, LLC; North Logan, UT).…”

“…The resulting laser excitation ablates neutrals that partition into the overhead, orthogonal electrospray where gas phase ions are generated through an ESI-like mechanism before mass analysis. , Conventionally, IR-MALDESI operates at a spatial resolution of 150 μm and employs a Gaussian laser beam. A custom diffractive optical element (DOE) has recently been incorporated into the IR-MALDESI source and characterized to convert the Gaussian beam to a top-hat square beam; however, the resulting spot size increased to nearly 190 μm.…”

“…5,6 Conventionally, IR-MALDESI operates at a spatial resolution of 150 μm and employs a Gaussian laser beam. A custom diffractive optical element (DOE) has recently been incorporated into the IR-MALDESI source and characterized to convert the Gaussian beam to a top-hat square beam; 7 however, the resulting spot size increased to nearly 190 μm.…”

Prototyping an ionization source for non‐engineers

“…and to change the beam shape, respectively. 14,15 The electrospray construct was relatively simple compared with the other components because the emitter assembly is constructed Finally, the source enclosure must be large enough to fit all the required components of the source and position them at an appropriate point relative to the inlet of the mass spectrometer. Because the source will need to uncouple and recouple from the mass spectrometer, it should be made from a rigid material.…”

Automatic z-Axis Correction for IR-MALDESI Mass Spectrometry Imaging of Uneven Surfaces

“…The source setup was described in detail elsewhere. 30 The two optics used in this work, a Schwarzschild-like reflective objective 31 and a diffractive optical element, 32 were optimized previously. All the experiments were conducted on the NextGen source coupled to an Orbitrap Exploris 240 mass spectrometer (Thermo Fisher Scientific, Bremen, Germany).…”