High-resolution atmospheric pressure infrared laser desorption/ionization mass spectrometry imaging of biological tissue

Abstract

An atmospheric pressure laser desorption/ionization mass spectrometry imaging ion source has been developed that combines high spatial resolution and high mass resolution for the in situ analysis of biological tissue. The system is based on an infrared laser system working at 2.94 to 3.10 μm wavelength, employing a Nd:YAG laser-pumped optical parametrical oscillator. A Raman-shifted Nd:YAG laser system was also tested as an alternative irradiation source. A dedicated optical setup was used to focus the laser beam, coaxially with the ion optical axis and normal to the sample surface, to a spot size of 30 μm in diameter. No additional matrix was needed for laser desorption/ionization. A cooling stage was developed to reduce evaporation of physiological cell water. Ions were formed under atmospheric pressure and transferred by an extended heated capillary into the atmospheric pressure inlet of an orbital trapping mass spectrometer. Various phospholipid compounds were detected, identified, and imaged at a pixel resolution of up to 25 μm from mouse brain tissue sections. Mass accuracies of better than 2 ppm and a mass resolution of 30,000 at m/z = 400 were achieved for these measurements.

Influence of C-Trap Ion Accumulation Time on the Detectability of Analytes in IR-MALDESI MSI

“…Initially conceived of by Castaing and Slodzian in the 1960’s 1 , MSI began as a relatively crude technique for biological ion imaging 2 that through technological advancements 3–9 now combines the high specificity and sensitivity of mass spectrometric detection with high spatial resolution for unparalleled measurement of analyte distribution within biological samples. MSI has been shown to be sensitive for analysis of metabolomics 10,11 , proteomics 10,12–14 , lipidomics 15–18 , as well as targeted analysis of pharmaceutical drugs 19–23 .…”

“…Hence, requisite ion inject time is inversely proportional to laser repetition rate, with a 20 Hz repetition rate mid-IR laser requiring more than 100 ms to accumulate ions generated from two laser pulses. As with other successful pulsed infrared MSI approaches 9 , it is desirable to fix the number of pulses and the inject time during MSI acquisition at individual voxels, and as a result methods to control trapped ion populations like such as AGC cannot be utilized. AGC uses a short pre-scan to dynamically set an ion accumulation time in order to maintain the trapped ion population at a target number and reduce space charging and saturation effects.…”

Mass Spectrometry Imaging of Biomolecular Information

“…1), allowing to decrease the laser spot size to 12 µm x 8 µm without signal loss. High performance MALDI MSI at atmospheric pressure on orbital trapping mass spectrometers has been described with a spot size as small as 5 µm and a pixel size down to 3 µm in diameter 4,19,23,[25][26][27][28][29][30][31] . Instrumentation and methodology of this 'AP-SMALDI' method with high resolution in mass and space was reviewed recently 4 .…”

IR-MALDESI Mass Spectrometry Imaging at 50 Micron Spatial Resolution

“…To circumvent this issue, researchers may use transmission geometry IR laser ablation, where the laser is focused on the opposite side of the sample being analyzed [11]. Alternatively, Römpp and coworkers used an optic with a central drilled hole to allow the ablated material to pass through [12]. The laser beam propagation factor M 2 is a defining parameter detailing the beam quality, with a perfect Gaussian beam having an M 2 of 1.…”