Plasma Ionization
Source for Atmospheric Pressure
Mass Spectrometry Imaging Using Near-Field Optical Laser Ablation
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Abstract
Mass spectrometry imaging (MSI) at
ambient pressures with submicrometer
resolution is challenging, due to the very low amount of material
available for mass spectrometric analysis. In this work, we present
the development and characterization of a method for MSI based on
pulsed laser ablation via a scanning near-field optical microscopy
(SNOM) aperture tip. SNOM allows laser ablation of material from surfaces
with submicrometer spatial resolution, which can be ionized for further
chemical analysis with MS. Efficient ionization is realized here with
a custom-built capillary plasma ionization source. We show the applicability
of this setup for mass spectrometric analysis of three common MALDI
matrices, α-4-hydroxycyanocinnamic acid, 3-aminobenzoic acid,
and 2,5-dihydroxybenzoic acid. Although the ultimate goal has been
to optimize sensitivity for detecting material ablated from submicrometer
diameter craters, the effective lateral resolution is currently limited
by the sensitivity of the MS detection system. In our case, the sensitivity
of the MS was about 1 fmol, which allowed us to achieve a spatial
resolution of 2 μm. We also characterize the analytical figures
of merit of our method. In particular, we demonstrate good reproducibility,
a repetition rate in the range of only a few seconds, and we determined
the amount of substance required to achieve optimal resolution and
sensitivity. Moreover, the sample topography is available from SNOM
scans, a parameter that is missing in common MSI methods