MSI LDPI: How does transmission laser desorption break through the optical diffraction limit?

In the field of optical microscopy, the Abbe diffraction limit is an inescapable law of physics. In traditional reflective optical systems, the diameter of the focused spot is d ≈ λ/2NA—when NA = 0.5 and the wavelength is 355 nm, the theoretical minimum spot size is approximately 355 nm. However, in practical mass spectrometry imaging, the spot size achieved by reflective laser desorption is often only in the 50–80 μm range, which is far larger than the diffraction limit.

Why is this?

The issue lies not with diffraction, but with engineering implementation. In reflective systems, the laser strikes the sample at an oblique angle, causing the focused spot to form an elliptical projection on the sample surface; even with a well-focused Gaussian beam, the obliquity factor increases the actual desorption area by a factor of 2–5. Transmissive laser desorption, however, takes a different approach.

Advantages of the transmission geometry

Neo Source INSTRUMENT’s MSI LDPI imaging source employs a transmission laser optical path: the laser is incident from the rear of the sample substrate, passes through the transparent substrate, and is then absorbed by the sample film. There are three aspects of this configuration worthy of closer examination.

Geometric symmetry. The laser is incident perpendicular to the substrate, and the focused spot maintains circular symmetry on the sample surface, eliminating the ‘stretching’ effect caused by oblique projection. This means that the actual desorption area ≈ the optical focal area, and no integration compensation is required for the angle of incidence.

Refractive index compression. This is a physical detail that is easily overlooked. When the laser passes through the transparent substrate (typical refractive index n ≈ 1.5) and reaches the substrate–sample interface, the near-field effect at the interface causes the effective spot size to shrink further, by a factor of approximately 1/n—that is, after passing through the substrate, the actual excited region within the sample layer is reduced by about one-third compared to the focused spot in air. This is also one of the key reasons why transmission-based schemes can achieve resolutions of 2–3 μm, rather than simply by using an objective lens with a higher numerical aperture (NA).

Control of ablation depth. In the transmission configuration, laser energy is deposited within a range of several hundred nanometres at the substrate–sample interface. Compared to the reflection configuration, where a significant amount of energy is wasted on reflection and scattering at the sample surface, the transmission configuration offers higher energy utilisation and more controllable ablation thresholds. The MSI LDPI engineering scheme achieves ‘micro-ablation’ by adjusting the laser energy density to a level slightly above the sample’s ablation threshold, thereby removing only an extremely thin layer at the sampling point without damaging the surrounding areas.

From 4 μm to 2 μm: Crossing the threshold for single-cell analysis

In a cover article published in *Analytical Chemistry* (2024, 96, 14), Professor Pan Yang’s research group at the University of Science and Technology of China reported a t-AP-LDI/PI imaging method that utilises transmission laser desorption coupled with post-laser photoionisation to achieve lipid imaging with a spatial resolution of 4 μm on mouse brain tissue sections.

In this 4 μm resolution image, the lipid heterogeneity within individual neuronal cell bodies (typically 10–50 μm in diameter) has been ‘visualised’ at the subcellular scale for the first time— —the neuronal cell membrane is rich in phosphatidylcholine (PC), whilst cholesterol esters are distributed relatively uniformly within the cytoplasm; the spatial distribution of these two components exhibits a clear boundary contrast at 4 μm resolution.

The engineering behind this achievement was no mean feat. Increasing the resolution from 20 μm to 4 μm reduced the sampling area per pixel by a factor of 25 (area being proportional to the square of the diameter), meaning that the ionic signal per pixel also decreased by a factor of 25. Signal attenuation is not a linear problem, but follows a square-law relationship. MSI LDPI compensates for this loss in two ways: the high energy utilisation of the transmission configuration reduces signal loss at the onset, whilst post-photoionisation re-ionises the neutral molecules produced by desorption, converting the vast majority of ‘waste’ into usable signal.

The next step in resolution

Is sub-micrometre resolution achievable? From an engineering perspective, the theoretical limit for a 355 nm UV laser passing through an oil-immersion objective with NA = 1.4 is approximately 127 nm. However, the experimental bottleneck is no longer optical, but rather the physico-chemical process of ablation: thermal diffusion distance (for pulses in the order of 100–300 nm/ns) and the matrix-assisted effect naturally set a practical lower limit on resolution of approximately 200–500 nm. The current 2–3 μm resolution of MSI LDPI is closer to an engineering limit than a physical one; a path towards sub-micrometre resolution does exist through the use of short-pulse lasers (picosecond/femtosecond) and optimised substrate materials.

The spatial resolution of mass spectrometry imaging is not merely a numbers game. The leap from 20 μm to 2 μm marks the transition from ‘tissue regions’ to ‘individual cells’—once MSI is capable of tracking the spatial distribution of metabolites at the single-cell scale, the cellular uptake, excretion and metabolic partitioning of drugs will truly enter an era of observability.

Related product: Neo Source INSTRUMENT MSI LDPI laser/photochemical ionisation mass spectrometry imaging source, with a spatial resolution of 2–3 μm, compatible with Agilent, AB SCIEX and Thermo mass spectrometers.