Imprint imaging: simply ‘press’ the sample to map the molecular distribution

Imprint imaging (also known as impression imaging) is a method of spatial sampling that does not require complex pre-treatment. The procedure is straightforward: samples such as plant leaves, petals, medicinal herbs and tissues are pressed directly onto a substrate (such as a microscope slide or polymer film); the metabolites on the surface of the sample are transferred to the substrate surface upon contact, retaining their original relative spatial positions; The substrate is then scanned point by point using an imaging ion source, yielding a spatial distribution map of the molecules. This method bypasses the preparation of ultrathin frozen sections, making it particularly suitable for samples such as leaves and medicinal herbs that are difficult to section or do not require sectioning in the first place.

Imprint imaging addresses a practical challenge: not all samples are suitable for sectioning. Plant leaves are thin and tough, with high water content, making them prone to fragmentation and deformation during cryosectioning; some medicinal materials are hard or have an uneven texture, making sectioning both time-consuming and difficult to ensure consistent thickness. Imprint sampling simplifies ‘sample preparation’ to a single ‘press’, resulting in minimal sample damage and high throughput, which is well-suited to the screening requirements of ‘multiple samples and multiple tissue sites’ in plant metabolism and natural products research.

Neo Source INSTRUMENT’s MSI DPI utilises imprint imaging in numerous plant studies, with its advantages primarily stemming from the detection gains offered by the DESI/PI dual-mode compared to DESI alone. Case studies on the official website include: sage imprints, where the DESI/PI mode detected a rich array of monoterpenes, diterpenes and triterpenes with a resolution of up to 200 μm; Ginkgo biloba imprinting, which revealed additional flavonoids, ginkgolic acids and cashew phenols, whilst also showing the distribution of secretory cavities; tea leaf imprinting, which identified additional amino acids, catechins and flavonoids, with the signal for theanine increased by 14.5-fold and that for caffeine by 61-fold, thereby enabling the reconstruction of most metabolites in the synthetic pathway from chalcones to catechins.

Imprinting experiments on a variety of samples collectively demonstrate the high detection sensitivity and extensive ionisation coverage of DESI/PI for plant metabolites. Imprint imaging and section imaging are not mutually exclusive but rather complementary: sections preserve the complete internal cross-sectional structure, whilst imprints provide rapid, section-free surface distribution; the combination of the two allows for a more comprehensive characterisation of the spatial specificity of metabolites. Both MSI DPI and MSI LDPI can be retrofitted to mass spectrometers from Agilent, AB SCIEX and Thermo.