Laser Ablation Technique Images Non-Flat Surfaces

A custom-built laser-based technique for mass spectrometry enables the imaging of samples with non-flat surfaces. The tool uses a distance sensor to record a height profile of the sample before the actual chemical imaging. Since most samples encountered in chemical ecology do not have flat surfaces, the novel tool could be useful in measuring the distribution of chemical compounds in these samples in order to answer ecological questions more accurately.

Custom-built laser source for mass spectrometry imaging: By means of improved laser ablation electrospray ionization (LAESI), the surfaces of non-flat samples — for example this coarse piece of savoy cabbage — can now be chemically analyzed. Courtesy of Benjamin Bartels, Max Planck Institute for Chemical Ecology.

A laser ablation electrospray ionization (LAESI) source was customized by researchers at the Max Planck Institute for Chemical Ecology to accommodate the topography of non-flat sample surfaces. Using a confocal distance sensor, the team recorded a height profile of the surface of the sample prior to the actual ionization experiment. The recorded height profile could be used to correct the distance between the focusing lens of the laser and the sample’s surface, ensuring that an essential parameter for laser probing was kept constant throughout an experiment on a sample with a 3D structure.

“The biggest challenge in analytics is preserving the constitution of a sample throughout the analytical process. More often than not, sample preparation influences the result by altering the sample’s chemical constitution. Typical preparation steps include sectioning a sample into thin, flat slices because flatness is required to guarantee optimal laser focus, a key parameter in reliable analysis," said researcher Benjamin Bartels.

Height profile of a piece of savoy cabbage (4 x 4 mm). The maximum difference in height is 2.38 mm. Courtesy of Benjamin Bartels, Max Planck Institute for Chemical Ecology.

Adapting the LAESI technique to non-flat surfaces has opened up the possibility of performing chemical imaging of samples with pronounced 3D shapes, while maintaining the reliability of classical measurements.

“This means that we can now investigate molecular distributions on a much bigger range of accessible surfaces," said Bartels. "I am thinking of insect exoskeletons or microbial colonies within their environment. We can now also compare the contents of different trichomes of a leaf.”

The researchers evaluated the system through a metabolic profiling of radish leaves, chosen due to their pronounced surface features and known content of specialized metabolites. After the ionization experiments, light microscopy imaging was performed to evaluate ablation crater size and position. Reproducible ablation mark diameters of 69 ± 7 µm on average were demonstrated. Mass spectrometric imaging capability was also demonstrated using radish leaf samples.

The researchers would like to implement further improvements and refinements to their technique. They believe that their approach could be easily adapted to other desorption/ionization methods.

The research was published in RSC Advances (doi: 10.1039/c6ra26854d).