Table 1. Ionization source characteristics.
Information about the resolution, type of information standardly collected, and sample considerations for each of the ionization sources reviewed herein. Pixels are two-dimensional datapoints, Voxels are three-dimensional datapoints. Abbreviations: DESI – Desorption Electrospray Ionization; LAESI – Laser Ablation Electrospray Ionization; MALDI – Matrix Assisted Laser Desorption/Ionization; SALDI – Surface Assisted Laser Desorption/Ionization; SIMS – Secondary Ion Mass Spectrometry; LESA – Liquid Extraction Surface Analysis; LA-DART - Laser Ablation Direct Analysis in Real-Time; IR-MALDESI - Infrared Matrix Assisted Laser Desorption Electrospray Ionization. *It is worth noting that this lateral resolution has not been achieved using a natural products based sample.54 **This lateral resolution was achieved using a radio frequency plasma oxygen primary ion source and has identified elements in biological samples but has yet to be applied to a natural products based sample.55
| Ionization source | Spatial resolution | Molecular information notes | Sample type/limitations considerations |
|---|---|---|---|
| DESI/nanoDESI (ambient) | 150 microns/50microns routinely 10–15 microns* | Pixel | Solid, frozen liquid |
| LAESI (ambient) | 150 microns | Voxel | Fresh sample |
| MALDI (vacuum) | 5 microns | Pixel | Dried sample in matrix/matrix signal interference |
| SALDI (vacuum) | 10 microns | Pixel | Sample transfer is necessary |
| SIMS (high vacuum) | 5 microns – 0.037 microns** | Secondary ions, Pixel | Dried sample/May ionize both inorganic and organic molecules which may complicate discovery efforts Can requires extensive sample preparation due to high vacuum environment |
| LESA (ambient) | 1 mm | Pixel | Fresh sample/Low spatial resolution |
| LA-DART (ambient) | 50 microns | Pixel | Fresh sample/Low spatial resolution |
| IR-MALDESI (ambient) | 50–100 microns | Voxel | Frozen sample/Compatible with high salt content |