Mass spectrometry imaging (MSI) was introduced more than five decades ago with secondary ion mass spectrometry (SIMS) and a decade later with laser desorption/ionization (LDI) mass spectrometry (MS). Large biomolecule imaging by matrix-assisted laser desorption/ionization (MALDI) was developed in the 1990s and ambient laser MS a decade ago. Although SIMS has been capable of imaging with a moderate mass range at sub-micrometer lateral resolution from its inception, laser MS requires additional effort to achieve a lateral resolution of 10 lm or below which is required to image at the size scale of single mammalian cells. This review covers untargeted large biomolecule MSI using lasers for desorption/ionization or laser desorption and post-ionization. These methods include laser microprobe (LDI) MSI, MALDI MSI, laser ambient and atmospheric pressure MSI, and near-field laser ablation MS. Novel approaches to improving lateral resolution are discussed, including oversampling, beam shaping, transmission geometry, reflective and through-hole objectives, microscope mode, and near-field optics.
We have developed a mid-infrared laser ablation sampling technique for nano-flow liquid chromatography coupled with tandem mass spectrometry proteomic profiling of discrete regions from biological samples. Laser ablation performed in transmission geometry was used to transfer material from 50-μm thick tissue sections mounted on a glass microscope slide to a capturing solvent. Captured samples were processed using filter-aided sample preparation and enzymatically digested to produce tryptic peptides for data-dependent analysis with an ion trap mass spectrometer. Comparison with ultraviolet laser capture microdissec- tion from neighboring regions on the same tissue section revealed that infrared laser ablation transfer has higher reproducibility between samples from different consecutive sections. Both techniques allowed for proteomics investigation of different orga- nelles without the addition of surfactants.
F. Donnarumma, F. Cao, & K. K. Murray, J. Am. Soc. Mass Spectrom. (2015) DOI: 10.1007/s13361-015-1249-0
We have developed a laser ablation sampling technique for matrix-assisted laser desorption ionization (MALDI) mass spectrometry and tandem mass spectrometry (MS/MS) analyses of in-situ digested tissue proteins. Infrared laser ablation was used to remove biomolecules from tissue sections for collection by vacuum capture and analysis by MALDI. Ablation and transfer of compounds from tissue removes biomolecules from the tissue and allows further analysis of the collected material to facilitate their identification. Laser ablated material was captured in a vacuum aspirated pipette-tip packed with C18 stationary phase and the captured material was dissolved, eluted, and analyzed by MALDI. Rat brain and lung tissue sections 10 μm thick were processed by in-situ trypsin digestion after lipid and salt removal. The tryptic peptides were ablated with a focused mid-infrared laser, vacuum captured, and eluted with an acetonitrile/water mixture. Eluted components were deposited on a MALDI target and mixed with matrix for mass spectrometry analysis. Initial experiments were conducted with peptide and protein standards for evaluation of transfer efficiency: a transfer efficiency of 16% was obtained using seven different standards. Laser ablation vacuum capture was applied to freshly digested tissue sections and compared with sections processed with conventional MALDI imaging. A greater signal intensity and lower background was observed in comparison with the conventional MALDI analysis. Tandem time-of-flight MALDI mass spectrometry was used for compound identification in the tissue.
C. A. Seneviratne, S. Ghorai & K. K. Murray, Rapid Commun. Mass Spectrom. in press
RATIONALE: Ambient mass spectrometry can detect small molecules directly, but complex mixtures can be a challenge. We have developed a method that incorporates small molecule separation based on laser desorption with 80 capture on a solid-phase microextraction (SPME) fiber for injection into a gas chromatography/mass spectrometry 81 (GCMS) system.
METHODS: Samples on a metal target were desorbed by a 3 μm mid-infrared laser focused to a 250 μm spot and 1.2 mJ pulse energy. The desorbed material was aspirated into a metal tube suspended 1 mm above the laser spot and captured 84 on a SPME fiber. The collected material was injected into a GC/MS instrument for analysis.
RESULTS: We have developed a versatile approach for ambient laser desorption sampling onto SPME for GC/MS analysis. The performance of the laser desorption SPME capture GC/MS system was demonstrated for small molecule 87 standards, a mixture of nitroaromatic explosives, and collected cigarette smoke.
CONCLUSIONS: The utility of ambient laser desorption sampling onto SPME for GC/MS was demonstrated. The performance of the method was evaluated by preparing calibration standards of caffeine over a range from 200 to 90 1000 ng. Laser desorption ambient sampling of complex mixtures was accomplished using SPME GC/MS.