Molecular imaging using mass spectrometry (MS)[1-4]
is rapidly gaining momentum as a powerful technique for chemical characterization of biological materials and real-time identification of tissues in biological and clinical applications.[5-10]
Imaging MS is attractive because of its high sensitivity, speed of analysis, and unprecedented chemical specificity that enables simultaneous detection and identification of a broad range of complex molecules in biological samples.
Spatially localized sampling in MS is traditionally achieved using secondary ion mass spectrometry (SIMS),[12-17]
matrix-assisted laser desorption ionization (MALDI)[1, 5-7, 10]
or other laser-based ionization techniques.
Despite its success, MALDI imaging requires sample pre-treatment and matrix deposition prior to analysis. In addition, experiments typically are performed in vacuum, which may affect the spatial distribution of the analyte molecules in the sample. Ambient pressure surface ionization methods have been developed to overcome these limitations.[3, 18-19]
Ambient imaging MS of biological tissues with a spatial resolution of 100-500 μm has been performed using desorption electrospray ionization (DESI),[20-22]
laser ablation electrospray ionization mass spectrometry (LAESI),[23-24]
and infrared laser ablation metastable-induced chemical ionization.
Recently, imaging MS with spatial resolution of 20 μm was achieved using matrix-assisted laser ionization in transmission geometry.
High spatial resolution (~10 μm) was achieved using scanning microprobe atmospheric pressure MALDI, which enabled the first ambient, MALDI-based histology imaging.
However, the application of a matrix was recognized as a principal limitation of this approach. Similar spatial resolution of was reported for tissue imaging using matrix-free nanostructure initiator mass spectrometry (NIMS).
We have developed a nanospray desorption electrospray ionization (nano-DESI) technique[29-30]
capable of achieving fast, spatially resolved analysis of complex mixtures of soluble organic and biological molecules on substrates without sample pre-treatment. Similar to DESI
and liquid microjunction surface sampling probe (LMJ-SSP),
this technique utilizes liquid extraction and ionization of an analyte deposited on a substrate. Here, we demonstrate the first application of nano-DESI to the ambient imaging of biological tissues.
shows schematics of the nano-DESI source; a photograph of the probe taken during one of the tissue-imaging experiments is shown in . Nano-DESI analysis is performed by bringing a liquid bridge formed between two glass capillaries in contact with analyte deposited on a substarate. The primary capillary maintains the solvent flow to the liquid bridge, while the nanospray capillary removes the solvent containing the dissolved analyte from the bridge and generates charged droplets at the mass spectrometer inlet through a self-aspirating nanospray.
This geometry provides an important advantage for imaging applications over the coaxial geometry utilized in the LMJ-SSP technique.
Specifically, it enables exquisite control of the size of the liquid bridge between the capillaries that can be varied from several milimiters for some applications that require sampling from larger areas on a surface and made smaller than 10 μm for high-resolution imaging experiments. shows a trace on a rhodamine film left by the nano-DESI probe operated in the imaging mode. The width of the white line in the figure representing the sampled area in less than 8 μm indicating the potential of this technique for high-resolution imaging applications. The size of the liquid bridge is determined by the solvent flow rate, as well as the size and relative position of the capillaries. Previously, we showed nano-DESI's ability to sample an analyte from a small area (<100 μm in diameter) on a surface with little or no hysteresis.
In this study, we demonstrate that with proper positioning of the capillaries, nano-DESI enables ambient imaging of tissue samples with a spatial resolution better than 12 μm without sample pretreatment.
Figure 1 a) Schematic drawing of the nanoDESI ion source and b) photograph of the nanoDESI probe taken during ambient imaging of a tissue sample on a glass slide (note that the liquid bridge is not visible to the eye); c) optical image of a trace left by the nanoDESI (more ...)