Laser microdissection and atmospheric pressure chemical ionization mass spectrometry coupled for multimodal imaging

Improvement in spatial resolution of atmospheric pressure molecular chemical imaging is required to resolve distinct surface features in the low micrometer and sub-micrometer scale. Laser capture microdissection systems have the capability to focus laser light to a few micrometers. This type of system, when employed for laser ablation (LA) mass spectrometry (MS)-based chemical imaging, has the potential to achieve high spatial resolution with multimodal optical and chemical imaging capability.

A commercially available laser capture microdissection system was coupled to a modified ion source of a mass spectrometer. This design allowed for sampling of laser-ablated material via a transfer tube directly into the ionization region. Ionization of the ablated material was accomplished using atmospheric pressure chemical ionization (APCI).

Rhodamine 6G dye of red permanent marker ink in a laser etched pattern as well as cholesterol and phosphatidylcholine in a cerebellum mouse brain thin tissue section were identified and imaged from the mass spectral data. Employing a spot diameter of 8 µm using the 10× microscope cutting objective and lateral oversampling resulted in a pixel size of about 3.7 µm in the same dimension. Distinguishing between features approximately 13 µm apart in a cerebellum mouse brain thin tissue section was demonstrated in a multimodal fashion co-registering optical and mass spectral images.

A LA/APCI-MS system was developed that comprised a commercially available laser microdissection instrument for transmission geometry LA and a modestly modified ion source for secondary ionization of the ablated material. The set-up was successfully applied for multimodal imaging using the ability to co-register bright field, fluorescence and mass spectral chemical images on one platform.