Atmospheric pressure matrix-assisted laser desorption ionization (AP-MALDI) method has been used for imaging of biological samples. The sampling protocol of AP-MALDI method involves heating of the samples to improve desorption and ionization of volatile organic compounds, which damages the biological samples and the limits the spatial resolution of the acquired images. Researchers at Department of New Biology, Companion Diagnostics and Medical Technology Research Group, DGIST, Daegu, Republic of Korea, Graduate School of Analytical Science and Technology (GRAST), Chungnam National University, Daejeon, Republic of Korea and KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul, Republic of Korea have reported a high spatial resolution atmospheric pressure mass spectrometric imaging method (referred as AP-nanoPALDI MS) for subcellular imaging of living biological tissue slices with a sampling depth down to several tens of μm.
The AP-nanoPALDI MS system used for high-resolution imaging of biological samples consists of a mass analyzer, a sampling stage, a femtosecond (fs) laser oscillator, an AP plasma equipment, and airflow-assisted ion transport equipment. The schematic of the AP-nanoPALDI MS system and imaging device are shown in Fig. 1.
Fig. 1 Schematic of the AP-nanoPALDI mass spectrometric analysis using a combination of fs laser oscillator, atmospheric pressure plasma jet, airflow-assisted ion transfer equipment and imaging systems
The focused fs laser enables desorption of the neutral molecules while the non-thermal AP helium plasma jet device helps to maintain a helium plasma medium above the sample, thus promoting desorption and ionization of the sample. The airflow-assisted ion transfer tube facilitates effective transport of molecules and ions to the mass analyzer. Since biological samples do not effectively absorb the NIR light, rod-shaped gold nanorods modified by polyethylene glycol (mPEG-AuNRs) are embedded inside the tissues, which served as hot spots, rapidly converts the absorbed photon energy into thermal energy, promotes absorption of the NIR light, enhanced the extent of desorption of neutral molecules, increased the intensity of the mass spectra and favours acquisition of high-quality MS images. Analysis of MS images of a mouse hippocampal tissue slice indicates that most of the strong ion signals are under m/z = 500 and the detected ions can be assigned to particular lipids and metabolites such as adenine, cholesterol and monoacylglycerol ions.
Fig. 2 (a) Optical images and (b, c) mass spectrometric images of a mouse hippocampal tissue slice
In spite of the continuous irradiation with a large number of laser shots, the use of mPEG-AuNRs and non-thermal AP plasma jet as post-ionization source prevents thermal damage of the biological specimens. In the absence of AuNRs, mass spectra could not be recorded. Since the images acquired from live tissue slices provide plenty of spatial information for metabolites, the AP-nanoPALDI MS method can also be used for tissue-based drug screening, which is evidenced by a decrease in cholesterol level in hippocampus tissues treated with methyl β-cyclodextrin. The AP nanoPALDI MS imaging technique can be applied for label-free bioimaging applications and tissue-based drug screening.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Jae Young Kim et al., Atmospheric pressure mass spectrometric imaging of live hippocampal tissue slices with subcellular spatial resolution, Nature Communications, 8 (2017) 2113, DOI: 10.1038/s41467-017-02216-6
Recent Advancements in Science 