Elemental Bioimaging by Means of Fast Scanning Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry

One of the most common setups for elemental bioimaging, the hyphenation of a laser ablation (LA) system and an inductively coupled plasma mass spectrometer (ICP-MS), was expanded by adding full scan mass spectrometric information as another dimension of information. While most studies deal with the analysis of typically not more than up to 10 isotopes per scan cycle, a fast scanning quadrupole mass analyzer was utilized to record the full mass spectrum of interest in this work. Mass-to-charge ratios from 6 to 250 were observed within one cycle. Besides the x- and y-position on the ablated sample and the intensity, the m/z-ratio served as fourth variable for each pixel of the obtained data, closing thereby the gap between "inorganic" and "organic" mass spectrometric imaging techniques. The benefits of this approach include an improved control of interferences, the discovery of unexpected elemental distributions, the possibility to plot isotopic ratios, and to integrate the intensities of a certain number of mass channels recorded for each isotope, thus virtually increasing sensitivity. The respective data are presented for dried droplets as well as embedded animal and human tissue slices. Limits of detection were calculated and found to be in accordance with counting statistics. A dedicated software macro was developed for data manipulation prior to common evaluation and image creation.

Laser ablation based bioimaging with simultaneous elemental and molecular mass spectrometry: towards spatially resolved speciation analysis

RATIONALE

Biological functions of metals are not only specified by the element itself, but also by its chemical form and by its organ, cell and subcellular location. The developed laser ablation based setup enables spatially resolved analysis with simultaneous elemental and molecular mass spectrometry (MS) and promises therefore localization, identification and quantification of metal or heteroelement-containing species in biological samples such as tissue sections.

METHODS

A UV laser ablation (LA) system is hyphenated in parallel both with an elemental and a molecular mass spectrometer via flow splitted transfer lines to simultaneously obtain data from both of the mass spectrometers. Elemental MS was performed using inductively coupled plasma (ICP)-MS, whereas atmospheric pressure chemical ionization (APCI)-MS with an orbitrap mass analyzer was utilized for molecular MS.

RESULTS

Simultaneous elemental and molecular MS imaging with high lateral resolution down to 25 µm was presented for the staining agents eosin Y and haematoxylin as well as for the chemotherapy drug cisplatin in thin tissue sections. For molecular MS, target compounds were identified by their exact masses and by characteristic fragment ions.

CONCLUSIONS

The first simultaneous elemental and molecular MS imaging approach based on laser ablation sampling was introduced for spatially resolved speciation analysis. The combination of the advantages of LA-ICP-MS such as low detection limits and high spatial resolution with information on the chemical identity promises not only localization of metals, but also identification of metal species in biological samples. Therefore, this novel technique opens up new possibilities to address complex challenges in life science research.

Ambient molecular imaging by laser ablation atmospheric pressure chemical ionization mass spectrometry

RATIONALE

The functions and properties of compounds are not only specified by their chemical structures, but also by their location inside a sample. Mass spectrometry is a powerful tool for imaging, whereby the kind of sample and compound depend on the used sampling and ionization methods. The developed laser ablation mass spectrometry method delivers high resolution images of small molecules in native samples.

METHODS

A UV laser ablation (LA) system was combined with an atmospheric pressure chemical ionization (APCI) mass spectrometer. The spatially resolved sampling was performed by focusing the 213 nm laser beam onto a sample. The fine aerosol generated by the ns pulsed laser irradiation was then transported to the APCI mass spectrometer by a nitrogen stream. In the APCI source, post-ionization was accomplished by a corona discharge. The resulting ions were detected with an orbitrap mass spectrometer.

RESULTS

The properties of the novel LA-APCI-MS setup are demonstrated by spatially resolved analysis of several samples including tablets, TLC plates and dried droplets. The target compounds are detected with high spatial and mass resolution. For higher molecular weight compounds like thyroxine, fragmentation was observed, whereas small molecules like caffeine stayed intact.

CONCLUSIONS

LA-APCI-MS is introduced as an ambient molecular mass spectral imaging method for molecules with high resolution in space and mass. The combination of two independent instruments offers flexible ion source and mass analyzer exchange and therefore LA-APCI-MS opens up new possibilities for molecular imaging under ambient conditions.