Robotic plasma probe ionization mass spectrometry imaging of non-plannar surfaces

EZEQUIEL MORZAN,; RACHEL V. BENNET; FACUNDO FERNANDEZ

Minneapolis

Congreso; American Society for Mass Spectrometry (ASMS) 2013; 2013

This is the first reported method for the 3D mass spectrometric imaging of irregular surfaces. Introduction Ambient ionization methods are increasingly used for mass spectrometry imaging as only minimal sample pretreatment is required. However, current imaging methods (ambient and vacuum) are generally limited to planar samples, such as thin, slide-mounted tissue slices. These methods allow creation of three-dimensional images either by removing layers of material and reimaging (e.g. laser ablation) or by stacking 2D images obtained in sequential experiments. However, no existing imaging method allows chemical probing of the surface of topographically complex, highly-curved objects. Here, we present a new MS imaging method combining acupuncture needles to remotely probe a 3D surface with a robotic arm followed by Direct Analysis in Real Time (DART) ionization to create a 3D chemical image of the sample surface. Methods A robotic arm (Lynxmotion) maneuvers to stab the surface of a sample remotely mounted outside the DART ionization region with an acupuncture needle, thus extracting material from the surface. The needle is then robotically inserted into the ionizing gas stream of the DART ion source (IonSense) allowing for desorption and ionization of the removed material. Ionic species produced at the ion source are detected using a quadrupole time-of-flight mass spectrometer (microTOF Q-II, Bruker). By systematically probing multiple points on the sample surface, the acquired mass spectra are correlated with the known x, y and z coordinates where the probe contacted the sample. MATLAB (MathWorks) is the then used to map the chemical information in three-dimensional space. Preliminary Data To demonstrate the capabilities of the proposed system, a hand-painted polystyrene hemisphere was imaged. The sphere was painted with three rows of eight equally-spaced spots of two dyes, Rhodamine 6G ([M]+, m/z 443) and Nile Blue Chloride ([M]+, m/z 318). Through manual positioning of the sample and triggering of the robot, all patches of dye on the hemisphere (20 in total) were successfully probed and sequentially analyzed. Ion intensities of the dye molecules for each time-point at which the needle was inserted into the DART ionizing stream, corresponding to a known x, y, and z coordinate on the sample, were mapped in three dimensions using MATLAB. Although a greater number of sample surface points would be required for complete surface imaging leading to more accurate contours, the experiment presented here successfully demonstrates the ability to directly sample an irregular surface using robotic plasma probe mass spectrometry. In addition to the model surfaces above described, we also present results on the distribution of pesticides, including malathion and isoproturon, that were unevenly applied to the surface of a cherry tomato sample.