Ambient Plasma Ionization/Imaging with Robotic 3D Microprobes and Dynamic Desorption Gradients

FACUNDO M. FERNÁNDEZ; RACHEL V. BENNET; EZEQUIEL M. MORZAN; JACOB O. HUCKABY; MARÍA EUGENIA MONGE; HENRICK I. CHRISTENSEN

Dallas

Congreso; 247th ACS National Meeting & Exposition, Nobel Laureate Signature Award for Graduate Education in Chemistry: Symposium in Honor of Livia S. Eberlin and R. Graham Cooks; 2014

American Chemical Society

Ambient ionization techniques are increasingly being employed for Mass Spectrometry Imaging (MSI) given their fast measurement times and minimal sample pretreatment, but are currently still limited to planar surfaces. In this work we introduce a method of imaging complex or irregularly-shaped surfaces in the open air . A machine vision system-controlled robotic arm that holds an acupuncture needle pokes the surface at given x,y,z coordinates thus probing the sample, which is then introduced into an ambient DC plasma for subsequent MS detection. The precision and sophistication of the robotic arm allow for accurate sampling (x,y,z) coordinates to be recorded. A 3-D surface chemical image can then be created by mapping the recorded MS data with the spatial coordinates. This system has successfully been used to image simple model surfaces, such as a hemisphere painted with two dyes, as well as the detection of pesticide on cherry tomato surfaces. A dynamic sample introduction approach to plasma probe ambient ionization is also presented. In contrast to the classical static analysis (leaving the probe at a fixed location in front of the plasma source), this procedure is based on programing a dynamic probe trajectory in the gap between the ion source outlet and mass spectrometer inlet to exploit the thermal and flow gradients generated by the plasma dynamics. In doing so, different analytes with different vapor pressure are differentially desorbed obtaining a transient characteristic desorption profile for each one. By comparing the profiles of target ions with other co-aligned peaks, correlations can be made between precursor and fragment ion pairs. This method can then be used to identify structurally-useful fragments produced during the ion generation process.