PE(14:0/PGE2)

All MSI experiments were performed on a hybrid linear ion trap 21 T FT-ICR mass spectrometer at the National High Magnetic Field Laboratory (NHMFL) at Florida State University (Tallahassee, FL). A Velos Pro linear ion trap (Thermo Scientific, San Jose, CA) was combined with NHMFL-designed external linear quadrupole ion trap, quadrupole ion transfer optics and a novel dynamically harmonized ICR cell, which is operated at 7.5 V trapping potential[1]. Briefly, the cell uses 120° cell segments for ion excitation and detection, for improved excitation electric field, detection sensitivity and reduced third harmonic signals[2][3]. The commercial ion source and stacked ring ion guide were replaced with an elevated-pressure MALDI ion source incorporating a dual-ion funnel interface (Spectroglyph LLC, Kennewick, WA) as has been described previously[4]. Voltages within the funnels were 625 kHz, 150 V peak-to-peak (first, high-pressure ion funnel) and 1.2 MHz, 90 V peak-to-peak (second, low-pressure ion funnel). An electric field gradient of ∼10 V/cm was maintained within the dual-funnel system, with a gradient of 100 V/cm between the sample and the funnel inlet. The system was equipped with a Q-switched, frequency-tripled Nd:YLF laser emitting 349 nm light (Explorer One, Spectra Physics, Mountain View, CA). The laser was operated at a repetition rate of 1 kHz and pulse energy of ∼1.2 μJ. Pressure within the ion source was set to 10 mbar in the first ion funnel and 2 mbar in the second ion funnel. MALDI stage motion was synchronized with ion accumulation using the Velos trigger signal indicating commencement of the ion trap injection event, as previously described[4]. The mass spectrometer was operated with an ion injection time of 250 ms and automatic gain control (AGC) was turned off. A transient duration of 3.1 s was used for ultrahigh mass resolving power analyses, resulting in a total time of 4s per pixel. Spectra were obtained in both positive and negative mode, at 100 μm spatial resolution. Total number of pixels per brain section were approximately 22 000 and 24 h of experimental time. A Predator data station was used for ion excitation and detection[5]. Refs: [1] Hendrickson CL, Quinn JP, Kaiser NK, Smith DF, Blakney GT, Chen T, Marshall AG, Weisbrod CR, Beu SC. 21 Tesla Fourier Transform Ion Cyclotron Resonance Mass Spectrometer: A National Resource for Ultrahigh Resolution Mass Analysis. J Am Soc Mass Spectrom. 2015 Sep;26(9):1626-32. doi:10.1007/s13361-015-1182-2. Epub 2015 Jun 20. PMID:26091892. [2] Hendrickson CL, Beu SC, Blakney GT, Kaiser NK, McIntosh DG, Quinn JP, Marshall AG. In Optimized cell geometry for Fourier transform ion cyclotron resonance mass spectrometry, Proceedings of the 57th ASMS Conference on Mass Spectrometry and Allied Topics, Philadelphia, PA, May 31 to June 4; Philadelphia, PA, 2009. [3] Chen T, Beu SC, Kaiser NK, Hendrickson CL. Note: Optimized circuit for excitation and detection with one pair of electrodes for improved Fourier transform ion cyclotron resonance mass spectrometry. Rev Sci Instrum. 2014 Jun;85(6):066107. doi:10.1063/1.4883179. PMID:24985871. [4] Belov ME, Ellis SR, Dilillo M, Paine MRL, Danielson WF, Anderson GA, de Graaf EL, Eijkel GB, Heeren RMA, McDonnell LA. Design and Performance of a Novel Interface for Combined Matrix-Assisted Laser Desorption Ionization at Elevated Pressure and Electrospray Ionization with Orbitrap Mass Spectrometry. Anal Chem. 2017 Jul 18;89(14):7493-7501. doi:10.1021/acs.analchem.7b01168. Epub 2017 Jun 28. PMID:28613836. [5] Blakney GT, Hendrickson CL, Marshall AG. Predator data station: A fast data acquisition system for advanced FT-ICR MS experiments. Int. J. Mass Spectrom. 2011;306 (2-3), 246- 252. doi:10.1016/j.ijms.2011.03.009.

The human colorectal adenocarcinoma sample was excised during a surgical operation performed at the Imperial College Healthcare NHS Trust. The sample and procedures were carried out in accordance with ethical approval (14/EE/0024).