Research Article Received: 16 December 2009

Accepted: 11 February 2010

Published online in Wiley Interscience:

(www.interscience.com) DOI 10.1002/jms.1729

Direct analysis of lipids in mouse brain using electrospray droplet impact/SIMS Daiki Asakawa,a Kentaro Yoshimura,b Sen Takedab and Kenzo Hiraokaa∗ Electrospray droplet impact (EDI)/secondary ion mass spectrometry (SIMS) is a new desorption/ionization technique for mass spectrometry in which highly charged water clusters produced from the atmospheric-pressure electrospray are accelerated in vacuum by 10 kV and impact the sample deposited on the metal substrate. EDI/SIMS was shown to enhance intact molecular ion formation dramatically compared to conventional SIMS. EDI/SIMS has been successfully applied to the analysis of mouse brain without any sample preparation. Five types of lipids, i.e. phosphatidylcholine (PC), phosphatidylserine, phosphatidylinositol (PI), galactocerebroside (GC) and sulfatide (ST), were readily detected from mouse brain section. In addition, by EDI/SIMS, six different regions of the mouse brain (cerebral cortex, corpus callosum, striatum, medulla oblongata, cerebellar cortex and cerebellar medulla) were examined. While GCs and STs were found to be rich in white matter, PIs were rich in gray matter. c 2010 John Wiley & Sons, Ltd. Copyright
Keywords: EDI/SIMS; MALDI; direct analysis; mouse brain; lipids

Introduction Mass spectrometry is a powerful analytical tool in a wide variety of scientific fields because of its high sensitivity and ability to determine molecular structures. In particular, electrospray ionization (ESI)[1] and matrix-assisted laser desorption/ionization (MALDI)[2,3] are important soft ionization methods. Furthermore, MALDI-MS techniques have been developed for the analysis of biological tissues. MALDI-MS achieves mass spectral profiling, and visualizing the distribution of many kinds of compounds, such as proteins,[4,5] lipids,[6,7] drugs and their metabolites.[8,9] In the MALDI imaging, sample surface is scanned by focused laser, and the recorded mass spectra of different sampling locations are used to construct images that show concentration profiles for analyte ions. However, even in the optimized MALDI instrument, the diameter of laser spot is 50 µm,[10] which is insufficient for imaging of cells. Recently, MALDI microscope imaging instrument has been reported.[11,12] In this method, the ions pass through the timeof-flight mass spectrometer forming an ion optical image on a position sensitive detector. Using mass spectrometric microscope, lateral resolution do not depend on the laser spot size. However, in the MALDI analysis of biological tissues, matrix must be applied in solution as a spray. The dissolvation of analytes into the matrix droplet will limit the achievable lateral resolution of MALDI imaging. In fact, lateral resolution of MALDI image is easily influenced by the sample preparation methods.[13,14] The secondary ion mass spectrometry (SIMS) is also a powerful technique for structural characterization and imaging for various kinds of samples, including biological tissues.[15 – 18] The most popular primary ion source is the liquid metal ion source, which can produce a metal ion beam, such as Ga+ . SIMS image was obtained by scanning the sample through the focused primary ion beam. SIMS has the advantage of very high spatial resolution (