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Technical Note
Ammonium sulfate improves detection of hydrophilic quaternary ammonium compounds through decreased ion suppression in matrixassisted laser desorption/ionization imaging mass spectrometry Eiji Sugiyama, Noritaka Masaki, Shoko Matsushita, and Mitsutoshi Setou Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b02672 • Publication Date (Web): 22 Oct 2015 Downloaded from http://pubs.acs.org on October 23, 2015
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Analytical Chemistry
Ammonium sulfate improves detection of hydrophilic quaternary ammonium compounds through decreased ion suppression in matrix-assisted laser desorption/ionization imaging mass spectrometry
Eiji Sugiyama, Noritaka Masaki, Shoko Matsushita and Mitsutoshi Setou*
Department of Cell Biology and Anatomy, Hamamatsu University School of Medicine, Hamamatsu, Japan
*Corresponding author: Mitsutoshi Setou, Department of Cell Biology and Anatomy, Hamamatsu University School of Medicine, 1-20-1 Handayama, Higashi-ku Hamamatsu, Shizuoka 431-3192, Japan. Fax: +81- 53-435-2473. E-mail:
[email protected] 1
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ABSTRACT Hydrophilic quaternary ammonium compounds (QACs) include derivatives of carnitine (Car) or choline, which are known to have essential bioactivities. Here we developed a technique for improving the detection of hydrophilic QACs using ammonium sulfate (AS) in matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS). In MALDI mass spectrometry for brain homogenates, the addition of AS greatly increased the signal intensities of Car, acetylcarnitine (AcCar), and glycerophosphocholine (GPC) by approx. 300-, 700- and 2500-fold. The marked improvement required a higher AS concentration than that needed for suppressing the potassium adduction on phosphatidylcholine and 2,5-dihydroxybenzoic acid. Adding AS also increased the signal intensities of Car, AcCar, and GPC by approx. 10-, 20-, and 40-fold in MALDI-IMS. Consequently, the distributions of five hydrophilic QACs (Car, AcCar, GPC, choline, and phosphocholine) were simultaneously visualized by this technique. The distinct mechanism from other techniques such as improved matrix application, derivatization, or post-ionization suggests the great potential of AS addition to achieve higher sensitivity of MALDI-IMS for various analytes.
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Analytical Chemistry
INTRODUCTION Hydrophilic quaternary ammonium compounds (QACs) include derivatives of carnitine (Car) or choline, which are known to have essential bioactivities for energy production, lipid metabolism, and neurotransmission.1-5 The metabolisms of those molecules are highly controlled by several transporters, and thus the gene deletion of those transporters leads directly to lethal diseases such as systemic carnitine deficiency3 or respiratory failure due to perturbations of cholinergic signaling.6 Moreover, the metabolisms of these molecules are closely associated.5,7 For instance, a recent study suggested that therapeutic hypothermia provides its effect through a metabolic change resulting in a decrease of acetylcholine and an increase of Car in the same area in the brain.5 These insights invoke the need for simultaneous analyses of the distribution of these molecules for the elucidation of their significant changes in various biological conditions. Several studies have visualized the distribution of individual hydrophilic QACs using
matrix-assisted
laser
desorption/ionization
imaging
mass
spectrometry
(MALDI-IMS), which provides label-free simultaneous analyses of multiple molecular distributions on tissue.8,9 However, the sensitivity of MALDI-IMS for examining hydrophilic QACs is still not satisfactory. For instance, the signal obtained from 3
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acetylcarnitine (AcCar) in the brain is quite low and the signal-to-noise ratio (S/N) reported by Pirman et al. is not adequate for analyzing its metabolic change.10,11 Moreover, further high sensitivity will be needed for the detection of small amounts of analytes at high spatial resolution (
