Imaging Analysis of Phosphatidylcholines and Diacylglycerols Using Surface-Assisted Laser Desorption/Ionization Mass Spectrometry with Metal Film Formed by Mist Chemical Vapor Deposition

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) has been widely used for analyses of biomolecules and industrial materials. Surface-assisted laser desorption/ionization (SALDI) is studied to complement the ionization ability for the MALDI/MS. In this study, lab-made mist chemical vapor deposition (mist CVD) system was used to produce metal films as ionization assistance materials for SALDI/MS. The system could give Ag film from inexpensive silver trifluoroacetate solution rapidly and simply under atmospheric pressure. Phosphatidylcholines could be detected high sensitively and diacylglycerols (DAGs) could not be detected in MALDI/MS. In the SALDI/MS and the MS imaging with Ag film by mist CVD, both the phosphatidylcholines and the DAGs could be detected and the localized images. In the Ag film-SALDI/MS of lipids, not only Ag-adducted ions but also Na- and K-adducted ions were detected. The Ag film formed by the mist CVD to act as an ionization-assistance material and a cationization agent in SALDI would be useful in MS imaging of biological tissue sections.

Improvement in Ionization Efficiency Using Metal Oxide Nanoparticles in Laser Desorption/Ionization Mass Spectrometry of a Cancer Drug

Mass spectrometry imaging (MSI) without labeling has the potential for faster screening in drug development. Matrix-assisted laser desorption/ionization (MALDI) is typically used, but it has a large matrix size and uneven drug distribution. Surface-assisted laser desorption/ionization (SALDI) using nanoparticles (NPs) may overcome these issues. Here, the influence of NPs, solvent ratio, and order of dropping of NPs on SALDI-MSI of protoporphyrin IX (PpIX), a cancer drug, are reported. A solution of PpIX in a 50% aqueous solution of 50% acetonitrile at a concentration of 10 μM was used. The NPs include ZnO, Fe₃O₄, and four types of TiO₂. The NPs were fabricated by dissolving them on an aqueous 90% acetonitrile solution. Mass spectra were obtained with a time-of-flight mass spectrometer using a Nd:YAG laser at a 355-nm wavelength. The signal intensity using TiO₂ at a 0.5 mg/mL concentration in 50% acetonitrile was increased by 1.6-fold compared to that without TiO₂. Changing the solvent to 90% acetonitrile gave a uniform TiO₂ distribution and a 9-fold increase in the signal intensity for PpIX. Among the four types of TiO₂ with different particle sizes and crystal structures, TiO₂ with a smaller particle size and a rutile crystal structure produced the highest signal intensity. Forming a layer on top of the PpIX also resulted in an increased signal intensity. Hence, SALDI using TiO₂ provides effective ionization of the drug. In the future, we plan to investigate a spray method for the ionization of PpIX using TiO₂ for the MSI of various drugs.

UV-Absorbing Ligand Capped Gold Nanoparticles for the SALDI-MS Analysis of Small Molecules

We report that modifying the surface of gold nanoparticles (Au NPs) with 2-mercaptopyridine-3-carboxylic acid (MPyCA) enhances surface-assisted laser desorption/ionization (SALDI) performance in the analysis of small molecules. The MPyCA ligand has a strong UV absorbance at the wavelengths of the typical MALDI laser at 337 nm, resulting in efficient thermal/energy transfer from the Au NPs to analytes during pulse-laser irradiation. In addition, the MPyCA ligand contains carboxylic acid and pyridine groups, providing affinity to various analytes through acid-base interactions. Irganox1010, glucose and meropenem were utilized as model analytes to evaluate SALDI performance because these molecules are generally ionized with difficulty by conventional MALDI-MS. Our results demonstrate that the MPyCA-Au NP based SALDI-MS could detect Irganox1010, glucose and meropenem with stronger ion peaks for these molecules compared to MALDI-MS using CHCA. The limit of detection (LOD) for meropenem was much lower in the case of SALDI (LOD=1 ng/mL) compared to MALDI (LOD=10 μg/mL).

Imaging of Neurotransmitters and Small Molecules in Brain Tissues Using Laser Desorption/Ionization Mass Spectrometry Assisted with Zinc Oxide Nanoparticles

Inorganic nanostructured materials such as silicon, carbon, metals, and metal oxides have been explored as matrices of low-background signals to assist the laser desorption/ionization (LDI) mass spectrometric (MS) analysis of small molecules, but their applications for imaging of small molecules in biological tissues remain limited in the literature. Titanium dioxide is one of the known nanoparticles (NP) that can effectively assist LDI MS imaging of low molecular weight molecules (LMWM). TiO₂ NP is commercially available as dispersions, which can be applied using a chemical solution sprayer. However, aggregation of NP can occur in the dispersions, and the aggregated NP can slowly clog the sprayer nozzle. In this work, the use of zinc oxide (ZnO) NP for LDI MS imaging is investigated as a superior alternative due to its dissolution in acidic pH. ZnO NP was found to deliver similar or better results in the imaging of LMWM in comparison to TiO₂ NP. The regular acid washes were effective in minimizing clogging and maintaining high reproducibility. High-quality images of mouse sagittal and rat coronal tissue sections were obtained. Ions were detected predominately as Na⁺ or K⁺ adducts in the positive ion mode. The number of LMWM detected with ZnO NP was similar to that obtained with TiO₂ NP, and only a small degree of specificity was observed.

Interpol review of controlled substances 2016-2019

This review paper covers the forensic-relevant literature in controlled substances from 2016 to 2019 as a part of the 19th Interpol International Forensic Science Managers Symposium. The review papers are also available at the Interpol website at: https://www.interpol.int/content/download/14458/file/Interpol%20Review%20Papers%202019.pdf.