Simple Pretreatment for the Analysis of Additives and Polymers by Surface-Assisted Laser Desorption/Ionization Mass Spectrometry Using a Through-Hole Alumina Membrane as a Functional Substrate

Quantitative analysis capability of DIUTHAME mass spectrometry verified by acetylcholinesterase enzyme-catalyzed reaction assays

Matrix-assisted laser desorption/ionization (MALDI) is advantageous for mass spectrometry applications where throughput is important. However, quantitative analysis is essentially problematic for MALDI-MS whose results depend on the intrinsically stochastic microcrystalline state of the matrix. High-throughput screening (HTS) of drug candidates is a typical example that requires high throughput. The application of MALDI-MS to HTS, which is quantitative analysis, imposes restrictions on designing an experimental system. Surface-assisted laser desorption/ionization (SALDI) methods, which do not depend on matrix crystal formation, are expected to be applied to quantitative analyses such as HTS. A recently developed one type of SALDI methods, desorption ionization using through hole alumina membrane (DIUTHAME), possesses a distinct feature that the surface microstructure effective for ionization is formed by through holes. In this study, the quantitative analysis capability of DIUTHAME was verified by applying DIUTHAME to enzyme-catalyzed reaction measurements, which are also used for HTS. Quantitative DIUTHAME-MS was conducted on various conditions of acetylcholinesterase-catalyzed reaction solutions containing cow milk as a substitute of biological media. Even for the enzyme-catalyzed reaction solutions containing complex additives that make the quantitative analysis extremely difficult, DIUTHAME based on the through hole structure enables quantitative measurements of the analytes by applying the reaction solutions to the back side of the laser exposed surface. In comparison with those obtained by MALDI-MS, the results obtained by DIUTHAME-MS showed less variability of data and delivered a better linearity of the Lineweaver-Burk plots and a more reasonable value of the Michaelis constant. Accordingly, it was demonstrated that DIUTHAME-MS possesses the quantitative analysis capability much better than that of MALDI-MS.

Matrix-Free High-Resolution Atmospheric-Pressure SALDI Mass Spectrometry Imaging of Biological Samples Using Nanostructured DIUTHAME Membranes

Applications of mass spectrometry imaging (MSI), especially matrix-assisted laser desorption/ionization (MALDI) in the life sciences are becoming increasingly focused on single cell analysis. With the latest instrumental developments, pixel sizes in the micrometer range can be obtained, leading to challenges in matrix application, where imperfections or inhomogeneities in the matrix layer can lead to misinterpretation of MS images. Thereby, the application of premanufactured, homogeneous ionization-assisting devices is a promising approach. Tissue sections were investigated using a matrix-free imaging technique (Desorption Ionization Using Through-Hole Alumina Membrane, DIUTHAME) based on premanufactured nanostructured membranes to be deposited on top of a tissue section, in comparison to the spray-coating of an organic matrix in a MALDI MSI approach. Atmospheric pressure MALDI MSI ion sources were coupled to orbital trapping mass spectrometers. MS signals obtained by the different ionization techniques were annotated using accurate-mass-based database research. Compared to MALDI MSI, DIUTHAME MS images captivated with higher signal homogeneities, higher contrast and reduced background signals, while signal intensities were reduced by about one order of magnitude, independent of analyte class. DIUTHAME membranes, being applicable only on tissue sections thicker than 50 µm, were successfully used for mammal, insect and plant tissue with a high lateral resolution down to 5 µm.

MoS2-Assisted LDI Mass Spectrometry for the Detection of Small Molecules and Quantitative Analysis of Sulfonamides in Serum

A two-dimensional MoS₂ nanosheet was prepared by a chemical exfoliation method and served as an excellent matrix for the detection of small molecules by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). In comparison with organic matrices (CHCA, 3-AQ) and a graphene matrix, we found that a MoS₂ matrix showed better performance in analysis of amino acids, peptides, fatty acids, and sulfonamides. A systematic comparison of the MoS₂ matrix with both ion modes showed that mass spectra produced in negative ion mode featured a corresponding single deprotonated ion, which was rather different from the complex multiple alkali metal addition peaks present in positive ion mode. In addition, better sensitivity and reproducibility were obtained in negative ion mode. The ionization mechanism of MoS₂ as a matrix in negative ion mode was further discussed. The deproton peak intensity of the analyte fatty acid decreased after the addition of the hole-scavenger KSCN, indicating that the ionization of the fatty acid was caused by the Auger complex effect of MoS₂ and electron injection. Experiments have shown that the MoS₂ matrix detects small molecules with good repeatability and can perform semiquantitative analysis of sulfonamides. Finally, the MoS₂ matrix was employed for quantitative determination of sulfamethoxine in serum samples by an internal standard method. This MoS₂-assisted laser desorption/ionization mass spectrometry (MoS₂-assisted LDI MS) method provides a simple, rapid, high-throughput approach to evaluate the drug levels in the patient serum and can achieve convenient drug therapy monitoring.

Surface-assisted Laser Desorption/ionization Mass Spectrometry Analysis of the Glycolipid Biosurfactants, Mannosylerythritol Lipids, Using an Ionization-assisting Substrate

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is a promising tool for the screening of glycolipid-type biosurfactants (BSs) from a crude extract of microbial products. However, it is unsuitable for the detection of lower molecular weight products because the observed ions are overlapped with matrix-derived ions at lower mass range. In this study, we applied a "matrix-free" surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) analysis using a through-hole alumina membrane as an ionization-assisting substrate. Using this method, we could detect a variety of lower molecular weight products in an extract of a glycolipid BS producer with good sensitivity. In addition, the culture solution could be analyzed directly by this method.

Desorption ionization using through-hole alumina membrane offers higher reproducibility than 2,5-dihydroxybenzoic acid, a widely used matrix in Fourier transform ion cyclotron resonance mass spectrometry imaging analysis

RATIONALE

DIUTHAME (desorption ionization using through-hole alumina membrane), a recently developed matrix-free ionization-assisting substrate, was examined for reproducibility in terms of mass accuracy and intensity using standard lipid and mouse brain sections. The impregnation property of DIUTHAME significantly improved the reproducibility of mass accuracy and intensity compared with 2,5-dihydroxybenzoic acid (DHB).

METHODS

Frozen tissue sections were mounted on indium tin oxide-coated glass slides. DIUTHAME and DHB were applied to individual sections. Subsequently, a solution of a phosphatidylcholine standard, PC(18:2/18:2), was poured onto the DIUTHAME and matrix. Finally, the samples were subjected to laser desorption ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry. The reproducibility was tested by calculating the mean ± standard deviation values of mass errors and intensities of individual ion species.

RESULTS

Analysis of the PC(18:2/18:2) standard showed significantly (p < 0.01) lower mass error for DIUTHAME-MS than for MALDI-MS. Endogenous PC(36:4) analysis in mouse brain section also showed significantly (p < 0.05) lower mass errors for DIUTHAME-MS. Furthermore, we investigated the mass error of some abundant lipid ions in brain sections and observed similar results. DIUTHAME-MS displayed lower signal intensity in standard PC analysis. Interestingly, it offered higher signal intensities for all the endogenous lipid ions. Lower fluctuations of both mass accuracies and signal intensities were observed in DIUTHAME-MS.

CONCLUSIONS

Our results demonstrated that DIUTHAME-MS offers higher reproducibility for mass accuracies and intensities than MALDI-MS in both standard lipid and mouse brain tissue analyses. It can potentially be used instead of conventional MALDI-MS and mass spectrometry imaging analyses to achieve highly reproducible data for mass accuracy and intensity.

Feasibility of Acetylcholinesterase Reaction Assay Monitoring in DIUTHAME-MS

DIUTHAME (desorption ionization using through hole alumina membrane) is a novel matrix-free laser desorption/ionization method that enables highly reproducible acquisition of mass spectra. This study aims to evaluate the applicability of DIUTHAME to the acetylcholinesterase reaction assay (AChE assay) commonly used in high-throughput screening (HTS) in the drug discovery process. The commercially available 9-ch DIUTHAME chip was applied to a series of AChE assays prepared with different reaction times. Numerous positive-mode TOF mass spectra were acquired from the raster-scanned sample spots of the AChE assays to analyze the progress of the enzyme reaction and to perform mass spectrometry imaging of the sample spots. The reaction kinetics plots obtained by DIUTHAME were found to reflect the time course of reaction progression as much as those obtained by MALDI and were found to have less error variation than MALDI. DIUTHAME allowed the already irradiated sample spots to be reused to reproduce the reaction kinetics plots by the second measurement conducted a week after, whereas MALDI was not able to properly reproduce the kinetics plots by remeasuring the irradiated sample spots. In DIUTHAME, which does not require a matrix, the experimental procedure for measuring AChE assay becomes extremely simple. DIUTHAME is potentially able to provide more precise AChE reaction kinetics plots than what have been accomplished by MALDI.