Graphite Sheet-Assisted Laser Desorption Ionization-Mass Spectrometry for Small Organic Compound Analysis

Carbon-based nanopowders have been used as ionization materials for laser desorption ionization-mass spectrometry (LDI-MS) and are very efficient at detection in low m/z regions. In this study, we aimed to develop a new sheet-type graphite material that possessed a randomly grooved nanostructured surface consisting of developed sp2-conjugated atomic carbon to facilitate the desorption/ionization of small compounds in LDI-MS. The graphite sheet exhibited higher UV absorption and provided higher ionization efficiency and survival yield in the LDI-MS detection of a thermometer ion, 4-chloro-benzopyridinium, than those of highly oriented graphite plates. These properties demonstrate that the present graphite sheet is suited for use as an LDI-MS material. Graphite sheet-assisted LDI-MS successfully detected various substances, including amino acids, peptides, and polyethylene glycol polymers, with higher ion intensities and less noise than those associated with conventional organic matrix-assisted LDI-MS (MALDI-MS). Furthermore, graphite sheet-assisted LDI-MS analysis provided more peaks (252 peaks) derived from soy sauce than those obtained by MALDI-MS (36 peaks) and required fewer preparation processes (dilution and air-dried) compared with previously established graphite carbon black-assisted LDI-MS (171 peaks) in the positive mode. This study demonstrates that graphite sheet-assisted LDI-MS has the potential for small organic compound analyses in the biomedical and food science fields.

Boric acids decorated polymers with Au nanoparticle anchor assisted laser desorption/ionization for qualitive and quantitative analysis of hydroxytyrosol in red wines

Hydroxytyrosol possesses a variety of biological and pharmacological activities that are beneficial to human health. However, the methodologies for its detection always suffered from problems. In this work, the gold nanoparticle modified polymer decorated with boric acids (pMBA/VPBA@Au) was synthesized and used both as the adsorbent and matrix to enrich and ionize small molecule substances through surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). The pMBA/VPBA@Au displayed a low detection limit (8 × 10-6 M) and high selectivity (1:100) for the enrichment of hydroxytyrosol, and the linear correlation curve between the concentration of hydroxytyrosol and the intensity of MS had a good correlation (10-4-10-2 M, R2 = 0.997). Additionally, the pMBA/VPBA@Au was used to quantify hydroxytyrosol in red wines, and the contents were 0.053-0.094 μg/mL. In general, a simple and novel method for the detection of hydroxytyrosol by SALDI-MS using boric acid functionalized polymer was developed for the first time, showing a good practical application value.

MALDI-2 Mass Spectrometry for Synthetic Polymer Analysis

Synthetic polymers are ubiquitous in daily life, and their properties offer diverse benefits in numerous applications. However, synthetic polymers also present an increasing environmental burden through their improper disposal and subsequent degradation into secondary micro- and nanoparticles (MNPs). These MNPs accumulate in soil and water environments and can ultimately end up in the food chain, resulting in potential health risks. Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) has the potential to study localized biological or toxicological changes in organisms exposed to MNPs. Here, we investigate whether MALDI-2 postionization can provide a sensitivity enhancement in polymer analysis that could contribute to the study of MNPs. We evaluated the effect of MALDI-2 by comparing MALDI and MALDI-2 ion yields from polyethyleneglycol (PEG), polypropylene glycol (PPG), polytetrahydrofuran (PTHF), nylon-6, and polystyrene (PS). MALDI-2 caused a signal enhancement of the protonated species for PEG, PPG, PTHF, and nylon-6. PS, by contrast, preferentially formed radical ions, which we attribute to direct resonance-enhanced multiphoton ionization (REMPI). REMPI of PS led to an improvement in sensitivity by several orders of magnitude, even without cationizing salts. The improved sensitivity demonstrated by MALDI-2 for all polymers tested highlights its potential for studying the distribution of certain classes of polymers in biological systems.

Preparation of core-shell MOF@MOF nanoparticle as matrix for the analysis of rhubarb anthraquinones in plasma by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

A core-shell structure UiO-66-(OH)₂@UiO-66-NH₂ (MOF@MOF) nanoparticle was synthesized through a simple hydrothermal method and employed as an adsorbent and laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) matrix for the quantitative analysis of rhubarb anthraquinones (RAs). The properties of the materials were characterized by field emission scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray powder diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, and Brunauer-Emmett-Teller. The results indicate that MOF@MOF is regular octahedral structure with a size distribution of about 100 nm, having large BET specific surface area (920 m²/g). Using the MOF@MOF as a matrix shows lower background interference, higher sensitivity, and better storage stability than that of traditional matrices. The MOF@MOF matrix exhibits excellent salt tolerance even under a NaCl concentration of 150 mM. Then, the enrichment conditions were optimized, and the adsorption time of 10 min, adsorption temperature of 40 °C and adsorbent amount of 100 μg were selected. In addition, the possible mechanism of MOF@MOF as an adsorbent and matrix was discussed. Finally, the MOF@MOF nanoparticle was employed as a matrix for the sensitive MALDI-TOF-MS analysis of RAs in spiked rabbit plasma, and the recoveries are in the range of 88.3-101.5% with RSD ≤9.9%. In short, the novel MOF@MOF matrix has demonstrated its potential in the analysis of small-molecule compounds in biological samples.

Evaluation and optimization of the influence of silver cluster ions on the MALDI-TOF-MS analysis of polystyrene nanoplastic polymers

In the analysis of polystyrene nanoplastics (PSNs), a nonpolar polymer (NP), using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), silver salts were used as cationization reagents and simultaneously brought the potential problems of silver clusters that interfered with the PSN signal of MS. To detect PSNs, silver trifluoroacetate (AgTFA) and silver nitrate (AgNO₃) were mixed with five polar matrices, namely 2-(4-hydroxyphenylazo) benzoic acid (HABA), dithranol (DI), sinapic acid (SA), trans-3-indoleacrylic acid (IAA), and 2,5-dihydroxybenzoic acid (DHB), and three nonpolar matrices, namely pyrene (PRN), anthracene (ATH) and acenaphthene (ACTH). The results showed that silver salt cluster ions were detected in the range of m/z 1000-4000. Five polar matrices with silver salts produced silver clusters, which interfered with the signals in the mass spectrum of PSNs, but the combination of these matrices with copper II chloride (CuCl₂) salt did not produce copper-related clusters. However, the use of nonpolar matrices such as PRN, ATH or ACTH significantly decreased the signals of silver salt cluster ions, and this alteration of matrix types is considered a promising optimization approach for silver cluster ions. The nonpolar matrix conditions were optimized without producing silver cluster ions and the optimal detection conditions were found to be under nonpolar matrices (e.g., pyrene) with silver salts (e.g., AgTFA). The results suggest that when polar matrices, such as HABA, DI, SA, IAA, and DHB, are combined with silver salts in MALDI-TOF-MS analysis, silver-related clusters are detected in the range of m/z 1000-4000. Inhibition of the production of silver cluster ions can be achieved by the use of a nonpolar matrix (e.g., PRN) or polar matrix (e.g., DHB) with copper salts.

To PEGylate or not to PEGylate: Immunological properties of nanomedicine's most popular component, polyethylene glycol and its alternatives

Polyethylene glycol or PEG has a long history of use in medicine. Many conventional formulations utilize PEG as either an active ingredient or an excipient. PEG found its use in biotechnology therapeutics as a tool to slow down drug clearance and shield protein therapeutics from undesirable immunogenicity. Nanotechnology field applies PEG to create stealth drug carriers with prolonged circulation time and decreased recognition and clearance by the mononuclear phagocyte system (MPS). Most nanomedicines approved for clinical use and experimental nanotherapeutics contain PEG. Among the most recent successful examples are two mRNA-based COVID-19 vaccines that are delivered by PEGylated lipid nanoparticles. The breadth of PEG use in a wide variety of over the counter (OTC) medications as well as in drug products and vaccines stimulated research which uncovered that PEG is not as immunologically inert as it was initially expected. Herein, we review the current understanding of PEG's immunological properties and discuss them in the context of synthesis, biodistribution, safety, efficacy, and characterization of PEGylated nanomedicines. We also review the current knowledge about immunological compatibility of other polymers that are being actively investigated as PEG alternatives.

A study of the application of graphite MALDI to the analysis of lanthanides and deconvolution of the isobaric species observed

Matrix-assisted laser desorption/ionisation mass spectrometry has always suffered from matrix interference at low-masses making it an unsuitable method for the analysis of low molecular weight analytes. In recent years, there has been considerable interest in the use of graphite as a matrix. In this study, we demonstrate the application of colloidal graphite for the analysis of lanthanides in the positive ion mode. Positive ion mode is of academic interest as spectra are dominated by lanthanide cations, oxides, hydroxides and carbides with the metal having been reduced to oxidation state I, II or III. The ratios of the different ions are considered in terms of redox potentials of the lanthanides and rates of reaction with oxygen. Positive ion mode is shown to be useful as a rapid technique for confirming which metal(s) are present in a sample which can have an application in environmental monitoring, for example. Demonstration of a least squares approach to deconvolution is applied for the complete separation and relative quantification of the different isobaric species observed due to the complex isotope distributions of some lanthanides.

Atmospheric pressure chemical ionisation mass spectrometry for the routine analysis of low molecular weight analytes

The routine analysis of low molecular weight analytes by mass spectrometry is often complicated by the lability of the analyte's functional groups and/or the lack of moieties that can be easily charged. If a molecule is too labile this precludes analysis by techniques such as electron ionisation or chemical ionisation as the analyte will undergo thermal decomposition prior to ionisation as well as spontaneous fragmentation during the ionisation process. If the analyte has a low propensity to form ions in electrospray ionisation (i.e., lacks acidic or basic sites) then often no analyte related ions are observed. In this paper, the robustness and versatility of the established method of atmospheric pressure chemical ionisation is demonstrated for the analysis of low molecular weight analytes. The utility of the technique is demonstrated through the analysis of 30 reference standards of varying functionality, and further by the analysis of 75 synthetic samples which were problematic when analysed by electron or electrospray ionisation. The resulting spectra are dominated by intact molecular species ([M+H]⁺ and M⁺ in positive ion mode and [M - H]^- and [M + Cl]^- in negative ion mode) along with logical neutral losses reminiscent of what you might expect from the analyte's structure (losses of H₂O from alcohols or CO from aldehydes etc). This paper presents atmospheric pressure chemical ionisation as an essential tool for broadening the chemical space of successful analyses for any routine mass spectrometry service laboratory of facility.