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1. Rapid and simple determination of average molecular weight and composition of synthetic polymers via electrospray ionization‐mass spectrometry and a Bayesian universal charge deconvolution

   https://doi.org/10.1002/rcm.9478  

   Keating, Addie R. ; Wesdemiotis, Chrys ( February 2023 , Rapid Communications in Mass Spectrometry)

   Simple, affordable, and rapid methods for identifying the molecular weight (MW) distribution and macromolecular composition of polymeric materials are limited. Current tools require extensive solvent consumption, linear calibrations, and expensive consumables. A simple method for the determination of average MW (M_n,M_w) and chain end groups is demonstrated for synthetic homopolymer standards using direct injection electrospray ionization‐mass spectrometry (ESI‐MS) and an open‐sourced charge deconvolution (CDC) algorithm.

   Five homopolymer standards in the 1–7 kDa MW range were analyzed using direct‐injection ESI‐MS on a quadrupole/time‐of‐flight mass spectrometer. The samples investigated, viz. two poly(ethylene oxide) (PEO) and two poly(styrene sulfonic acid) (PSS) standards with narrow polydispersity and one poly(d,l‐alanine) (pAla) standard with undefined polydispersity, were chosen to illustrate challenges with ESI‐MS quantitation. Using the UniDec program, weight average MWs (M_w) obtained from the charge‐deconvoluted spectra were compared to the reportedM_wdata of the standards from size exclusion chromatography (SEC) measurements.

   The MW data derived for the PSS, PEO, and pAla standards agreed well with the corresponding reportedM_wor MW range values. The method was able to provide MW, degree of polymerization (DP), and polydispersity index (PDI) information for polymers with narrow (PSS, PEO) as well as broader (pAla) molecular weight distribution; this feature provides an advantage over MW analysis via matrix‐assisted laser desorption/ionization (MALDI) for ESI‐compatible materials. PSS standards differing in average MW by only a few repeat units could be confidently distinguished. Additionally, the oligomeric resolution observed for all samples studied unveiled chain‐end information not available through chromatographic analysis.

   Overall, the free and easy‐to‐use UniDec CDC algorithm provides a simple, alternative method to measuring MW and DP for polymeric materials without high solvent consumption, expensive ionization sources, or calibration curves. Information about the masses of individual oligomers and the possibility to further characterize these oligomers using tandem mass spectrometry and/or ion mobility techniques constitutes additional benefits of this approach vis‐à‐vis traditional MW and PDI elucidation through SEC.

    

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2. Enhancing Metabolite Coverage for Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging Through Multiple On-Tissue Chemical Derivatizations

   https://doi.org/10.1007/978-1-0716-2030-4_14  

   O’Neill, K.C. ; Dueñas, M.E. ; Larson, E. ; Forsman, T.T. ; Lee, YJ. ( January 2022 , Methods in molecular biology)

   Lee, YJ (Ed.)

   The ability to study and visualize metabolites on a cellular and sub-cellular level is important for gaining insights into biological pathways and metabolism of multicellular organisms. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) is a powerful analytical tool for metabolomics experiments due to its high sensitivity and small sampling size. The spatial resolution in MALDI-MSI is mainly limited by the number of molecules available in a small sampling size. When the sampling size is low enough to achieve cellular or subcellular spatial resolution, signal intensity is sacrificed making poorly ionized metabolites difficult to detect. To overcome this limitation, on-tissue chemical derivatization reactions have been used to enhance the desorption/ionization efficiency of selected classes of compounds by adding a functional group with a permanent positive charge or one that can be easily ionized. By utilizing several chemical derivatizations in parallel, metabolite coverage can be drastically improved. This chapter outlines methodology for sample preparation and data analysis for on-tissue chemical derivatization using various derivatization reagents. 

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3. Mass Spectrometry Imaging of Bio‐oligomer Polydispersity in Plant Tissues by Laser Desorption Ionization from Silicon Nanopost Arrays

   https://doi.org/10.1002/ange.202015251  

   Samarah, Laith Z. ; Tran, Tina H. ; Stacey, Gary ; Vertes, Akos ( March 2021 , Angewandte Chemie)

   Mass spectrometry imaging (MSI) enables simultaneous spatial mapping for diverse molecules in biological tissues. Matrix‐assisted laser desorption ionization (MALDI) mass spectrometry (MS) has been a mainstream MSI method for a wide range of biomolecules. However, MALDI‐MSI of biological homopolymers used for energy storage and molecular feedstock is limited by, e.g., preferential ionization for certain molecular classes. Matrix‐free nanophotonic ionization from silicon nanopost arrays (NAPAs) is an emerging laser desorption ionization (LDI) platform with ultra‐trace sensitivity and molecular imaging capabilities. Here, we show complementary analysis and MSI of polyhydroxybutyric acid (PHB), polyglutamic acid (PGA), and polysaccharide oligomers in soybean root nodule sections by NAPA‐LDI and MALDI. For PHB, number and weight average molar mass, polydispersity, and oligomer size distributions across the tissue section and in regions of interest were characterized by NAPA‐LDI‐MSI.

    

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4. Mass Spectrometry Imaging of Bio‐oligomer Polydispersity in Plant Tissues by Laser Desorption Ionization from Silicon Nanopost Arrays

   https://doi.org/10.1002/anie.202015251  

   Samarah, Laith Z. ; Tran, Tina H. ; Stacey, Gary ; Vertes, Akos ( March 2021 , Angewandte Chemie International Edition)

   Mass spectrometry imaging (MSI) enables simultaneous spatial mapping for diverse molecules in biological tissues. Matrix‐assisted laser desorption ionization (MALDI) mass spectrometry (MS) has been a mainstream MSI method for a wide range of biomolecules. However, MALDI‐MSI of biological homopolymers used for energy storage and molecular feedstock is limited by, e.g., preferential ionization for certain molecular classes. Matrix‐free nanophotonic ionization from silicon nanopost arrays (NAPAs) is an emerging laser desorption ionization (LDI) platform with ultra‐trace sensitivity and molecular imaging capabilities. Here, we show complementary analysis and MSI of polyhydroxybutyric acid (PHB), polyglutamic acid (PGA), and polysaccharide oligomers in soybean root nodule sections by NAPA‐LDI and MALDI. For PHB, number and weight average molar mass, polydispersity, and oligomer size distributions across the tissue section and in regions of interest were characterized by NAPA‐LDI‐MSI.

    

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5. Toward understanding the ionization mechanism of matrix‐assisted ionization using mass spectrometry experiment and theory

   https://doi.org/10.1002/rcm.8382  

   Lee, Chuping ; Inutan, Ellen D. ; Chen, Jien Lian ; Mukeku, Mutanu M. ; Weidner, Steffen M. ; Trimpin, Sarah ; Ni, Chi‐Kung ( March 2019 , Rapid Communications in Mass Spectrometry)

   Matrix‐assisted ionization (MAI) mass spectrometry does not require voltages, a laser beam, or added heat to initiate ionization, but it is strongly dependent on the choice of matrix and the vacuum conditions. High charge state distributions of nonvolatile analyte ions produced by MAI suggest that the ionization mechanism may be similar to that of electrospray ionization (ESI), but different from matrix‐assisted laser desorption/ionization (MALDI). While significant information is available for MAI using mass spectrometers operating at atmospheric and intermediate pressure, little is known about the mechanism at high vacuum.

   Eleven MAI matrices were studied on a high‐vacuum time‐of‐flight (TOF) mass spectrometer using a 266 nm pulsed laser beam under otherwise typical MALDI conditions. Detailed comparisons with the commonly used MALDI matrices and theoretical prediction were made for 3‐nitrobenzonitrile (3‐NBN), which is the only MAI matrix that works well in high vacuum when irradiated with a laser.

   Screening of MAI matrices with good absorption at 266 nm but with various degrees of volatility and laser energies suggests that volatility and absorption at the laser wavelength may be necessary, but not sufficient, criteria to explain the formation of multiply charged analyte ions. 3‐NBN produces intact, highly charged ions of nonvolatile analytes in high‐vacuum TOF with the use of a laser, demonstrating that ESI‐like ions can be produced in high vacuum. Theoretical calculations and mass spectra suggest that thermally induced proton transfer, which is the major ionization mechanism in MALDI, is not important with the 3‐NBN matrix at 266 nm laser wavelength. 3‐NBN:analyte crystal morphology is, however, important in ion generation in high vacuum.

   The 3‐NBN MAI matrix produces intact, highly charged ions of nonvolatile compounds in high‐vacuum TOF mass spectrometers with the aid of ablation and/or heating by laser irradiation, and shows a different ionization mechanism from that of typical MALDI matrices.

    

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