The Role of Cationization Reagents on Generation of Multiply Charged Ultra-High Molecular Weight Polystyrene MALDI Ions

Avinash Adhikrao Patil^1*, Thị Khánh Ly Lại¹, Wen-Ping, Peng¹

¹Physics, National Dong Hwa University, Hualien, Taiwan

* Presenter:Avinash Adhikrao Patil, email:avi.smash4@gms.ndhu.edu.tw

Polymers play an important role in industry fields. Mass spectrometry is an effect tool for characterization of those polymers. The mass analysis of ultra-high molecular weight (UHMW, Mn > 1000 kDa) linear polymers has drawn the attention in its applications in mist control, drag reduction, and reducing agrichemical spray drift. So far the use of matrix-assisted laser desorption/ionization–mass spectrometry (MALDI–MS) for characterization of those polymers is not well studied. The primary ionization mechanism in the analysis of synthetic polymers is known to be gas-phase cationization, which involves the attachment of transition metal, alkali metal, and other cations. The phenyl rings in polystyrene(PS), a non-polar hydrocarbon, are the most likely place for cations to attach. It is found the effective cation generation is a key to ionize polystyrene polymers and also to form multiply charged polystyrene ion signals. As the molecular mass increases, the number of charge states increases as well, which could be correlated to the role of cationizing reagents during MALDI ionization process.
Here we examine the influence of various cations (Ag+, Cu+ and Cs+) on the MALDI ionization process of ultra-high mass polystyrene polymers. The silver salts, e.g. silver nitrate (AgNO₃) and silver trifluoroacetate (AgTFA), copper salts, e.g. copper(II) chloride (CuCl₂), cupric acetate (CuAc), and copper(II) acetylacetonate (CuAcAc), and cesium chloride (CsCl) salt are examined in this study. MALDI mass spectra were analyzed using linear ion trap mass spectrometers (LIT-MS). For 200 kDa PS sample, up to three charge states were observed using AgTFA, and ions up to two charge states were observed using AgNO₃, CuCl₂, and CuAc, CuACAc without dimer, but only singly charged monomer and dimer ions were observed using CsCl additive. With the 650 kDa PS sample, ions with up to four charge states were observed using AgTFA, up to three charge states with AgNO₃, CuAc, CuACAc, and up to two charge states with CuCl₂, but only singly charged ions were observed using CsCl additive. The charge distribution trends to remain the same for higher mass 900 kDa PS with increasing charge states by one for AgTFA, AgNO₃, and CuCl₂; however, doubly charged ions are the most intense peaks when using AgTFA and AgNO₃. But singly charged ions are more intense when using CuAc additives. Surprisingly, for a 2000 kDa PS sample, AgTFA or AgNO₃ generates multiply charged ions without singly charged ions signal, whereas CuAc can generate multiply charged ions along with singly charged ions. The tendency to form multiply charged PS ions using transition/alkali metal based cationizing reagents fells in line with the order of AgTFA > AgNO₃ > CuAc > CuACAc > CuCl₂ > CsCl.

Keywords: Polymer, Matrix assisted laser desorption/ionization, Cationization Reagents, MALDI mass analysis, Linear Ion Trap mass spectrometry