Enhancing mass analysis of ultra-high molecular weight polystyrene: a comparative study of copper and silver salts with MALDI mass spectrometry

Abstract

This study presents a report on the use of copper (Cu) salts in polymer mass spectrometry for samples exceeding several thousand daltons, demonstrating their efficiency and cost-effectiveness for analyzing ultra-high molecular weight (UHMW) polystyrene (PS). Using matrix-assisted laser desorption/ionization (MALDI) coupled with a home-built linear ion trap mass spectrometer (LIT-MS) under optimized conditions, polystyrene (PS) with masses up to two million daltons was successfully detected. This method using MALDI LIT-MS surpasses the previous upper detection limits observed with silver (Ag) and cesium (Cs) salts using a DCTB matrix. In the MALDI time-of-flight (TOF) mass spectrometry system, these salts were ineffective for detecting PS masses in the range from 660 kDa to 1.1 million Da, respectively (Rapid Commun. Mass Spectrom., 2015, 29, 1039–1046). In this study, we demonstrate for the first time that both Ag and Cu salts can successfully analyze UHMW PS samples up to 2 million Da using the trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]-malononitrile (DCTB) matrix. However, Ag salts predominantly generate multiply charged state ion signals with no control over charge states. In contrast, Cu salts demonstrate superior versatility in modulating charge states, with copper(I) salts achieving higher charge states of up to +5 and copper(II) salts generating lower charge states of up to +3. This targeted control simplifies analysis and enhances the method's flexibility, making Cu salts particularly valuable for analyzing both individual PS samples and mixtures. Additionally, Cu salts are compatible with both DCTB and all-trans-retinoic acid (RA) matrices, with DCTB delivering superior performance. Comparative analysis with Ag salts further validates Cu salts as optimal cationizing reagents for UHMW PS analysis, offering a powerful tool for mass distribution measurements in polymer research and advancing analytical methods in this field.