Effects of matrix‐assisted laser desorption/ionization experimental conditions on quantitative compositional analysis of ethylene oxide/propylene oxide copolymers

Abstract

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry has the potential to become a valuable tool for the compositional analysis of copolymers. For a copolymer composed of structurally very similar building blocks with minor chain length changes, one would expect the relative peak intensities observed in the MALDI mass spectra to reflect its composition, at least within a narrow mass range. However, we show that variations in experimental conditions in MALDI can have a significant effect on the mass spectral appearance of a copolymer. The effects of concentration, laser power, type of matrices and solvents on mass spectra of an ethylene oxide/propylene oxide copolymer are illustrated. These somewhat surprising results show that great care needs to be exercised when interpreting copolymer spectra for compositional analysis, even for copolymers with structurally similar monomers. This work also points out that further studies are needed to better understand and optimize spectral acquisition conditions for reliable copolymer compositional analysis by MALDI.

Improved matrix‐assisted laser desorption/ionisation sample preparation of a partially depolymerised cellulose derivative by continuous spray deposition and interfacing with size‐exclusion chromatography

“…Since cellulose derivatives are not homopolymers it can be expected that the fragments obtained after depolymerisation will vary in hydrophobicity. Variation in hydrophobicity could cause discrimination effects in MALDI‐TOFMS analysis 44, 45. Therefore, application of appropriate separation techniques, such as for instance reverse‐phase HPLC, could give additional characterisation data.…”

Mass spectra of copolymers

“…Chen et al analyzed a block copolymer with units of ethylene oxide (A) and propylene oxide (B) obtained with a mixture of two initiators (Chen et al, 2000). They recorded the MALDI–TOF spectrum of the sample, and noted that the peaks are due to ions of the type H‐(CH 2 ) 16 O‐A m B n ‐H,,,Na + and H‐(CH 2 ) 18 O‐A m B n ‐H,,,Na + .…”

“…They prepared the sample with three different analyte‐to‐matrix ratios, and once again, differences were easy to detect. Figure 13 reports a section of the MALDI–TOF spectrum when the analyte‐to‐matrix ratio is 1:500, 1:50, and 1:10 (Chen et al, 2000). There are strong differences indeed.…”

“…Section of the MALDI–TOF spectrum, when the analyte‐to‐matrix ratio is 1:500 ( A ), 1:50 ( B ), and 1:10 ( C ). Reproduced from Chen et al (2000) with permission from John Wiley & Sons, Ltd. (copyright 2000).…”

“…The best theoretical spectrum possesses a surperb resolution (artificially enhanced). It cannot be excluded that the method based on statistics can help to solve isobaric interferences in MALDI–TOF mass spectra of copolymers with units of ethylene oxide (EO) and propylene oxide (PO), which were described in detail by many authors (Schriemer, Whittal, & Li, 1997; Chen et al, 2000, 2001; Gallet et al, 2002).…”