A New Method for the Analysis of the Substitution Pattern of Hydroxyethyl(methyl)-Celluloses Along the Polysaccharide Chain

Potential of ion mobility mass spectrometry in cellulose ether analysis: substitution pattern of hydroxyethyl celluloses

Ion mobility mass spectrometry (ESI-tims-ToF-MS, syringe pump infusion) has been applied to glucose and oligosaccharide ethers derived from hydroxyethyl-methyl celluloses (HEMC) and hydroxyethyl celluloses (HEC) after permethylation and partial depolymerization: by hydrolysis without or with subsequent reductive amination with m-amino benzoic acid (mABA) or by reductive cleavage. As model compounds without tandem substitution methoxyethylated methylcellulose was used. Regioisomeric glucose ethers were separated according to their ion mobility, and positions of substitution could be assigned. Glucose ethers including isomers with tandem substitution showed additional signals with a smaller collision cross-section (CCS) than core-substituted isomers. Positional isomers of cellobiose ethers were only partly resolved due to too high complexity but showed a characteristic fingerprint that might allow classifying samples. Relative intensities of signals of glucose ether isomers could only be quantified in case of ABA derivatives with its fixed charge, while sodium adducts of methoxyethyl ethers showed an influence of the MeOEt position on ion yield. Results were in very good agreement with reference analysis. [M + Na]+ adducts of α- and β-anomers of glucose derivatives were separated in IM, complicating position assignment. This could be overcome by reductive cleavage of the permethylated HE(M)C yielding 1,5-anhydroglucitol-terminated oligosaccharides, showing the best resolved fingerprints of the cellobiose ethers of a particular cellulose ether. With this first application of ion mobility MS to the analysis of complex cellulose ethers, the promising potential of this additional separation dimension in mass spectrometry is demonstrated and discussed.

Comparing 13C methyl and deuterated methyl isotopic labeling for the quantification of methyl cellulose patterns using mass spectrometry

The methyl substitution along and among the polymer chains of methyl cellulose (MC) is commonly analyzed by ESI-MS after perdeuteromethylation of the free-OH groups and partial hydrolysis to cello-oligosaccharides (COS). This method requires a correct quantification of the molar ratios of the constituents belonging to a particular degree of polymerization (DP). However, isotopic effects are most pronounced for H/D since their mass difference is 100%. Therefore, we investigated whether more precise and accurate results could be obtained for the methyl distribution of MC by MS of ¹³CH₃ instead of CD₃-etherified O-Me-COS. Internal isotope labeling with ¹³CH₃ makes the COS of each DP chemically and physically much more similar, reducing mass fractionation effects, but at the same time requires more complex isotopic correction for evaluation. Results from syringe pump infusion ESI-TOF-MS with ¹³CH₃ and CD₃ as isotope label were equal. However, in the case of LC-MS with a gradient system, ¹³CH₃ was superior to CD₃. In the case of CD₃, the occurrence of a partial separation of the isotopologs of a particular DP resulted in slight distortion of the methyl distribution since the signal response is significantly dependent on the solvent composition. Isocratic LC levels this problem, but one particular eluent-composition is not sufficient for a series of oligosaccharides with increasing DP due to peak broadening. In summary, ¹³CH₃ is more robust to determine the methyl distribution of MCs. Both syringe pump and gradient-LC-MS measurements are possible, and the more complex isotope correction is not a disadvantage.

Composition mapping of highly substituted cellulose-ether monomers by liquid chromatography-mass spectrometry and probability-based data deconvolution

Cellulose ethers (CEs) are semi-synthetic polymers produced by derivatization of natural cellulose, yielding highly substituted products such as ethyl hydroxyethyl cellulose (EHEC) or methyl ethyl hydroxyethyl cellulose (MEHEC). CEs are commonly applied as pharmaceutical excipients and thickening agents in paints and drymix mortars. CE properties, such as high viscosity in solution, solubility, and bio-stability are of high interest to achieve required product qualities, which may be strongly affected by the substitution pattern obtained after derivatization. The average and molar degree of substitution often cannot explain functional differences observed among CE batches, and more in-depth analysis is needed. In this work, a new method was developed for the comprehensive mapping of the substitution degree and composition of β-glucose monomers of CE samples. To this end, CEs were acid-hydrolyzed and then analyzed by gradient reversed-phase liquid chromatography-mass spectrometry (LC-MS) using an acid-stable LC column and time-of-flight (TOF) mass spectrometer. LC-MS provided monomer resolution based on ethylene oxide, hydroxyl, and terminating methyl/ethyl content, allowing the assignment of detailed compositional distributions. An essential further distinction of constitutional isomer distributions was achieved using an in-house developed probability-based deconvolution algorithm. Aided by differential heat maps for visualization and straightforward interpretation of the measured LC-MS data, compositional variation between bio-stable and non-bio-stable CEs could be identified using this new approach. Moreover, it disclosed unexpected methylations in EHEC samples. Overall, the obtained molecular information on relevant CE samples demonstrated the method's potential for the study of CE structure-property relationships.

Impact of instrumental settings in electrospray ionization ion trap mass spectrometry on the analysis of O-methoxyethyl-O-methyl cellulose: a comprehensive quantitative evaluation

The hydroxyethyl substitution along and among the polymer chains of respective cellulose ethers (HEC and HEMC) can be analyzed by ESI-IT-MS after permethylation of the free OH-groups, partial hydrolysis, and mABA labeling. This method requires the correct quantification of the molar ratios of the constituents belonging to a particular degree of polymerization (DP) with respect to their numbers of MeOEt and Me groups without any discrimination along the MS analysis pathway. The influence of the chemistry on the ionization and the impact of the voltages controlling the ion transport (Cap Exit, Octopoles) and the ion storage efficiency (Trap Drive, TD) on a relative quantification were studied using binary equimolar mixtures of cellobiose with increasing number of methoxyethyl and decreasing number of methyl groups (Δ m/z 88, 2× MeOEt). No suppression effects were observed in concentration-dependent measurements. Choice of Cap Exit is especially crucial for low m/z with less MeOEt residues. An equation describing the relationship between Oct 2 DC, m/z, and TD_max (TD at maximum intensity) was established from the experimental data and applied to calculate TD_max for higher DPs (larger COS). Optimized conditions allowed to determine the correct molar ratio of binary mixtures. Measurements of overlapping m/z segments and subsequent interrelation of the data gave complete substitution profiles for MeOEt/Me celluloses in accordance with reference data. The study generally makes aware of potential erroneous quantification in ESI-IT-MS analysis using internal standards of similar chemistry or in relative quantification of analytes, even for those with related structures.

Analysis of the Heterogeneities of First and Second Order of Cellulose Derivatives: A Complex Challenge

The complexity of the substituent distribution in polysaccharide derivatives is discussed and defined. The challenges regarding analytical characterization that results from various interrelated categories of distributions, including molecular weight, chemical composition, and microstructure, are outlined. Due to these convoluted levels of complexity, results should always be interpreted with carefulness. Various analytical approaches which have been applied to starch and cellulose derivatives are recapped, including enzymatic, mass spectrometric, and chromatographic methods. The relation of heterogeneities of first and second order among and along the polysaccharide chains is addressed. Finally, examples of own analytical work on cellulose ethers are presented, including the MS analysis of methyl cellulose (MC) blends and fractionation studies of fully esterified MC, especially its 4-methoxybenzoates by gradient HPLC on normal phase. Preparative fractionation according to the degree of substitution (DS) allows follow-up analysis in order to get more detailed information on the substituent distribution in such sub-fractions.

A new method for the quantification of monosaccharides, uronic acids and oligosaccharides in partially hydrolyzed xylans by HPAEC-UV/VIS

A new method for the chemical characterization of xylans is presented, to overcome the difficulties in quantification of 4-O-methyl-α-D-glucuronic acid (meGlcA). In this regard, the hydrolysis behavior of xylans from beech and birch wood was investigated to obtain the optimum conditions for hydrolysis, using sulfuric acid. Due to varying linkage strengths and degradation, no general method for complete hydrolysis can be designed. Therefore, partial hydrolysis was applied, yielding monosaccharides and small meGlcA containing oligosaccharides. For a new method by HPAEC-UV/VIS, these samples were reductively aminated by 2-aminobenzoic acid. By quantification of monosaccharides and oligosaccharides, as well as comparison with borate-HPAEC and (13)C NMR-spectroscopy, we revealed that the concentrations meGlcA are significantly underestimated compared to conventional methods. The detected concentrations are 85.4% (beech) and 76.3% (birch) higher with the new procedure. Furthermore, the quantified concentrations of xylose were 9.3% (beech) and 6.5% (birch) higher by considering the unhydrolyzed oligosaccharides as well.

Critical investigation of the substituent distribution in the polymer chains of hydroxypropyl methylcelluloses by (LC-)ESI-MS

Three hydroxypropyl methylcellulose samples (HPMC1-3, DS(Me) = 1.45, 1.29, and 1.36; MS(HP) = 0.28, 0.46, and 0.84) were analyzed with respect to their methyl and hydroxypropyl substitution pattern in the polymer chains. Ionization yield of HPMC oligomers in electrospray ionization ion trap mass spectrometry (ESI-IT-MS) is strongly influenced by the hydroxypropyl pattern. Therefore, a sample derivatization procedure, as well as suitable measurement conditions that enable relative quantification were elaborated. Analysis was performed by negative ESI-IT-MS after per(deutero)methylation, partial depolymerization, and reductive amination with m-aminobenzoic acid. Measurement parameters like solvent, trap drive, and voltages of the ion transportation unit were studied with regard to the suitability for quantitative evaluation. Using direct infusion of the samples, strong influence of trap drive and octopole settings was observed. Optimized measurement conditions were used for the determination of the HP pattern of the permethylated samples by direct infusion. The methyl pattern was determined from the perdeuteromethylated samples by high-performance liquid chromatography-electrospray tandem mass spectrometry. For HPMC1, substituents were both found to fit the random distribution model. The other two samples showed pronounced heterogeneity which could be interpreted in more detail by extracting methyl subpatterns depending on the number of HP groups.