Institute of Food Chemistry, Technische Universität Braunschweig, Schleinitzstr. 20, 38106, Braunschweig, Germany.
Anal Bioanal Chem. 2022 Jan;414(3):1279-1296. doi: 10.1007/s00216-021-03767-w. Epub 2021 Dec 16.
Exact quantification of the molar ratios of isotopologous mixed O-methyl-O-methyl-d-cellooligosaccharides (COS) comprising all combinations from fully methylated to fully deuteromethylated constituents within an individual degree of polymerization (DP) is the key step in the analysis of the substituent distribution over the polymer chains in methyl celluloses (MC). Deuteromethylation of MC is performed to level chemical differences, but due to a m/z range of 3 DP·ΔMe/Me-d, bias during MS measurement cannot certainly be excluded. Therefore, ionization, ion transportation, and ion storage were studied with an electrospray ionization ion trap mass spectrometer (ESI-IT-MS) using binary equimolar mixtures of per-O-Me- and per-O-Me-d-COS, defining the border cases of a particular Me/Me-d-profile. Reference data of their molar ratio were determined after reductive amination with m-amino benzoic acid by HPLC-UV. COS of DP2-6 were measured as their sodium adducts at c = 10 M by syringe pump infusion. The impact of the RF voltage of the ion trap (TD), the octopole RF and DC voltages, and the Cap Exit potential on absolute and relative ion intensities were studied. Adapting the Cap Exit voltage was essential for correct quantification of DP2, while all COS of higher DP behaved insensitive with respect to bias. To check whether any bias occurs in the electrospray ionization process of the isotopologs, concentration-dependent measurements were performed with optimized instrumental settings for each DP. Intensity ratios IR = I (Me-d)/I (Me) did not show any concentration-dependent trend and no selective ion suppression. Its decrease with DP observed under usually applied standard conditions (smart mode) is a consequence of discrimination according to m/z and can be overcome by appropriate instrumental settings of Oct 2 DC and TD. IR between 0.971 ± 0.008 and 1.040 ± 0.009 with no trend for DP (2-6) were obtained by averaging all measurements in the range 2 · 10 to 2 · 10 M total concentration. The DP-related optimized settings were applied to two MCs and compared with the results obtained under so far applied standard conditions.
准确量化具有单个聚合度 (DP) 内所有完全甲基化到完全氘甲基化成分的同系混合 O-甲基-O-甲基-d-纤维素低聚糖 (COS) 的摩尔比是分析纤维素甲基醚 (MC) 中支链取代分布的关键步骤。MC 的氘甲基化是为了消除化学差异，但由于 m/z 范围为 3 DP·ΔMe/Me-d，因此在 MS 测量过程中不能排除偏差。因此，使用电喷雾电离离子阱质谱仪 (ESI-IT-MS) 研究了离子化、离子传输和离子存储，使用了二元等摩尔比的全-O-Me-和全-O-Me-d-COS 混合物，定义了特定 Me/Me-d-谱的边界情况。摩尔比的参考数据通过用间氨基苯甲酸进行还原胺化后，由 HPLC-UV 确定。通过注射器泵输注，将 DP2-6 的 COS 作为它们的钠盐，以 c = 10 M 的浓度进行测量。研究了离子阱 (TD) 的射频电压、八极 RF 和 DC 电压以及 Cap Exit 电势对绝对和相对离子强度的影响。调整 Cap Exit 电压对于正确定量 DP2 至关重要，而所有更高 DP 的 COS 对偏差表现出不敏感。为了检查同系物在电喷雾电离过程中是否存在任何偏差，在每个 DP 的优化仪器设置下进行了浓度依赖性测量。强度比 IR = I (Me-d)/I (Me) 没有显示出任何浓度依赖性趋势，也没有选择性离子抑制。在通常应用的标准条件下 (智能模式) 观察到的随着 DP 的降低是根据 m/z 进行区分的结果，可以通过适当调整八极 RF 2 DC 和 TD 来克服。在 2 · 10 至 2 · 10 M 总浓度范围内进行所有测量的平均值为 0.971 ± 0.008 至 1.040 ± 0.009，IR 随 DP 无变化 (2-6)。通过将优化的 DP 相关设置应用于两种 MC，并将其与迄今为止应用的标准条件下获得的结果进行比较。
