Electrospray ionization-mass spectrometry (ESI-MS) of low-charging synthetic polymers typically produces mass spectra exhibiting a bias toward the low-mass region of the polymer mass distribution. To examine the origin(s) of this ionization bias, narrow dispersity polystyrene polymers (Đ < 1.10) were prepared with ionizable carboxylic acid end-groups at one or both chain termini. The mixture complexity was further reduced through preparative size-exclusion chromatography (SEC), and these well-defined polymers were subjected to negative ion ESI-MS on a high-resolution instrument with a mass-to-charge (m/z) range up to 8000. Incorporation of one carboxylic acid end-group facilitated the generation of singly charged [M - H]- ions across the entire range of the mass analyzer. The comparison of mass spectra with size-exclusion chromatograms of the same polymer revealed an ionization bias toward lower masses, which was partially overcome through fractionation, modification of electrospray solvent, and increased declustering potentials. Incorporation of a second ionizable moiety within polymers of equivalent size facilitated multiply charged [M - 2H]2- ion formation with significantly improved ionization efficiency, spectral coverage of the molar mass distribution, and minimal cluster ion formation. These findings indicate that increased charging of polymers through multiple, well-defined sites of ionization can enhance volatilization and ionization of higher-mass polymers. Generation of higher-molecular-weight polymers in low-charge states-while possible under ideal conditions-competes ineffectively with either nonspecific, multiple-charging of similar sized polymers or ionization of the smaller polymers in the distribution.

中文翻译：

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用羧酸端基官能化的合成聚合物的电喷雾电离质谱。

低电荷合成聚合物的电喷雾电离质谱 (ESI-MS) 通常产生的质谱显示偏向于聚合物质量分布的低质量区域。为了检查这种电离偏向的起源，制备了窄分散聚苯乙烯聚合物 (Đ < 1.10)，在一个或两个链末端带有可电离的羧酸端基。通过制备型尺寸排阻色谱 (SEC) 进一步降低了混合物的复杂性，并且这些定义明确的聚合物在质荷比 (m/z) 范围高达到 8000。一个羧酸端基的结合促进了整个质量分析器范围内单电荷 [M - H]- 离子的产生。质谱图与相同聚合物的尺寸排阻色谱图的比较揭示了偏向于较低质量的电离偏差，这部分通过分馏、电喷雾溶剂的改性和增加的去簇电位来克服。在同等大小的聚合物中加入第二个可电离部分促进了多电荷 [M - 2H] 2 - 离子的形成，显着提高了电离效率、摩尔质量分布的光谱覆盖率和最小的簇离子形成。这些发现表明，通过多个明确定义的电离位点增加聚合物的电荷可以增强更高质量聚合物的挥发和电离。