G-quadruplex secondary structures are naturally found in genome sequences and play essential roles in regulating a wide variety of important biological processes. Although stabilizing effects of monovalent cations (e.g., K⁺ and Na⁺) has been recognized during the past decades, a general and reliable analytical method for accurate characterization of specific interactions of K⁺/Na⁺ with G-quartets is still not well established. In the present study, we demonstrate a practical strategy that combined the use of a nanoscale ion emitter, a low-flow drying gas and a volatile salt (trimethylammonium acetate) to almost totally suppress the nonspecific cationic adduction to G-quadruplexes during the ionization process. Our combined strategy takes full advantage of the ultrasmall initial charged droplets when employing a nanoscale ion emitter, the maximum uneven fission of charged droplets under the gentle desolvation conditions, and the effective shielding of the negatively charged phosphate groups by trimethylammonium ions, to eventually producing ions of G-quadruplexes free of non-specific K⁺/Na⁺ adduction. For the first time, the accurate binding states as well as the quantitative binding constants between K⁺/Na⁺ and G-quadruplexes can be directly obtained even in the presence of tens of millimolar non-volatile salts, which has long been a notorious challenge in mass spectrometry.

G-四链体二级结构天然存在于基因组序列中，在调节多种重要的生物过程中发挥着重要作用。尽管在过去的几十年中已经认识到单价阳离子（例如 K ⁺和 Na ^{+ ）的稳定作用，但是一种通用且可靠的分析方法可以准确表征 K +} /Na ^{+的特定相互作用}与 G-四重奏的关系还没有很好地建立。在本研究中，我们展示了一种实用的策略，该策略结合使用纳米级离子发射器、低流量干燥气体和挥发性盐（乙酸三甲基铵）来几乎完全抑制电离过程中 G-四链体的非特异性阳离子加成。 . 我们的组合策略充分利用了采用纳米级离子发射器时的超小初始带电液滴、温和去溶剂化条件下带电液滴的最大不均匀裂变以及三甲基铵离子对带负电的磷酸基团的有效屏蔽，最终产生离子不含非特异性 K ⁺ /Na ^{+的 G-四链体}内收。^{第一次，即使在数十毫摩尔的非挥发性盐存在下，也可以直接获得 K +} /Na ⁺和 G-四链体之间的准确结合状态以及定量结合常数，这一直是一个臭名昭著的挑战在质谱。