Parchami Razieh, Tabrizchi Mahmoud
Department of Chemistry, Isfahan University of Technology, Isfahan, 84156-83111, Iran.
Rapid Commun Mass Spectrom. 2021 Jun 30;35(12):e9090. doi: 10.1002/rcm.9090.
The observed drift times of monoatomic ions, including alkali metal ions and halide anions, are not fully consistent with their size. When the effect of mass is included through the Mason-Schamp equation, the deviation gets worse so that the trend of the experimental collisional cross-sections becomes completely opposite to what is expected. This is attributed to the stronger local electric field around smaller ions. The strong electric field in the vicinity of a small ion leads to strong ion-neutral interactions and creates a drag force against ion motion. The smaller the ions, the stronger the interaction, because of the higher charge density.
In view of this, a modified equation is introduced to describe the relationship between the observed drift times or ion mobilities and the cross-sections of small ions. Here, for small ions with high charge density, the experimental collision cross-section is expressed as the effective collision cross-section, Ω = σ (1 + α/r ), that takes into account both intrinsic ion size, σ , and the ion-molecule interactions through a correction term of α/r , which is proportional to the charge density.
A linear fit of the drift times of alkali metal ions and halide anions to the proposed equation showed relative deviations of <8.2%. The model successfully predicted the drift time of other small diatomic ions with reasonable error.
The proposed model can be used as a simple and efficient relationship in predicting the effective cross-section of small ions.
包括碱金属离子和卤化物阴离子在内的单原子离子的观测漂移时间与其大小并不完全一致。当通过梅森 - 尚普方程纳入质量效应时，偏差会变得更糟，以至于实验碰撞截面的趋势与预期完全相反。这归因于较小离子周围更强的局部电场。小离子附近的强电场会导致强烈的离子 - 中性相互作用，并产生阻碍离子运动的阻力。离子越小，由于电荷密度越高，相互作用越强。
鉴于此，引入了一个修正方程来描述观测到的漂移时间或离子迁移率与小离子截面之间的关系。在这里，对于具有高电荷密度的小离子，实验碰撞截面表示为有效碰撞截面，Ω = σ (1 + α/r )，它既考虑了离子的固有大小σ ，又通过α/r 的修正项考虑了离子 - 分子相互作用，该修正项与电荷密度成正比。
碱金属离子和卤化物阴离子的漂移时间与所提出方程的线性拟合显示相对偏差<8.2%。该模型成功地以合理的误差预测了其他小双原子离子的漂移时间。
所提出的模型可作为预测小离子有效截面的一种简单而有效的关系。
