University of Warsaw, Faculty of Chemistry, Pasteura 1, 02-093 Warsaw, Poland.
Talanta. 2011 Dec 15;87:295-301. doi: 10.1016/j.talanta.2011.09.023. Epub 2011 Oct 4.
Membrane ion-selective magnesium electrodes are commonly used to determine ionized magnesium concentration in blood serum and intracellular fluid by potentiometric clinical analyzers. The selectivity of these electrodes against calcium ion is typically insufficient to avoid calcium interference in blood serum analysis. For this reason the selectivity coefficient for calcium ion has to be studied to make possible any mathematical corrections for calcium ion influence. Existing methods relate to the thermodynamic concept of ISE response which suggest a single constant value of the selectivity coefficient and slope that are stable over the concentration ranges of calcium and magnesium ions in the samples. Unfortunately, this rarely happens, and we rather observe dependences on solution and membrane composition, readout time, matrices (anticoagulant, vial coats) that justify usage of apparent selectivities and slopes. To get the practical insight into the response of magnesium ion-selective electrodes a novel method for estimating the selectivity coefficients and the slope of the electrode characteristics is proposed. This method is an effective starting point for selecting electrodes and designing transient signal software in a potentiometric clinical analyzer. The method allows obtaining the ionized magnesium concentration in blood serum with minimal possible error by addressing the assessed targets, i.e. apparent selectivity and slope. The method is based on computer simulation and on the Nicolsky-Eisenman equation. Usually only a few iterations are needed to obtain stable congruent results. The method presented is particularly useful in conditions where is not possible to obtain calibration curve, which is typical for clinical analyzer where at most three point calibration is performed.
膜离子选择性镁电极常用于通过电势临床分析仪来测定血清和细胞内液中离子化镁的浓度。这些电极对钙离子的选择性通常不足以避免血清分析中的钙离子干扰。因此，必须研究钙离子的选择性系数，以便对钙离子的影响进行任何数学修正。现有的方法与 ISE 响应的热力学概念有关，该概念表明选择性系数和斜率的单一恒定值在样品中钙离子和镁离子的浓度范围内是稳定的。不幸的是，这种情况很少发生，我们更观察到依赖于溶液和膜组成、读数时间、基质（抗凝剂、小瓶涂层），这证明了表观选择性和斜率的使用是合理的。为了深入了解镁离子选择性电极的响应，提出了一种估计选择性系数和电极特性斜率的新方法。该方法是在电势临床分析仪中选择电极和设计瞬态信号软件的有效起点。该方法通过解决评估目标（即表观选择性和斜率），可以以最小的可能误差获得血清中离子化镁的浓度。该方法基于计算机模拟和 Nicolsky-Eisenman 方程。通常，只需几次迭代即可获得稳定的一致结果。该方法在无法获得校准曲线的情况下特别有用，这在临床分析仪中很常见，因为临床分析仪最多只能进行三点校准。
