Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, TX 76019, USA.
Rapid Commun Mass Spectrom. 2012 Dec 30;26(24):2885-96. doi: 10.1002/rcm.6413.
The detection of metal cations continues to be essential in many scientific and industrial areas of interest. The most common electrospray ionization mass spectrometry (ESI-MS) approach involves chelating the metal ions and detecting the organometallic complex in the negative ion mode. However, it is well known that negative ion mode ESI-MS is generally less sensitive than the positive ion mode. To achieve greater sensitivity, it is necessary to examine the feasibility of detecting the chelated metal cations in positive ion mode ESI-MS.
Since highly solvated native metal cations have relatively low ionization efficiency in ESI-MS, and can be difficult to detect in the positive ion mode, a tetracationic ion-pairing agent was added to form a complex with the negatively charged metal chelate. The use of the ion-pairing agent leads to the generation of an overall positively charged complex, which can be detected at higher m/z values in the positive ion mode by electrospray ionization linear quadrupole ion trap mass spectrometry.
Thirteen chelating agents with diverse structures were evaluated in this study. The nature of the chelating agent played as important a role as was previously determined for cationic pairing agents. The detection limits of six metal cations reached sub-picogram levels and significant improvements were observed when compared to negative ion mode detection where the metal-chelates were monitored without adding the ion-pairing reagent (IPR). Also, selective reaction monitoring (SRM) analyses were performed on the ternary complexes, which improved detection limits by one to three orders of magnitude.
With this method it was possible to analyze the metal cations in the positive ion mode ESI-MS with the advantage of speed, sensitivity and selectivity. The optimum solution pH for this type of analysis is 5-7. Tandem mass spectrometry (MS/MS) further increases the sensitivity. Speciation is straightforward making this a broadly useful approach for the analysis of metal ions.
金属阳离子的检测在许多科学和工业领域仍然至关重要。最常见的电喷雾电离质谱（ESI-MS）方法涉及螯合金属离子，并以负离子模式检测有机金属络合物。然而，众所周知，负离子模式 ESI-MS 通常不如正离子模式敏感。为了实现更高的灵敏度，有必要研究在正离子模式 ESI-MS 中检测螯合金属阳离子的可行性。
由于高度溶剂化的天然金属阳离子在 ESI-MS 中的电离效率相对较低，并且在正离子模式下难以检测，因此添加了一种四价离子配对试剂来与带负电荷的金属螯合物形成复合物。使用离子对试剂会导致生成整体带正电荷的络合物，通过电喷雾线性四极离子阱质谱法，可以在正离子模式下以更高的 m/z 值检测到这些络合物。
本研究评估了 13 种具有不同结构的螯合剂。螯合剂的性质与之前确定的阳离子配对试剂一样重要。六种金属阳离子的检测限达到皮克以下水平，与不添加离子对试剂（IPR）监测金属-螯合物的负离子模式检测相比，有了显著的提高。此外，还对三元络合物进行了选择性反应监测（SRM）分析，将检测限提高了一到三个数量级。
通过这种方法，可以在正离子模式 ESI-MS 中分析金属阳离子，具有速度快、灵敏度高和选择性好的优点。这种类型分析的最佳溶液 pH 值为 5-7。串联质谱（MS/MS）进一步提高了灵敏度。形态分析简单明了，这是一种广泛有用的金属离子分析方法。
