Moini Mehdi
Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas 78712, USA.
Anal Chem. 2007 Jun 1;79(11):4241-6. doi: 10.1021/ac0704560. Epub 2007 Apr 21.
A robust, reproducible, and single-step interface design between low flow rate separation techniques, such as sheathless capillary electrophoresis (CE) and nanoliquid chromatography (nLC), and mass spectrometry (MS) using electrospray ionization (ESI), is introduced. In this design, the electrical connection to the capillary outlet was achieved through a porous tip at the capillary outlet. The porous section was created by removing 1-1.5 in. of the polyimide coating of the capillary and etching this section by 49% solution of HF until it is porous. The electrical connection to the capillary outlet is achieved simply by inserting the capillary outlet containing the porous tip into the existing ESI needle (metal sheath) and filling the needle with the background electrolyte. Redox reactions of water at the ESI needle and transport of these small ions through the porous tip into the capillary provides the electrical connection for the ESI and for the CE outlet electrode. The etching process reduces the wall thickness of the etched section, including the tip of the capillary, to 5-10 microm, which for a 20-30 microm i.d. capillary results in stable electrospray at approximately 1.5 kV. The design is suitable for interfacing a wide range of capillary sizes with a wide range of flow rates to MS via ESI, but it is especially useful for interfacing narrow (<30 microm i.d.) capillaries and low flow rates (<100 nL/min). The advantages of the porous tip design include the following: (1) its fabrication is reproducible, can be automated, and does not require any mechanical tools. (2) The etching process reduces the tip outer diameter and makes the capillary porous in one step. (3) The interface can be used for both nLC-MS and CE-MS. (4) If blocked or damaged, a small section of the tip can be etched off without any loss of performance. (5) The interface design leaves the capillary inner wall intact and, therefore, does not add any dead volume to the CE-MS or nLC-MS interface. (6) Bubble formation due to redox reactions of water at the high-voltage electrode is outside of the separation capillary and does not affect separation or MS performances. The performance of this interface is demonstrated by the analyses of amino acids, peptide, and protein mixtures.
介绍了一种在低流速分离技术（如无鞘毛细管电泳（CE）和纳升液相色谱（nLC））与使用电喷雾电离（ESI）的质谱（MS）之间稳健、可重复且单步的接口设计。在这种设计中，通过毛细管出口处的多孔尖端实现与毛细管出口的电连接。多孔部分是通过去除毛细管1 - 1.5英寸的聚酰亚胺涂层并使用49%的氢氟酸溶液蚀刻该部分直至其多孔而形成的。只需将包含多孔尖端的毛细管出口插入现有的ESI针（金属鞘）中并用背景电解质填充该针，即可实现与毛细管出口的电连接。水在ESI针处的氧化还原反应以及这些小离子通过多孔尖端进入毛细管，为ESI和CE出口电极提供了电连接。蚀刻过程将蚀刻部分（包括毛细管尖端）的壁厚减小到5 - 10微米，对于内径为20 - 30微米的毛细管，这会在约1.5 kV时产生稳定的电喷雾。该设计适用于通过ESI将各种毛细管尺寸和各种流速与MS连接，但对于连接窄内径（<30微米）的毛细管和低流速（<100纳升/分钟）尤其有用。多孔尖端设计的优点包括：（1）其制造可重复、可自动化，且不需要任何机械工具。（2）蚀刻过程减小了尖端外径并使毛细管一步变为多孔。（3）该接口可用于nLC - MS和CE - MS。（4）如果堵塞或损坏，可以蚀刻掉一小部分尖端而不会有任何性能损失。（5）接口设计使毛细管内壁保持完整，因此不会给CE - MS或nLC - MS接口增加任何死体积。（6）由于水在高压电极处的氧化还原反应形成的气泡在分离毛细管外部，不会影响分离或MS性能。通过对氨基酸、肽和蛋白质混合物的分析证明了该接口的性能。
