Micro-scale analytical plasmas for liquid chromatography detection
Micro-scale analytical plasmas for liquid chromatography detection
复制标题
用于液相色谱检测的微量分析等离子体
DOI：
10.1007/s00216-004-2929-y
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发表时间：
2005
影响因子：
4.3
通讯作者：
R. K. Marcus
中科院分区：
文献类型：
--
作者：
J. Venzie;R. K. Marcus
随着色谱技术的发展和新的分离技术的应用，必须开发新的检测器技术以跟上步伐。色谱分析师希望检测器对多种物质敏感，在初始支出和持续操作方面成本低，占地面积小（如果可能的话），并且不产生需要昂贵处理的废物。然而，也许最重要的是，检测装置应该与分离移动的相组成和流速（气体或液体）直接兼容。虽然已经有数千篇研究出版物描述了气相色谱和液相色谱（GC和LC）与等离子体源的耦合，但微波诱导等离子体原子发射检测器（MIP-AED）[1]可能是第一个商业实用的色谱检测器，因为它在移动的相组成和流速方面与毛细管GC输出自然耦合。分析仪器的一个主要趋势是走向现场便携性和“微型”规模;这与分离技术非常相似。最近，一些研究小组已经向前迈进，并推出了不同版本的微型分析等离子体用作色谱检测器[2-12]。这些装置专门用作光发射源，Niemax及其同事[10]除外，他们采用二极管激光器测量分析物原子吸光度。如果使用“微”作为具有亚毫米尺度上的某种维度的等离子体的宽松定义，则参考文献[2-4，7，9，10，13]的等离子体是真正的微等离子体。这些小型放电装置在大气压力或略低于大气压力下工作，需要很少的功率。表1列出了不同仪器报告的样品形式和元素沿着检测限（如报告）。大多数微型/微等离子体被设计为接受气态形式的样品，因此非常适合GC应用[2，4-7，10]。在这种情况下，维持等离子体气体和GC载气（例如氦气）是同一种气体。由于液体基质的存在增加了等离子体的能量需求，LC微等离子体检测器的情况变得复杂。在传统的等离子体和火焰光谱法中，雾化、去溶剂化和解离的步骤通过使用雾化器和喷雾室来增强。这可能是一个非常低效的过程，不适合低流量分离。即使在直接注射雾化器[14]与ICP源结合的情况下，传统等离子体源及其相关光谱仪的功率、尺寸等也限制了其作为一般LC检测器的可接受性。直接LC检测的另一种方法是使用移动的相本身作为等离子体回路中的电极，类似于GC情况下的He。1959年，Couch和Brenner [15]报道了在一个设计用于进行大规模电解的装置中，在盐水溶液和悬浮在液体表面上的钨电极之间产生辉光放电。他们描述了当铜和铟的溶液构成液体阴极时的元素特定发射，尽管当使用其他阳离子（例如Na，Li和U）时没有观察到可分配的发射。这个想法最终导致了Cserfalvi和Mezei [5]，Marcus和Davis [3]以及Jenkins和Manz [9]设计的原子发射仪器，所有这些仪器都使用导电溶液作为放电电路中的电极之一。Cserfalvi和Mezei的仪器[5]，电解质阴极辉光放电（ELCAD），利用电解质的垂直喷泉，在其上...
As the general field of chromatography has evolved and new separation techniques have been implemented, new detector technology has had to be developed to keep pace. Chromatographers desire detectors that are sensitive to multiple species, low cost in terms of initial outlay and continuing operation, small footprint (if possible), and do not create waste requiring expensive handling. Perhaps most importantly, though, is the fact that the detection means should be directly compatible with the separation mobile phase composition and flow rate (be it gas or liquid). While there have been thousands of research publications describing the coupling of gas and liquid chromatography (GC and LC) to plasma sources, the microwave induced plasma–atomic emission detector (MIP–AED)[1] was perhaps the first commercially practical chromatographic detector by virtue of its natural coupling to capillary GC output in terms of mobile phase composition and flow rate. A major trend in analytical instrumentation has been towards field portability and ‘‘micro’’scaling; much in parallel with separation technologies. Recently, a few research groups have stepped forward and introduced different versions of miniature-scale analytical plasmas for use as chromatographic detectors [2–12]. These devices have been used exclusively as optical emission sources with the exception of Niemax and co-workers [10] who have employed a diode laser to measure analyte atomic absorbance. If one uses a loose definition of ‘‘micro’’as a plasma having some dimensionality on the sub-millimeter scale, the plasmas of references [2–4, 7, 9, 10, 13], are truly microplasmas. These small discharges operate at or just below atmospheric pressure and require very little power. Table 1 lists the sample form and elements reported for the different instruments along with the limits of detection if reported. Most mini/microplasmas are designed to accept the sample in the gaseous form and are thus well suited to GC applications [2, 4–7, 10]. In this case, the sustaining plasma gas and the GC carrier gas (eg helium) are one in the same. The case for an LC microplasma detector is complicated by the fact that the presence of the liquid matrix increases the energy requirements of the plasma. In conventional plasma and flame spectroscopy, the steps of nebulization, desolvation, and dissociation are augmented by the use of nebulizers and spray chambers. This can be a very inefficient process not suitable to lowflow separations. Even in the case of direct-injection nebulizers [14] in conjunction with ICP sources, the power, size, etc. of conventional plasma sources and their associated spectrometers have limited their acceptance as general LC detectors. Another approach to direct LC detection is to use the mobile phase itself as an electrode in the plasma circuit, analogous to He in the GC case. In 1959, Couch and Brenner [15] reported generation of a glow discharge between an aqueous salt solution and a tungsten electrode suspended over the surface of the liquid in a device designed to perform large-scale electrolysis. They described element-specific emission when solutions of copper and indium made up the liquid cathode, though no assignable emission was observed when other cations (eg, Na, Li, and U) were used. This idea eventually lead to the atomic emission instruments designed by Cserfalvi and Mezei [5], Marcus and Davis [3] and Jenkins and Manz [9] all of which make use of a conducting solution as one of the electrodes in a discharge circuit. Cserfalvi and Mezei’s instrument [5], the electrolyte cathode glow discharge (ELCAD), makes use of a vertical fountain of electrolyte over which is …