CE Interfaces

Fig. 1 The inductively coupled plasma-mass spectrometer (ICP-MS) used as a detector for high-performance liquid chromatography (HPLC). The liquid sample passes through the capillary into a nebulizer where it is changed into an aerosol. The aerosol passes through a spray chamber and into the plasma. The analytes pass into the mass spectrometer. The CE interface is not in detail in this figure.

A basic understanding of the nebulizer function and the types of nebulizers is necessary to successfully interface CE to the ICP-MS. Nebulization, as previously described, is the process to form an aerosol, i.e., to suspend a liquid sample into a gas in the form of a cloud of droplets. The quality of any nebulizer is based on many different parameters including mean droplet diameter, droplet size distribution, span of droplet size distribution, droplet number density, and droplet mean velocity. There are numerous nebulizers commercially available for the use with ICP-MS systems, and their detailed description can be found elsewhere.Pneumatic designs, both concentric and cross flow, are the most popular for CE interfaces with the occasional use of the ultrasonic nebulizer (USN). Figure 2 shows some typical nebulizers. The pneumatic nebulizer is either a concentric design (Fig. 2A), where both the gas stream and the liquid flow in... 

Although there has been limited use with CE interfaces, the direct injection nebulizer (DIN) was first described by Shum et al. - and later used by Liu et al. for CE (Fig. 2E). In this design, the nebulizer introduces the sample very near the plasma inside the ICP torch and eliminates the spray chamber assembly. Close to 100% analyte transport efficiency can theoretically be obtained with the DIN, but the nebulizer is restricted to very low liquid flow rate and thus is well matched to CE interfacing. This design does induce local plasma cooling due the lack of desolvation and detection limits are only slightly improved over other nebulizer designs. 

OCN), which is a variation of the pneumatic concentric nebulizer built from flexible capillary mbes, was used in an interface. The OCN has had little application in CE interfaces, owing to its generally lower sensitivity performance when compared to other pneumatic nebulizers used with ICP-MS detection.The direct injection nebulizer (DIN), previously described in The Nebulizer, was used by Liu et al. in a CE interface. The electrophoretic capillary was directly inserted through the central sample introduction capillary of the DIN. A platinum grounding electrode was positioned into a three-port connector. This connector contained the DIN sample introduction capillary as well as a make-up buffer flow. These alternative nebulizers have been successfully used in CE interfaces, but the pneumatic designs dominate the interface systems reported in the literature. 

In a previously mentioned work by Kirlew et al., electrophoretic separations of Se, Se As As, and dimethylarsinic acid were performed using various ultrasonic nebulizer (USN) interfaces. Using the optimized CE interface conditions and a borate run buffer at pH 8, a separation was accomplished within 10 min. Electrokinetic injections gave better sensitivities for the analytes as compared to hydrostatic sample injection. In the Kirlew study, arsenate and selenite ions had very similar migration times, but these analytes were easily resolved by the multielement capability of the ICP-MS detector. An electropherogram of this work is shown in Fig. 5. In an application to field samples. Van Holderbel ... 

An estimation of the heat removed is complex since it not only involves latent heat of fusion, but sensible heat effects that may not be insignificant where large systems are involved. A further complication arises where natural convection in the water at the water ce interface occurs, i.e. modifying the simple conduction concept implied in Equation 9.5. 

Kulka, S., G. Quintas, and B. Lendl. 2006. Automated sample preparation and analysis using a sequential-injection-capillary electrophoresis (SI-CE) interface. Analyst 131 739-744. 

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