Flow-injection assemblies limitations

Since ISEs can be used in continuous flow systems or in flow systems with sample injection (flow injection analysis, FIA)21 their application is wide, not limited to discrete samples. Analysis time becomes shorter, with faster recycling. Additionally, in flow systems the experimental assembly and data analysis can be controlled automatically by microcomputer, including periodic calibration. Another development is the use of sensors for the detection of eluents of chromatographic columns in high-pressure liquid chromatography (HPLC). Miniaturization has permitted an increase in the use of sensors in foods, biological tissues, and clinical analyses in general. 

Hapten monolayer electrode sensor assembly was used to detect triazine in a flow injection analysis mode. The interaction of the electrode with different antibody concentrations resulted in the formation of an antibody-antigen (Ab-Ag) complex which insulated the electrode towards the [Fe(CN)6] /Fe(CN)6] " redox probe and diis in turn resulted in no charge transfer. The extent of insulation depends on the antibody concentration and the time of exposure to the antibody solution. The decrease in amperometric response of the antigenic monolayer to corresponding antibody solution for a fixed time produces a quantitative measurement of the antibody concentration. Typical responses obtained for cyanazine-hapten monolayer electrode to different antibody concentrations is shown in Figure 4. The lowest detection limit achieved for cyanazine sensor was 4.0 pg/ml at a response time of few minutes and a less-than 2% cross-reactivity to atrazine, simazine and other metabolites. 

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. 

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