Flow injection analysis-direct current

For the routine determination of analytes in the quality control of the production of speciality chemicals, a combination of direct current plasma emission spectroscopy (DCP-OES) with flow injection analysis (FIA) has been used. Results obtained for the determination of boron, copper, molybdenum, tungsten and zinc in non-aqueous solutions have been published by Brennan and Svehla [3], The principle has been extended to other analytes, carrier liquids, and solvents, and the details of a fully automatic system have been described by Brennan et al. [4]. 

Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) is used for multi-element determinations in blood and tissue samples. Detection in urine samples requires extraction of the metals with a polydithiocarbamate resin prior to digestion and analysis (NIOSH 1984a). Other satisfactory analytical methods include direct current plasma emission spectroscopy and determination by AAS, and inductively coupled argon plasma spectroscopy-mass spectrometry (ICP-MS) (Patterson et al. 1992 Shaw et al. 1982). Flow injection analysis (FIA) has been used to determine very low levels of zinc in muscle tissue. This method provides very high sensitivity, low detection limits (3 ng/mL), good precision, and high selectivity at trace levels (Fernandez et al. 1992b). 

The combination of ASV with the flow injection analysis (FIA) improves the reproducibility and versatility of the anodic stripping procedure. For a conventional operation the detection limit is of 10" M. The disadvantage of the FIA, namely the short deposition time, can be compensated by repeated reversals of the flow direction [174]. The multiple passage of the same sample along the mercury-plated carbon fiber electrode increases the effective deposition time and thus the stripping current response. 

With a method in hand for routinely constructing cytochrome c oxidase modified electrodes that exhibited direct electron transfer between the electrode and the oxidase, amperometry was used to detect reduced cytochrome c in solution at the oxidase-modified electrodes in a flow injection analysis format [69]. The dialysis cell was equipped with a wall jet inlet to direct cytochrome c solution past the oxidase-modified electrodes. Figure 12 shows the current response for three sequential reduced cytochrome c injections. Control experiments conducted at bilayer modified electrodes containing no oxidase showed current responses that are about 2% of those shown in Figure 12. This response may be due to changes in electrode capacitance and/or cytochrome c reacting at bilayer defect sites on the electrode. QCM measurements showed that no cytochrome c incorporated into the bilayer. However, cytochrome c was electrostatically held at the surface of the bilayer membrane at lower ionic strength [69]. 

The data are referenced to the most efficient molecule, in this case Verapamil. The MRM sensitivity is the signal obtained from one product ion which reflects typical assay conditions. The sampling efficiency refers to the number of ions entering the vacuum system optics. The relative sampling efficiency directly reflects the relative ionization efficiency because the transfer efficiency will be very similar for all compounds. The relative sampling and MRM efficiencies are similar but not identical because some compounds concentrate the majority of the ion current in a single product ion. Data acquired by flow injection analysis at 200 pL/min in 50/50 MeOH/HaO, 0.1% formic acid. For the chemical structures see the Appendix or Ref. 57. 

This approach will not be practical for some time to come. The fundamental properties of surfactants (micelle formation, enrichment at interfaces) mean that the activity of a surfactant will usually differ from its absolute concentration (1). Just as serious is the technical problem that current surfactant-selective electrodes suffer from response which varies with their past and recent history they are also sensitive to the concentration of nonsurfactant ions. The result is that quantitative applications use electrodes not in direct measurements relating potential to concentration, but as indicators of the end point of a titration. In this latter application, it is not important that the electrode potential be exactly reproducible, but only that the potential change sharply as the surfactant concentration changes. For the titration of an anionic surfactant with a cationic surfactant, the electrode used for end point detection can be chosen to respond to either surfactant. Because of the drift in electrode potential, titrations must be conducted to an inflection in the titration curve rather than to a specific millivolt value. Details of the potentiometric titration methods can be found earlier in this chapter. The electrodes have also been demonstrated as detectors for flow injection analysis. 

One of the benefits of electrochemical batch injection analysis is that dilution of the sample with electrolyte is not necessary, see below. A sample of volume =sl00p.L is injected directly from a micropipette, tip diameter 0.5 mm, over the centre of a macroelectrode exactly as in a wall-jet system. This is equivalent to a flow injection system with zero dispersion. During the injection, and after a short initial period to reach steady-state, the hydrodynamics is wall-jet type and a time-independent current is registered. BIA was first devised using amperometric, e.g., [31], and potentiometric, e.g., [34], detection. A typical amperometric trace is shown in Fig. 16.5. By using a programmable, motorised electronic... 

Emission spectroscopic techniques such as inductively coupled plasma optical emission (ICP-OES) and direct current plasma optical emission (DCP-OES). include the analysis of copper in biological materials (Delves et al.. 1983. Roberts et al., 1985). These techniques, with suitable sample preparation, have sufficient low bias and precision for clinical work but are more expensive and more complex than AAS (Herber et al.. 1982). Flow injection-ICP-OES will be mentioned below. 

Mass spectra are similarly meant to complement existing data bases. Mass spectra are very suitable to identify additives in solvent extracts of food contact polymers. For substances that can be analysed by GC the mass spectra were measured on two GC-MS systems to generate classical electron-impact spectra. However many additives cannot be analysed by GC for reasons of e.g. low volatility or thermal instability. For these substances the rapidly developing technique of LC-MS is suitable. A limitation of current LC-MS instrumentation is that most ionisation interfaces do not give consistent and library-searchable spectra. One exception to this is the particle-beam interface which gives classical electron-impact spectra. So for additives that could not be analysed by GC, then direct source injection or flow-injection particle beam MS analysis was applied. These spectra were recorded twice in the same laboratory (separated by several weeks) to check for consistency. 

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