Amperometric detectors pulsed potential

Potential of zero charge, 20, 23, 25, 66 Potential scanning detector, 92 Potential step, 7, 42, 60 Potential window, 107, 108 Potentiometry, 2, 140 Potentiometric stripping analysis, 79 Potentiostat, 104, 105 Preconcentrating surfaces, 121 Preconcentration step, 121 Pretreatment, 110, 116 Pulsed amperometric detection, 92 Pulse voltammetry, 67... 

The pulsed amperometric detector (PAD) developed by Johnson and co-workers using an Au or Pt electrode has permitted the direct detection of aliphatic alcohols including carbohydrates, amines, and sulfur compounds. Fouling of these electrodes is prevented by application of both positive (to eliminate sample adsorption) and negative (to reduce any metal oxide) reactivation step potentials on the order of 100 ms before resetting the potential for detection of the analyte. The analytical current is usually sampled near the end of the detection potential pulse to permit decay of the charging current. The oxidation of these aliphatic compounds such as carbohydrates is facilitated in basic solution at about pH 12, so postcolumn addition of 0.1 Af NaOH or the use of a polymeric column with a basic mobile phase is required. Detection limits of alcohols and carbohydrates are at the 10 ppb level. Alka-nolamines, amino acids, and sulfur compounds other than sulfonic acids and sulfones can also be detected. 

Although tocopherols and tocotrienols can be detected by UV absorbance at 280 nm, fluorescence detection (excitation 294 nm and emission 326 nm), as shown in Figure 11.3, has proven to be a much more sensitive method. Electrochemical detection such as pulsed amperometric and coulometric (Uspitasari-Nienaber, 2002) has also proven to be sensitive and potentially valuable for the quantitative analysis of tocopherols and Tocotrienols (Abidi, 2000), especially for tocol analysis in blood and serum samples. HPLC mass detectors such as flame-ionization detectors, evaporative light-scattering detectors, and charged aerosol detectors have proven to be valuable for the quantitative analysis of many types of lipids, but because tocols have... 

Several metal oxides (platinum, gold," nickel, copper, ) and cobalt phtalo-cyanine have been employed as surface bound mediators for carbohydrate detection. In a dc amperometric mode of operation detectors based on these mediators exhibit a significant loss of response with time and/or exposure to analyte. Various potential pulse programs have circumvented this stability problem, but at the expense of sensitivity and complexity of the instrumentation. Silver electrodes coated with electrogenerated silver oxide exhibit electrocatalytic activity with respect to carbohydrate oxidation. This paper describes our efforts to utilize an electrode as a carbohydrate detector in a dc amperometric mode. 

Pulsing the applied potential at the working electrode using simple waveforms has long been employed in EC analysis and was also proposed in some of the earliest patents for EC detectors for liquid chromatography. In the early 1980s, Johnson and co-workers at Iowa State University began an extensive series of studies on pulsed amperometric detection at platinum electrodes in a flow injection system of simple... 

The separation of monosaccharides on CarboPac PAl under isocratic conditions shown above - especially the separation of epimers - can only be carried out with a very dilute NaOH eluent (c = 1 mmol/L). A sodium hydroxide concentrate (c = 0.3 mol/L) must be added to the column effluent before it enters the detector cell. Subsequently, pulsed amperometric detection with the conventional pulse sequence of four different potentials (see Section 8.1.2.2) is applied. 

The amperometric detection uses less than 10% of the analyte in the flow cell, unlike the coulometric detector, and can be operated in a pulsed mode (cydic voltammetry, with a gold working electrode) in addition to the constant potential mode. The pulsed mode helps cleaning the working electrode. 

The separation of monosaccharides on CarboPac PAl under isocratic conditions shown above — especially the separation of epimers — can ordy be carried out with a very dilute NaOH eluant (1 mmol/L). An NaOH concentrate (0.3 mol/L) must be added to the column effluent before it enters the detector cell. Subsequently, pulsed amperometric detection with the conventional pulse sequence of three different potentials is applied. Post-column addition of an NaOH concentrate is necessary to raise the eluant pH to about 13, a value that is optimal for pulsed amperometric detection. The addition of NaOH can be done pneumatically via a metal-free manifold connected to a reaction coil of corresponding dimension in which the column effluent and the NaOH concentrate are mixed. Alternatively, a pump with a corresponding pulse damper can be used. Application of the new quadruple pulse sequence (see Section 7.1.2.2) has rendered the post-column addition of an NaOH concentrate obsolete. However, the linear range is not that large in comparison with the detection at pH 13. 

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