Pulsed Amperometric Detection PAD

Reductive cleaning by a large negative potential pulse, or Ejed (-1.6 to -2.0 V) for a period of t ed- In addition, any Au ions in the diffusion layer are reduced to elemental gold, which greatly extends the Ufe of the electrode 

Oxidative cleaninghy a positive potential pulse, or Eoxd (-t-0.6 to +0.8 V) for a period of t xd 

Reductive activation and adsorption by a large negative step to Eads (-0.8 to +0.1 V) for tads prior to the next detection cycle. 

This last step can also be used to increase adsorption, or preconcentration, of the analyte to the electrode surface. 

Parameter PAD Potential (V) Time (min) vs. Ag/AgCI reference IPAD Time (min) Potential (V) vs. pH reference Int ration 

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Pulsed amperometric detection (PAD), introduced by Johnson and LaCourse (64, 65) has greatly enhanced the scope of liquid chromatography/electrochemistry (66). This detection mode overcomes the problem of loss of activity of noble metal electrodes associated with the fixed-potential detection of compounds such as carbohydrates, alcohols, amino acids, or aldehydes. Pulsed amperometric detection couples tlie process of anodic detection with anodic cleaning and cathodic reactivation of a noble metal electrode, thus assuring a continuously cleaned and active... 

Oligosaccharides were isolated preparatively by high-pH anion exchange chromatography carried out on a LC-system (Dionex Corporation, Sunnyvale CA) equipped with a CarboPac PA-1 column (9 x 250 mm), coupled to a Spectra System AS 3500 auto sampler. The detection was carried out using Pulsed Amperometric Detection (PAD-II). 

The quality of this library was examined by monosaccharide compositional analysis. A portion of the trisaccharide library was treated with aqueous trifluroacetic acid at 100°C for 4 h. Analysis of the resulting mixture of monosaccharides on a Dionex HPLC system with a PA1 column and pulsed amperometric detection (PAD) showed that galactose, glucose, and mannose were present in approximately the required ratio. 

Amperometric detection is a very sensitive technique. In principle, voltammetric detectors can be used for all compounds which have functional groups which are easily reduced or oxidized. Apart from a few cations (Fe , Co ), it is chiefly anions such as cyanide, sulfide and nitrite which can be determined in the ion analysis sector. The most important applications lie however in the analysis of sugars by anion chromatography and in clinical analysis using a form of amperometric detection know as Pulsed Amperometric Detection (PAD). 

Copper electrodes have been used to determine amino acids and carbohydrates [10]. Metal oxide electrodes (including thin-film semiconductors) show some promise, but nothing of substance has yet been published with regard to LCEC. Pulsed amperometric detection (PAD) takes advantage of metal oxides formed in situ. This approach is discussed later. 

For the liquid chromatographic detection of carbohydrates and alcohols, a pulsed waveform of the type illustrated in Figure 27.16A is employed the technique is called pulsed amperometric detection (PAD). In this case, the potential is stepped to El, where oxidation of the compound takes place. The current is sampled for a short time at the end of the pulse (16.7 ms), where charging current is at a minimum. The potential is then stepped to E2, where the electrode undergoes oxidative cleaning. Last, the oxide-free surface is regenerated at potential E3. 

Figure 27.16 Potential-time (E - t) waveforms. Processes Elt anodic detection E2, oxidative cleaning E3, cathodic reactivation. Waveforms are (A) pulsed amperometric detection (PAD) with a short current sampling period, (B) PAD with a long current integration period, and (C) integrated PAD with a long integration period. [Reproduced with permission from Ref. 31.]...

Pulse amperometric detection (PAD) has been used for the detection on a PDMS chip. This method is useful for analysis of underivatized compounds, such as carbohydrates, amino acids, and sulfur-containing antibiotics, which easily caused electrode fouling. In PAD, a high positive potential (1.4-1.8 V) is first applied in order to clean the electrode (e.g., Au) surface. This is followed by a negative potential step (-0.5 V) to reactivate the electrode surface. A third moderate potential (+0.5 to +0.7 V) is applied for detection of the target analyte [752]. 

The refractive index detection (RID), often used in high-performance liquid chromatography, is an interesting detection method in CE with a laser light source and a limit of detection (LOD) in the micromolar range. Electrochemical detection (ECD) and pulsed amperometric detection (PAD) of sugars are common and effective methods used in HPLC. Some recent communications show that the sensitivity of these detection methods in CE have an approximately 1000-fold better LOD than RID. Unfortunately, these de-... 

Pulsed amperometric detection (PAD) Amino acids are not generally considered to be electrochemically active because products of the oxidation accumulate on the electrode surface and prevent it from participating in any further electrochemical processes. This problem can be overcome if PAD is employed. Amino acids are generally detected using a platinum electrode under alkaline conditions (0.25 mol 1 NaOH) using a triple-pulse waveform with Ei, E2, and E3 at 0.50, — 0.89, and 0.70 V, respectively. Due to the basic conditions required for the detection of amino acids, a base-stable anion-exchange column must be employed. Detection limits of 50 pmol have been obtained for phenylalanine and methionine using this technique. 

Figure 4 Typical potential step sequence in (A) pulsed amperometric detection (PAD) and (B) integrated pulsed amperometric detection (IPAD) of peptides.

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