Pulsed Amperometric Detector PAD

The principles of the PAD were discussed in Chapter 4. Perhaps the major use of the PAD in anion chromatrography has been for the detection of carbohydrates. At pH around 11 or higher sugars become anionic and can be separated by anion chromatography. For many years a somewhat awkward post-column derivatization reaction was used for detection of carbohydrates after a chromatographic separation, but the use of a PAD now provides a simple and direct detection method. 

All carbohydrates (aldoses and ketoses) and polyalcohols produce a large anodic peak response at ca. +0.15 V. The peak for glucose corresponds to a reaction with n approaching 10 equiv/mol for fluid velocities typical of flow-through detection cells. This n value is consistent with an oxidative cleavage for the C1-C2 and Cs-C bonds to form two moles of formate and one mole of dicarboxylate dianion [24]. 

Either a gold or platinum electrode may be used in a PAD, although gold electrodes are more popular. One reason is that dissolved oxygen contributes a cathodic response at a platinum electrode over the entire useful range for anodic detection. Serious oxygen interference at the gold electrode can be avoided by careful selection of detection potential. 

A simple but useful example of pulsed-amperometric detection is shown in Fig. 6.21 where glucose, fructose and a trace of sucrose are determined in honey by anion chromatography. Much more complex samples can be resolved using gradient elution. This is demonstrated in Fig. 6.22 where 18 carbohydrates were separated. Elution of the later peaks is speeded up by gradually reducing the eluent pH to inhibit ionization of the carbohydrates. However, post-column addition of 0.4 M sodium hydroxide was needed to restore the effluent to a pH sufficiently alkaline for effective pulsed amperometric detection. 

All carbohydrates (aldoses and ketoses) and polyalcohols produce a large anodic peak response at about -t-0.15 V. The peak for glucose corresponds to a reaction 

3 = fucose f = rhamnose g = arabinose h = glucose I = xylose j = fructose k = sucrose I = unknown m = maltose n = maltotriose o = maltotetraose p = malto-pentaose q = maltohexaose r = malto-heptaose (from Ref [32] with permission). 

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With the development of the pulsed amperometric detector (PAD) [13] a new detector cell was also designed. It is schematically shown in Fig. 6-10. To facilitate replacing the... 

Pellicular anion-exchanger (CarboPac PA-1) NaOH, 150-160 mmol r for most neutral sugars, 10-15 mmol 1 for amino sugars None RT Pulsed amperometric detector (PAD)... 

The various types of detectors that can be used in ion chromatography are discussed in Chapter 4. Variable wavelength UV-Vis detectors are extremely useful for detection of sample ions with sufficient absorbance at the analytical wavelength. Various electrochemical detectors, including the pulsed amperometric detector (PAD), offer excellent selectivity and sensMvily. 

Conductivity, direct absorbance or a differential refractometer are the most common forms of detection for lEC, PAD and ELSD. A pulsed amperometric detector (PAD) or, more recently, an evaporative light-scattering detector (ELSD) is appropriate for detection of carbohydrates. Both non-suppressed and suppressed conductivity have been used extensively. The need to incorporate a low concentration of a strong acid into the eluent has been an impediment to direct conductivity detection. 

With the development of the pulsed amperometric detector (PAD) [32] a new detector cell was also designed. It is schematically shown in Fig. 7-25. To facilitate the replacement of the working electrode, the detector cell consists of two blocks separated by a spacer. One part houses the reference electrode and the capillary connections, the other houses the working electrode. This second cell block may be replaced as a whole. Two stainless steel connectors at the inlet and outlet boreholes serve as a counter electrode. 

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. 

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