Pulsed amperometric detector for

Dionex also offers visible, fluorescence, and pulsed amperometric detectors for use with the series 4000i. Dionex also supply a wide range of alternative instruments, e.g., single channel (20101) and dual channel (2020i). The latter can be upgraded to an automated system by adding Autoion 100 or Autoion 300 controllers to control two independent ion chromatograph systems. Dionex also supply 20001 series equipped with conductivity pulsed amperometric, UV/visible, visible, and fluorescence detectors. 

Tabata, S. and Dohi, Y., An assay for oligo-(l—>4) —5(1—>4)-glucantransferase activity in the glycogen debranching enzyme system by using HPLC with a pulsed amperometric detector, Carb. Res., 230,179, 1992. 

In the literature, isocratic solvent systems are found for monosaccharides, and gradient systems are described for oligosaccharides. Table 3.4 gives some gradient systems for carbohydrate separation Table 3.5 illustrates programs for pulsed amperometric detectors. 

The equipment widely used for the detection of carbohydrates in the HPLC method is the differential refractive index (RI) detector. The principle involved in this detection depends on the continuous measurement of the variation of the RIs of the mobile phase containing the samples with little or no chromophores such as carbohydrates, lipids, and other polymer compounds that do not absorb UV light. RI detection method presents high degree of reproducibility and is very convenient for the analysis of polysaccharides. However, other detectors such as evaporative light scattering detector and pulsed amperometric detector have been used for the detection of polysaccharides [100]. 

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)... 

Thirteen free and conjugated bile acids (e.g., cholic, taurocholic, deoxycholic, lithocholic, taurolithocholic, glycodeoxycholic acids) were baseline resolved on a 35 C PLRP-S column (pulsed amperometric detector at +0.03V vs. Ag/AgCl). A 55-min 20/60/30-> 29/51/20 acetonitrile/water/water (0.5 M NaOH) gradient was used [1066]. The presence of 0.1 M hydroxide is essential for the amperometric detection of the hydroxyl fimctional group on the bile acids. However, the authors... 

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. 

More recently, the availability of high performance anion exchange columns stable at high pH conditions and the development of the pulsed amperometric detector (PAD) (78,79) have introduced a most powerful method for the HPLC of higher oligosaccharides. Such a system was introduced by Rochline and Pohl (80), and commercialized into the Dionex system. 

The detection system employed, a pulsed amperometric detector, permits remarkable sensitivity (100 ppb), and provides the most sensitive workable commercially available detector yet developed for HPLC of under vatized carbohydrates. The drawback is that it requires strongly alkaline conditions for optimum carbohydrate oxidation (and detection). Eluants therefore contain high concentrations of non-volatile salts (typically sodium acetate and sodium hydroxide) and further structural elucidation or identification by e.g. mass spectroscopy and/or NMR requires prior desalting. The use of an anionic micromembrane suppressor downstream of the detector, thus converting the sodium hydroxide and sodium acetate to water and acetic acid, respectively, has been found satisfactory for NMR at 500 MHZ (86). However, with the inherent insensitivity of NMR and the low capacity of pellicular HPAEC columns, preparation for more sensitive analytical methods, e.g. MS, is desirable. Derivatization of fractionated oligosaccharides (either by methylation techniques or reductive coupling of 4-amino-ben oic acid ethyl ester, ABBE) and subsequent... 

Also using amperometric detection, Chen, et al. developed a three-dimensionally adjustable amperometric detector for MCE and applied to separate aromatic amines and nitroaromatic pollutants. Amperometry, and related pulse amperometric detection modes, are very attractive because provide enhanced selectivity related to conductimetric detection. 

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. 

Electrochemical detectors, which are based on the electrochemical oxidation or reduction of the analyte, can be applied to the analysis of selected compounds such as phenols. It is physically simple, but is very sensitive for catecholamines. However, the adsorption of reacted molecules on the surface of the electrodes can reduce the conductivity. To overcome this problem a pulsed voltage is applied, which cleans the electrode surface between measurements. This pulsed amperometric detection is also sensitive for carbohydrates. 

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). 

All of the fat-soluble vitamins, including provitamin carotenoids, exhibit some form of electrochemical activity. Both amperometry and coulometry have been applied to electrochemical detection. In amperometric detectors, only a small proportion (usually <20%) of the electroactive solute is reduced or oxidized at the surface of a glassy carbon or similar nonporous electrode in coulometric detectors, the solute is completely reduced or oxidized within the pores of a graphite electrode. The operation of an electrochemical detector requires a semiaqueous or alcoholic mobile phase to support the electrolyte needed to conduct a current. This restricts its use to reverse-phase HPLC (but not NARP) unless the electrolyte is added postcolumn. Electrochemical detection is incompatible with NARP chromatography, because the mobile phase is insufficiently polar to dissolve the electrolyte. A stringent requirement for electrochemical detection is that the solvent delivery system be virtually pulse-free. 

Because the sensitivity of the detector decreases with decreasing analyte ionization, the pH of the mobile phase should be chosen to maximize solute dissociation. For example, anions with pKa values above 7 are not detectable by conductivity detection. However, conductivity detection is often the preferred method for organic acids with carboxylate, sulfonate, or phospho-nate functional groups, since the pKa values are below 5. For cations, most aliphatic amines have pKa values around 10 and are readily detected by conductivity detection. The pKa values of aromatic amines, however, are in the range 2 to 7, which is too low to be detected by suppressed conductivity. Sensitivity by nonsuppressed conductivity is also poor, so these amines are monitored by UV absorption or pulsed amperometric detection. 

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