Organic acids conductivity detectors

It is appropriate to refer here to the development of non-suppressed ion chromatography. A simple chromatographic system for anions which uses a conductivity detector but requires no suppressor column has been described by Fritz and co-workers.28 The anions are separated on a column of macroporous anion exchange resin which has a very low capacity, so that only a very dilute solution (ca 10 4M) of an aromatic organic acid salt (e.g. sodium phthalate) is required as the eluant. The low conductance of the eluant eliminates the need for a suppressor column and the separated anions can be detected by electrical conductance. In general, however, non-suppressed ion chromatography is an order of magnitude less sensitive than the suppressed mode. 

A conductivity detector measures the electrical conductivity of the HPLC eluent stream and is amenable to low-level determination (ppm and ppb levels) of ionic components such as anions, metals, organic acids, and surfactants. It is the primary detection mode for ion chromatography. Manufacturers include Dionex, Alltech, Shimadzu, and Waters. 

In most cases, the separation of alcohols, usually methanol, ethanol, and glycerol, is carried out contemporaneously with the separation of sugars and organic acids, and almost always the desire is to quantify all these analytes. It is seen, therefore, that the mobile phase is often an aqueous acid solution, even though only water may be used (5,9). Sulphuric acid is the one most frequently used, although phosphoric acid is preferred by some, since it is less corrosive on the components of the HPLC system (10). The concentration of sulphuric acid normally varies between 0.004 N and 0.01 N or more. The choice of acid may, however, be dictated by other considerations. This is the case, for example, with the use of a conductivity detector, which requires an appropriate conductivity suppressor system. If such a device is not available for a particular... 

Organic acids can also be quantified using HPLC linked to a conductivity detector. This has one advantage over UV detection in that only charged species are measured, which means that the method is liable to fewer interferences. Depending on the actual approach chosen, it is sometimes possible to detect other anions, such as Cl-, S04 and P04, in the same ran. If the conditions given in Dionex application no. 21 are used, this allows the organic acids to be separated without interferences from the fully ionised anions such as Cl-, S04 (Anon, n.d.d). 

The method involves chromatographic separation of water soluble analytes and detection of separated ions by a conductivity detector. It can also be used to analyze oxyhalides, such as perchlorate (C104) or hypochlorite (CIO) weak organic acids, metal ions, and alkyl amine. The analytes that can be determined by ion chromatography are listed in Table 1.11.1. 

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. 

Many GC detectors exist, but not all are suitable for phytochemicals. The thermal conductivity detector (TCD) is considered a universal detector and is appropriate for most analytes as long as the thermal conductivity of the carrier gas is different from that of the analytes. During the early development phase of GC, TCD was an easy choice because thermal conductivity measuring devices were already in use (Colon and Baird, 2004). Ionization detection arrived with its improved trace determinations and replaced TCD in many applications. While TCD is still used for some food applications (Allegro et ak, 1997 Sun et ak, 2007) and in the past was used for phenolic acids (Blakely, 1966), currently it is not generally used for phytochemicals. Rather, the flame ionization detector (FID) is better-suited due to its selectivity for organic compounds and superior measuring ability for trace measurements. 

Figure 14.4 Separation of organic acids (as anions) and inorganic anions in wine (reproduced with permission of Dionex). Conditions column, 25cm x 4mm i.d. and precolumn stationary phase, lonPac AS11-HC mobile phase, 1.5 ml min nonlinear gradient from 1 mM to 60mM NaOH and from 0 to 20% methanol temperature, 30°C conductivity detector after packed suppressor. Peaks 1 = lactate 2 = acetate 3 = formate 4 = pyruvate 5 = galacturonate 6 = chloride 7 = nitrate 8 = succinate 9 = malate 10 —tartrate 11—fumarate 12 —sulfate 13 = oxalate 14 = phosphate 15 —citrate 16 = isocitrate 17 = c/s-aconitate 18 = frans-aconitate.

This type of detector is used in ion chromatography for the detection of inorganic anions (e.g., S04, PO43 ), some inorganic cations (e.g Ca +, Mg2+), and some ionised organic acids. This is due to the fact that all ions are electrically conducting. Conductivity detectors are based on the conductance of an eluent prior to and during the elution of the analyte from the column. 

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