Eluent background conductivities

The chromatograms in Fig. 8.4 show the effect of enhancement columns. Each enhancement column increases the carbonic acid peak height, but the baseline conductance is also increased somewhat. A pre-column packed with anion-exchange resin in the OH" form was then placed between the pump and loop injector to remove completely and continuously the carbon dioxide in the eluent. This arrangement resulted in a significant decrease in eluent background conductance, as shown in Fig. 8.4D. An almost linear calibration plot was obtained from 0.05 to 5.0 mM bicarbonate. The detection limit was estimated to be 1.45 pM. 

On the contrary, eluents with high background conductance (e.g., NaOH) are amenable for indirect conductivity detection, where analyte elution is accompanied by a negative conductance change [247]. 

Suppressed conductivity detection is the most common mode of detection and differs from the previous approach for the use of an additional device, called suppressor, whose function is to reduce the background conductivity of the eluent prior to the conductivity cell and to increase the signal of the analyte. 

Conductivity detector is the most common and useful detector in ion exchange chromatography. However UV and other detectors can also be useful [10]. Conductivity detection gives excellent sensitivity when the conductance of the eluted solute ion is measured in an eluent of low background conductance. Therefore when conductivity detection is used dilute eluents should be preferred and in order for such eluents, to act as effective competing ions, the ion exchange capacity of the column should be low [1]. 

Okada [1] has described a redox suppression for the ion exclusion chromatography of carboxylic acids with conductiometric detection. The reaction between hydriodic acid (the eluent) and hydrogen peroxide (the precolumn reagent) is used as the redox suppressor for ion exclusion chromatography of carboxylic acids. The suppressor is useful with highly acidic eluents and reduces background conductance more effectively than a conventional ion exchange suppressor. 

An electrochemical detector is destructive. It requires (1) a working electrode (where oxidation or reduction takes place), (2) an auxiliary electrode, and (3) a reference electrode (to regulate voltage and compensate for changes in background conductivity of the eluent). When an active substance flows into the electrochemical cell and a potential difference is applied between the working and reference electrodes, the electrolysis of the analyte yields a current (detector signal) that is a function of the applied potential. The three steps in the process are ... 

Fig. 2 Conductivity detection of anions in nonsuppressed (single column) ion chromatography using an eluent of (a) low background conductance (direct detection) and (b) high background conductance (indirect detection). The direction of the arrow indicates the increase of conductivity.

The principle of conductivity suppression is the reduction of background conductivity by converting the eluent to a less conductive medium (H2O) through acid-base neutralization while the analyte ions conductivity is increased, by converting them to a more conductive medium Anions are converted to their acid forms and cations to their hydroxide forms. These reactions lead to higher S/N ratios, thus significantly improving baseline stability and detection limits. 

Ion chromatography (IC) was introduced in 1975 by Small, Stevens, and Baumann [14] as a new analytical method. Within a short period of time, ion chromatography developed from a new detection scheme for a few selected inorganic anions and cations to a versatile analytical technique for ionic species in general. For a sensitive detection of ions via their electrical conductance, the effluent from the separator column was passed through a suppressor column. This suppressor column chemically reduces the background conductance of the eluent, while at the same time increasing the electrical conductance of the analyte ions. 

The most commonly employed detector in ion chromatography is the conductivity detector, which is used with or without a suppressor system. The main function of the suppressor system as part of the detection unit is to chemically reduce the high background conductivity of the electrolytes in the eluent, and to convert the sample ions into a more conductive form. In addition to conductivity detectors, UV/Vis, ampero-metric and fluorescence detectors are used, all of which are described in detail in Chapter 6. 

Eluents for conductivity detection with chemical suppression of the background conductivity... 

The versatile mixture of sodium carbonate and sodium bicarbonate, on the other hand, finds widespread application, since the elution power and the selectivity resulting there from are determined over a wide range solely by the concentration ratio of these two compounds. A great variety of inorganic and organic anions can be separated with this eluent combination. As the product of the suppressor reaction, the carbonic acid is only weakly dissociated, so that the background conductivity is very low. 

As an alternative to carbonate/bicarbonate systems, amino adds (a-aminocarb-oxylic acids) may be used as an eluent [47,48]. Their dissociation behavior is depicted in Fig. 3-36. At alkaline pH, amino acids exist in the anionic form due to the dissociation of the carboxyl group and, thus, may act as an eluent ion. The product of the suppressor reaction is the zwitterionic form with a correspondingly low background conductance. This depends on the isoelectric point, pi, of the amino acid. 

The residual dissodation of the zwitter ion and the background conductance of the eluent is even lower than for the carbonate/bicarbonate system, if amino acids are selec-... 

Table 3-8. Background conductances of the eluents used by Fritz et al. [19] in their introductory paper.

Eluents of the second kind should have a low background conductivity to enable a sensitive conductivity detection of the anions to be analyzed. The selection of eluents... 

A suppressor system is used for the sensitive detection of ions via their electrical conductance. Its function is to chemically reduce the background conductivity of the electrolyte used as eluent before it enters the conductivity cell. Therefore, the suppressor system may be regarded as part of the detection system. 

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