Ionic conductivity background phases

Although the ionic strength of the eluant may remain the same throughout the run, the background conductivity can decrease due to the changing dielectric constant. These baseline changes can be compensated either by chemical means or by computer baseline subtraction. Often, mobile phase ion chromatography is used to elute ions which are very... 

This is the classical ion chromatography detector and measures the eluate conductivity, which is proportional to ionic sample concentration (provided that the cell is suitably constructed). Its sensitivity decreases as the specific conductivity of the mobile phase increases. The active cell volume of 2 gl is very small. Good conductivity detectors have automatic temperature compensation (conductivity is highly temperature-dependent) and electronic background conductivity suppression. The linear range is not large. 

Stationary phases for ion chromatography have a lower exchange capacity than the ones used for classical ion-exchange separations as described in Chapter 12. Therefore, the ionic strength of the eluent can be low and 1 mM solutions are not uncommon. Diluted mobile phases have low conductivity which facilitates detection. However, even with mobile phases of low electrolyte concentration, the background conductivity is too high to allow detection without special techniques. Two principles... 

However, the added H increases the background conductivity and reduces the detection sensitivity for weak acids that need to be in the ionic form. With a proper choice of a stationary phase, the molecular RCO2H will be retained more strongly than RCO2 and the equilibrium in Eq. (8.4) will be shifted accordingly. 

Aliphatic tricarboxylic acids such as citric acid, isocitric acid, tricarballylic acid, and aconitic acid exhibit a remarkably high affinity toward the stationary phase of carbonate-selective anion exchangers. Hence, low-ionic strength car-bonate/bicarbonate buffer solutions are not particularly suitable as eluents. However, when a sodium hydroxide solution at a comparatively high concentration (c 0.08 mol/L) is used, tricarboxylic acids may be eluted. When the detection of these compounds is carried out via electrical conductivity, a high-capacity suppressor system such as micromembrane (MMS) or self-regenerating suppressors must be used to reduce background conductivity. 

---