Scope of Anion Separations

Anions of strong acids may be separated in acidic solution (as in Fig. 6.10) or at a basic pH. Weak acid anions require a basic solution to exist in the anionic form. Separation of borate, silicate, sulfide, cyanide as well as the anions of two stronger acids is shown in Fig. 6.11. An alkaline solution of sodium benzoate was used as the eluent. Carbonate may also be separated under alkaline conditions. Separation of these anions by suppressed IC is usually not attempted because they arc converted to the non-conducting molecular form by the acidic suppressor. 

Although a very large number of organic anions may be separated by ion chromatography, non-suppressed conductance is not always a viable detection method. Anions of small organic acids can usually be detected but the conductivity of more 

The term sensitivity will be used here in its technically correct context as the change in detector signal per unit concentration. It should not be confused with detection limits that are dependent on baseline noise. (Baseline noise is dependent on the signal magnitude, temperature variations, the electronics, etc. and varies from instrument to instrument.) In this section we shall examine the factors that determine the sensitivity that can be attained in anion chromatography. 

Experimental measurements of the conductance of various eluents were made by Gjerde and Fritz [13].The eluent was pumped through a Wescan model 213 conductivity detector until a steady reading was obtained. Most of the measurements were made at 22.0-22.3 °C, although a few were at a slightly higher or lower temperature. 

The cell constant was measured at 22.3 °C with a 1.00 x lO solution of potassium chloride. It was found to be 33.0 cm as calculated from the equation k = GK. where k is the known specific conductance of the potassium chloride in pS, G is the conductance in pS, and K is the cell constant. 

Indirect conductivity detection with a sodium hydroxide eluent offers excellent sensitivity. As an example, formate (limiting equivalent conductance = 55 S cm equiv ) replaces an equivalent amount of the more conductive hydroxide anion limiting equivalent conductance = 198 and thus gives a lower signal when the formate passes through the detector. 

the conductance G was measured for several different eluents. For comparison, the conductances of the same eluents were calculated from a table of limiting ionic equivalent conductances, with the equation  

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