Eluent carboxylic acid salts

The benzoate salt is one of the two most useful carboxylic acid salts for eluents. The other is phthalate. Benzoate salts are useful for separation of acetate, bicarbonate, fluoride, chloride, nitrite, nitrate, and other early-eluting anions. Divalent anion and other late-eluting anions such as thiocyanate and perchlorate are not eluted effectively by benzoate. The concentration of a benzoate eluent that should be used depends on the type and capacity of anion-exchange resin used, but is typically 0.5 to... 

In nonsuppressed anion chromatography, the conductivity of the analyte anion is higher than that of the eluent, so conductivity increases when analyte emerges from the column. Detection limits are normally in the mid-ppb to low-ppm range but can be lowered by a factor of 10 by using carboxylic acid eluents instead of carboxylate salts. 

Figure 6.63 Separation of aromatic carboxylic acids on OmniPac-PAXlOO with a salt gradient. Eluent NaCI/NaOH/MeCN (80 20 v/v) gradient linear, 0.2 mmol/L NaOH -H 50 mmol/L NaCI to 1.6 mmol/L NaOH -H400 mmol/L NaCI in 20min flow rate 1 mL/min detection UV (254 nm) peaks (1) benzoate, (2)...

Conversely, in samples with high TDS (in excess of 15000ppm), the salt peak may swamp out the early-eluting organic acid peaks. One way to avoid this is by serial dilution of the sample. However, if the difunctional carboxylic acids are in low concentration, they may not be detected at all. Another method is to use a stripper column that removes the strongly charged species from the eluent stream before it passes through the detector. How-... 

It was demonstrated that carbon dioxide or carbonic acid in a 1-butanol/water eluent equilibrates with the resin to form an adsorbed layer that is responsible for conversion of carboxylate salts to the molecular form. 

---