Keyword: organic acids

Organic acids of various kind can be separated and quantified with suppressed ion chromatography (IC). The benefits of this mode include limited influence from neutral matrix components and high sensitivity with suppressed conductivity detection, or even with UV absorption, provided that the structure contains some double bond or similar structural feature. The separation selectivity is often adequate for many organic acids, although column selection might be challenging, and it can be especially hard to accomplish separation of multiple small, singly charged organic acids of similar structure. For more simple mixtures, isocratic carbonate eluents can be employed, but for complex samples, hydroxide gradients tend to be the dominating eluent choice.

HILIC (hydrophilic interaction chromatography) is an attractive separation technique for many organic acids but often require more retentive columns with bonded hydrophilic stationary phases rather than plain bare silica. HILIC of organic acids is particularly powerful when combined with mass spectrometry detection, although ion fragmentation patterns of carboxylic acids may be difficult to interpret. Challenges in this mode may arise from adduct formation with salt components, especially for samples containing more complex ionic matrixes.

Although ion exclusion chromatography (IEC / ICE) is a rather dated separation technique, it may provide complementary selectivity for small, singly charged carboxylic acids. However, the separation efficiency tends to be significantly lower, and the peak capacity more limited, typically reserving this mode for the simpler mixtures with higher analyte concentrations.

Also reversed-phase (RP) might accomplish adequate separation of organic acids, especially if they are singly charged and not too small and hydrophilic. Eluents then typically need to contain low amounts of organic solvents, making some hydrophobic stationary phases unsuitable since they might experience dewetting, thereby expelling eluent from the pores, resulting in drastic retention loss (aka “phase collapse”).

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