Topic: 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”).

To learn more, browse the application examples below or contact Diduco to discuss your specific analysis.

Applications

Oral L-carnitine is a dietary supplement that has been shown to reduce fatigue in elderly people with low muscular endurance, but its effects on athletes during physical activity are not well-supported. Although this endogenous compound is well-known for its involvement in the mitochondrial oxidation of long-chain fatty acids, no scientific evidence has been found that carnitine in isolation enhances fat-burning.
Separation of organic acids can be a challenging task due to their hydrophilic nature and structural diversity. One of the more traditional approaches to this analysis is ion exclusion chromatography, which offer complementary selectivity to other separation techniques, especially for the monovalent acids which tend to be more strongly retained in this mode.
Many medicines and dietary supplements are bases that carry a positive charge, and consequently these small molecules are accompanied by negatively charged counterions. Such anionic excipients may influence the properties of the pharmaceutical formulation and can also influence the effect of the active ingredient since different ion pairs can have different dissolution rates or abilities to penetrate cell walls.
Gradient elution is a powerful tool when aiming to separate many compounds having a wide range of different chemical characteristics. Hydroxide gradients are the most common in ion chromatography since they can be converted into pure water with nearly zero conductivity.
Many medicines and dietary supplements are bases that carry a positive charge, and consequently these small molecules are accompanied by negatively charged counterions. Such anionic excipients may influence the properties of the pharmaceutical formulation and can also influence the effect of the active ingredient since different ion pairs can have different dissolution rates or abilities to penetrate cell walls.
Small organic acids are an important group of chemical compounds that needs to be analysed and quantified in many different industries. This application shows ion chromatography of a variety of organic acids using a non-optimized isocratic carbonate-based eluent.
Separation of inorganic anion standards at eluent conditions recommended by the column manufacturer.
Separation of inorganic anion standards at eluent conditions recommended by the column manufacturer.

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Tobias Jonsson
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E-mail: tobias@diduco.com

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