High-purity Carboxylic Acids for Trace-level Determinations

Sigma-Aldrich^® offers new carboxylic acids used for trace analysis techniques such as ion-chromatography, AAS and polarography

Michael Jeitziner, Market Segment Manager Analytical Reagents & Standards michael.jeitziner@sial.com

Introduction

Organic acids are weak acids that don’t dissociate completely in water. They are often used in trace analysis applications as buffer and complexing agents for improving the separation. Further on, ascorbic acid is a highly effective antioxidant and therefore used as a stabiliser in various analytical applications. The most important fields of application are in metallurgy, environmental analysis, food analysis, toxicology, and clinical analysis. Sigma-Aldrich has recently introduced three organic acids with extremely low levels of inorganic impurities.

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Hydride Generation-Atomic Absorption Spectroscopy (HG-AAS)

Metalloids like antimony, arsenic and selenium are mainly analysed by AAS using the hydride generation technique, which is easily connectable to various detection systems. The detection limits using this technique are below 1.0 μg/L.

A mixture of potassium iodide and ascorbic acid is most often used for the prereduction of As(V) to As(III), whereas potassium is the reducing agent, and ascorbic acid the antioxidant for stabilising the potassium iodide solution.¹

Several heavy metal ions, such as iron(III), chromium(VI) and copper(II), can suppress the arsenic or antimony hydride generation. These interferences can be eliminated by the addition of potassium iodide and ascorbic acid.²

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Ion Chromatography (IC) applications

IC is a technique that is used to separate and quantify ppt-%-levels of common anions and cations in aqueous samples based on the charge properties of the ions.

Citric acid, oxalic acid and other carboxylic acids in dissociated form chelate metal ions. By using different organic acids, a competitive complex formation takes place.

Besides their complex forming properties, the carboxylic acids also act as buffer substances. Their strength depends on their pK_A values (see Table 1).

Table 1. Acid dissociation constants at 25 °C in water³

Depending on the separation problem, variation of the pH value, the use of a complexing agent and/or an increase in column temperature are powerful tools to broaden the scope of cation chromatography.

L-Ascorbic acid (vitamin C) is added to reduce Fe(III) to Fe(II)^4-7 for the determination of iron using cation chromatography. Further on, it can also be used in polarography to eliminate the interfering effect of iron(III) ions⁸ or for the speciation analysis of Fe(III) and Fe(II) ions.

Citric acid and oxalic acid form strong complexes especially with di- and trivalent cations. Tables 2 and 3 illustrate the effect of citric and oxalic acid on different analytes.

Table 2. “Effect of eluent composition on IC column retention times”. The original work can be downloaded from http://products.metrohm.com (search for 8.000.6015EN)

Table 3. Effect of different concentrations of citric acid and oxalic acid on the retention time of various cations using the Metrosep C 2 – 150 separation column

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