o na cond., suppressed conductivity; na, not available; APCI, atmospheric pressure — chemical ionization; ES, electrospray; MS, mass spectrometer; UV, ultraviolet (detection). a Detection limits vary with sampling time.
have also been investigated recently for their use in IEC with promising results.157'158 This material is inert towards organic solvents and allows for the addition of high amounts of solvents as eluent modifiers. These are commonly used to reduce retention times of strongly retained acids such as aromatic acids. In the past, the use of solvents as eluent modifiers was accompanied by problems such as swelling when using sulfonated cross-linked polymers. Another approach to reduce swelling in the presence of organic solvents is the use of highly crosslinked copolymers instead of the normally used copolymers with low crosslinkage.158 However, these newer type columns have not yet found widespread use in routine analysis.
Eluent choice is determined by factors such as acidity, solvation properties, polarity, and especially detection mode. Eluent choice and detection mode are therefore discussed together. Traditionally UV detection predominates, although suppressed conductivity detection using tetrabutylammonium hydroxide as the regenerant and also nonsuppressed conductivity detection are becoming more common. Eluents applied in IEC are usually aqueous solutions of mineral acids or organic acids. The relatively low pH of the eluent leads to a shift in the dissociation equilibrium of weak organic acids towards their nonionized form, thus obtaining narrow peaks. Mineral acids such as sulfuric acid are preferably used as eluents in connection with direct UV detection, whereas strong organic acids, especially aliphatic sulfonic acids (e.g., methanesulfonic or octanesulfonic acid) are used as eluents in combination with suppressed conductivity detection.130 Nonsuppressed conductivity detection requires eluents with low background conductivity and, hence, weak organic acids such as benzoic acid are preferred. Other weak organic acids more recently suggested include Ce organic acids,105 C7 organic acids,159 and benzoic acid with |3-cyclodextrin.160 It has also been shown that nonacidic eluents (e.g., polyvinylalcohol/water,161 butanol,162 or sucrose/methanol163) can be applied successfully to the separation of organic acids in conjunction with nonsuppressed conductivity detection.
Organic solvents such as acetonitrile164 or various alcohols165 are used as eluent modifiers to reduce tailing and retention times of more hydrophobic analytes such as aromatic organic acids. The eluent modifiers compete with the analytes, thus reducing interaction between the polymer and the more hydrophobic analytes. Gradient elution is usually not applied in IEC since it has been found that concentration gradients give only very little benefit.164 However, gradients with increasing modifier amounts achieved better and faster separation of more hydrophobic organic acids than elution under isocratic conditions.164
Compound identification in IEC is accomplished by retention time comparison and coelution and/or incomplete separation of organic acids can be expected due to the low separation efficiency of IEC compared to GC, for example. Organic acid pairs known to coelute are fumaric/acetic acid and also succinic/glycolic acid.64,155 Ideally the identity of a compound should be confirmed with an independent analytical method. It is therefore surprising that only few of the environmental methods listed in Table 13.6 even touch on this issue.29,59,111
An interesting approach to lowering detection limits is the use of a concentrator column in place of a sample loop. IEC columns cannot be used as a concentrator since the organic acids would not be retained during the injection process, given that water acts as an eluent. However, an anion exchange concentrator has been combined successfully with an ion exclusion analytical column achieving detection limits of 7 to 10 /¿g/l.85 The difficulty in combining these two techniques lies in the choice of eluent which must be able to remove the anions from the concentrator while being suitable for IEC. This is accomplished by using methanesulfonic acid — at pH 9 for the removal of the dissociated analytes from the concentrator and at pH 2.7 for their separation on the IEC column. Unfortunately, the use of anion exchange concentrators is largely restricted to samples with relatively low ionic strength due to potential breakthrough of the organic acids.
AEC has also been coupled with IEC to achieve two-dimensional separations for the quantification of organic acids and inorganic anions.166 Though AEC/IEC has not found widespread use for routine analysis of environmental samples since this system is complex to operate and prone to contamination problems.166 Newer AEC methods are capable of determining inorganic anions and organic acids in one run, at least for matrices such as precipitation samples
Was this article helpful?