detail elsewhere.149,150 Direct and indirect UV detection are rarely used with AEC due to their inferior sensitivity, whereas amperometric detection has recently been gaining some interest.129-131 Some matrices such as ice or drinking water contain very low concentrations of organic acids (low /Ag/l). In order to achieve the necessary detection limits, methods for these matrices make use of either concentrators or large volume injections.
On-line concentrators are elegant tools to increase method sensitivity without using time- and
labor-intensive pretreatment steps. , , A concentrator column is a short, medium- to high-capacity strong anion exchanger which is installed instead of a sample loop. Volumes of up to 10 ml of aqueous sample have been loaded onto concentrators. Inorganic anions and carboxylate anions are retained on the concentrator during the loading process due to the minimal elution power of water on this resin. Trapped anions are then transferred onto the analytical column by using a hydroxide eluent which also facilitates analyte separation on the analytical column during the chromatographic process. Loading and transfer process are performed countercurrent to each other so that the sample band entering the analytical column remains narrow, thus ensuring acceptable peak shapes in the final chromatogram.130 Autosampler use is recommended for achieving reproducible injections and for overcoming the increased backpressure caused by the concentrator. Carry-over can be avoided by flushing transfer lines in between injections. Employing a concentrator works well for samples with low organic acid and low inorganic anion concentrations such as ice.18,84 Detection limits for organic acids can be as low as 1 /Ag/l with the added benefit that inorganic anions may be quantified at the same time. However, the use of concentrators is limited by the fact that the analytes have to be retained quantitatively. Problems arise when inorganic anions are present even in moderate amounts (e.g., drinking water) since these can cause breakthrough of the more weakly retained carboxylate anions during the loading process, thus leading to irreproducible results. Breakthrough of carboxylate anions should always be considered possible when employing a concentrator — especially in the method development phase and later, if significant changes in the sample matrix are experienced. However, if interfering anions such as chloride and sulfate are removed prior to injection, preconcentration may be applied successfully to the analysis of organic acids.45
An alternative for increasing method sensitivity is the injection of larger sample volumes.43'58'60'62'63'72'86'87 If the capacity of the analytical column is sufficiently high, volumes of up to 1 ml may be injected via a sample loop without significant peak broadening. This "relaunch" effect ensures that the analytes are collected as a relatively small sample band at the start of the column during injection, which is a prerequisite for achieving sharp peaks.129-131 Method detection limits for this approach are in the low /Ag/l range for samples such as drinking water and snow. Again, higher concentrations of inorganic anions may interfere with the analysis, this time by coelution and masking of specific organic acids. Careful column selection and optimization of the gradient can alleviate this problem to a certain extent.86,87 If coelution still occurs, another option is to utilize the "heart-cut" technique which has been successfully used for drinking water analysis of coeluting sulfate and oxalate. In brief, a large volume injection is followed by gradient separation of most carboxylate and inorganic anions. The detector effluent is then redirected to a concentrator for the time window of the coeluting peaks. Sulfate and oxalate are retained on the concentrator and then reinjected onto the same analytical column using sodium hydroxide for the transfer. Separation is achieved by gradient elution optimized for these anions.43
Peak identification and potential coelution of organic acids are not always given due consideration, although AEC has much inferior separation efficiencies compared to GC. Ideally, organic acid identification, which is typically done by retention time comparison, should be confirmed with another independent method. Several publications investigating rain and atmospheric precipitation validated their AEC results with either IEC or
CZE.30,51,58,59 Other publications completely ignore the possibility of coelution of different organic acids, although it has been shown to occur.43'64'72'86'87'116 Separation of coeluting acids may be achieved by taking a structured approach to improving separation by using capacity factors.64
Overall, AEC is a well-developed separation technique which is widely applied for organic acid analysis. Some of the current trends include the implementation of eluent generators;149 development of and increased accessibility to ion chromatograph/MS systems which are now commercially available;149,151 miniaturization of complete ion chromatographic systems;152 and the development of monolithic columns which have been used for fast analysis of inorganic
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