Application of Multidimensional Matrix Elimination Ion Chromatography for Bromate Analysis

Jun 01, 2011
Volume 24, Issue 6, pg 322–332

Ion chromatography (IC) is routinely used for ion analysis. However, the presence of high concentration of matrix ions in some samples can pose a challenging analytical problem especially when pursuing trace level analysis. Often the matrix ions interfere with the analysis of the target analytes causing peak broadening and affecting the sensitivity of peaks of interest in a deleterious fashion. In this article, we discuss an inline two dimensional matrix elimination ion chromatography (MEIC) method to analyse samples containing high levels of matrix ions. The method was successfully applied for the analysis of trace level of bromate ion in the presence of large amount of interfering matrix ions in a variety of environmental water samples.

Ion chromatography (IC) with suppressed conductivity detection is a primary method established for analysing ions in aqueous sample matrices such as drinking water and waste water. In fact, water analysis is the biggest application of IC.1,2 The technique provides an easy method for analyte quantification and is convenient for contaminant monitoring. Over the years, with the introduction of low noise suppressor technology, in-line eluent generation, continuously-regenerated trap columns (CR-TC) and carbonate removal devices (CRDs) for the removal of carbonate from the eluent and the sample, the detection limit of IC has been improved greatly, facilitating the direct analysis of trace level analytes.3

For the analysis of trace- level analytes in a low ionic strength matrix, such as ultra pure water, a simple large loop injection or standard pre-concentration method is adequate to improve the method detection limit. However, for samples which contain a high ionic strength matrix, such as in some drinking waters and waste waters, the determination of trace level analytes becomes more challenging. The presence of high levels of matrix ions in these samples can overload the separator column, causing the analyte peak shape to smear or distort. Furthermore, the matrix ions can co-elute with the analytes of interest making analysis difficult with a direct injection of the sample. The analysis becomes even more problematic when a concentrator column is used as a high concentration of matrix ions can act as an eluent and elute the analytes off the concentrator column.

For samples containing a relatively high level of analyte (concentration in the mg/L regime) a simple sample dilution can be used to lower the matrix ion concentration and minimize the matrix effect. However, if the analyte concentration is low, such as in the low μg/L regime, the sample dilution will also lower the analyte concentration. which makes trace analysis difficult.

One strategy adopted was to pursue post-column derivatization to selectively derivatize the peak of interest to increase its response, relative to the underivatized matrix ions.4,5 Usually a UV/vis detection scheme was commonly used with this strategy. However, many of the analytes commonly encountered in drinking water samples do not possess good chromophores for UV/vis detection, and therefore are not suitable for this approach. Furthermore, many of the post-column reactions require harsh reaction conditions and toxic reactants. Additionally, derivatization is an additional processing step and can be prone to error.

Another approach used to address the challenge of trace analysis in the presence of high matrix ion concentrations, included the use of higher capacity columns that provided a better peak capacity to handle the large amount of matrix ions. Although this method was successful to some extent, the increased capacity also required the use of a higher eluent strength and increased the overall run time. Furthermore, based on the capacity of available commercial columns, this method was not suited for analysing the wide range of matrix ion concentrations present in some drinking and waste water samples.

Another approach was to pursue sample pretreatment solutions such as using a solid phase extraction (SPE) cartridge.6 For example, a pretreatment step with a silver cartridge can selectively remove chloride from the water samples. However, the SPE methods are only selective to some ions and can be labour intensive, inconsistent and prone to errors (operator or operation related). The SPE phase also needs to be chosen judiciously so that only the matrix ions are eliminated without affecting the target analytes. For example, a silver form cartridge could be used for removing sulphides. However, it should be noted that this phase also removes thiosulphate making sulphur speciation difficult.7

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