I am really pleased to be blogging about this topic as we just received a new patent for 2-Dimensional Ion Chromatography (2-D IC). Patent US 8,101,422, Multidimensional Chromatography Apparatus and Method was awarded to our scientists Dr. Kannan Srinivasan (left) and Dr. Rong Lin (right) who work here in the ion chromatography & sample prep arena in the Sunnyvale California lab! This patent discusses a 2-D IC matrix elimination application for the analysis of trace ionic contaminants in drinking water samples that are difficult to analyze due to interferences from high levels of matrix ions. The 2-D IC technique is approved in US EPA methods for analysis of trace bromate (EPA 302) and trace perchlorate (EPA 314.2). Analysis of 9 haloacetic acids (HAAs) is also feasible by suppressed conductivity using the new 2-D method which is presently under development.
The new 2-D methods were developed to provide a easy to use, sensitive, automated analysis platform for analyzing trace ions in the presence of matrix ions. The use of suppressed conductivity detection (great overview by Dr. Joachim Weiss, our Technical Director)simplifies the ion analysis without the need for post column derivatization methods which are difficult to implement. In general, 2-D methods eliminate labor intensive, manual sample prep routines geared for removing the matrix ions. For this reason, more and more 2-D methods are being approved by the US EPA Office of Ground Water and Drinking Water for routine compliance monitoring. The simplicity of Reagent-Free IC (RFIC) combined with 2-D analysis is superior for trace analysis in samples containing high levels of matrix ions.
Matrix ions not only interfere with chromatography but also impact detection. For example, interfering ions consume column capacity resulting in either excessive peak band broadening or completely obscure some trace analytes making their detection very difficult. Although EPA Methods 317 and 326 for bromate use selective visible detection for bromate that do not detect other anions, they still suffer from peak band broadening due to the presence of relatively high concentration of matrix ions and result in overall lower sensitivity. Post column methods generally are also problematic due tothe overall method complexity in terms of the post column setup, system complexity, post column reagent stability, and complex implementation.
2-D IC Matrix Elimination Advantages
There are several advantages of the 2-D matrix diversion approach:
- Initial sample loading onto the 4 mm column allows a large loop injection (larger sample mass) due to the high capacity of the analytical column.
- The trace analyte can be efficiently trapped onto a concentrator column in the second dimension. The suppressed effluent with hydroxide eluent is water and provides the ideal environment for ion-exchange retention and focusing.
- The second dimension column has a smaller cross-sectional area relative to the first dimension, thereby enhancing the detection sensitivity.
- There is a potential of combining two different chemistries in two dimensions, which enables a selectivity that is not possible when using a single chemistry dimension
- RFIC™ ensures that both channels are easy to use.
Capillary Ion Chromatography Makes 2-D Even More Sensitive
Sensitivity is always important, and we have seen how 2-D enhances sensitivity with concentration sensitive detectors such as conductivity. Because the 2-D IC technique uses a larger column in the first dimension and a smaller column in the second dimension, analyte peaks become taller due to the smaller cross sectional area of the second dimension column. The sensitivity enhancement is thus proportional to the ratio of the operational flow rates of the two dimensions.
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For example, a 4 mm column uses a 1 mL/min flow rate and a 2 mm format uses 0.25 mL/min. Next, 1 divided by 0.25 = 4 so we expect to see, and do, indeed, see a fourfold increase in peak area from a 2-D IC experiment using suppressed conductivity detection. If we make the second dimension an even smaller capillary format of 0.4 mm andwith an optimal flow rate of 0.01 μl/min. This gives a concentration factor of 1/0.01 = 100. Thus, there is a 100-fold increase in sensitivity going to a capillary format!
Reagent Free Ion Chromatography Advantages
RFIC allows users to just add water for ease of use plus the use of hydroxide eluents allows for increased sensitivity
2-D IC provides matrix elimination, added selectivity, and signal enhancement
2-D IC with Capillary IC increases sensitivity 100 fold!
Next blog post on two application notes using the 2-D IC technique.
Hope you enjoyed learning about this great technique! Let us know if you are looking for other applications using this technique in the Comments box below.