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Geochemistry Ion Chromatography Purpose: Become Familiar with The Geochemistry Ion Chromatography Purpose: Become familiar with the principles and methods of ion chromatography Generate a calibration curve Determine the anionic concentrations of natural water samples Theory Chromatography is an experimental technique used for analyzing and/or separating mixtures of chemical substances. There are many different kinds of chromatography, among them paper chromatography, gas chromatography, liquid chromatography, and ion-exchange chromatography. An ion chromatograph is an example of the latter kind of chromatography. All chromatographic methods share the same basic principles and mode of operation. In every case, a sample of the mixture to be analyzed (the analyte) is applied to some stationary fixed material (the adsorbent) and then a second material (the eluent) is passed through or over the stationary phase. The compounds contained in the analyte are then partitioned between the stationary adsorbent and the moving eluent. The success of the method depends on the fact that different materials adhere to the adsorbent with different forces. Some adhere to the adsorbent more strongly than others and are therefore moved through the adsorbent more slowly as the eluent flows over them. Other components of the analyte are less strongly adsorbed on the stationary phase and are moved along more quickly by the moving eluent. So, as the eluent flows through the column, the components of the analyte will move down the column at different speeds and therefore separate from one another, as shown in the diagram below. Now if we monitor the end of the column, at some point we will observe molecules or ions of the fastest moving substance (least tightly bound to the adsorbent) emerging from the column - usually in a narrow band if things are working right. Shortly thereafter we see molecules of the second fastest moving substance emerge from the column and so forth. If we now attach a detector to analyze the output at the end of the column we should be able to determine whenever a group of analyte molecules emerge from the column. This is, in practice, what is done in most commercial chromatography instruments. Each time analyte molecules/ions emerge from the chromatography column the detector generates a measurable signal which is usually printed out as a peak on the chromatogram. The chromatogram is a record of detector output Vs time as the analyte passes through the chromatography system. It usually consists of a series of several peaks corresponding to the different times in which components of the analyte mixture emerge from the column. The number of peaks corresponds to the minimum number of different substances (compounds or ions) contained in the analyte. If the analyte is found to display only a single peak, it is an indication that it is composed of only a single component, i.e., it is pure, although rigorous confirmation of purity may require additional testing. The Instrument The Injector and the Eluent For anion chromatography the eluent is a dilute aqueous solution of sodium bicarbonate and sodium carbonate, each about 1-4 mM in concentration. The water used for the preparation of the eluent is of the highest purity - Type I water which is virtually free from any contaminating ions. The sample is injected into the instrument through the injection system. The eluent, which the pump draws from the eluent jar and forces through the system, then carries the analyte onto the ion exchange column under a pressure of about 1000 psi (ca 65 atm). Figure 1. ISC-2000 in Environmental Geology Lab The Ion Exchange Column The ion exchange column (sometimes called the analytical column) is tightly packed with the stationary adsorbent. This adsorbent is usually composed of tiny polymer beads which have positive charge centers (actually ammonium type cations) attached to their surfaces with covalent chemical bonds. When just the pure eluent is flowing through the column these positive centers on the polymer surface attract the negatively charged bicarbonate and carbonate ions in the eluent. But as the anions contained in the analyte sample begin to enter the column, these anions, which are also attracted to the positive centers on the polymer surface, may replace (i.e., they exchange with) the bicarbonate and carbonate ions stuck on the polymer surface. The analyte anions and the eluent anions compete with each other for the positive centers on the polymer surface. As a result of these interactions a rather complex equilibrium is set up as the analyte is carried through the column by the eluent. How tightly the competing anions stick (i.e., how strongly they are attracted) to the surface of the polymer beads depends on the size of the anion and its charge. Usually, the greater the charge on the anion the more strongly it is attracted to the surface of the polymer beads (so, for example, sulfate ions are more tightly held to the polymer beads than chloride ions). Also, larger anions generally move more slowly through the column than smaller anions (e.g., bromide ions usually are more tightly bound to the polymer beads than chloride ions). So now we have a situation in which the eluent is trying to move the anions of the analyte forward through the column, but the polymer beads are trying to hold the anions in place by attracting them to the positive charge centers. Obviously, anions can only move forward through the column when they are in solution. So the speed at which a given type of anion moves through the column is dependent on the relative amount of time the ion is in solution. If an ion is only weakly attracted to the positive centers on the surface of the polymer beads, it will spend relatively more of its time in the solution phase than an anion that is strongly attracted to the polymer surface. So these anions move through the column more quickly. The more strongly the anion is attracted to the surface of the polymer beads, the more slowly it moves through the column. The result is that what was injected as a homogeneous solution of analyte anions at the top of the column, separates into bands of different kinds of ions as they travel through the column. While the details of what goes on inside the column is more complicated than described above, perhaps this description will provide you with a qualitative understanding of how different anions can be separated on such a column. Detection and the Suppresser Column The detector that is most commonly used in ion chromatography is a conductivity cell which measures the conductance of the solution passing through it. The conductance of a solution is proportional to the concentration of the ions dissolved in the solution. Because we use conductivity measurements to detect the different anions as they emerge from the column, it is essential to pass the eluent through a suppresser column before entering the detector. Here’s why. The eluent contains relatively high concentrations (1-5 mM) of sodium bicarbonate and sodium carbonate - both strong electrolytes. If such a solution were passed directly into a conductivity detector (which measures the electrical conductance of the eluent solution), a large signal would be obtained just from the presence of these eluent ions. The actual concentration of the analyte anions of interest is usually only a tiny fraction of the concentration of the bicarbonate and carbonate ions contributed by the eluent itself. Usually the analyte anions of interest are present in the parts per million level or less; however the bicarbonate and carbonate ions on the eluent are present in much greater concentration – approximately 100 times as large as the anions of interest. Thus the bicarbonate and carbonate ions impart a large conductivity to the solution entering the detector and the additional conductivity imparted to the solution of the eluent by the presence of a tiny amount of an anion of interest coming from the sample is extremely small. That is, we would have a very hard time measuring the difference in the conductivity of the pure eluent itself and the conductivity of the eluent containing a tiny amount of an anion of interest. In other words, the sensitivity of the measurement would be too low to be of value. To get around this problem, after emerging from the column and before entering the conductivity detector, we pass the solution through a suppresser column which effectively removes all the bicarbonate and carbonate ions from the solution, leaving only the analyte ions of interest dissolved in the solution. When the resulting solution is now passed through the conductivity cell, we can readily detect the signal due to the presence of a small amount of the analyte ion, because in this case the reference signal we are comparing it to is that of pure water which has an exceedingly low conductivity. So by removing the large quantities of the eluent anions, bicarbonate and carbonate, we greatly increase the sensitivity of the measurement. Indeed, using suppression, it is possible to detect ion concentrations as low as parts per billion in the analyte sample. The suppresser column essentially is another ion exchange column. Although there are different designs for such columns, we may think of these columns also as being packed with small polymer beads, but these beads carry acidic protons (H+1) on their surface. As the solution from the analytical column flows into the suppresser column, the carbonate and bicarbonate ions, both of which are basic, combine chemically with the protons on the polymer surface forming carbonic acid which , being unstable in aqueous solution, decomposes to carbon dioxide gas and water. In this way the carbonate and bicarbonate ions are removed from the solution: HCO3-1 + H+1 ----> [H2CO3] ---> H2O + CO2 CO3-2 + 2 H+1 ----> [H2CO3] ---> H2O + CO2 The spaces that are left on the polymer surface by the departure of the protons are filled by sodium ions provided by the eluent solution (i.e., sodium ions undergo exchange with protons on the surface of the polymer beads).
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