DEFINICION DE LAS HIPOTESIS ESPECIFICAS. 1

This is the most widely used type of IC in power plants. It involves an ion exchange process in the separator column, and it is useful for separating both inorganic and organic anions and cations.

The first step in the analysis is injection of a known volume of the water sample into a flowing liquid, called the eluent or mobile phase (see Figure 13-1). The eluent carries the sample into a separator column that is packed with resin beads made from materials such as polymethacrylate or polystyrene-based resin crosslinked with divinyl benzene.

The resin is formed with exchange functions or separation groups that interact either with negatively or positively charged ions, depending on whether analysis of anions or cations is desired. When analysis of anions is needed, the exchange function is generally a quaternary ammonium group whereas, in cation chromatography, it is a sulfonate group. An anion exchange resin, for example, may be pre-treated with a bicarbonate (HCO₃^-) solution to completely convert the fixed -N⁺R₃ groups to the bicarbonate form (resin-N⁺R₃ HCO₃^-).

As the water sample passes through the column, the ions interact with the resin functional groups in different ways depending on their unique chemical and physical properties. For instance, an anion exchange resin in bicarbonate form can exchange a bicarbonate ion for, say, a chloride ion from the flowing water sample:

resin−N R HCO⁺ _{3 3}⁻+Cl⁻→resin−N R Cl⁺ ₃ − +HCO₃⁻ Equation 13-1

Adsorption processes may also be involved in the process (especially for hydrophobic ions). The resin will bind the chloride ion temporarily, but eventually the eluent, flowing behind the sample, reverses the procedure by providing a replacement bicarbonate ion and causing the chloride ion to be released or eluted into the eluent. As a result, each type of ion is retained for a discrete, reproducible period of time (retention time) which is characteristic of that ion.

This causes a temporal separation of the ions in the eluent so that, as it flows out of the bottom of the separator column, first one ion appears, then another, and so on, as illustrated in Figure 13-2.

In this idealized example, the dead time, tm, is the time taken for a compound which does not interact with the separator column to migrate through the column with the eluent. The gross retention time, t_ms, is the time taken from first introducing the eluent at the top of the column to achieving the peak concentration of an interactive ion at the bottom of the column (peak 1 in Figure 13-2). The net retention time, t_s, is equal to the gross retention time less the dead time (t_s = t_ms – t_m). Another ion may appear at a greater net retention time, as represented by peak 2 in Figure 13-2.

Figure 13-1

Schematic of a Single Column Ion Chromatograph

By injecting standard solutions of each ion of interest into the chromatograph, the retention times are established for each, which allows a correlation to be made subsequently between the t_s value and the type of ion present.

The eluent then flows to a detector that allows the concentration peaks in Figure 13-2 to be detected by one of a number of methods. The detector is coupled to a microprocessor-controlled instrument that monitors the retention time and the height of (or area under) the concentration peak. Comparing the sample data with calibration data, the computer then identifies and quantifies the dissolved ions in the original water sample. The most widely used detector is a conductivity cell, but amperometric, fluorescence, and ultraviolet/visible absorbance detectors are also used on occasion.

Figure 13-2

Illustration of How Ions Elute (Leave the Separator Column) at Different Rates Resulting in a Separation of Ionic Species in the Flowing Eluent

When the detector is based on conductivity, the higher the concentration of the ions in the eluent, the higher is the conductivity reading. The precise correlation between conductivity and

concentration is dependent on the relative mobilities of the ions, and may be determined by calibration with standard samples.

Although the separated ions contribute to the conductivity, the eluent itself is often highly conductive. Consequently, the effects of the ions of interest would be overwhelmed unless methods are employed to suppress the eluent conductivity. One approach is to pass the effluent from the separator column through a second column—a suppressor column—before it reaches the detection system. The function of the second column is to convert the highly conductive eluent into a weakly dissociated solution of low conductivity. For instance, in the case of a sodium bicarbonate eluent carrying chloride anions, the suppressor column would exchange the sodium ions for protons to produce weakly dissociated carbonic acid. In this way, the ability of the conductivity cell to detect chlorides is increased dramatically.

The suppressor column is also invaluable when a technique called gradient elution is employed.

For ion chromatography to be a practical analytical tool, the analyte peaks must be completely separated prior to detection and quantification. Several anions such as fluoride, acetate, formate, chloride and nitrite have short retention times on the stationary phase and a weak eluent (low solution strength) must be used to separate the peaks. However, this same solution strength would cause exceedingly long retention times for anions such as nitrate and phosphate. This slow elution would limit the practicality of the test for on-line utilization. Gradient elution describes a process where the early ions are separated under a low solution strength and then the eluent concentration is increased (other analytical schemes even change the chemical nature of the eluent) to shorten retention time of the later analytes. This eluent change however increases

the specific conductivity of the flow to the detector and elevates the background conductivity.

Use of the suppressor column provides more stable and lower background conductivity to the detector which improves detection limits and response.

Gradient elution is further enhanced by a device called an eluent generator. This system

electrolytically produces high purity potassium hydroxide eluent from demineralized water. The eluent concentration can be changed by altering the carrier flow and the applied current. In addition to producing a highly repeatable eluent concentration, the KOH eluent is free of

carbonate ions to provide the exceptionally flat detector baseline seen in Figure 13-3 [9]. Acidic eluent can also be produced with the eluent generator.

Figure 13-3

Gradient Separation of Common Anions Using a Hydroxide Gradient [9]

Source: Reference 9, Courtesy Dionex Corp.

Problems can occur if the water sample contains foulants, such as corrosion product particulates.

In such cases, the sample should first pass through a so-called guard column before reaching the separator, as illustrated in Figure 13-1. The guard column removes these species and prevents fouling and interferences in the separator column. If the sample contains a significant

concentration of particulate matter, a 0.22 µm or 0.45 µm filter should be installed upstream of

Modern instruments require only 10 to 100 microliters of a water sample to identify ions present in the parts per billion (µg/L (ppb)) range. The detection limit can be reduced to a few parts per trillion (ppt) or less by enriching the samples in a concentrator column. This is done by passing relatively large volumes (25 milliliters or more) of sample through the column to accumulate the ions of interest before passing the eluent through the analytical column in the normal way.