Ion exchange chromatography

An ion chromatography workstation

The ion exchange chromatography ( IC ) often only ion chromatography , is an analytical tool in chemistry and biology. With the help of this chromatographic process, substances can be separated based on their charge. There are charged functional groups on a polymer matrix that have reversibly bound counterions (cations in the case of cation exchangers and anions in the case of anion exchangers).

Brief historical summary

The ion exchange is one of the oldest separation processes described in the literature. The predecessor of ion exchange chromatography is classic column chromatography , in which the sample is more or less split into individual components and collected with the help of an automatic fraction collector. Now the collected samples were often examined wet-chemically. The eluent volume was often several liters . The enormous increase in performance is due to Hamish Small. He developed reproducible ion exchange resins with low capacity and high chromatographic efficiency. This led to a reduction in the injection volumes to ten to 100 microliters, whereby the resolution could be increased, and very narrow signals were obtained. The automatic detection, which now enables continuous recording of the signals, and the introduction of conductivity detection represent important improvements .

The ion chromatographic system

The mobile phase is conveyed through the entire system with the pump. The sample to be analyzed is injected using a loop injector. The sample is first injected and then transported through the opening of the valve through the mobile phase to the separation system. Injection volumes of five to 100 microliters are typical. The most important part of the ion chromatographic system is the analytical separation column . The carrier materials are mostly quartz glass (coated), ethylene tetrafluoroethylene (ETFE), epoxy resins , divinylbenzene , polymers or polyetheretherketone (PEEK) with a low capacity of functional groups. The ion exchange is usually carried out at room temperature. The detector is used for the qualitative and quantitative detection of an analyte. Since this is a relative method, a calibration must be carried out for quantification. The conductivity detector is used most frequently , along with UV / VIS , amperometric and fluorescence detectors. The system can also be coupled to a mass spectrometer via an ionizer . The task of the suppressor connected upstream of the conductivity detector is to suppress the intrinsic conductivity of the eluent, which significantly improves the detection limit of the analyte. The eluent is usually removed from the analyte stream by the suppressor and discarded. Accordingly, this is only necessary when using a conductivity detector. Nowadays the detectors often have interfaces so that they can be connected to a PC. The evaluation is usually carried out using special chromatography software that also controls pumps, valves and suppressors.

The mobile phase and the eluent

All liquid or dissolved substances that pass through the ion chromatographic system with the help of the pump as a mixture of substances are referred to as mobile phase . The mobile phase usually consists of the substance to be analyzed (the ions to be analyzed are marked with “A” in the figure) and the eluent ( the eluent ions are marked with “E”). The task of the eluent is to detach the ions to be detected after they have been exchanged and bound to the stationary phase, so that these ions can be detected. The following general formula can be given for this equilibrium reaction:

${\displaystyle {\mathsf {y\ \cdot \ A_{M}^{x-}\ +\ x\ \cdot \ E_{S}^{y-}\Leftrightarrow y\ \cdot \ A_{S}^{x-}\ +\ x\ \cdot \ E_{M}^{y-}}}}$

"A" stands for the analyte ion, "E" for the elution, "S" for stationary phase, "M" for mobile phase, "x" for the amount of charge of the analyte ion and "y" for the amount of charge of the Eluentions.

The type of eluent that can be used depends on the type of detection used. The eluents for conductivity detection are subdivided as follows:

• Eluent for conductivity detection with chemical suppression of the basic conductivity and
• Eluent for conductivity detection with electronic compensation of the basic conductivity.

Furthermore, the affinities of eluent and solute (to be detected) ions for the stationary phase must be roughly the same.

The stationary phase and the separation column

The stationary phase is the resting phase, which is used as a carrier material in the separation column. In high-performance liquid chromatography, polymer- based ion exchangers are mainly used because, in contrast to silica gel-based columns, they are stable and work even in the alkaline range. Columns that work with support material based on silica gel have a somewhat higher chromatographic efficiency; however, they only work in the pH range of two to eight. The stationary phases in anion exchange chromatography can not only be distinguished by the type of their basic structure, but also by the pore size and the capacity of the carrier material. The ion exchange capacity is defined as the number of ion exchange groups per unit weight of the column packing material. The unit is milliequivalent per gram of resin. The higher the exchange capacity, the longer the retention time for the ion to be detected. This effect can be partially compensated for by adding additional eluent.

As an example, latex anion exchangers should be listed here, which have the following advantages: The substrate is relatively resistant to mechanical influences and it guarantees moderate back pressure. The latex particles have a small size, which results in a high chromatographic efficiency of the separation column. Such a separation column is subject to only slight swelling and shrinking processes due to the surface composition and functionalization. Inexpensive eluents can be used (e.g. sodium hydroxide solution ). The columns can be used for many separation problems. The selectivity is influenced by the factors of the degree of crosslinking of the latex polymer and the type of functional group on the latex polymer. The ion exchange capacity depends on the following factors:

• Particle size of the substrate (inversely proportional)
• Latex particle size (proportional)
• Degree of latex coverage on the substrate surface (proportional).

The suppressor

Suppressors are used when detection is carried out by means of conductivity measurement. Their task is to reduce the basic conductivity of the eluent, which is why the suppression takes place before the mobile phase enters the conductivity measuring cell. In the simplest case, a suppressor consists of an ion exchange column in hydrogen form , i.e. the exchange ions are H ⁺ or H ₃ O ⁺ .

Initially, so-called column suppressors were used, which had to be loaded (regenerated) periodically with H ⁺ . 25 years ago this process was replaced by continuously operating membrane suppressors. However, more and more column suppressors have recently been used again, with a system of 3 columns being used that are automatically used alternately for suppression and regenerated. The regeneration of suppressors is done either continuously by electrolysis of water or discontinuously by acids .

The following equations should clarify this exchange process:

• the eluent is sodium hydrogen carbonate (NaHCO ₃ ):

${\displaystyle \mathrm {Harz{-}SO_{3}H\ +\ NaHCO_{3}\longrightarrow \ Harz{-}SO_{3}Na\ +\ CO_{2}\ +\ H_{2}O} }$

• the eluent is sodium hydroxide solution (NaOH):

${\displaystyle \mathrm {Harz{-}SO_{3}H\ +\ NaOH\longrightarrow \ Harz{-}SO_{3}Na\ +\ H_{2}O} }$

The substances to be detected are converted into their corresponding acids in an analogous manner:

• Analysis substance is sodium chloride (NaCl):

${\displaystyle \mathrm {Harz{-}SO_{3}H\ +\ NaCl\longrightarrow \ Harz{-}SO_{3}Na\ +\ HCl} }$

• Analysis substance is sodium bromide (NaBr):

${\displaystyle \mathrm {Harz{-}SO_{3}H\ +\ NaBr\longrightarrow \ Harz{-}SO_{3}Na\ +\ HBr} }$

From these equations it follows that only the weakly dissociating carbonic acid or water reach the conductivity measuring cell as the reaction product of the suppressor reaction of the eluents. These products are almost not electrically conductive. In contrast, the reaction products of the analysis substances are very conductive because they largely dissociate in water :

${\displaystyle \mathrm {HCl\ +\ H_{2}O\longrightarrow \ H_{3}O^{+}\ +\ Cl^{-}} }$

${\displaystyle \mathrm {HBr\ +\ H_{2}O\longrightarrow \ H_{3}O^{+}\ +\ Br^{-}} }$

When using carbonate as an eluent, the basic conductivity can be further reduced by removing the carbonic acid. This can e.g. B. can be achieved by a second suppressor or the application of a slight negative pressure.

detector

The types of detection are divided into electrochemical and spectroscopic methods. The electrochemical processes include conductivity detection and amperometric detection. The choice of the type of detection depends on the separation process and the eluent required for this; Conductivity detection plays a central role here, as it can be used universally. The electrical conductivity is given in Siemens per centimeter and has the following formula:

${\displaystyle \chi ={\frac {l}{A\cdot R}}}$

where = length of the conductor, = cross section of the conductor, = resistance ${\displaystyle l\,}$ ${\displaystyle A\,}$ ${\displaystyle R\,}$

The conductivity is proportional to the concentration of the ions, the charge number of the ions and the mobility of the ions, so that a quantitative analysis is possible. Furthermore, the conductivity increases when the temperature increases, but since the temperature is kept constant, this fact can be neglected. The signal is sent from the detector to the computer for data processing.

evaluation

As with other forms of chromatography, the analytical evaluation is carried out by comparing the areas of peaks . As part of a calibration , the ratio of the peak area to the concentration of a desired substance is determined with the help of standard substances . The retention time of the substance is used to identify several analytes . For quantitative determination, there must be no overlapping of peaks of different substances. This is guaranteed either by a suitable choice of the chromatography system or by changing to a different detection system.

Sample analysis

This measurement was carried out with an IonPac CS18 cation exchange column from Dionex (Sunnyvale, USA). An aqueous solution of methanesulfonic acid served as the eluent . Almost complete separation of all components can be seen. Using the example of lithium (1) and trimethylamine (7), the different specific conductivity of the substances can be demonstrated (almost the same peak height with an approximately 40-fold difference in concentration). Calcium (9) and phenethylamine (16) show a slight tailing .

This measurement shows the chromatogram of anions in drinking water (Houston, USA) after addition of chlorite , bromate and chlorate ; an aqueous Na ₂ CO ₃ solution was used as the eluent . The measurement was carried out with a Metrosep A Supp 7-250 anion exchange column from Metrohm. There are great differences in concentration between the everyday anions chloride, fluoride and sulfate compared to added chlorite and bromate. By increasing the resolution, the tailing of these substances is more clearly visible. In this case, there is no separation between chloride and nitrite at the baseline level.

literature

• Hamish Small: Ion Chromatography. ISBN 0-306-43290-0
• Joachim Weiss: Handbook of Ion Chromatography , Dionex Corporation, ISBN 978-3527287017
• Joachim Weiss: Ion chromatography , ISBN 978-3527286980
• James S. Fritz and Douglas T. Gjerde: Ion Chromatography. ISBN 3527299149
• PR Haddad and PE Jackson: Ion Chromatography (Journal of Chromatography Library). ISBN 0444882324

Web links

Commons : Ion Chromatography  - collection of images, videos and audio files

• Ion chromatography Forschungszentrum Jülich, accessed on October 27, 2011
• Ion exchange chromatography on Chemgapedia

See also

• Ion exchanger
• Isoelectric point
• High performance liquid chromatography

Individual evidence

1. ↑ dionex.com: IonPac CS18 Cation-Exchange-Column (PDF; 584 kB), accessed on November 17, 2013.