Ion chromatography with pulsed amperometric detection (IC-PAD)
We use ion chromatography for the determination of mineral elements mainly of soil water samples and carbohydrates as plant metabolites from plant material.
The determination of carbohydrates is carried out with the aid of anion exchange chromatography. Since the pK values of sugars are between 12-14, they can be converted into the anionic form in a strongly alkaline environment and separated in the hydroxide form on a strongly basic anion exchanger. Sodium hydroxide in the concentration range c = 0.001 and 0.15 mol/L is suitable as an eluent. The hydroxide ions have two functions. On the one hand, they function as eluent ions and, on the other hand, they determine the pH value of the mobile phase. A change in the hydroxide ion concentration in the mobile phase thus has two different effects on the retention behaviour of the carbohydrates. While the dissociation of the carbohydrates and consequently their retention increases with increasing pH, the associated increase in the eluent ion concentration causes a reduction in retention. As long as the dissociation of the carbohydrates is not quantitative, these two effects compensate each other. With complete dissociation of the carbohydrates, a further increase in the hydroxide ion concentration only results in a reduction in retention.
When preparing the sodium hydroxide eluent, it must be ensured that it is free of carbonate. The carbonate ion has a much higher eluting power than the hydroxide ion, so even traces of carbonate in the eluent lead to a reduced dissolution. The eluent is therefore continuously flushed with helium.
A typical IC column is 250 mm long and has a diameter of 4.6 mm. The columns contain the anion exchanger made of pellicular latex, on which there are quaternary ammonium groups that serve as exchange sites for the anions. The term "exchanger" comes from the fact that ions repeatedly displace each other from the interaction sites. Latex" here refers to the colloidal solution of polymers, e.g. polymethacrylate derivatives.
A pump drives the eluent (e.g. NaOH 18 mMol) at a defined flow rate (e.g. 1 mL/min) through the IC system, which usually consists of an injection valve, separation column and detector. The injection valve contains the sample loop (volume 10 -100 µL) with the sample to be injected. The system is flushed with eluent until it is in stable equilibrium. Only then is the injection valve switched, the sample loop is flushed by the eluent and the sample is applied to the separation column.
The individual anions are retained on the separation column for different lengths of time due to different degrees of interaction with the column. This process is called retention. The separated ions therefore differ in their retention times. The separated ions are fed to a detector where they trigger a time-dependent signal (peak). If the course of signal intensity / time is visualised in an xy-diagram, the corresponding chromatogram results, which is used to evaluate the measurement.
The qualitative analysis is done by comparing the retention times, the quantitative evaluation is done by comparing the peak areas (if necessary also the peak heights) with standard solutions of known composition.
The carbohydrates are detected by pulsed amperometry. The measuring cell consists of a working electrode, reference electrode and counter electrode. If an electroactive substance (e.g. a carbohydrate anion) passes the flow cell, the anion is oxidised. An anodic current is generated that is proportional to the concentration of the carbohydrate species and can therefore be displayed as a chromatogram. To avoid deposits of the oxidised species on the electrode surface, rapidly changing working potentials are applied, thus removing possible reaction products from the electrode surface.