Ion Chromatography - Ion Chromatography Instrumentation

Ion Chromatography Instrumentation

A diagram of a basic ion chromatograph is shown in figure 12.

Figure 12. Basic Layout of an Ion Chromatograph

At the start there is a series of mobile phase reservoirs that can contain a range of different mobile phases that can be used individually, blended or for mobile phase programming purposes. Some instruments may have up to six reservoirs available but in many applications only two different mobile phases are required and in ninety per cent of all applications three reservoirs will be quite adequate. In general liquid chromatography, the reservoirs can be stainless steel but in ion chromatography where the mobile phases can have extreme pH values the reservoirs need to be made of glass or preferably a suitable plastic such as PEEK (polyether-ether-ketone). The advantage of PEEK is that it is also inert to many organic solvents that may need to be used in the mobile phase. In fact all components of an ion chromatograph that may come in contact with either phase of the distribution system should be constructed from appropriate inert material. This includes all mobile phase conduits, valves, pumps, sampling devices, columns, detector sensor cells etc. The solvent reservoirs are connected to a solvent selection valve and a solvent programmer where a particular solvent or particular solvent program can be selected. The solvent then passes from the selector/programmer to a high pressure pump. The mobile phase passes from the pump to the sampling device, usually a simple rotating valve that, on rotation, places the sample, in line with the mobile phase flow which, then, passes onto the column.

The exit flow from the column passes either to a ion suppressor (which will be discussed later) or directly to the detector. The detector is usually a electrical conductivity detector but the UV detector and other types of detector can also be used under certain circumstances. The output from the detector sensor is modified by the detector electronics and the electronically modified output, that is now linearly related to ion concentration, is either passed to a potentiometric recorder (now largely obsolete) or to an A/D converter and thence to a computer. The computer output is either observed on a monitor or presented in printed form by the computer printer. The individual components of the ion chromatograph will now be discussed in some detail.