For many ISE analyses most natural waters can be analysed directly without any sample preparation. In some cases, however, where the total salt content is high (or, more precisely, where the total Ionic Strength is greater than about 0.01 Molar for monovalent ions and 0.001M for divalent ions) it is necessary to add ISAB (Ionic Strength Adjustment Buffer) to all standards and samples. In other cases, where interference effects may be important, it is often possible to treat the sample chemically to remove or reduce the effect of the interfering ion (by precipitation or complexing). Pre-treatment is also required if the ISE has a limited pH range, or if the analysed ion may be subject to biological degradation or oxidation.

Addition of ISAB:

The reason for this is that ISEs only measure the activity or effective concentration of an ion in solution - i.e. the proportion of ions actually reaching the membrane. In solutions with a high salt content, whether of the ion of interest or of other species, the presence of many ions causes inter-ionic interactions which impede their motion. Thus the activity of the ion becomes less as the Ionic Strength increases and a smaller proportion of the ions reach the ISE membrane. In this situation, a spuriously low value will be recorded for the concentration. The purpose of adding ISAB is to raise the Ionic Strength of all standards and samples to a uniformly high level so that the activity coefficient is the same for the ions in both solutions; i.e. there is the same discrepancy between the concentration and the measured activity. Thus the concentration of the samples can be determined directly from the concentration of the standards.

It must be noted, however, that most recipes for adding ISAB only result in an increase of 0.1 mol/L and hence this procedure will only be effective if the samples have an ionic strength significantly less than this. Furthermore, at a total Ionic Strength of less than about 0.01 Molar for monovalent ions (0.001M for divalent ions) the difference between activity and concentration will not significantly effect the results and hence it is not necessary to add ISAB to dilute solutions - unless it is needed for other purposes - see below. Different ISABs are used for different ISEs and the recommended solutions and proportions are given in the electrode specification sheets.

Interference, pH, and Oxidation Effects:

In many cases the effect of these factors can be counteracted by adding the appropriate reagent and this is usually done by combining these reagents in a complex ISAB which is added to all standards and samples in order to ensure the same matrix effects in both.

Biological Degradation Inhibitor:

If samples are prone to biological degradation then the majority of "bugs" can be killed by adding a few drops of biological inhibitor to samples immediately after collection. This is made by dissolving 0.1g of phenylmercuric acetate in 20ml of dioxane and diluting to 100ml with de-ionized water.

The majority of analyses on plant material are carried out on samples which have been dried to ensure a uniform moisture content and then weighed and ashed to remove all organic material. In some cases, however, it may be necessary to analyse fresh material: e.g. fruit juices. In either case, the final analysis is performed on an aqueous solution which has been obtained by leaching, dissolving or diluting a known weight of sample in a known volume. It must be recognised that the ISE analysis will only yield the concentration of ions in this solution and whether or not this represents the concentration of the element in the original plant material depends on the solubility and degree of ionisation of the compounds containing this element.

Washing

It is usually necessary to wash plant material after collection to ensure that there is no contamination from soil, dust, fertilisers or animal products. In order to minimise any losses due to leaching of water-soluble constituents, this is best done quickly under a running tap and then briefly rinse in de-ionised water to remove any contamination from tap water.

Drying

The temperature at which this is carried out can vary with the type of material and the nature of the element to be determined. In general, plant material is coarsely chopped and laid in wire mesh baskets and dried in a forced - draught oven at between 30 and 60°C.

Grinding

For routine analysis the dried plant material is ground to pass through a 1mm mesh sieve and then sealed in air-tight containers and stored away from direct sunlight if not to be analysed immediately.

Dry Combustion (Ashing)

The organic material must be destroyed by dry combustion before analysis. This is done by taking a weighed sample of the dried material (say 2g) in a silica or platinum crucible and placing it in a muffle furnace at 500°C for at least 4 hours. When ashing is completed the residue is leached in 5Molar hydrochloric acid, ensuring that there are no losses due to effervescence, and then evaporated to dryness on a hot plate or heating mantle at about 102°C. It is left baking at this temperature for a further hour before dissolving the bulk of the residue in a small amount of concentrated HCl and then diluting with water. Any remaining residue is filtered off and the effluent is collected in a volumetric flask and diluted to the mark. This solution is then used for the ISE analysis.

NB: Do not use HCl if Chloride is to be analysed, or any ion for which Chloride is an interferent. In this case nitric acid can be used.

For most ISE analysis the soil must be dried and ground in order to obtain the concentration of the required element in a standard dried weight of sample and avoid any errors due to variable moisture content. In some applications, however, the concentration in the "field" or moist state is required. In this case the field sample should be rubbed through a 5-6 mm wire mesh sieve and any large stones discarded. The sample should be kept in a sealed plastic bag, to ensure no loss of moisture during transportation and storage, and preferably analysed soon after collection to minimise any biological degradation.

In either case, a known weight of soil sample is subsequently stirred and leached with water and the extract is filtered and diluted to a known volume. It must be recognised that the ISE analysis will only yield the concentration of ions in this solution and whether or not this represents the total concentration of the element in the soil depends on the solubility and degree of ionisation of the compounds containing this element. Nevertheless, this is actually an advantage in soil analysis because, in most cases, the soil is being analysed in order to assess the amount of water soluble material which is available for plant uptake.

Air Drying

The soil sample should be spread out in a thin layer on a metal tray. Any stones and fresh plant material should be removed, and any large clods broken up. If this is done with the fingers then plastic gloves should be worn to avoid contamination. The metal tray should then be placed in a current of air not exceeding 30°C and left until completely dry.

Grinding

When the sample is dry it can be ground in a pestle and mortar or proprietary grinding machine until it passes through a 2mm mesh sieve. For heavy soils it may be better to grind the sample whilst it is still partially moist and complete the drying process after grinding.