The potentiometric response is a function of the activity of the free, uncomplexed ions in the solution. The selectivity of the sensors originates from doping of the membrane with ionophore but it will also depend on other components, such as the lipophilic salt which provides the ionic sites and the type of plasticizer if the membrane is based on poly(vinyl chloride). Even optimized membranes based on the most selective ionophores, such as valinomycin, are not specific. No perturbation, such as current or voltage sweep is applied to the system and due to the steady state nature of the measurement, each sample can be characterized by only one, unique data point. The signal is an average of all the processes taking place at the membrane/sample interface. Analysis of selectivity coefficients, which describe the preference of the sensor towards the primary ion over others, indicates that in complex samples, with many interfering ions the response will be subject to considerable error. The problem can be solved by preparing calibration curves in the sample matrix, however one should expect much narrower linear range (as seen in the Fixed Interference Method) . Variations in the electrode systems, liquid/liquid junctions, calibration and measurement procedures are indicated in the guidelines for the analysis of blood samples as reasons why routine measurements can produce highly precise results, which might be far from being accurate .