1995excursion in the abundance of that isotope. The natural abundance of 34S is on the order of 4.0% and that of I5N is about 0.37%. It will, therefore, be possible to follow the dilution of 16N down to approximately a factor of 10 lower abundance than will be possible with Calculations based on these estimates suggest that 1 kg of %SO4 would label about 1 mile3 of air at the detection limit given a background level of about 8 pg/m3 sulfate. Sir:In recent years, the fabrication and study of various miniaturized versions of ion-selective electrodes (ISEs) have been vigorously pursued (1). Once fully developed, it is hoped that such electrochemical devices could be routinely used for continuous in vivo monitoring of blood electrolytes (e.g., Kt, Nat, C1-, etc.) during surgical procedures or at the bedside of patients in critical care units. Band and co-workers have reported considerable success in preliminary animal studies with simple scaled down versions of conventional ion-exchanger or neutral carrier based polymer membrane type ISEs (with internal electrolyte solutions) (2,3). Others have described the fabrication and performance of ion-selective field effect transistors (ISFETs) which are, in effect, solid-state devices (no internal reference electrolyte) (4-8). The ISFET has attracted considerable attention because it is envisioned that a single miniaturized solid-state chip could contain multiple gates and be used to sense several ions simultane-ISFETs which are not covered with polymeric ion-selective membranes can be used directly as pH sensors. The insulating gate materials are typically silicon nitride and silicon oxide which, by their own intrinsic surface properties, develop phase boundary potentials proportional to the logarithm of hydrogen ion activity of any solution they are in contact with. However, potassium ISFETs, for example, are usually prepared by coating the gate region of these pH devices with the same polymeric ion-selective membranes used in conventional potassium ISEs. Thus, once the gate region is covered with the polymeric membrane material, two potentials are generated: a Donnan potential at the interface of the membrane and the sample, dependent on the analyte ion activity in the sample, and an unknown potential at the interface of the membrane and the gate which is dependent on the activity of hydrogen ions present in this region. It is our belief that, since the hydrogen ion activity is not fixed at this membrane-gate interface, species which can enter this region by passing through the membrane can alter the pH and therefore interfere with the measurement of the analyte ions.In this correspondence, we provide preliminary data which strongly supports this view. Indeed, it is clearly shown that ously, ISFETs based on polymer ion-selective membranes are subject to positive interference by carbon dioxide and organic acids in the sample solution. As a model, we describe experimental results obtained with potassium ISFETs and nonselective ISFETs coated with polymer membranes containing...