O-18 exchange in suspensions of red blood cells: Determination of parameters of mass spectrometer inlet system

A membrane inlet system to a mass spectrometer (MIMS) allows us to monitor the abundance of O-18 in CO2 during the exchange of O-18 between HCO3- diss olved CO2 and H2O that occurs after dissolving O-18-iabelled NaHCO3 in aque ous solution. Using a mathematical model we quantitate intracellular carbon ic anhydrase activity and membrane permeabilities for HCO3-, CO2 and H2O of isolated cells or intact epithelia added to the solution. For these calcul ations it was necessary to determine (a) the relation between MS signal and [(COO)-O-18-O-16], (b) MIMS response kinetics and (c) CO2 leakage during t he experiment. The three parameters were determined by stepwise addition of HCI to HCO3- solution: (a) was found to be linear, (b) response times were 7.5 +/- 2s (10 degrees C), 4.5 +/- Is (20 degrees C), 3.5 +/- 0.6s (37 deg rees C), and (c) CO2 leakage was < 1 parts per thousand s(-1).