18O 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 18O in CO2 during the exchange of 18O between HCO3-, dissolved CO2 and H2O that occurs after dissolving 18O-labelled NaHCO3 in aqueous solution. Using a mathematical model we quantitate intracellular carbonic anhydrase activity and membrane permeabilities for HCO3-, CO2 and H2O of isolated cells or intact epithelia added to the solution. For these calculations it was necessary to determine (a) the relation between MS signal and [C18O16O], (b) MIMS response kinetics and (c) CO2 leakage during the experiment. The three parameters were determined by stepwise addition of HCl to HCO3- solution: (a) was found to be linear, (b) response times were 7.5 +/- 2s (10 degrees C), 4.5 +/- 1s (20 degrees C), 3.5 +/- 0.6s (37 degrees C), and (c) CO2 leakage was < 1/1000 s-1.