TRANSIENT ACID EFFECTS IN INDUCTIVELY-COUPLED PLASMA OPTICAL-EMISSIONSPECTROMETRY AND INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY

A change in sample acid concentration affects both transient lover sev eral minutes) and steady state ICP-OES and ICP-MS signal magnitudes, S teady state signals are depressed when the acid concentration is incre ased and vice versa. The transient effect appears as a signal 'undersh oot' or 'overshoot' of up to 50% followed by a time dependent (5-25 mi n) 'relaxation' to the steady state signal, The magnitude of the trans ient acid effect depends on the type and concentration of acid, Time-r esolved side-on (radial) ICP-OES, end-on (axial) ICP-OES, ICP-MS, aero sol size and transport rate measurements mere made. Similar acid effec ts mere observed in both ICP-OES and ICP-RIS, Following a change in sa mple acid concentration, the tertiary aerosol volumetric flux changed on a similar time scale as ICP-OES and ICP-MS signals whereas the prim ary aerosol flux did not. It is proposed that the transient effect is due to changes in the extent of aerosol evaporation in the spray chamb er, which in turn affects the analyte transport rate, There appears to be a competition between evaporation of solution on the walls of the spray chamber and evaporation of the sample aerosol, The relative rate s of aerosol and solution evaporation depend on their acid concentrati ons. The mater vapour pressure decreases as the acid concentration inc reases, Spray chambers conditioned with high acid concentrations will produce smaller amounts of water vapour from the solution on the spray chamber walls. When a low acid concentration sample is then sprayed i nto the spray chamber the aerosol evaporation mill be more extensive. With time, however, the acid concentration of the solution coating the malls decreases owing to dilution by the aerosol of lower acid concen tration. Then the extent of evaporation of newly introduced aerosol mi ll decrease until a steady state is reached (i.e., the acid concentrat ion of solution coating the walls equals the acid concentration in the aerosol), Transport rate measurements show an increase of 10% immedia tely after the sample is changed from a 25% to a 2% HNO3 matrix, Metho ds to minimize and potentially eliminate this effect are proposed.