Transient acid effects in inductively coupled plasma optical emission spectrometry and inductively coupled plasma mass spectrometry

Abstract

A change in sample acid concentration affects both transient (over several minutes) and steady state ICP-OES and ICP-MS signal magnitudes. Steady state signals are depressed when the acid concentration is increased andvice versa. The transient effect appears as a signal ‘undershoot’ or ‘overshoot’ of up to 50% followed by a time dependent (5–25 min) ‘relaxation’ to the steady state signal. The magnitude of the transient acid effect depends on the type and concentration of acid. Time-resolved side-on (radial) ICP-OES, end-on (axial) ICP-OES, ICP-MS, aerosol size and transport rate measurements were made. Similar acid effects were observed in both ICP-OES and ICP-MS. Following a change in sample acid concentration, the tertiary aerosol volumetric flux changed on a similar time scale as ICP-OES and ICP-MS signals whereas the primary aerosol flux did not. It is proposed that the transient effect is due to changes in the extent of aerosol evaporation in the spray chamber, 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 rates of aerosol and solution evaporation depend on their acid concentrations. The water vapour pressure decreases as the acid concentration increases. 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 into the spray chamber the aerosol evaporation will be more extensive. With time, however, the acid concentration of the solution coating the walls decreases owing to dilution by the aerosol of lower acid concentration. Then the extent of evaporation of newly introduced aerosol will decrease until a steady state is reached (i.e., the acid concentration of solution coating the walls equals the acid concentration in the aerosol). Transport rate measurements show an increase of 10% immediately after the sample is changed from a 25% to a 2% HNO₃ matrix. Methods to minimize and potentially eliminate this effect are proposed.