The effect of nitric acid concentration and nebulizer gas flow rates on aerosol properties and transport rates in inductively coupled plasma sample introduction

Abstract

An investigation of the influence of nitric acid on the aerosol generation and transport processes for an inductively coupled plasma (ICP) sample introduction system consisting of a cross-flow nebulizer and a Scott-type double pass spray chamber is described. Two important results are presented. (1) Aerosol and analyte transport rates decrease similarly when the nitric acid concentration is increased from 0 to 25% (v/v). This suggests that changes in analyte transport rates are due mainly to changes in aerosol transport through the spray chamber rather than changes in the relative analyte concentration in the tertiary aerosol compared with the bulk sample solution. (2) The relative decrease in transport rates as the nitric acid concentration is increased from 0 to 25% (v/v) is dependent on nebulizer gas flow rate. At low nebulizer gas flow rates, there is a dramatic decrease in the analyte and aerosol transport rates for a change from 0 to 2% (v/v) nitric acid and then little change as the nitric acid concentration is increased to 25% (v/v). At high nebulizer gas flow rates, there is a continuous decrease in the analyte and aerosol transport rates as the nitric acid concentration is increased from 0 to 25%. The nebulizer gas flow rate affects the ‘robustness’ of ICP-OES or ICP-MS signals to changes in acid concentration both through plasma condition ‘robustness’ and the susceptibility of the aerosol transport rate to variation in acid concentration. Changes in tertiary aerosol properties are more dramatically affected by changes in acid concentration than primary aerosol properties. Acid-dependent changes in the analyte and aerosol transport rates are likely most affected by changes in liquid aerosol density and evaporation rates. Evaporation is most efficient for 0% HNO₃ solutions and least efficient from 25% HNO₃ solutions and contributes to changes in the relative liquid density of aerosols. From the data described here, the acid-dependent changes in aerosol properties appear to occur predominantly during the transport of aerosol through the spray chamber. Similar effects should occur with other solutions (e.g. 5% w/w NaCl) whose density and water vapour pressure change significantly with concentration.

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