THE EFFECT OF NITRIC-ACID CONCENTRATION AND NEBULIZER GAS-FLOW RATES ON AEROSOL PROPERTIES AND TRANSPORT RATES IN INDUCTIVELY-COUPLED PLASMA SAMPLE INTRODUCTION

An investigation of the influence of nitric acid on the aerosol genera tion and transport processes for an inductively coupled plasma (ICP) s ample introduction system consisting of a cross-flow nebulizer and a S cott-type double pass spray chamber is described. Two important result s are presented. (1) Aerosol and analyte transport rates decrease simi larly when the nitric acid concentration is increased from 0 to 25% (v /v). This suggests that changes in analyte transport rates are due mai nly to changes in aerosol transport through the spray chamber rather t han changes in the relative analyte concentration in the tertiary aero sol 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 nebuliz er gas flow rates, there is a dramatic decrease in the analyte and aer osol transport rates for a change from 0 to 2% (v/v) nitric acid and t hen little change as the nitric acid concentration is increased to 25% (v/v). At high nebulizer gas flow rates, there is a continuous decrea se in the analyte and aerosol transport rates as the nitric acid conce ntration is increased from 0 to 25%. The nebulizer gas flow rate affec ts the 'robustness' of ICP-OES or ICP-MS signals to changes in acid co ncentration both through plasma condition 'robustness' and the suscept ibility of the aerosol transport rate to variation in acid concentrati on. Changes in tertiary aerosol properties are more dramatically affec ted 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 evaporat ion rates. Evaporation is most efficient for 0% HNO3 solutions and lea st efficient from 25% HNO3 solutions and contributes to changes in the relative liquid density of aerosols. From the data described here, th e acid-dependent changes in aerosol properties appear to occur predomi nantly during the transport of aerosol through the spray chamber. Simi lar effects should occur with other solutions (e.g. 5% w/w NaCl) whose density and water vapour pressure change significantly with concentra tion.