Micro flow system to perform programmable isotope dilution for inductively coupled plasma-mass spectrometry

A flow system using computer controlled devices to implement binary sampling of small volumes of sample and spike solutions was coupled to an inductively coupled plasma-mass spectrometer to perform programmable isotope dilutions. A computer running software in QuickBasic controlled the time-based sampling of sample and spike solution by switching on/off a set of solenoid valves. Mixing was improved by sampling 15 small aliquots of both solutions at a flow-rate of 65?lmin-1 through a capillary tubing connected to a microconcentric nebulizer (MCN). Each aliquot of 6?l containing the sample plus spike was introduced during 4s. Programming the sample to spike volumetric ratios in each aliquot allowed to perform different isotopic dilutions generated from a single enriched solution. The flow system was calibrated measuring the isotopic ratios of 112Cd/114Cd by binary sampling of natural abundance Cd solutions as sample and enriched 112Cd (97at.%) solution as spike. The programmed and the measured ratios in the range from 6 to 25 presented good correlation (0.997). For routine analysis, using an estimate analyte concentration, and the isotope ratio within the defined range, the software calculates the time delays to introduce the sample and spike volumes. The true and found concentrations for solutions containing from 2.0 to 20.0?gl-1 Cd attained good agreement at the 95% of confidence level. For these solutions the consumption of enriched 112Cd (97at.%) ranged from 33 to 168pg per determination. About 30 isotope dilutions per hour were achieved.