MINIMIZATION OF ACID EFFECTS AT LOW CONSUMPTION RATES IN AN AXIALLY VIEWED INDUCTIVELY-COUPLED PLASMA-ATOMIC EMISSION SPECTROMETER BY USINGMICRONBULIZER-BASED SAMPLE INTRODUCTION SYSTEMS

Two methods were applied in order to minimize acid effects at very low liquid uptake rates in an axially viewed ICP-AES system: aerosol solv ent elimination and direct injection in the plasma base. The latter ch oice eliminated the use of a spray chamber. Four different sample intr oduction systems were evaluated based on the use of micronebulizers, T he first system made use of a so-called microconcentric nebulizer (MCN ) associated with a double-pass Scott spray chamber. Two other systems were based on the use of a desolvation unit that consisted of a first and a second stage in which the solvent was partially removed. The so lvent elimination system was either two Liebig condensers connected in series or a porous PTFE membrane, In these cases two different MCNs w ere coupled. The last sample introduction system employed was a direct injection nebulizer (DIN) that eliminated the contribution of the spr ay chamber to the acid interference. The liquid flow rates ranged from 5 to 120 mu l min(-1), depending on the sample introduction system em ployed, Nitric, hydrochloric and sulfuric acid of 0.9 mol l(-1) were t ested and 3.6 mol l(-1) nitric acid solution was also studied in order to evaluate the acid concentration effect. The Mg II to Mg I line int ensity ratio was used to evaluate any possible changes in the plasma c onditions. The results showed that, for the MCN coupled to the spray c hamber, the lower the liquid flow rate, the greater was the acid inter ference. The extension of this effect was dependent on the MCN configu ration. For the desolvation systems and DIN the behavior was the oppos ite, the acid interference being more important as the liquid flow rat e increased. For the four acids employed, and at very low liquid how r ates (below 30 mu l min(-1)), the acid effect was eliminated by using a conventional desolvation system with a heating temperature of 160 de grees C, the condensation temperatures for the two condensers being 10 and 0 degrees C, respectively. The use of a membrane was found to be advisable because, under some conditions, acids and water gave the sam e analytical signal. As regards the DIN, it was able to eliminate the interference for 0.9 mol l(-1) nitric and hydrochloric acid. Neverthel ess, the two remaining solutions, i,e,, 0.9 mol l(-1) sulfuric and 3.6 mol l(-1) nitric acid, gave rise to a decrease in the signal of aroun d 20%.