A New Analytical Method to Quantify Ammonia in Freshwater with a Bulk Acoustic Wave Sensor

A new method to analyse ammonia in freshwater, based on a piezoelectric quartz crystal coated with the metalloporphyrin chloro[5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato] manganese(III) is presented. A 9 MHz quartz crystal coated on both faces with an amount of porphyrin produced a frequency decrease of 21.4 kHz, which allowed ammonia in a 10.00 mL sample to be quantified in concentrations between 5 and 70 mu g L-1, with a sensitivity of 0.60 Hz L mu g(-1), over a period of at least eight months. The proposed method has several advantages over the officially recommended indophenol spectrophotometric method: sample volume was reduced by a factor of 2.5, toxic reagents (phenol and sodium nitroprusside) were eliminated, analysing turbid samples presented no difficulty, and there was not only a significant time saving in solution preparation, but also in sample analysis time, which was reduced from 1 h to 2 min. No statistically significant differences (alpha = 0.05) were found both in the mean and precision of the results obtained for ammonia in water samples collected from domestic wells, analysed by this new method and by the indophenol spectrophotometric method. Furthermore, the proposed method would allow the individual quantification, with similar sensitivity, of amines and ammonia within a single analytical run.

Automated summary

A new method for analyzing ammonia in freshwater using a metalloporphyrin-coated piezoelectric quartz crystal is presented. This method offers several advantages over the traditional spectrophotometric method, including reduced sample volume, elimination of toxic reagents, ease of analyzing turbid samples, and significant time saving in solution preparation and sample analysis. The results obtained using this method showed no statistically significant differences compared to the results obtained using the traditional method.