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Aerosol Hygroscopic Growth as a New Factor for Trace and Ultra-Trace Determination of Phosphorous in Flame Containing Optical Trapping-Cavity Ring-Down Spectroscopy

Mohammad Mahdi Doroodmand and Fatemeh Ghasemi

A new method has been introduced based on aerosol hygroscopic growth as a new factor for trace and ultra-trace determination of phosphorous in flame containing optical trapping-cavity ring down aerosol extinction (emission) spectrometer (OT-CRD-AES). In this study, a cavity ring down has been designed using hydrogen and air as fuel and oxidant during introduction of the aerosols containing phosphorous species using an ultrasonic generator (humidifier) from an acidic solution by a flow rate of N2 , followed by detection of the Mie scattering using a charged coupling device (CCD) system. Parameters having strong influence during following scattering of the aerosols during their hygroscopic growth inside the humidified flame (H2 /air), include: influence and amount of Na+ as radiation buffer (as light source), flow rates of H2 , air and N2 , kind and concentrations of acid, evaluation of the aerosols inside flame, etc. These parameters were optimized using simplex and one at a time methods. Based on the figures of merit under optimized condition, two linear calibration curves with reverse slopes were evaluated between 10.0 - 250.0 ng mL-1 and 1.0 - 20.0 µg mL-1 with correlation coefficients (R2 ) the same as 0.999 and 0.998, respectively. The calibration sensitivities were also estimated to 57.46 and 0.348 (a.u), respectively, with detection limit of 5.0 ng mL-1. The mechanism of the radiation (Mie scattering) was also evidenced based on i) dependency of the scattered radiations to the quantity of an alkali ions such as Na+ as well as the humidity, ii) presence of acceptable correlation between the response of the cavity with turbidometry, iii) observation of blue shift from green (color related to the luminescence of HPO* in H2 /air flame) to blue (scattered radiation) and finally iv) effect of hydration number during stability and growth of the aerosols inside the flames. No serious interference was evaluated during analysis of at least 500-fold excess of various foreign species. However, the only observed interference was evaluated during introduction of 200-fold excess of SO4 2-. Good correlation was also evaluated between the results obtained from this technique and ion exchange chromatography during analysis of wastewater samples that clearly revealed the reliability of this method for phosphorous detection and determination at µg mL-1 and ng mL-1 levels.