Simultaneous determination of lead, cadmium and zinc in Metro Manila air particulates by anodic stripping voltammetry

摘要

Air particulate samples were collected from two monitoring stations in Metro Manila using a 'Gent' type dichotomous sampler for pollutant source apportionment studies. Samples were collected in two fractions: a fine fraction with aerodynamic diameter, d(sub p)<2.5 (mu)m and a coarse fraction with 2.5 (mu)m <d(sub p)<10 (mu)m. Zinc, cadmium, and lead were analyzed in these samples as markers for incinerators and vehicular emissions. Differential pulse anodic stripping voltammetry (DPASV) of the three toxic heavy metals was performed using a mercury film plated rotating glassy carbon working electrode, Ag/AgCl reference electrode, and glassy carbon rod auxiliary electrode. The heavy metals exhibited electrochemical behavior satisfactory for their simultaneous determination in the supporting electrolytes studied. They were detected at average stripping peak potentials of -1.10 V for zinc, -0.70 V for cadmium, and -0.50 V for lead. Background levels of these elements in the media studied were significant and were corrected for. Microwave digestion was employed in the decomposition of the sample. For the simultaneous digestion of 12 samples, microwave digestion at maximum power in 3.0 ml of acid (HNO(sub 3): HCL: HF, 4: 1: 1) for at least 20 minutes with subsequent heating at lower power settings for a total of 20 minutes more, effectively decomposed the sample with complete recovery of the elements. The digests were evaporated to near dryness to eliminate the troublesome effect of HF and HNO(sub 3) and to decrease acidity of the electrolytic solution to pH (>=) 2. At pH < 2 the high signal due to the evolution of hydrogen gas masked the zinc peak and interfered with its determination. At pH 2 the detection of zinc was improved by deposition at -1.30 V. Also this pH further addition of a supporting electrolyte was not necessary. At pH > 2, the addition of at least 0.01 M KCl was needed to improve sensitivity. The formation of Zn-Cu intermetallic compounds which interfered in the accurate quantitation of zinc was eliminated by addition of gallium as a 'third' element. The amount of gallium needed varied from sample to sample and was affected by the pH of the solution. The DPASV parameters found to be optimum for the analysis of the air particulate samples are as follows: pulse amplitude, 50 mV; scan rate, 10 mV/sec; E(sub dep) , - 1.30 V; t(sub dep), 2 min; and RDE rotation rate, 1500 rpm. Detection limits of 0.2 ppb for zinc, 0.6 ppb for lead, and 0.05 ppb for cadmium in the sample matrix were obtained. The standard addition method was found to be reliable for the quantitative determination of the analytes in the sample. All R(sup 2) values obtained were > 0.9900 at 95% confidence level. Validation of the established analytical methodology by analyzing certified reference standards and performing parallel analysis by GF-AAS and flame AAS showed acceptable accuracy of the DPASV measurements. (Author). 73 refs., 45 figs., 13 tabs.