ABSTRACT
A flow-injection system with a Chelite-S(R) cationic resin packed minicolumn is proposed for the determination of trace levels of mercury in agroindustrial samples by cold vapor atomic absorption spectrometry. Improved sensitivity and selectivity are attained since mercuric ions are on-line concentrated whereas other potential interferents are discarded. With on-line reductive elution procedure, concentrated hydrochloric acid could be replaced by 10% w/v SnCl(2), in 6 M HCl as eluent. The reversed-intermittent stream either carries the atomic mercury to the flow cell in the forward direction or removes the residue from reactor/gas-liquid separator to a discarding flask in the opposite direction. Concentration and volume of reagent, acidity, flow rates, commutation times and potential interfering species were investigated. For 120 s preconcentration time, the proposed system handles about 25 samples h(-1) (50.0-500 ng l(-1)), consuming about 10 ml sample and 5 mg SnCl(2) per determination. The detection limit is 0.8 ng l(-1) and the relative standard deviation (RSD) (n=12) of a 76.7 ng l(-1) sample is about 5%. Results are in agreement with certified value of standard materials at 95% confidence level and good recoveries (97-128%) of spiked samples were found.
ABSTRACT
A new strategy for minimization of Cu(2+) and Pb(2+) interferences on the spectrophotometric determination of Cd(2+) by the Malachite green (MG)-iodide reaction using electrolytic deposition of interfering species and solid phase extraction of Cd(2+) in flow system is proposed. The electrolytic cell comprises two coiled Pt electrodes concentrically assembled. When the sample solution is electrolyzed in a mixed solution containing 5% (v/v) HNO(3), 0.1% (v/v) H(2)SO(4) and 0.5 M NaCl, Cu(2+) is deposited as Cu on the cathode, Pb(2+) is deposited as PbO(2) on the anode while Cd(2+) is kept in solution. After electrolysis, the remaining solution passes through an AG1-X8 resin (chloride form) packed minicolumn in which Cd(2+) is extracted as CdCl(4)(2-). Electrolyte compositions, flow rates, timing, applied current, and electrolysis time was investigated. With 60 s electrolysis time, 0.25 A applied current, Pb(2+) and Cu(2+) levels up to 50 and 250 mg l(-1), respectively, can be tolerated without interference. For 90 s resin loading time, a linear relationship between absorbance and analyte concentration in the 5.00-50.0 mug Cd l(-1) range (r(2)=0.9996) is obtained. A throughput of 20 samples per h is achieved, corresponding to about 0.7 mg MG and 500 mg KI and 5 ml sample consumed per determination. The detection limit is 0.23 mug Cd l(-1). The accuracy was checked for cadmium determination in standard reference materials, vegetables and tap water. Results were in agreement with certified values of standard reference materials and with those obtained by graphite furnace atomic absorption spectrometry at 95% confidence level. The R.S.D. for plant digests and water containing 13.0 mug Cd l(-1) was 3.85% (n=12). The recoveries of analyte spikes added to the water and vegetable samples ranged from 94 to 104%.
ABSTRACT
A flow injection procedure for the separation and pre-concentration of inorganic arsenic based on the complexation with ammonium diethyl dithiophosphate (DDTP) and sorption on a C-18 bonded silica gel minicolumn is proposed. During the sample injection by a time-based fashion, the As(3+)-DDTP complex is stripped from the solution and retained in the column. Arsenic(V) and other ions that do not form complexes are discarded. After reduction to the trivalent state by using potassium iodide plus ascorbic acid, total arsenic is determined by electrothermal atomic absorption spectrometry (ETAAS). Arsenic(V) concentration can be calculated by difference. After processing 6 ml sample volume, the As(3+)-DDTP complexes were eluted directly into the autosampler cup (120 mul). Ethanol was used for column rinsing. Influence of pH, reagent concentration, pre-concentration and elution time and column size were investigated. When 30 mul of eluate plus 10 mul of 0.1% (w/v) Pd(NO(3))(2) were dispensed into the graphite tube, analytical curve in the 0.3-3 mug As l(-1) range was obtained (r=0.9991). The accuracy was checked for arsenic determination in a certified water, spiked tap water and synthetic mixtures of arsenite and arsenate. Good recoveries (97-108%) of spiked samples were found. Results are precise (RSD 7.5 and 6% for 0.5 and 2.5 mug l(-1), n=10) and in agreement with the certified value of reference material at 95% confidence level.
ABSTRACT
Periodate electrodes without inner reference solution based on tetraoctylammonium periodate plus solvent mediator (dibutyl phthalate or 2-nitrophenyl octylether) were constructed. Linear dynamic range, practical detection limit, slope, stability, selectivity coefficients, pH dependence, response time and lifetime were evaluated. A tubular version was further developed and coupled to a flow-injection system for glycerol determination in samples relevant to the industrial production of soaps, detergents and similar. The method involves glycerol oxidation by periodate with potentiometric evaluation of its consumption. The influence of oxidizing agent concentration (10(-5)-10(-2)M NalO(4)), ionic strength (0.0-1.0M Na(2)SO(4)) and mean resident time were investigated and the feasibility of using a single-fine manifold was discussed. The proposed system handles about 40 samples/hr, is very stable and suitable to industrial control. Results within the 1000 and 5000 mg/l range glycerol are precise (r.s.d. <0.005) and in fair agreement with conventional procedures. Baseline drift or noise is not observed and a thermostat water bath is not required. A noteworthy feature is the almost linear relationship between glycerol concentration and recorded peak height which is a consequence of combined effects of reaction kinetics and electrode Nernstian response.
