Um sistema de análises por injeção em fluxo é proposto para a determinação espectrofotométrica de sulfito em vinhos brancos. O método envolve conversão do analito a SO 2 , difusão gasosa através de uma membrana semi-permeável de Teflon ® , coleta em um fluxo alcalino (pH 8), reação com verde de Malaquita (VM) e monitorização a 620 nm. O projeto do sistema foi simplificado empregandose uma membrana tubular concêntrica. Foram investigados: concentrações dos reagentes, pH dos fluxos doador e receptor, intensidade iônica, temperatura, temporização, adição de agentes tensoativos e presença de interferentes em potencial presentes na matriz vinho. A sensibilidade analítica melhorou (ca. 100%) adicionando-se cloreto de cetil piridinio (CPC). O sistema proposto é robusto e deriva em linha base não é observada durante períodos de até 4 horas de operação. São requeridos apenas 400 µL de amostra e 0.32 mg VM por determinação. O sistema processa 30 amostras por hora, fornecendo resultados precisos (d.p.r. < 0.015 para 1.0 -20.0 mg L -1 SO 2 ) e concordantes com os obtidos por um procedimento alternativo.A flow-injection system is proposed for the spectrophotometric determination of sulphite in white wines. The method involves analyte conversion to SO 2 , gas diffusion through a Teflon ® semipermeable membrane, collection into an alkaline stream (pH 8), reaction with Malachite green (MG) and monitoring at 620 nm. With a concentric tubular membrane, the system design was simplified. Influence of reagent concentrations, pH of donor and acceptor streams, temperature, timing, surfactant addition and presence of potential interfering species of the wine matrix were investigated. A pronounced (ca. 100%) enhancement in sensitivity was noted by adding cetylpyridinium chloride (CPC). The proposed system is robust and baseline drift is not observed during 4 h operating periods. Only 400 µL of sample and 0.32 mg MG are required per determination. The system handles 30 samples per hour, yielding precise results (r.s.d. < 0.015 for 1.0 -20.0 mg L -1 SO 2 ) in agreement with those obtained by an alternative procedure.Keywords: concentric tubular membrane, sulfite, spectrophotometric flow analysis, micellar medium, gas diffusion
IntroductionGas diffusion is a very effective means to enhance selectivity in analytical chemistry, and was implemented in a flow-injection system in the pioneer work dealing with the development of an improved procedure for determination of total CO 2 in plasma. 1 Since then, the strategy has been often exploited, as it is efficiently accomplished in flow systems. 2,3 Generally, a gaspermeable membrane is placed between two liquid streams, one of them -the donor stream -containing the sample under processing. The analyte is converted to a volatile chemical species that is removed from the sample matrix and collected by other stream -the acceptor stream.Sulphite is an important preservative in the food and beverage industry, especially with regard to wine production 4-8 and its content should be strictly controlle...