Green analytical flow method for the determination of total sulfite in wine using membraneless gas–liquid separation with contactless conductivity detection

Abstract

A green analytical flow method was developed for the determination of total sulfite in white wine. The method employs the membraneless vaporization (MBL-VP) technique for gas–sample separation allowing direct analysis of wine. Sulfite in an aliquot of sample was converted to SO₂ gas via acidification. Dissolution of the gas into the water acceptor led to a change in the conductivity of the acceptor which was monitored using a ‘capacitively coupled contactless conductivity detector’ (C4D) flow cell. Only a minute amount of common acid (100 μL of 1.5 mol L⁻¹ H₂SO₄) is used. The MBL-VP unit was incorporated into the flow system to separate the SO₂ gas from the wine sample using the headspace above the donor and acceptor compartments as a virtual membrane. The method provides a linear working range (10–200 mg L⁻¹ sulfite) which is suitable for most wines with calibration equation y = (0.056 ± 0.002)x + (1.10 ± 0.22) and r² = 0.998. Sample throughput is 26 samples h⁻¹. The lower limit of quantitation (LLOQ = 3SD of blank per slope) is 0.3 mg L⁻¹ sulfite for 20 s diffusion time with good precision (%RSD = 0.8 for 100 mg L⁻¹ sulfite, n = 10). We also present a simple modification of the MBL-VP unit by the addition of a third cone-shaped reservoir to provide two acceptor zones leading to improvement in sensitivity of more than three-fold without use of heating to enhance the rate of diffusion of SO₂.