A simple and low-cost approach for microdistillation: Application to methanol determination in biodiesel exploiting smartphone-based digital images

Abstract

Although usual in analytical chemistry, separations of volatile analytes by distillation are time-consuming, whereas gas diffusion through membranes is characterized by low efficiency. These limitations directly hinder sample throughput and sensitivity. In this work, a simple and inexpensive approach is proposed for separation of volatile analytes by microdistillation. It consists of an eppendorf type tube containing the acceptor solution suspended inside a 50 mL Falcon^® capped tube containing the sample. The bottom of the sample tube is submitted to controlled heating in a water bath to promote analyte volatilization to the headspace, thus favoring its collection into the acceptor solution. The efficiency of the approach was evaluated by determination of methanol in biodiesel, whose threshold limit is 0.2% (m/m) according the current legislation. Measurements were based on discoloration of an acidic potassium permanganate solution (750 μL), caused by oxidation of methanol with consequent reduction to Mn(II). The analytical measurements were based on intensity of the reflected radiation acquired as digital images by the camera of a smartphone under controlled lighting. Values of the G channel were used as analytical parameter because of the complementarity with the color of the potassium permanganate solution. Under heating at 70 °C for 40 min, the transference of methanol to the acceptor solution was estimated at (98 ± 3)%. A linear response was achieved within 0.025% and 0.200% (m/m) methanol, with coefficient of variation (n = 10) and limit of detection (99.7% confidence level) estimated at 3.7% and 0.009% (m/m) respectively. The procedure requires only 100 μg of potassium permanganate and 200 μL of sample and generates as low as 950 μL of residue per determination. Because of the selectivity provided by microdistillation, the results obtained by the proposed approach agreed with the reference procedure at the 95% confidence level. The proposed approach is a simple, efficient, and cost-effective alternative for separation and determination of volatile species.

Keywords: Biofuels; Gas-liquid separation; In-situ analysis; Photometry; Point-of-care measurements; Volatile species.