Exploitation of pulsed flows for on-line dispersive liquid–liquid microextraction: Spectrophotometric determination of formaldehyde in milk

Abstract

Formaldehyde is often added to foods as a preservative, but it is highly toxic to humans, having been identified as a carcinogenic substance. It has also been used for the adulteration of milk in order to diminish the bacteria count and increase the shelf life of the product. Herein, we present a green dispersive liquid–liquid microextraction procedure in a flow-batch system for the determination of formaldehyde in milk. Pulsed flows were exploited for the first time to improve the dispersion of the extractant in the aqueous phase. The Hantzsch reaction was used for the derivatization of formaldehyde and the product was extracted with the ionic liquid (IL) trihexyltetradecylphosphonium chloride with methanol as the disperser. The flow-batch chamber was made of stainless steel with the facility for resistive heating to speed up the derivatization reaction. Spectrophotometric measurements were directly carried out in the organic phase using an optical fiber spectrophotometer. The limit of detection and coefficient of variation were 100μgL−1 and 3.1% (n=10), respectively, with a linear response from 0.5 to 5.0mgL−1, described by the equation A=0.088+0.116CF (mgL−1) in which A is absorbance and CF is formaldehyde concentration in mgL−1. The estimated recoveries of formaldehyde from spiked milk samples ranged from 91% to 106% and the slopes of the analytical curves obtained with reference solutions in water or milk were in agreement, thus indicating the absence of matrix effects. Accuracy was demonstrated by the agreement of the results with those achieved by the reference fluorimetric procedure at the 95% confidence level. The proposed procedure allows for 10 extractions per hour, with minimized reagent consumption (120μL of IL and 3.5μL acetylacetone) and generation of only 6.7mL waste per determination, which contribute to the eco-friendliness of the procedure.