Determination of 18 polychlorinated biphenyls in milk by dispersive solid-phase extraction based on zeolitic- imidazolate-framework composite microspheres prior to gas chromatography-mass spectrometry

Polychlorinated biphenyls （PCBs） are hazardous， persistent organic pollutants that are widely used industrially. Although the use of PCBs is banned in many countries， they are still present at trace levels in food and the environment. PCBs are highly chemically stable and lipophilic； hence， they are easily enriched and accumulate in the human body through milk and dairy products. PCBs residues pose serious threats to human health； therefore establishing a reliable enrichment method is an important objective. Sample pretreatment is required to efficiently extract target PCBs owing to sample-matrix complexity and their low contents. Efficient adsorbents form the cores of novel sample-pretreatment technologies， and designing new stable adsorbents is crucial for the further development of pretreatment techniques. Zeolitic imidazolate frameworks （ZIFs） are a family of metal-organic frameworks composed of imidazole linkers and metal ions. Their large surface areas， good stabilities， high porosities， and ease of modification are distinct advantages； consequently， ZIFs are widely used to adsorb organic pollutants. However， powdered ZIFs are difficult to separate and collect， which provides reuse challenges； hence， preparing ZIF composites with other functional materials is a highly effective way of addressing this challenge. Chitosan （CS） is an inexpensive and biodegradable natural polysaccharide that gelates easily. The structure of CS contains many free amino and hydroxyl groups that facilitate chemical modification and hybridization； consequently， CS is a matrix commonly used in composite materials. In this study， we prepared CS@ZIF-8 composite beads by the in-situ synthesis of ZIF-8 on chitosan through acid-solubilization/base-fixation. An analytical method for determining 18 PCBs in milk was developed using CS@ZIF-8 composite microspheres as the adsorbent for dispersive solid-phase extraction （DSPE） coupled with gas chromatography-mass spectrometry （GC-MS）.The CS@ZIF-8 composite microspheres were characterized by scanning electron microscopy （SEM）， Fourier-transform infrared （FT-IR） spectroscopy， X-ray diffractometry （XRD）， and nitrogen-adsorption-desorption experiments， which confirmed that the material had been successfully prepared. How adsorbent dosage， extraction and desorption times， and type and volume of the desorption solvent affect the extraction efficiency were investigated， with the following optimal extraction conditions determined： 20 mg of CS@ZIF-8 as the adsorbent， 30 min of extraction by shaking， and 8 min of ultrasonic desorption with 1 mL of n-hexane. The 18 PCBs exhibited good linearities in the 1-200 μg/L under these optimal conditions， with coefficients of determination （r2） exceeding 0.999. Detection limits （S/N=3） ranged between 0.06 and 0.24 μg/L， with quantification limits （S/N=10） of 0.19-0.79 μg/L. Repeatability experiments were performed by the addition of 100 μg/L of the 18 PCBs， which exhibited intra-day and inter-day precisions （n=6） of 2.5%-5.3% and 4.3%-5.9%， respectively， while inter-batch material precisions （n=3） ranged between 4.9% and 9.7%. The applicability of the developed method was investigated by selecting whole milk and skim milk as samples based on PCBs lipophilicity. Spiked recovery experiments were conducted at three concentrations （5， 20， and 100 μg/L）， with the 18 PCBs exhibiting spiked recoveries of 84.8%-114.3%. CS@ZIF-8 not only has a larger specific surface area than CS， but it also adsorbs PCBs through π-π interactions and hydrophobicity， leading to superior extraction efficiency. CS@ZIF-8 exhibited spiked recoveries exceeding 70% for all samples after four adsorption-desorption cycles during reproducibility testing. The developed method provides a simplified extraction process by eliminating the need for centrifugation or filtration steps that are usually associated with conventional DSPE. In addition， the developed method is highly sensitive， precise， and accurate， with adsorbent reusability a noteworthy feature， thereby supporting the simple and efficient detection of PCBs in milk samples.