Wooden toothpicks coated with nylon-6 coupled to DI-MS, a high throughput alternative for the determination of 3 drugs of abuse in saliva

 In our latest research, wooden toothpicks (WTs) coated with a commercial polyamide such as nylon-6 (N6) were used in a solid phase microextraction (SPME) procedure for the determination of methadone, cocaine, and methamphetamine in saliva. Bare WTs present weak hydrophilic and hydrophobic forces provided by cellulose and lignocellulose, respectively. Consequently, the N6 polymeric coating was used to reinforce the hydrogen bonds and hydrophobic forces with the target analytes.

The synthesis of N6-WTs is based on the dip coating technique, where the superficial -OH groups provided by the (ligno)cellulosic components of wood allow the coating of the WTs. This simple dipping-and-drying process consists of the immersion of the WTs in a N6 precursor solution (3 % (w/v) in formic acid), followed by the evaporation of the solvent to anchor the thin polymeric phase to the surface of the WTs. The synthesized sorbents were characterized by attenuated total reflection infrared spectroscopy (ATR-IR) to confirm the presence of the polymeric layer onto the WTs and by scanning electron microscopy (SEM) to evaluate the homogeneity of the deposition of N6.

The extraction procedure of multiple samples (up to 30 different samples at the same time) is achieved using HPLC vials as an extraction device (Fig 1). Once the N6-WTs are simultaneously conditionated with water at pH 10, each N6-WT is transferred to a HPCL with 1.5 mL of the sample at pH 10. The samples are stirred at 500 rpm for 30 min. Afterwards, the N6-WTs are washed with 1 mL of water at pH 10 at 500 rpm for 5 min. Finally, the N6-WTs are eluted with 150 μL of methanol in a HPLC insert into the vial and the extracts are analyzed by direct infusion mass spectrometry. The extraction, washing and elution steps are carried out simultaneously, providing a high sample throughput.

Figure 1. Extraction procedure.

The pH and the variables that affect significantly in the extraction of the analytes, namely extraction time, number of dips and dilution of the sample were optimized by a Box-Behnken design. The optimum values are pH 10, 30 min of extraction time, 2 dips and 1:4 dilution factor. Working under these optimal conditions, the limits of detection (LODs) provided by the method, 1.5 μg/L, were compared to the cut-off values established by the European project Driving Under the Influence of Drugs, Alcohol and Medicines in Europe (DRUID) that provides the limit in which a drug test is reflected positive. As the LODs provided by this method are lower than the cut-off values, it is possible to identify positive cases.

We encourage everyone who comes across this to read the full paper, which can be found at the Microchemical Journal webpage.

Reference

J. Millán-Santiago, M.T. García-Valverde, R. Lucena, S. Cárdenas, Polyamide-coated wooden tips coupled to direct infusion mass spectrometry, a high throughput alternative for the determination of methadone, cocaine and methamphetamine in oral fluid, Microchem. J. 162 (2021) 105843. doi:10.1016/j.microc.2020.105843.

About the author

Jaime Millán Santiago is a Ph.D. student in the FQM-215 Research Group of the Analytical Chemistry Department of the University of Córdoba. He finished the bachelor’s degree in Chemistry in 2019 and the master’s degree in Applied Chemistry in 2020. Currently, he is involved in the use of lignocellulosic materials for the determination of opioids in biofluids by mass spectrometry.

Keywords

Lignocellulosic materials, wooden toothpicks, dip coating, drugs of abuse, high sample throughput, mass spectrometry