Convenient Determination of Trace Methylcyclosiloxanes in Water by Vortex-Assisted Small-Scale Liquid-Liquid...

Volatile methylcyclosiloxanes, particularly octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6), are commonly found in the environment and have gained significant attention as emerging contaminants. However, their accurate quantification at trace levels can be challenging because of complex environmental matrices and background interferences.

VMCSs were extracted from water using vortex-assisted small-scale liquid-liquid extraction (LLE) with n-hexane as the extractant. Both the VMCS standards and the extracts were analyzed using gas chromatography-mass spectrometry (GC-MS). The mechanistic fragmentation of D4, D5, and D6 in EI-MS was studied through ¹³C-labeling experiments.

In EI-MS, the molecular ion readily eliminates a methyl radical to form the ion [M-CH₃]⁺ for D4, D5, and D6. For D4, the ion [M-CH₃]⁺ subsequently loses CH₄, while the ions corresponding to D5 and D6 tend to undergo cyclization and methyl-shift, resulting in the characteristic ions of (CH₃)₃Si⁺ (m/z 73) and [M-CH₃-CH₄]⁺. These characteristic ions were used as the quantitative or qualitative indicators in the developed GC-MS method for determining D4, D5, and D6, achieving limits of quantification of 0.72-0.99 ng/mL. Efficient extraction of VMCSs in water was achieved within 12 min at room temperature, using only 0.40 mL of n-hexane for each extraction operation. Isotope-spiked experiments demonstrated satisfactory recoveries: 107.9% ± 8.0% for D4, 97.2% ± 5.8% for D5, and 84.7% ± 5.6% for D6. Analysis of real water samples revealed concentrations of D4, D5, and D6 in surface water ranging from 0.05 to 0.09 ng/mL, 0.07 to 0.27 ng/mL, and 0.07 to 0.25 ng/mL, respectively.

We developed a convenient and accurate method for the determination of trace level of D4, D5, and D6 in water, which addresses the limitations of traditional LLME, and offers a simple and environmentally friendly detection protocol for quantifying trace VMCSs in water.