A Dual GC-MS and LC-MS/MS Analytical Strategy for Evaluating Free Chlorine Toxicity in Aqueous Samples

Chlorine gas is a highly toxic industrial chemical that poses a significant risk in accidental releases and deliberate terrorist attacks. Reliable analytical methods for the determination of chlorine in aqueous samples are therefore required for forensic and environmental investigations. This study aimed to develop a rapid and selective mass spectrometry-based method for chlorine determination following derivatization.

Chlorine in aqueous samples was derivatized with methyl 2-(3,4,5-trimethoxyphenyl)acetate (MTMPA) at 50°C for 15 min, yielding methyl 2-(2-chloro-3,4,5-trimethoxyphenyl)acetate (Cl-MTMPA). The derivatization product was analyzed by gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-tandem mass spectrometry (LC-MS/MS). While LC-MS/MS analysis was performed directly, GC-MS analysis required an additional liquid-liquid extraction step. Reaction conditions were optimized to maximize derivatization efficiency.

The derivatization reaction provided a high yield of Cl-MTMPA under the optimized conditions. GC-MS and LC-MS/MS analyses produced a common major fragment ion at m/z 215.0. Further structural confirmation by liquid chromatography-high-resolution tandem mass spectrometry (LC-HRMS/MS) revealed a characteristic product ion at m/z 215.0473 generated from the precursor ion at m/z 275.0681. Calibration curves covering acute toxicologically relevant chlorine concentrations (5-200 mg/L) showed excellent linearity, with correlation coefficients (R²) exceeding 0.99.

A rapid and selective derivatization-based method was developed in which the resulting chlorine derivative is readily analyzed by both GC-MS and LC-MS/MS. The method affords reliable analyte identification through characteristic fragmentation and excellent quantitative performance, making it suitable for forensic toxicology and environmental investigations involving chlorine exposure.