Abstract:Establishing an accurate determination method for trace iodine in carbonaceous-siliceous shale is crucial for predicting iodine release potential during hydraulic fracturing in shale gas development, formulating groundwater pollution prevention strategies , and evaluating iodine resources in carbonaceous-siliceous shale . This study optimized a microwave digestion-ICP-MS method using a five-factor four-level non-interaction orthogonal experiment combined with a 2×2 interaction orthogonal experiment, systematically evaluating the effects of key factors such as hydrofluoric acid dosage and aqua regia dosage on iodine extraction efficiency. Results showed that hydrofluoric acid dosage (1.2 mL) was the dominant factor; under optimal conditions, iodine recovery ranged from 91.2% to 103.1% with a detection limit as low as 0.005 μg/g. Adjusting pH to 7.5～8.5 with ammonia water significantly inhibited iodine volatilization (RSD<5%), shortened pretreatment time to 30 minutes, and reduced memory effect error to less than 5%. Method validation indicated that retesting results of soil and stream sediment reference materials were within the standard range (RSD 0.62%～4.97%); actual sample determination showed that iodine content in Chongqing carbonaceous-siliceous shale was 0.33～0.95 μg/g, consistent with regional geological background. This method provides efficient technical support for shale gas development iodine pollution prevention and iodine resource exploration in carbonaceous-siliceous shale.