Abstract: Medium-chain chlorinated paraffins (MCCPs) are complex mixtures consisting of numerous homologues with carbon chain lengths of C₁₄-C₁₇. MCCPs with chlorine contents ≥ 45 wt% have been listed in Annex A of the Stockholm Convention on persistent organic pollutants. Accurate characterization of MCCP carbon and chlorine distribution has become increasingly important for compliance assessment and export inspection. However, conventional gas chromatography coupled with electron capture negative ionization low-resolution mass spectrometry (GC-ECNI-LRMS) suffers from limited mass resolution. Moreover, its quantitation relies heavily on calibration standards with chlorine contents similar to those of the samples. These limitations often lead to systematic errors in quantitative analysis and limited information on the carbon and chlorine distribution of MCCPs. To overcome these limitations, three MCCP mixtures with known carbon chain ratios (C₁₄:C₁₅:C₁₆:C₁₇=1:1:1:1) and chlorine contents were synthesized. Gas chromatography coupled with Orbitrap high-resolution mass spectrometry (GC-Orbitrap HRMS) was employed to analyze the mixtures. Characteristic fragment ions [M-Cl]^- of individual homologues were monitored at a mass resolution of ≥ 60,000. The resulting chromatographic and mass spectrometric data provided accurate information for detectable homologues and served as the basis for subsequent quantitative modeling. Chlorination of paraffins proceeds through stochastic radical substitution reactions. Therefore, the mole fractions of homologues with the same carbon chain length but different chlorine substitution numbers were assumed to follow a Gaussian distribution. Based on this assumption, a genetic algorithm was developed to reconstruct the mass distribution of individual homologues and to determine the relative response factors (RRFs) of detectable homologues. During optimization, the algorithm simultaneously minimized the variation of RRFs and the error of the calculated mass. The relative error between calculated and actual carbon skeleton molar masses remained below 0.1%, whereas the deviation between calculated and measured total chlorine contents was less than 0.9%. These results demonstrate that the proposed model accurately describes the composition of MCCP mixtures and provides reliable quantitative parameters for subsequent inversion. The optimized RRFs were then combined with Gaussian distribution inversion to enable estimation of homologues that produced no detectable instrumental signals. As a result, complete carbon and chlorine distributions could be reconstructed from partially observed mass spectrometric data, overcoming one of the major limitations of conventional ECNI-based analysis.
The proposed method was further evaluated using three commercial MCCP standards with nominal chlorine contents of 42%, 52%, and 57%. The reconstructed distributions showed that C₁₄ homologues accounted for 60.3%-68.4% of the total mass, whereas the combined abundance of C₁₆ and C₁₇ homologues was below 13%. A consistent underestimation of low-chlorinated components was revealed when chromatographic peak areas alone were used for calculation. This bias became increasingly pronounced as chlorine substitution decreased. Three industrial MCCP products were subsequently analyzed to evaluate the applicability of the method to real samples. The measured chlorine contents ranged from 49.0% to 55.0%, in good agreement with values determined by the flask combustion method. Regulated homologues with chlorine contents of ≥ 45 wt% accounted for 71.3%, 92.4%, and 93.7% of the total product mass, respectively. Nevertheless, only 50.6%-83.6% of these regulated components generated directly detectable mass spectrometric signals. These results clearly demonstrate that direct instrumental detection alone cannot fully characterize the composition of industrial MCCP products. The proposed strategy provides a quantitative framework for resolving MCCP carbon and chlorine distributions and supports accurate assessment of regulated homologues in commercial products under increasingly stringent international regulations.

Key words: medium-chain chlorinated paraffins, gas chromatography-Orbitrap high-resolution mass spectrometry, genetic algorithm, carbon and chlorine distribution, precise quantification, persistent organic pollutants