Methoxsalen is a naturally occurring photoactive substance found in the seeds of the Ammi majus (Umbelliferae) plant. It belongs to a group of compounds known as psoralens or furocoumarins. The chemical name of methoxsalen is 9-methoxy-7H-furo [3,2-g] [1]-benzopyran7-one. It has an empirical formula of C12H8O4 and a molecular weight of 216.19 g mole−1. Photochemotherapy (PUVA) is treatment involving the use of psoralen, like methoxsalen, an exogenous photosensitizer followed by ultraviolet A (UVA) irradiation.

The manufacturing process of methoxsalen is seven step syntheses. In synthetic stage five a key raw material dimethyl sulfate (DMS) is used as methylating agent. It is used mainly for converting active-hydrogen compounds such as phenols, amines and thiols to the corresponding methyl derivatives. Dimethyl sulfate is probably carcinogenic to humans so IARC classified DMS in Group 2A. It is a potent genotoxic chemical which can directly alkylate DNA both in vitro and in vivo. Because of the known carcinogenicity and genotoxicity, the presence of residual dimethyl sulfate in methoxsalen drug substance must be controlled as per European Medicines Agency (EMA), International Conference on Harmonization and Food and Drug Administration (FDA) guidelines. EMA and FDA guidelines proposed the use of the “threshold of toxicological concern” (TTC) concept for the limit of genotoxic/carcinogenic impurities. The concentration limit, in ppm, of genotoxic impurity in drug substance, is the ratio of TTC in μg per day to the expected dose of drug substance in a gram per day. Considering the recommended daily maximum dose of 0.070 g methoxsalen per day, dimethyl sulfate must be limited to less than 21.4 μg/g in drug substance. So it is necessary to develop sensitive, accurate and robust analytical method.

Methoxsalen is official drug substance in United State Pharmacopoeia (USP). Dimethyl sulphate is not controlled in the USP monograph. During literature survey several analytical methods are found reported of estimation of dimethyl sulfate. Dimethyl sulfate is checked at workplace atmosphere by GC with electrolytic conductivity detector (sulphur mode) at trace level using stainless steel Chromosorb WHP column. DMS is estimated at workplace atmosphere by thin layer chromatography (TLC) method derivatizing with 4-nitrophenol. The LOQ of DMS is reported 40 ppm which is very high because TLC technique has limitations in sensitivity point of view. Quantification of dimethyl sulfate by GCMS is reported by derivatizing with pentafluorobenzenethiol. This report suggests that solution stability and diluent study need to be optimized and mass detector is necessary to get DMS sensitivity at trace level. Few more methods are reported by extracting the DMS in a solvent and tested by GC-MS. DMS quantification by ion chromatography technique with conductivity detector is also reported by separating ionic compounds using Allsep anion exchange column. Literature suggests that to achieve such a low level of limit of quantification hyphenated techniques with sophisticated instruments like GC-MS or sulphur specific detector or LC with conductivity detector is required. It might hardly possible for quality control laboratory of pharmaceutical industry to perform the testing routinely with GC-MS because of the cost and maintenance of a mass detector is very high. So in present research, analytical method is developed to quantify the DMS in methoxsalen drug substance with flame ionization detector. Due to low sensitivity of FID, pre-column derivatization was tried to achieve the quantification of analyte at trace level. The proposed analytical method was validated as per International conference on harmonization guidelines ICH Q2-R1.