Determination of trace levels of dimethyl sulfate in the presence of monomethyl sulfate by gas chromatography–mass spectrometry

• MMS decomposes in the hot GC injector into DMS leading to overestimated DMS levels.
• Proper measures to suppress DMS formation from MMS in the GC injector.
• GC–MS method development for the determination of DMS in the presence of MMS.
• Validation of the developed GC–MS method.

During a routine determination of dimethyl sulfate in technical materials using gas chromatography–mass spectrometry (GC–MS), we found that residual monomethyl sulfate originating from a prior methylation reaction with dimethyl sulfate decomposed in the hot GC injection system to yield dimethyl sulfate and sulfuric acid. This thermal reaction leads to false positive or overestimated residue levels of dimethyl sulfate, accompanied by bad chromatographic peak shapes and poor precision and accuracy values. This short communication describes proper measures to counteract this problem and presents a fast, reliable and validated GC–MS method that is capable of determining dimethyl sulfate residues in the presence of monomethyl sulfate in technical materials using a simple dissolve-and-inject approach. Applying deuterated dimethyl sulfate as internal standard and with a sample weight of 25 mg, a limit of detection of 0.24 mg kg−1 and a limit of quantification of 0.48 mg kg−1 was achieved along with a linear range of 0.48–208.6 mg kg−1. The method offers a precision of 9.1% and an accuracy of 96.5% at the limit of quantification and a precision of 3.6% and an accuracy of 93.8% at a dimethyl sulfate level of 1 mg kg−1.