An analytical method for determining exposure to 2,2'-dichlorodiethyl sulfide (sulfur mustard, HD) has been enhanced. The method is based on the cleavage of adducted HD (protein-hydroxyethylthioethyl esters) to produce thiodiglycol. Following cleavage, a deuterated internal standard is added, and the analytes are extracted, derivatized, and analyzed by gas chromatography-negative ion chemical ionization-mass spectrometry. Inclusion of a concentration step, addition of solid sodium bicarbonate to neutralize excess derivatization reagent, and optimization of method and instrument conditions provided dramatic increases in signal-to-noise ratio. A five-day precision and accuracy study was conducted, including interday and intraday unknown analysis. Linearity was verified by a R(2) > 0.9995 for all five curves evaluated. The precision and accuracy of the assay were demonstrated to be excellent by evaluation of the interday and intraday unknown samples (< 10% relative standard deviation and relative error in most cases). Statistical treatment of the method blanks and calibration results demonstrated a reduction in the limit of quantitation from 25 nM (HD, human plasma, in vitro) to 1.56 nM. Sample and calibration stability through the analytical sequence was established by the inclusion of continuing calibration verification standards (< 5% error). Short-term sample stability was verified by reinjection of a calibration set after 18 days (R(2) = 0.9997). Quantitative agreement with the previous method was supported by the analysis of a 50 nM standard protein sample (HD, rat plasma) with both methodologies (< 1% error).