In previous work involving half-mask respirators, NIOSH found a correlation between fit factor (a quantitative measure of respirator performance) and total Freon-113 exposure dose (TFED), the amount of Freon-113 in a subject's end-exhaled breath measured 30 minutes after a simulated workplace test. Better detection methods for Freon-113 need to be developed in order to investigate the correlation for other classes of respirators. In this presentation we will describe improved strategies for measuring TFED using gas chromatography-mass spectrometry (GC-MS) with a preconcentration system. This presentation will describe column selection, detection limits, and precision of the analytical method. The effect of critical environmental parameters such as the background level of Freon-113 in the room will also be assessed. The use of GC/MS with preconcentration for exhaled breath analysis and trace detection may find suitability in other applications such as biomarker discovery, industrial hygiene and environmental monitoring.