Temperature-programmed desorption for membrane inlet mass spectrometry

We present a novel technique for analyzing volatile organic compounds in air samples using a solid adsorbent together with temperature-programmed desorption and subsequent detection by membrane inlet mass spectrometry (TPD-MIMS). The new system has the advantage of a fast separation of compounds prior to the detection by MIMS. The gaseous sample is simply adsorbed on the adsorbent, which is then rapidly heated from 30 °C to 250 °C at a rate of 50 °C/min. Trapped organic compounds are released from the adsorbent into a helium stream at different temperatures depending on the strength of the interaction between the individual compound and the adsorbent. The helium stream carries the desorbed compounds to a membrane inlet (90 °C) equipped with a thin (25 µm) silicone membrane. The thin membrane and the high temperature of the membrane inlet allows most volatile compounds to diffuse through the membrane into the mass spectrometer in a few seconds. In this fashion we could completely separate many similar volatile compounds, for example toluene from xylene and trichloroethene from tetrachloroethene.
Typical detection limits were at low or sub-nanogram levels, the dynamic range was 3 orders of magnitude, and the analysis time for a mixture was about 3–4 minutes.