Fast heating rates of microhotplate sensors (10⁶ K/s) have been applied for temperature programmed sensing (TPS), an atmospheric pressure technique, in which temperature modulation of the sensing film leads to characteristic electrical conductance signatures for a particular gas analyte. The TPS signatures are governed by adsorption, desorption and reaction rates which control the surface coverages of chemical species. We present an ultra-high vacuum technique to measure these kinetic parameters, during desorption from a microhotplate, which provides great promise for relating fundamental surface phenomena to sensing performance.

The technique was demonstrated for isothermal temperature programmed desorption of benzoic acid from a reduced SnO₂ covered microhotplate at surface temperatures ranging from 296 to 347K. Measurable signal intensity for femtomolar quantities of adsorbates was achieved by rapidly desorbing the benzoic acid over timescales ranging from about 2 to 700 ms. Models indicate that the effects of the microhotplate heating time constant (≈0.2 ms) should have a negligible effect on the desorption spectrum under these conditions. However, the effects of pumping in the mass spectrometer could not be neglected. Therefore, the data were analyzed using models that included both pumping and desorption rate effects. Qualitative and quantitative analysis of the data using zero, first, and second order kinetic models indicate that benzoic acid desorption is best represented by first order kinetics. The first order pre-exponential factor and the desorption energy in the zero coverage limit are determined to be 1×10¹⁷ s^-1 and 97 kJ/mol respectively from desorption of 10⁸ molecules, corresponding to an initial coverage of 10¹² cm^-2 (≤0.005 ML).

The ability to heat the microhotplates faster than typical reaction time constants makes them particularly useful for studying phenomena such as kinetic branching ratios or the partial adsorption of adsorbates on energetically heterogeneous surfaces. We will report results of isothermal desorption studies of the mechanisms for the chemiresistive response of both TiO₂ and SnO₂ microsensors to 2-propanol. Experiments were conducted under various conditions of surface oxidation. We monitored mass spectral peaks and the electrical resistances of the sensing films during isothermal desorption, and we will discuss the role of surface heterogeneity and competing reaction kinetics in the interpretation of these data.