Femtomolar isothermal desorption using microhotplate sensors

Abstract

The authors describe a technique that utilizes the fast heating rates (106K∕s) of a microhotplate sensor along with a calibrated thermal desorption system to determine the initial coverage and kinetic parameters using isothermal desorption on a millisecond time scale. Models for isothermal desorption including both pumping and desorption rate effects are presented for zero, first, and second order kinetics. Analysis of the first order model illustrates the domain of the desorption, pumping speed, and heating rate time constants that permit the desorption parameters to be estimated from the mass spectrometer signal. The technique is demonstrated using isothermal temperature programed desorption of benzoic acid from a single SnO2 covered microhotplate at surface temperatures ranging from 296to347K. The data indicate that desorption is best represented by first order kinetics. The first order preexponential factor and the desorption energy in the zero coverage limit are determined to be 1×1017s−1 and 97kJ∕mol, respectively, from desorption of 108molecules which corresponds to an initial coverage of 1012cm−2 (⩽0.005 ML).