Unambiguous identification of low concentration chemical mixtures can be performed by broadband enhanced infrared absorption (BEIRA). Here we propose and numerically study a corrugated parallel plate waveguide (CPPW) with gradient grooves which is capable of directly converting transmission modes to surface plasmon modes and could hence serve as a powerful chemical sensor. Such a waveguide can be designed to exhibit a wide pass band covering an extended portion of a molecule absorption spectrum. Broadband sensing of toluene and ethanol thin layers is demonstrated by calculating the transmission coefficient of the waveguide and is shown to correspond exactly to their infrared spectra. In addition, the upper limit and the lower limit of the bandgap are mainly dependent on the minimum and maximum groove height, respectively, which provide an effective way of tuning the working frequency of the device in order to support surface plasmon modes within a desired frequency range according to a specific application.