  We present and characterize a 2-D imaging spectrometer based on a
virtually-imaged phased array (VIPA) disperser for rapid, high-resolution
molecular detection using mid-infrared (MIR) frequency combs at 3.1 and 3.8 \mu
m. We demonstrate detection of CH4 at 3.1 \mu m with >3750 resolution elements
spanning >80 nm with ~600 MHz resolution in a <10 \mu s acquisition time. In
addition to broadband detection, rapid, time-resolved single-image detection is
demonstrated by capturing dynamic concentration changes of CH4 at a rate of
~375 frames per second. Changes in absorption above the noise floor of 5\times
10-4 are readily detected on the millisecond time scale, leading to important
future applications such as real time monitoring of trace gas concentrations
and detection of reactive intermediates.
