Abstract: Methods of measuring a sample characteristic and accessories for infrared (IR) spectrometers are provided. An accessory includes an input port and an output port having an optical path therebetween, a surface plasmon resonance (SPR) structure for contacting a sample, a mirror system, and an optical element for producing collimated light. The SPR structure produces internally reflected light responsive to broadband IR light, modified by a SPR between the SPR structure and the sample. The mirror system directs the broadband IR light from the input port to the SPR structure and directs the internally reflected light from the SPR structure to the output port, producing output light indicative of a characteristic of the sample associated with the SPR. The optical element is disposed along the optical path between the input port and the output port.