The invention relates to an add-on system for a unit mainly adapted for attenuated total reflectance infrared (ATR-IR) spectroscopy. The add-on system enables time-resolved in-situ measurements of different sample types in an easy, simple and inexpensive way. The add-on system includes a cap (300G,300H,300J) to be attached to an ATR plate (200) to form a sample cavity (302) enclosing the sample (204). The cap (300G,300H,300J) comprises openings for allowing gas to enter and exit the sample cavity (304,306,308), cap sealing means (310) for providing a tight sealing between the ATR plate (200) and the cap (300B,300H,300J), a source of light, e.g. a light emitting diode (418) or an optical fibre (416) connectable to a laser, and a light source containing cavity.