Abstract: The invention relates to a method for the analysis of the qualitative and/or quantitative composition of fluids with at least one light source, one interaction space area in which the light interacts with the fluid, and one detection device for detecting the interaction between the fluid and the light. The device and the method are characterised in that a photonic the gap structure is provided creating a greater reduced group velocity in the interaction space area and thus an increased dwell time for the light in the interaction space area. This means that the interaction space area can be considerably reduced in size, which allows a compact design of the device.