A cavity ring-down spectrometer having a light-conveying structure, a mirror and an isolator. The structure may form a resonator cavity, and the resonator cavity may include a sample sub-cavity and a mirror sub-cavity. A preconcentrator medium may be within a sample sub-cavity, and the medium may adsorb a sample substance entering the sample sub-cavity. A heater may heat the medium to desorb the sample through which a light can propagate. The mirror may be within the mirror sub-cavity, and the mirror may direct light through the mirror sub-cavity and the sample sub-cavity. The isolator may reduce, inhibit or prevent the sample substance in the sample sub-cavity from entering the mirror sub-cavity and affecting the mirror in the mirror sub-cavity.