Abstract: A compact, ultra-sensitive, inexpensive NIR spontaneous Raman spectrometer is presented. High sensitivity is achieved by the use of a multi-pass cell configuration combined with the electromotive properties of silicon crystal surface. A thin layer of silicon oxide chemisorbs molecules, which stick to its surface without altering their spectroscopic signatures. This new Raman spectrometer may be used to detect less than 40 ng (?0.5 n mol) of ammonium nitrate deposited on the silicon surface with the signal-to-noise ratio better than 50 during 0.1 s recording time and for an illuminated area of 2x10?8 m2. These results may be useful in many areas, for example the foundation of an extended project to record the dissolved NO3? ions in a large river such as the Mississippi.