A further test of the spectrograph is the evaluation of its potential for the study of the hydrogen containing minerals and in particular the vibration of the OH bonds of water and hydroxyl ions.  The figure below shows the spectrum for a hornblende crystal. A very small broad band between 3600 and 3700 cm^-1 appears, it should be the OH stretching vibration of hydroxyl but the sharp peaks at 2700 and 2950 cm^-1 are unknown. These two artifacts also appear on a gypsum spectrum below. Their intensity is lower on the gypsum spectrum simply because the Raman intensity of the OH band is much higher for gypsum than for hornblende. After a few experiments, these two peaks were identified as spurious laser lines in the UV present here in the second order of the ruled  grating. Using a red filter in front of the laser suppress completely these lines and give a pure Raman spectrum for CH and OH region. These lines do not appear in the plastic spectra due to the high intensity of CH in Raman. Normally, the laser had a band pass filter in front of it to isolate laser line but after consulting the manufacturer data sheet of this filter, I realized that it is not efficient in the UV. So now, the red filter is always in place when recording spectra in the range 2500 - 4000 cm^-1. The last spectrum on this page is a clean Raman spectrum of the crystal water in a gypsum with the red filter in place.

High frequency OH band of a hornblende with spectral artifacts.

OH band of a gypsum (water molecules in the crystal) with two spurious lines at low frequency.

Same gypsum crystal with the addition of a red order filter.

The two second order lines coming from the laser disappeared.