Sulfate ion in inorganic salts and in simple crystal environments is exactly tetrahedral. Because of this high symmetry there are only two infrared active modes of vibration. A very strong band at around 1100 cm-1 is composed mostly of a S--O stretch, and a moderately strong band at around 650 cm-1 is mainly composed of an umbrella type motion of the other three oxygen atoms -- largely a bending vibration. In a more complicated crystal environment these bands may be shifted (up to about 50 cm-1), broadened (out to a FWHH around 100 cm-1) or split.
Unfortunately, these two regions, 1100 cm-1 and 650 cm-1 co-incide with those where a fairly complicated set of rather strong infrared absorptions appear for most organic molecules and biomolecules. Although the two sulfate signals are quite strong in themselves, they would be easily overlooked if they were broadened and hidden under a lot of organic molecule peaks
It looks like I will need to narrow my scan window and decrease the slit width to see if there are any differences. Thank you.