In theory, you can obtain the spectrum of your solid suspension using the aqueous suspension without any sample preparation using a transmission cell. But, usually these cells are made of NaCl, and KBr and these materials easily dissolve in water. Thus, you need to make special liquid cell using IR transparent materials like AgCl, AgBr, or CeBr which are all water insoluble. Another big problem is that water is a very strongly absorbing solvent in IR (if you use Raman, using aqueous suspension is very easy as water is nearly transparent in Raman, but IR is opposite). Therefore, unless the concentration of your suspension is more than 10%, and you are extremely experienced, using aqueous suspension directly is not recommended. I have experience using an aqueous solution of 0.5%, but only those very strong bands could be studied with this concentration.

An easier method is to centrifuge the suspension and get rid of the solvent. Then make a KBr pellet for the transmission study after drying the collected powder. This will give you a beautiful spectrum. If your suspension is like a colloid and difficult to precipitate, you can add the aqueous suspension with KBr powder. The KBr powder then dissolves. You can then dry this mixture in a vacuum oven to get rid of water. At this point, KBr crystals reappear. After very careful drying (KBr likes to hold onto water strongly and water interferes with the IR spectrum appearance), you can then press the powder mixture into a KBr pellet and do the transmission study. Make sure the amount of aqueous suspension added is well controlled so that the final powder mixture is in the following ratio (KBr: 250-300 mg, your target material: 0.5-1.5 mg). This will give you an ideal intensity spectrum with the strongest absorbance typically less than 1.0 absorbance unit. A IR intensity less than 0.7 absorbance unit is highly quantitative, but too weak a band suffers from the noise. So, intensities 0.5-1.0 absorbance unit is easiest and most accurate to study. Measuring the amount of material is difficult for unknown sample. Then, you can do opposite. Make a sample with known amount of KBr and liquid suspension, and record the spectrum. If the intensity of the strongest band is around 1.0 absorbance unit, use it. If not, calculate proportionately and figure out the amount of KBr and liquid suspension to be used.

In the past, I have used ATR-FTIR for this purpose (spectrum 100 perking elmer). A background correction can be done by using a blank suspension matrix. you might get enough interaction to see the functional groups without making pellets