Methods and apparatus are provided for determining weight percent of solids in a suspension using Raman spectroscopy. The methods can be utilized to acquire Raman spectral data from the suspension and to determine weight percent of solids in a process being carried out, for example, in a vessel, without the need to remove samples for analysis. The weight percent of the solids can be determined with a desired accuracy in a relatively short time, typically 10 minutes or less. The acquired Raman spectral data may be processed by chemometric software using, for example, a Partial Least Squares algorithm and data pretreatment to provide a predicted value of weight percent solids. In some embodiments, the invention is used to determine the weight percent of microparticles of a diketopiperazine in an aqueous solution.