<p>Mass spectrometry (MS) alone or coupled with high-performance liquid chromatography (HPLC) or gas chromatography (GC) is a versatile analytical tool that is routinely employed for identification of unknown compounds in complex mixtures. MS operates by separating ionized analytes based on their mass-to-charge (<i>m/z</i>) ratios. If the analyte can be ionized without complete fragmentation, MS provides molecular weight information and, if performed at high resolution, elemental compositions for the ionized analytes. Tandem mass spectrometry (MSⁿ, n <u>></u> 2 where each MS step corresponds to an ion isolation or separation event) also provides structural information of ionized analytes. With this approach, structural information of the ionized analytes is obtained by isolating the ionized analytes of interest and subjecting them to fragmentation experiments, such as collision-activated dissociation (CAD). The ions of interest can also be isolated and allowed to react with gaseous molecules to generate product ions (ion-molecule reactions). </p> The experiments described in this dissertation focused on the development of tandem mass spectrometry methods based on CAD and/or gas-phase ion-molecule reactions for the differentiation of acyl, <i>N</i>- and <i>O</i>-glucuronide drug metabolites and for identification of primary carbamates as potentially mutagenic impurities. Further, by using a previously published method titled Distillation, Precipitation, Fractionation Mass Spectrometry (DPF MS), the chemical compositions of five crude oil samples, including heavy, medium, and light crude oils with different API gravities, were determined. Additionally, the gravimetric percentages of different compound classes found in these crude oils are reported as well as the correlations found between API gravities and the chemical compositions of crude oils.