Halogenated Organic Compounds (HOCs) are typically monitored via targeted mass spectrometry, potentially identifying a fraction of contaminants actually present in environmental samples. With thousands of new anthropogenic compounds introduced to the environment annually, novel and proactive alternatives to targeted approaches are needed in order to comprehensively characterize the diversity of known and unknown compounds likely to cause adverse effects. Nontargeted analyses, based on comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC_GC/TOF-MS), are an effective, tractable solution for monitoring typically monitored, typically unmonitored, and unknown compounds. Chapter 1 employed GC_GC/TOF-MS to develop a method that characterized HOCs in the eggs of a sentinel seabird, the California Black skimmer (Rynchops niger) (n=4). Our study identified 111 HOCs; 84 compounds were regularly detected via targeted approaches, while 27 were typically unmonitored or unknown. Typically unmonitored compounds of interest included tris(4-chlorophenyl)methane (TCPM), tris(4-chlorophenyl)methanol (TCPMOH), heptachloro-1'-methyl-1,2'-bipyrrole (MBP), and four halogenated unknown compounds that could not be further identified. The presence of these compounds in eggs suggests they are persistent, bioaccumulative, and maternally transferred. Chapter 2 deployed the method developed in Chapter 1. First, we expanded our analysis to 21 eggs to validate previously detected compounds (n=41). Second, we explored detection patterns via Principal Components Analysis and Pearson's Correlations to assist in describing HOCs and unknowns. Finally, the 21 eggs represented random samples of presumed viable and non-viable eggs in order to explore relationships between egg viability and contaminant detection and abundance. Our results validated the method developed in Chapter 1, with detections of TCPM, TCPMOH, and p,p'-DDE in all 21 eggs; MBP and DDMU in 20 eggs; and Unknowns 2A and 2B in 19 and 18 eggs, respectively. Pearson's correlation matrices revealed that Unknowns 2A and 2B were positively correlated with DDMU, potentially revealing information about their parent compound. No significant relationships between egg viability and contaminants were found via Fisher's Exact tests (p=0.153) or Discriminant Functions Analysis (p=0.803). Our results highlight the necessity of employing nontargeted analytical tools to assess true contaminant burdens in organisms, and demonstrate the utility of using environmental sentinels to proactively identify novel contaminants for future monitoring.