We tested whether Elemental Analysis (EA) used to quantify organic matter from marine samples also include plastic-derived carbon. Estuarine sediment was used as the natural, background sedimentary organic carbon pool due to its higher carbon content compared to suspended POM and DOM in seawater. Microplastic samples were mixed with sediment to produce a range of weight percentages of MPs and analyzed by EA. Data set contains raw data from the EA conducted on pure plastic, pure sediments and admixtures.