The measurement of the bulk purity of single-walled carbon nanotubes (SWNTs) is an important outstanding problem. We report a solution-phase near-IR (NIR) spectroscopic study of a range of carbon materials with an emphasis on SWNTs, and we show that NIR spectroscopy is an extremely powerful tool in the assessment of the carbonaceous purity of SWNTs. We demonstrate the applicability of Beer's law for all of the carbon materials included in this study within a range of concentrations in dimethylformamide (DMF), and we are able to solve for the effective extinction coefficients. By analyzing the areal absorptivities of the second interband transition of semiconducting SWNTs for a number of samples of differing purities, we are able to derive an absolute molar extinction coefficient for the carbonaceous impurities in SWNT samples. We demonstrate significant progress toward the establishment of an absolute scale for the bulk carbonaceous purity of SWNTs.