Quantifying the masses of light elements C, N, and O within individual marine phytoplankton is critical for understanding complex biological and environmental systems. Traditional methods for measuring light elemental masses require chains of assumptions or aggregating thousands of cells in culture rather than wild specimens. Using a total-reflection Wolter mirror to achromatically focus soft X-rays, we developed a method to measure quantitative masses of light elements in a single, fixed phytoplankton in just a few hours. This technique involves measuring image stacks by scanning the photon energy across an elemental K-edge with spectro-ptychography. The absolute masses are then extracted from the absorption edge jumps. This presentation will describe the development of our achromatic focusing soft X-ray ptychographic system, present recent work verifying the quantitative mass extraction, and highlight its capability for rapid, high-resolution measurements. These methods enable elemental quantification within single cells and already have sufficient resolution for mapping elemental spatial distributions, offering further opportunities for studying phytoplankton composition.