A copper substitution approach for the selective analysis of chlorophyll-derived porphyrins in modern geological and environmental samples.

The input of chlorophyll provides key information for reconstructing paleoproductivity and tracing diagenesis in geological matrices. However, accurate analysis is hampered as chlorophyll readily suffers from demetallation and carbon-chain cleavage, leading to a complex mixture of modified Mg(II) porphyrins and porphins. Here, we developed a copper substitution pretreatment that selectively converts native chlorophyll, its derived Mg(II) porphyrins and porphins (chlorophyll-derived porphyrins) into stable Cu(II) porphyrins, thus enabling a comprehensive qualitative and quantitative analysis of the chlorophyll-derived pool. The method was evaluated using model compounds and ultra-performance liquid chromatography coupled with ultra-high-resolution mass spectrometry (UPLCOrbitrap MS). It demonstrated >95% conversion of chlorophyll a with CuCl2 and 89% for porphin with Cu sheets, whereas non-target Ni(II), V(IV), and Fe(III) porphyrins remained unaffected. Applied to lake sediments, the pretreatment enabled specific analysis of chlorophyll-derived porphyrins against complex geological matrix. This strategy unifies chlorophyll-derived porphyrins into a single detectable form, offering a selective approach for the accurate reconstruction of initial chlorophyll input and composition in geological samples.