CuFe₂O₄ magnetic nanocrystal clusters as a matrix for the analysis of small molecules by negative-ion matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

CuFe₂O₄ magnetic nanocrystal clusters (CuFe₂O₄ MNCs) were proposed as a new matrix for small molecule analysis by negative ion matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) for the first time. We demonstrated its advantages over conventional organic matrices in the detection of small molecules such as amino acids, peptides, nucleobases, fatty acids, and steroid hormones. A systematic comparison of CuFe₂O₄ MNCs with different ionization modes revealed that MS spectra obtained for the CuFe₂O₄ MNC matrix in the negative ion mode was only featured by deprotonated ion peaks with a free matrix background, which was different from the complicated alkali metal adducts produced in the positive ion mode. The developed method was found relatively tolerant to salt contamination and exhibited good reproducibility. A detection limit down to the subpicomolar level was achieved when testosterone was analyzed. In addition, by comparison of the MS spectra obtained from bare Fe₃O₄ and MFe₂O₄ MNC (M = Co, Ni, Cu, Zn) matrices, two main factors of MFe₂O₄ MNC matrices were revealed to play a vital role in assisting the negative ion desorption/ionization (D/I) process: doping transition metals into ferrite nanocrystals favoring laser absorption and energy transfer and a good match between the UV absorption of MFe₂O₄ MNCs and the excitation of nitrogen laser source facilitating LDI efficiency. This work creates a new branch of application for MFe₂O₄ MNCs and provides an alternative solution for small molecule analysis.