N-Butyl-4-hydroxy-1,8-naphthalimide: A new matrix for small molecule analysis by matrix-assisted laser desorption/ionization mass spectrometry

Rationale: The matrix plays an essential role in defining detection limits and ionization yields of analytes in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) analysis. Small molecule MALDI-MS analyses commonly suffer from the high background interference generated from matrices. Moreover, the inhomogeneous crystallization of some matrices, such as 2,5-dihydroxybenzoic acid (DHB), is to the detriment of the quality or repeatability of detection. We have found that N-butyl-4-hydroxy-1,8-naphthalimide (BHN) can provide improved performance as a matrix for small molecule analysis. Methods: BHN was evaluated in the low-mass region for its ionization efficiency, repeatability and background interference using O-acetyl-L-carnitine hydrochloride, A beta 35-40, A beta 35-42, and oxytocin as the model analytes. In addition, the modification effects of BHN on DHB were investigated for the in situ analysis of endogenous compounds in rat brain slices using Fourier transform ion cyclotron resonance (FTICR)-MS. Results: BHN is capable of ionizing small molecules, including O-acetyl-L-carnitine hydrochloride and peptides, with high repeatability and low background interference signals. A low concentration of BHN (3 mM) modifies the crystallization state of DHB but still retains its ionization performance. The determination of small molecules desorbed from tissue slices was significantly improved by using a binary matrix of DHB and BHN, yielding superior signal-to-noise ratio and signal intensities. Conclusions: The new matrix BHN has exhibited suitability for the analysis of small molecules. Compared with the conventional matrices, CHCA and DHB, BHN provides a clean background in the low-mass region. In combination with DHB, the ability of BHN to form highly homogenous crystalline particles shows the clear beneficial effects of BHN for the reproducibility of MS detection.