Quantitative Mass Spectrometry Imaging Protocols for Spatially Heterogeneous Samples

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) is a promising tool for the spatial quantitation of endogenous and exogenous compounds directly in biological tissue sections. However, precise quantitation may be hampered due to matrix effects and variations in ionization efficiency, especially in spatially heterogeneous samples such as brain tissue. In this study, we developed and implemented two advanced MALDI-MSI protocols to address these limitations by employing a standard addition approach. The protocols involved the homogeneous spraying of standard solutions onto tissue sections to minimize the matrix effects associated with heterogeneous samples. The first method utilized spraying of deuterated analogues of neurotransmitters across all tissue sections for normalization, while calibration standards were applied in a quantitative manner to consecutive tissue sections. The second method employed two stable isotope-labeled compounds: one for calibration and the other for normalization. Both methods were applied to quantify neurotransmitters and their metabolites, e.g., dopamine, norepinephrine, and 3-methoxytyramine, in rodent brain tissue. The results showed strong linearity between signal intensities and analyte concentrations across brain tissue sections with values comparable to those obtained using high-performance liquid chromatography-electrochemical detection. The standard addition approach significantly enhanced the quantitation accuracy by accounting for tissue-specific matrix effects, providing a robust method for the spatial quantification of neurotransmitters in complex brain tissue environments.